WO2021195692A1 - A bi-directional ac-dc power converter – inverter - Google Patents

A bi-directional ac-dc power converter – inverter Download PDF

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Publication number
WO2021195692A1
WO2021195692A1 PCT/AU2021/050260 AU2021050260W WO2021195692A1 WO 2021195692 A1 WO2021195692 A1 WO 2021195692A1 AU 2021050260 W AU2021050260 W AU 2021050260W WO 2021195692 A1 WO2021195692 A1 WO 2021195692A1
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Prior art keywords
node
directional
input
output
module
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PCT/AU2021/050260
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French (fr)
Inventor
Swamidoss Sathiakumar
Yiyang LI
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The University Of Sydney
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Priority claimed from AU2020900963A external-priority patent/AU2020900963A0/en
Application filed by The University Of Sydney filed Critical The University Of Sydney
Publication of WO2021195692A1 publication Critical patent/WO2021195692A1/en

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Classifications

    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M3/00—Conversion of dc power input into dc power output
    • H02M3/02—Conversion of dc power input into dc power output without intermediate conversion into ac
    • H02M3/04—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters
    • H02M3/10—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M3/145—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
    • H02M3/155—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
    • H02M3/156—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only with automatic control of output voltage or current, e.g. switching regulators
    • H02M3/158—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only with automatic control of output voltage or current, e.g. switching regulators including plural semiconductor devices as final control devices for a single load
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M3/00—Conversion of dc power input into dc power output
    • H02M3/02—Conversion of dc power input into dc power output without intermediate conversion into ac
    • H02M3/04—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters
    • H02M3/10—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M3/145—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
    • H02M3/155—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J7/00—Circuit arrangements for charging or depolarising batteries or for supplying loads from batteries
    • H02J7/02—Circuit arrangements for charging or depolarising batteries or for supplying loads from batteries for charging batteries from ac mains by converters
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J7/00—Circuit arrangements for charging or depolarising batteries or for supplying loads from batteries
    • H02J7/34—Parallel operation in networks using both storage and other dc sources, e.g. providing buffering
    • H02J7/35—Parallel operation in networks using both storage and other dc sources, e.g. providing buffering with light sensitive cells
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/0067—Converter structures employing plural converter units, other than for parallel operation of the units on a single load
    • H02M1/007—Plural converter units in cascade
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M3/00—Conversion of dc power input into dc power output
    • H02M3/02—Conversion of dc power input into dc power output without intermediate conversion into ac
    • H02M3/04—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters
    • H02M3/10—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M3/145—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
    • H02M3/155—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
    • H02M3/156—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only with automatic control of output voltage or current, e.g. switching regulators
    • H02M3/158—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only with automatic control of output voltage or current, e.g. switching regulators including plural semiconductor devices as final control devices for a single load
    • H02M3/1584—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only with automatic control of output voltage or current, e.g. switching regulators including plural semiconductor devices as final control devices for a single load with a plurality of power processing stages connected in parallel
    • H02M3/1586—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only with automatic control of output voltage or current, e.g. switching regulators including plural semiconductor devices as final control devices for a single load with a plurality of power processing stages connected in parallel switched with a phase shift, i.e. interleaved
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
    • H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
    • H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/53—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
    • H02M7/537—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters
    • H02M7/5387—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H02M7/66—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal
    • H02M7/68—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters
    • H02M7/72—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/79—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
    • H02M7/797—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/28—Arrangements for balancing of the load in a network by storage of energy
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/0067—Converter structures employing plural converter units, other than for parallel operation of the units on a single load
    • H02M1/0074—Plural converter units whose inputs are connected in series
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy
    • Y02E10/56—Power conversion systems, e.g. maximum power point trackers

Definitions

  • the present invention relates to power converters and inverters, in particular with bi directional operation.
  • the present invention has been developed primarily as a bi-directional power converter-inverter for use in a bi-directional multi-mode power flow system for interfacing renewable energy sources and energy storage elements such as batteries with a conventional AC power grid.
  • renewable energy sources and energy storage elements such as batteries with a conventional AC power grid.
  • the invention is not limited to this particular field of use and may also be applicable to other fields, for example island mode power systems, electric vehicles and the like.
  • PV photovoltaic
  • PV panels are direct current (DC) and at voltages much lower than a conventional main grid.
  • the mains grid is also alternating current (AC).
  • solar energy is not dispatchable, rather solar power generation fluctuates throughout the day and with changing weather conditions.
  • a conventional boost converter may comprise of a single inductor, diode and switch. Whilst low in cost and simple in design it cannot satisfy modern industrial and domestic demands due to its limited voltage gain and efficiency as a function of duty cycle of the converter operation. When the duty cycle increases, the portion of the power transferred to the load side increases. Therefore, when the duty cycle is too large, the loss on each component will increase and this will lead to a decrease in the efficiency of the converter. Cascading multiple converters in series or parallel is often done to overcome these deficiencies but this may be expensive and introduces switching interference. Topologies with switched capacitors, multiple input inductors or which use diodes to switch operation modes of inductors cause power loss across such further components. To improve voltage gain, the component count of the power converter typically is increased.
  • a bi-directional power converter for interfacing an energy storage element between a DC input side and a DC output side, the converter including: a reference node and an input node for receiving an input voltage potential; a boost node, a primary node, a bi-directional node, a secondary node, and an output node for interfacing with an output voltage potential; an input inductor connected between the input node and the boost node; an input diode connected between the boost node and the primary node; a parallel diode connected between the boost node and the bi directional node; the energy storage element connected between the primary node and the reference node; a primary inductor connected between the primary node and the bi-directional node; an output diode connected between the bi-directional node and the secondary node; a secondary inductor connected between the secondary node and the output node; a boost switch connected between the bi-directional node and the reference node; and a bi-directional switch
  • the energy storage element includes at least one of a battery and a capacitor. More preferably the energy storage element is a battery with a capacitor connected in parallel.
  • the bi-directional power converter preferably further includes an input switch connected between the boost node and the reference node, and a parallel switch for selectively disconnecting the parallel diode from the input node, wherein the input switch is operable to track a maximum power point of a photovoltaic panel across the input node and the reference node.
  • the bi-directional power converter Preferably varying the coupling between the primary inductor and the secondary conductor varies at least one of the boost to the input voltage potential and the buck to a voltage potential applied to the output node.
  • the bi-directional power converter preferably further includes an H-bridge connected between the output node and the reference node to interface the DC output side with an AC grid by selectively inverting the DC output voltage potential or to rectifying an AC grid voltage potential.
  • the bi-directional power converter further includes filter capacitors between the input node and the reference node and between the output node and reference to node to respectively filter the input voltage and output voltage.
  • a bi-directional power flow system for interfacing an energy storage module with a direct current (DC) input module and a DC output module, the system including: the DC input module for receives a DC input voltage potential; the energy storage module is coupled to the DC input module; the DC output module for interfaces with a DC output voltage potential; a converter module for interfacing the energy storage module to the DC output module, the converter module including a pair of magnetically coupled inductors and a first switching arrangement; and a bi directional module interfaced with the DC output module, the converter module and the energy storage module, the bi-directional module including a second switching arrangement; wherein the first switching arrangement is operable to step the DC input voltage potential and the energy storage module voltage potential up to the DC output voltage potential at the DC output module; and wherein the second switching arrangement is operable to step an applied DC voltage potential at the DC output module down to the voltage potential of the energy storage module.
  • DC direct current
  • the DC input module includes a third switching arrangement operable to track a maximum power point of a source of the DC voltage potential.
  • the first switching arrangement is preferably interfaced with the third switching arrangement.
  • the bi-directional power flow system further includes a DC-AC (alternating current) inverter and rectifier module, wherein the DC-AC inverter and rectifier module is coupled with the DC output module.
  • DC-AC alternating current
  • FIGURE 1 is a concept diagram of a PV panel interfaced with the AC grid by a bi directional charging system.
  • FIGURE 2 is a functional modules diagram of a bi-directional power converter- inverter according to the invention.
  • FIGURE 3 is a schematic circuit diagram of a bi-directional power converter according to the invention.
  • FIGURE 4 is a schematic of a function and control scheme diagram of a simulation of the bi-directional converter of FIGURE 3 with a single phase inverter.
  • FIGURE 5 is a schematic of the output side voltage waveform toward the boost converter operation of FIGURES 3 and 4.
  • FIGURE 6 is a schematic of the 240Vrms 50Hz voltage waveform output of the single phase inverter from the output of the bi-directional converter of FIGURE 5.
  • FIGURE 7 is a schematic of a function and control scheme diagram for the single phase inverter rectification of 240Vrms AC to DC.
  • FIGURE 8 is a voltage waveform of a 240Vrms AC mains voltage.
  • FIGURE 9 is voltage waveforms of a rectified 340V DC voltage.
  • FIGURE 10 is a schematic of a function and control scheme diagram of the bi directional converter of FIGURE 3.
  • FIGURE 11 is a schematic diagram of the voltage waveform across a battery during charging.
  • FIGURE 12 is a schematic circuit diagram of another bi-directional converter according to the invention with another inverter.
  • FIGURE 13 is a schematic of a function and control scheme diagram of the bi directional converter and inverter of FIGURE 12
  • FIGURE 14 is a table of the charging current of a battery and the power drawn from a PV panel during MPPT tracking with the bi-directional converter of FIGURE 12 or 13.
  • FIGURE 15 is a schematic diagram of the voltage waveform of the switching signal and duty cycle of a boost switch of a bi-directional converter of FIGURE 12 or 13.
  • FIGURE 16 is a schematic diagram of the voltage waveform of the potential across the output of the bi-directional converter of FIGURE 12 or 13.
  • FIGURE 17 is a schematic diagram of the voltage waveform of the AC voltage potential across the load of the inverter of FIGURE 12 or 13.
  • FIGURE 18(a) is a schematic diagram of the waveforms of the current flowing from the solar panel via the input diode and the current flowing from the AC side via the primary inductor for a bi-directional switch duty cycle of 15%.
  • FIGURE 18(b) is a schematic diagram of the waveforms of the current flowing from the solar panel via the input diode and the current flowing from the AC side via the primary inductor for a bi-directional switch duty cycle of 20%.
  • FIGURE 1 is the “100” series
  • FIGURE 2 is the “200” series and so on.
  • like features across circuit topologies FIGURES 3 and 12, for example, may be indicated by like references.
  • battery 324 in FIGURE 3 is also a battery 1224 in FIGURE 12.
  • FIGURE 1 is a schematic block diagram of a bi-directional power flow system, according to the invention. It features a bi-directional power converter 110 interfaced with a photovoltaic (PV) panel 112 to drive DC loads and / or to also interface with a AC mains grid 116 via a bi-directional inverter/rectifier 118. Alternatively, the bi-directional converter and inverter may be used to drive AC loads 120. Furthermore, the system includes a battery 122 to store excess energy for later use, when demand requires it.
  • the bi-directional charging capability allows energy to be stored in a battery from the PV panel 112 during the day as well as from the AC grid 116. As energy prices are generally cheaper during the day, storing electrical energy from the AC grid 116 during the day, for later evening use when demand requires, is a cheap and efficient utilisation of energy.
  • AC mains or utility grid 116 may also include a local or domestic AC grid. For example, to a suburban street, apartment block or an individual house.
  • FIGURE 2 is a schematic diagram to the functional modules of the bi-directional converter and inverter power system of FIGURE 1. The circuit diagrams and control schemes are described with respect to FIGURES 3 to 11 and 12 to 17.
  • the bi-directional converter 110 includes a DC input module 210 suitable for receiving a PV panel 112.
  • the DC input module 210 is connected to a MPPT module 212 to extract a maximum amount of power from the PV panel 112.
  • the MPPT module 212 is connected to an energy storage element 214 which is preferably a battery or battery bank in parallel with a capacitor.
  • the energy storage element 214 is connected, via a boosting module 216, to a DC output section 218.
  • the DC input module 210 is interleaved or interfaced with the energy storage element 214 and the boosting module 216 so as to form a boost converter module 220 for boosting an input voltage potential to the output section 210 with a high step-up gain ratio and efficient power transfer.
  • the DC output section 218 may be connected to DC loads to drive them with the boosted input voltage. Additionally, the DC output section may be connected to a bi-directional DC/AC module 222 suitable for inverting DC power and rectifying AC power as required. The DC/AC module 222 is connected to an AC output section 224 for driving AC loads 120 and to interface with the AC grid 116.
  • a bi-directional switching arrangement 226 is interfaced with the output section 218 and the boosting module 220 to form a bi-directional module 228 for converting power from the output section 218 to the energy storage element 214.
  • the bi-directional module 228 can step down an AC grid voltage rectified via the bi-directional DC/ AC module 222 to charge the energy storage element.
  • the bi-directional module 228 may charge the energy storage element via the external DC power source.
  • FIGURE 3 is a schematic circuit diagram to a bi-directional power converter 310.
  • the converter 310 converts an input DC potential or voltage source 312 on a first input side 314 of the converter to a different potential or voltage at a potential output 318.
  • the input to output voltages 312, 318 may be stepped up or stepped down, that is the bi-directional converter 310 may operate as a boost converter or a buck converter.
  • the converter 310 also stores electrical energy in and discharges energy from an energy storage element 322 which is preferably a battery 324.
  • the energy storage element 322 may also include a high energy capacity capacitor 326 in parallel with the battery.
  • the provision of battery as opposed to the conventional capacitor enables long term high capacity energy storage.
  • the additional capacitor 358 in parallel with the battery 324 improves the response time of the energy storage element allowing rapid charging and discharging during a boosting operation of the converter.
  • the bi-directional converter 310 has three degrees of freedom for control. These are provided by a bi-directional loop switch 356 (S2), a boost converter switch 352 (S1) and varying a coupling between a primary inductor 340 (Lp) and a secondary inductor 348 (Ls). Accordingly, the bi-directionality of the converter 310 allows in the first direction the energy storage element to be charged from a first power source such as a solar panel 112 at a first input side 314 or in the second direction from a second power source 318 derived from the AC grid 120 applied to the second input side 320.
  • the bi directional converter may be switched or re-configured in operation to receive the second power source via the output side 320 alternatively operating as a second input 320.
  • the first input voltage 312 is connected across an input node 328 and a reference node 330.
  • An input inductor 332 (L1) connects the input node 328 to a boost node 334 which is connected to a primary node 336 by an input diode 338 (D1).
  • the energy storage element 322 is connected across the primary node 336 and the reference node 330.
  • a primary inductor 340 (Lp) is connected between the primary node 336 and a bi directional node 342 which is in turn connected to a secondary node 344 by an output diode 346 (D3).
  • a secondary inductor 348 (LS) is connected between the secondary node 344 and an output node 350.
  • the output voltage 318 is connected across the output node 350 and the reference node 330.
  • the output node 350 and the reference node 330 also correspond to the output voltage potential 318 or output side 320 of the bi-directional converter 310 operating in the first direction or first mode of operation.
  • the reference node 330 may also be referred to as the ground rail 330 when extending to the input side 314 of the bi-directional converter 310.
  • the primary inductor 340 (Lp) is magnetically coupled with the secondary inductor 348 (Ls) to form a transformer.
  • each inductor is coiled around a common ferrite or other magnetisable core to form the transformer.
  • the inductance of each inductor and the corresponding properties of the core, if used, in the converter 310 is preferably selected to ensure continuous current mode (CCM) operation under various load conditions.
  • a first switch also referred to as a boost switch 352 (S1), is connected between the bi-directional node 342 and the reference node 330 to selectively short the primary inductor 340 across the energy storage element 322 to inductively charge the primary inductor which in turn induces a charge in or induces a voltage potential across the secondary inductor 348.
  • S1 boost switch 352
  • a parallel diode 354 (D2) also connects the boost node 334 to the bi-directional node 342 allowing the input inductor 332 (L1) to be selectively shorted across the input node 328 and the ground rail reference node 330 upon pulse width modulation operation of the boosting switch 352 (S1 ) to thereby charge the input inductor 332 (L1 ) via the input voltage 312.
  • This first configuration enables the bi-directional converter 310 in the first direction mode to efficiently boost the combined voltage potentials of the energy storage element 322 and the first input voltage / source 312, 314 with a high gain when boost converting or stepping up the voltage potential to the output side 320.
  • excess energy from first input voltage / source 312, 314 can be stored in the energy storage element 322, 324 and, in particular, used to charge the battery 324.
  • respective filter capacitors 358, 360 are respectively connected in parallel with the first input 312, 314 and output / second input 318, 320 of the bi-directional converter 310.
  • Pulse width modulation operation of the boost switch 352 (S1) may be used to operate the bi-directional converter 310 to boost or step-up the voltage potential from the first power source of the input side 312, 314 and the energy storage element 322, 324 to drive a load at the output side 320 with a boosted voltage relative to the first input side 312, 314 and the energy storage element 322 such as the battery 324.
  • the voltage gain ratio from the first input side 312, 314 to the output side 318, 320 of the bi-directional converter 310 operating in the first direction mode may be defined as: [0063] Where V out is the output voltage, V in is the input voltage, N is the turns ratio of the two magnetically coupled inductors 340, 348 (Lp, Ls) and D is the pulse width modulation (PWM) duty cycle of the boosting switch as a percentage of the time it is on per switching period.
  • PWM pulse width modulation
  • the bi-directional converter 310 may be operated as a boost converter to draw power only from the battery 324 to drive a load at the output side 320 with a boosted voltage in comparison to a lower potential or voltage of the battery 324.
  • the voltage gain ratio from the battery 324 potential to the output side 318, 320 potential may be defined as:
  • the bi-directional converter 310 may also operate in a second direction or mode from a second power source 318 such as derived from an AC grid 116.
  • the bi-directional converter 310 may also be operated as a buck converter to provide a step-down voltage potential relative to the second power source 318 at the second input side 320.
  • the further functionality of the bi-directional converter is described in detail below.
  • a second switch also referred to as a bi-directional switch 356 (S2 of FIG. 3), is connected across the output diode 346 (D3) between the secondary node 344 and the bi directional node 342.
  • the bi-directional switch 356 (S2) enables a bi-directional loop circuit configuration 342, 356, 344 where a voltage potential as a second power source 318 applied to the previously termed output side 320 now operating as a second input side 320, can be converted in a step-down, buck conversion manner to charge the energy storage element 322, e.g. the battery 324.
  • the bi-directional switch 356 operation can be re-configured to operate the magnetically coupled inductors 340, 344 (Lp, Ls) in a buck converter or step-down voltage potential mode to charge the battery.
  • the use of the re-configured operation of the coupled inductors 340, 344 providing a substantial voltage potential step down ratio suitable for optimally charging the battery 624. Examples of the substantial step down ratio are given below with respect to FIGURES 8 to 11.
  • the voltage gain or step-down ratio from the second input side 320 to the battery 324 may be defined as:
  • bi-directional converter 310 is connected to a DC bus with a DC output voltage of 340VDC rectified from the AC grid.
  • Typical batteries for use with the bi-directional converter may range from 12V to 40V DC.
  • a large step down ratio can be provided.
  • the single, uni-directional converters of the prior art cannot provide in the one circuit device a high gain boost converter and then be re-configured in operation to be a buck converter with a large step-down ratio.
  • an energy storage element 322, 324 may also used advantageously with the bi-directional converter 310 as described herein.
  • FIGURE 4 is a schematic of a function and control scheme diagram of a simulation 410 of the bi-directional converter 310 of FIGURE 3 with a single phase inverter 412.
  • the simulation was done using Matlab / Simulink.
  • the inverter 412 is undertaken with the four switches 414, 416, 418, 420 (S1 to S4 of FIG. 4) shown in FIGURE 4.
  • a simulation was conducted to a solar PV array 112 as the first power source 312 at an unregulated nominal voltage of 12V with a battery 324 rated at 40V.
  • the bi-directional converter 310 was operated as a boost converter by controlling boost switch 352 and keeping the bi-directional switch 356 open.
  • FIGURE 5 is a schematic of the output side 318, 320 voltage waveform 510 to the boost converter 310 operation of FIGURES 3 and 4.
  • the vertical axis 512 full scale is 450V with increments of 50 V.
  • the horizontal time axis 514 has a full scale of 0.2s in increments of 0.02s.
  • the gain of the bi-directional converter in boost mode is at least 10 relative to the battery and at least 30 to the nominal voltage of the solar PV array 112. The gain may be varied by changing the pulse width modulation (PWM) of the driving signal to the boost switch 352 (S1).
  • PWM pulse width modulation
  • the gain may also be advantageously varied by varying the coupling of the primary and secondary inductors 340, 344 (Lp, Ls) as well as the inductance values of the individual inductors.
  • Maximum Power Point Tracking of the bi-directional converter 310 to a PV array 112 is detailed with respect to FIGURE 14.
  • FIGURE 6 is a schematic of the 240Vrms 50Flz voltage waveform 610 output of the single phase inverter 412 from the output 510, 320 of the bi-directional converter 310 of FIGURE 5.
  • the vertical axis 612 full scale is 400V with increments of 100V.
  • the horizontal time axis 614 has a full scale of 0.2s in increments of 0.02s.
  • the 240Vrms waveform is suitable for feeding into the mains grid 116.
  • the unique high gain is advantageous for interfacing low voltage PV panels and batteries with the relatively high voltage mains grid.
  • FIGURE 7 is a schematic of a function and control scheme diagram for the single phase inverter 412 rectifying the 240Vrms AC to DC suitable for the bi-directional converter 310.
  • the bi-directional converter 310 being operated in the buck converter / step-down mode as detailed with respect to the control scheme of FIGURE 10.
  • the rectification is achieved by operation of the switch pair S1 and S4 of the H-bridge of FIGURE 4 complementary to the switch pair S2 and S3.
  • the switching operation may be performed, for example, by 50kHz pulses with a 50% duty cycle wherein switches S1 and S4 have an inverse duty cycle to S2 and S3.
  • FIGURES 8 and 9 are respective voltage waveforms 810, 910 of the 240Vrms mains and the rectified 340VDC.
  • the vertical axis 612 of FIGURE 8 is as for FIGURE 6.
  • the vertical axis 912 for FIGURE 9 has a full scale of 500V with 100V increments.
  • the common horizontal time axis 914 has a full scale of 0.5 s in increments of 0.05s.
  • An initial voltage spike 916 from switching and inrush current to the inductors may be reduced by a soft starting procedure.
  • FIGURE 10 is a schematic of a function and control scheme diagram of the bi directional converter 310 of FIGURE 3 in the buck converter or step-down mode to charge the battery 324.
  • the bi-directional switch 356 is kept closed for the second mode of operation of the bi-directional converter.
  • the control scheme used is advantageously making use of both inputs 312, 320, respectively: the DC first input source 312 from for example a solar PV array 112 and the second input source 318 derived the utility grid 116.
  • the charging current to the battery 324 will be the sum of current from the input sources 312, 318.
  • the switching duty cycle of bi-directional switch 356 can be selectively operated to vary the current charging the battery from input source 318.
  • charging from input source 312 can operate independently in parallel. This is provided in part by input diode 338 and parallel diode 354 which block reversing current from input source 318 ensuring each charging operation can work independently.
  • FIGURE 11 is a schematic diagram of the voltage waveform 1110 across the battery 324 during charging from the two power sources 312, 318 of the first and second input sides 314, 320.
  • the vertical axis 1112 has a full scale of 80V with increments of 10V.
  • the horizontal time axis 1114 as a full scale of 0.2s with increments of 0.02s.
  • the bi-directional converter 310 is able to hold the terminals of a battery at a voltage just above the nominal battery voltage to charge the battery from both inputs simultaneously. From FIGURES 9 and 11 a substantial step-down ratio of approximately 9 is achieved and maintained. It will be readily appreciated that bi-directional converter 310 can also charge the battery 324 in the absence of either input source 312, 318.
  • FIGURE 12 is a schematic circuit diagram of another bi-directional converter 1210 with another inverter 1211.
  • the converter 1210 includes all the functionality of the bi directional converter 310 described with respect to FIGURES 3 to 11.
  • the bidirectional converter 1210 and inverter 1211 have additional features to interface a PV panel 1212, 112 panel to the AC grid 116.
  • the features of converter 1210 which correspond with those of the above bi-directional converter 310 of FIGURES 3 to 11 are correspondingly named.
  • the PV Panel 1212 is connected across the input node 1228 and the reference node 1230 as a DC power source 312 to provide a DC input voltage.
  • a further switch, referred to as an input switch 1216 (Q3) is connected between the boost node 1234 and the reference node 1230.
  • Another additional switch referred to as a parallel switch 1218 (Q4) is connected from the boost node 1234 to the parallel diode 1254 (D1 in FIG. 12) for selectively disconnecting the parallel diode 1254 (D1) from the boost node 1234.
  • FIGURE 13 is a schematic of a function and control scheme diagram of the bi directional converter 1210 and inverter 1211 of FIGURE 12.
  • FIGURE 13 also provides further details to the MPPT control of power transfer from the PV panel 1212 to the battery 1224 and bi-directional converter 1210.
  • the inventors constructed a prototype of the circuit of FIGURES 12 and 13.
  • the circuit component values were as follows:
  • circuit components may be selected by a person skilled in the art to provide a bi-directional converter 1210 operating to at least 1 kW.
  • MOSFET/IGBT switching devices may be used together with a control scheme developed with the MyRio processor kit by National Instruments.
  • the bi-directional converter 1210 of FIGURES 12 and 13 can step up the voltage of the PV Panel 1212 for MPPT and also invert the DC output voltage with the inverter 1211 to output an AC voltage.
  • the inverter 1211 being a H-bridge 1219 connected across the output node 1250 and reference node 1230 of the bi-directional converter 1210. Additionally, power from the 240V AC grid 116 may be rectified to 340V DC by the inverter / rectifier 1211 and stepped down by the bi-directional converter 1210 to be stored in the battery 1224.
  • FIGURE 14 is a table of the charging current of the battery 1224 and the power drawn from the unregulated PV panel 1212 with increasing light intensity on the PV panel 1212.
  • the circuit of FIGURES 12 and 13 in MPPT operation was used to provide the results in FIGURE 14.
  • the circuit was tested to the MPPT operation by the parallel switch 1218 (Q4 in FIG. 12), the boost switch 1252 (Q1 in FIG. 12) and the bi-directional switch 1256 (Q2 in FIG. 12) being kept open to isolate the battery 1224 from the AC source 116.
  • the input switch 1216 (Q3 in FIG. 12) is then operated to track a Maximum Power Point of the PV panel 1212 and to charge the battery 1224.
  • the table of FIGURE 14 shows: in the first column the battery charging current 1410, in the second column the voltage potential 1412 across the PV panel output, in the third column the power draw 1414 from the PV panel 1212 and in the fourth column the duty cycle 1416 of the input switch 1216 (Q3).
  • the results of the FIGURE 14 table show the duty cycle 1416 of the input switch 1216 (Q3) being controlled automatically to correspond with the increased insolation intensity onto the PV panel 1212.
  • a maximum power point for a light intensity 1418 is obtained at approximately 3.1W at a Q3, 1216 switch duty cycle of approximately 45%.
  • a PV panel operating at a maximum power point can be used to charge a battery with the present invention whilst also charging the battery via power from the AC source 116.
  • variations of the switching duty cycle of the bi-directional switch 1256 do not substantially impact the operation of PV panel charging.
  • the charging current to the battery 1224 will be the sum of current from the PV panel 1212 and from the rectified, stepped down AC grid 116.
  • the switching duty cycle of the input switch 1216 (Q3) is operated to drawn maximum power from the PV panel.
  • the switching duty cycle of bi directional switch 1256 (Q2) can be selectively operated to vary the current charging the battery from the AC grid 116 to supplement the charging current from the PV panel.
  • Charging from the PV panel 121 and the AC grid can operate independently in parallel. This is provided in part by input diode 1238 and parallel diode 1254 which block reversing current from the AC grid as well as by parallel switch 1218 (Q4) which selectively isolates the PV panel.
  • the inverter / rectifier 1211 has a H-bridge 1219 switching arrangement of two sets of series switches in parallel across the output node 1250 and reference node 1230 of the bi directional converter 1210 as shown in FIGURE 12. That is, a first set 1221 of series switches S1 , 1264 and S2, 1266 is in parallel to a second set 1223 of series switches S3, 1268, and S4, 1270. Between switches S1 and S2 the active terminal 1225 of the AC mains grid 116 is connected via a filter inductor 1227 (Lf). Between switches S3 and S4 the neutral terminal 1229 of the AC mains grid 116 is connected.
  • An AC filter capacitor 1262 (Cf) is connected across the active 1225 and neutral 1229 terminals to the AC mains grid 116.
  • the filter inductor 1227 (Lf) and AC filter capacitor 1262 (Cf) form a filter to achieve a smooth AC waveform when inverting a DC voltage to supply or feed into the AC grid 116.
  • the filter 1227, 1262 is configured to filter out frequencies other than 50Hz to 60Hz.
  • the bi-directional converter 1210 and inverter 1211 circuit of FIGURES 12 and 13 may also be configured to charge the battery 1224 from the AC mains grid source 116. Part of the bi-directional converter 1210 to the battery 1224 is used together with the filter capacitor 1260 (C2) and the H-bridge 1219 of the inverter / rectifier 1211. In use the switches S1 , 1264 and S4, 1270 of the H-bridge 1219 are turned ON and OFF together and S2, 1266 and S3, 1268 are switched complementary to S1 , 1264 and S4, 1270. The switching control signals are at a frequency of 50kHz with a 50% duty cycle.
  • the bi-directional converter 1210 and inverter 1211 can boost convert the voltage potential from the battery 1224 then invert it to AC to apply to a resistive load with a low pass filter, instead of the AC mains grid.
  • FIGURES 15 to 17 are to results where there is no DC input such as a PV panel or otherwise to the bi directional converter 1210.
  • the bi-directional converter 1210 and inverter 1211 of FIGURES 12 and 13 was tested using boost switch Q1 , 1252 with inverter switches, S1 , S2, S3 and S4, 1262-1270.
  • FIGURE 15 is a schematic diagram of the voltage waveform 1510 of the switching signal and duty cycle of boost switch Q1 , 1252.
  • the vertical voltage scale is in divisions of 2v and the horizontal time scale is in divisions of 10 ps.
  • FIGURE 16 is a schematic diagram of the voltage waveform 1610 of the potential across the output of the bi-directional converter at filter capacitor C2, 1260.
  • the vertical voltage scale is in divisions of 20v and the horizontal time scale is in divisions of 10ms.
  • FIGURE 17 is a schematic diagram of the voltage waveform 1710 of the AC voltage potential across the load of the inverter 1211.
  • the vertical voltage scale is in divisions of 20v and the horizontal time scale is in divisions of 10ms.
  • FIGURES 18(a) and 18(b) are schematic diagrams of the waveforms of the current flowing from the solar panel via the input diode D2 (1238) and the current flowing from the AC side via the primary inductor Lp (1240) to the battery 1224 for bi-directional converter of FIGURES 12 or 13.
  • the bi-directional switch Q2 (1256) has a duty cycle of approximately 15%.
  • the bi-directional switch Q2 duty cycle is approximately 20%.
  • the charging current of the battery 1224 is the sum of the currents through D2 1238 and Lp 1240. Each current varies differently during operations. It was found that the D2 current was a larger proportion of the battery charging current.
  • the nominal voltage of the battery is a lead-acid battery having a nominal voltage of between 12 and 14 volts.
  • the bi-directional converter is capable of boosting the battery voltage, via the magnetically coupled inductors, with a gain ratio of 3 resulting in the DC output voltage waveform 1610 of approximately 40 volts.
  • the bi-directional converter can transform the DC voltage to an AC voltage as displayed in Figure 17.
  • the AC output voltage waveform 1710 is a relatively smooth sinewave with only minor switching losses during inversion.
  • the converter advantageously includes three degrees of freedom for control, namely the boosting switching, bi-directional switching and coupling between inductors. These controllable degrees of freedom allow the converter to direct power bi-directionally across a wide variety of voltage gains and also facilitates MPPT of a PV panel and controlled battery charging from both the input and output sides.
  • the invention provides a high gain and high step- down ratio bi-directional power converter-inverter AC-DC power flow system with a minimum of components.
  • the minimum of components provides substantial advantages to a compact, versatile solution for interfacing a PV panel and battery with each other in island mode applications and / or additionally interfacing with the grid.
  • the minimum of components also provides significant cost and fabrication savings, essential in suburban, household and remote area power provision.
  • any one of the terms comprising, comprised of or which comprises is an open term that means including at least the elements/features that follow, but not excluding others.
  • the term comprising, when used in the claims should not be interpreted as being limitative to the means or elements or steps listed thereafter.
  • the scope of the expression a device comprising A and B should not be limited to devices consisting only of elements A and B.
  • Any one of the terms including or which includes or that includes as used herein is also an open term that also means including at least the elements/features that follow the term, but not excluding others. Thus, including is synonymous with and means comprising.
  • exemplary is used in the sense of providing examples, as opposed to indicating quality. That is, an “exemplary embodiment” is an embodiment provided as an example, as opposed to necessarily being an embodiment of exemplary quality.
  • Coupled when used in the claims, should not be interpreted as being limited to direct connections only.
  • the terms “coupled” and “connected,” along with their derivatives, may be used. It should be understood that these terms are not intended as synonyms for each other.
  • the scope of the expression a device A coupled to a device B should not be limited to devices or systems wherein an output of device A is directly connected to an input of device B. It means that there exists a path between an output of A and an input of B which may be a path including other devices or means.
  • Coupled may mean that two or more elements are either in direct physical or electrical contact, or that two or more elements are not in direct contact with each other but yet still co-operate or interact with each other.

Abstract

The present disclosure relates to power converters and inverters, in particular with bi-directional operation for use in a bi-directional multi-mode power flow system. A bi-directional power converter (110) may be interfaced with a photovoltaic (PV) panel (112) to drive DC loads and/or to also interface with an AC mains grid (116) and drive AC loads (120) via a bi-directional inverter/rectifier (118). Furthermore, the system includes a battery (122) to store excess energy. The bi-directional capability allows energy to be stored from the PV panel (112) as well as from the AC grid (116). The bi-directional power converter (110) may include several functional modules and switching arrangements interleaved together to provide the bi-directional functionalities, to provide Maximum Power Point Tracking (MPPT) of the PV panel (112), to boost the panel voltage to suitable levels to drive DC loads and to interface with the AC grid (116).

Description

A BI-DIRECTIONAL AC-DC POWER CONVERTER - INVERTER
FIELD OF THE INVENTION
[0001] The present invention relates to power converters and inverters, in particular with bi directional operation.
[0002] The present invention has been developed primarily as a bi-directional power converter-inverter for use in a bi-directional multi-mode power flow system for interfacing renewable energy sources and energy storage elements such as batteries with a conventional AC power grid. However, it will be appreciated that the invention is not limited to this particular field of use and may also be applicable to other fields, for example island mode power systems, electric vehicles and the like.
DISCUSSION OF THE PRIOR ART
[0003] Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.
[0004] With the increasing shortage of traditional fossil fuels and the environmental impact of their consumption there is an increasing need for energy alternatives to bolster power generation and reduce pollution. A viable alternative energy source is from the sun, using photovoltaic (PV) solar panels to convert solar energy to electric energy. Solar energy is also widely available in urban areas as well as in areas where the mains power grid does not extend or is unreliable.
[0005] However, the output of PV panels is direct current (DC) and at voltages much lower than a conventional main grid. The mains grid is also alternating current (AC). Additionally, solar energy is not dispatchable, rather solar power generation fluctuates throughout the day and with changing weather conditions.
[0006] Accordingly, in order to fully utilise solar energy, power converters, such as a conventional boost converter, are needed to boost the PV panel output voltage. Additionally, the DC output voltage must be inverted to an AC voltage before being connecting or otherwise feeding into the grid. Furthermore, in order to match demand with the fluctuating solar power generation it is desirable that excess energy from the PV panel be stored for later use, generally with a battery system.
[0007] A conventional boost converter may comprise of a single inductor, diode and switch. Whilst low in cost and simple in design it cannot satisfy modern industrial and domestic demands due to its limited voltage gain and efficiency as a function of duty cycle of the converter operation. When the duty cycle increases, the portion of the power transferred to the load side increases. Therefore, when the duty cycle is too large, the loss on each component will increase and this will lead to a decrease in the efficiency of the converter. Cascading multiple converters in series or parallel is often done to overcome these deficiencies but this may be expensive and introduces switching interference. Topologies with switched capacitors, multiple input inductors or which use diodes to switch operation modes of inductors cause power loss across such further components. To improve voltage gain, the component count of the power converter typically is increased.
[0008] Commercially available solar power converter-inverters are able to step-up the voltage of PV panels, perform Maximum Power Point Tracking (MPPT) to draw maximum power from the panel, and invert the panel output into AC before injecting power to the grid. In order to match demand, conventional systems store the excess power generated from the PV panel throughout the day in battery storage systems.
[0009] These conventional systems, whilst able to interface PV panels with the grid and store excess solar energy are not capable of drawing power from the grid to charge their battery systems, for example when grid energy prices are low or “off-peak”. Additionally, conventional converters and inverters cannot satisfy modern industrial and domestic demands due to their limited voltage gain. Attempts to alleviate these shortcomings by cascading converters, introduces losses and inefficiencies due to the excess components and interference.
[0010] Accordingly, there remains a need for a compact, efficient and high gain bi directional power converter and inverter arrangement that can interface a PV panel to the main grid with a battery or other electrical energy storage backup.
SUMMARY OF THE INVENTION
[0011] It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative. [0012] It is an object of preferred embodiments of the present invention to provide a high gain bi-directional power converter with minimum components and to provide a bidirectional AC-DC power flow system.
[0013] According to one aspect of the present invention there is provided a bi-directional power converter for interfacing an energy storage element between a DC input side and a DC output side, the converter including: a reference node and an input node for receiving an input voltage potential; a boost node, a primary node, a bi-directional node, a secondary node, and an output node for interfacing with an output voltage potential; an input inductor connected between the input node and the boost node; an input diode connected between the boost node and the primary node; a parallel diode connected between the boost node and the bi directional node; the energy storage element connected between the primary node and the reference node; a primary inductor connected between the primary node and the bi-directional node; an output diode connected between the bi-directional node and the secondary node; a secondary inductor connected between the secondary node and the output node; a boost switch connected between the bi-directional node and the reference node; and a bi-directional switch connected between the secondary node and the bi-directional node; wherein the primary inductor is magnetically coupled with the secondary inductor; wherein the boost switch is operable to selectively short the input inductor across the input voltage and to selectively short the primary inductor across the energy storage element, to boost the input voltage potential to the output node; and wherein the bi-directional switch is operable to selectively bypass the output diode and to alternatively buck the output voltage from the output node to the energy storage element.
[0014] Preferably the energy storage element includes at least one of a battery and a capacitor. More preferably the energy storage element is a battery with a capacitor connected in parallel.
[0015] The bi-directional power converter preferably further includes an input switch connected between the boost node and the reference node, and a parallel switch for selectively disconnecting the parallel diode from the input node, wherein the input switch is operable to track a maximum power point of a photovoltaic panel across the input node and the reference node.
[0016] Preferably varying the coupling between the primary inductor and the secondary conductor varies at least one of the boost to the input voltage potential and the buck to a voltage potential applied to the output node. [0017] The bi-directional power converter preferably further includes an H-bridge connected between the output node and the reference node to interface the DC output side with an AC grid by selectively inverting the DC output voltage potential or to rectifying an AC grid voltage potential.
[0018] Preferably the bi-directional power converter further includes filter capacitors between the input node and the reference node and between the output node and reference to node to respectively filter the input voltage and output voltage.
[0019] According to another aspect of the present invention there is provided a bi-directional power flow system for interfacing an energy storage module with a direct current (DC) input module and a DC output module, the system including: the DC input module for receives a DC input voltage potential; the energy storage module is coupled to the DC input module; the DC output module for interfaces with a DC output voltage potential; a converter module for interfacing the energy storage module to the DC output module, the converter module including a pair of magnetically coupled inductors and a first switching arrangement; and a bi directional module interfaced with the DC output module, the converter module and the energy storage module, the bi-directional module including a second switching arrangement; wherein the first switching arrangement is operable to step the DC input voltage potential and the energy storage module voltage potential up to the DC output voltage potential at the DC output module; and wherein the second switching arrangement is operable to step an applied DC voltage potential at the DC output module down to the voltage potential of the energy storage module.
[0020] Preferably the DC input module includes a third switching arrangement operable to track a maximum power point of a source of the DC voltage potential.
[0021] The first switching arrangement is preferably interfaced with the third switching arrangement.
[0022] Preferably the bi-directional power flow system further includes a DC-AC (alternating current) inverter and rectifier module, wherein the DC-AC inverter and rectifier module is coupled with the DC output module.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:
[0024] FIGURE 1 is a concept diagram of a PV panel interfaced with the AC grid by a bi directional charging system.
[0025] FIGURE 2 is a functional modules diagram of a bi-directional power converter- inverter according to the invention.
[0026] FIGURE 3 is a schematic circuit diagram of a bi-directional power converter according to the invention.
[0027] FIGURE 4 is a schematic of a function and control scheme diagram of a simulation of the bi-directional converter of FIGURE 3 with a single phase inverter.
[0028] FIGURE 5 is a schematic of the output side voltage waveform toward the boost converter operation of FIGURES 3 and 4.
[0029] FIGURE 6 is a schematic of the 240Vrms 50Hz voltage waveform output of the single phase inverter from the output of the bi-directional converter of FIGURE 5.
[0030] FIGURE 7 is a schematic of a function and control scheme diagram for the single phase inverter rectification of 240Vrms AC to DC.
[0031] FIGURE 8 is a voltage waveform of a 240Vrms AC mains voltage.
[0032] FIGURE 9 is voltage waveforms of a rectified 340V DC voltage.
[0033] FIGURE 10 is a schematic of a function and control scheme diagram of the bi directional converter of FIGURE 3.
[0034] FIGURE 11 is a schematic diagram of the voltage waveform across a battery during charging.
[0035] FIGURE 12 is a schematic circuit diagram of another bi-directional converter according to the invention with another inverter.
[0036] FIGURE 13 is a schematic of a function and control scheme diagram of the bi directional converter and inverter of FIGURE 12
[0037] FIGURE 14 is a table of the charging current of a battery and the power drawn from a PV panel during MPPT tracking with the bi-directional converter of FIGURE 12 or 13.
[0038] FIGURE 15 is a schematic diagram of the voltage waveform of the switching signal and duty cycle of a boost switch of a bi-directional converter of FIGURE 12 or 13.
[0039] FIGURE 16 is a schematic diagram of the voltage waveform of the potential across the output of the bi-directional converter of FIGURE 12 or 13. [0040] FIGURE 17 is a schematic diagram of the voltage waveform of the AC voltage potential across the load of the inverter of FIGURE 12 or 13.
[0041] FIGURE 18(a) is a schematic diagram of the waveforms of the current flowing from the solar panel via the input diode and the current flowing from the AC side via the primary inductor for a bi-directional switch duty cycle of 15%.
[0042] FIGURE 18(b) is a schematic diagram of the waveforms of the current flowing from the solar panel via the input diode and the current flowing from the AC side via the primary inductor for a bi-directional switch duty cycle of 20%.
[0043] In the figures the reference numerals are prefixed by the figure number. For example, FIGURE 1 is the “100” series, FIGURE 2 is the “200” series and so on. In addition, like features across circuit topologies FIGURES 3 and 12, for example, may be indicated by like references. For example, battery 324 in FIGURE 3 is also a battery 1224 in FIGURE 12.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0044] FIGURE 1 is a schematic block diagram of a bi-directional power flow system, according to the invention. It features a bi-directional power converter 110 interfaced with a photovoltaic (PV) panel 112 to drive DC loads and / or to also interface with a AC mains grid 116 via a bi-directional inverter/rectifier 118. Alternatively, the bi-directional converter and inverter may be used to drive AC loads 120. Furthermore, the system includes a battery 122 to store excess energy for later use, when demand requires it. The bi-directional charging capability allows energy to be stored in a battery from the PV panel 112 during the day as well as from the AC grid 116. As energy prices are generally cheaper during the day, storing electrical energy from the AC grid 116 during the day, for later evening use when demand requires, is a cheap and efficient utilisation of energy.
[0045] It will be readily appreciated that AC mains or utility grid 116 may also include a local or domestic AC grid. For example, to a suburban street, apartment block or an individual house.
[0046] In order to facilitate Maximum Power Point Tracking (MPPT) of the PV panel 112, to boost the panel voltage to suitable levels to drive DC loads 114 and to interface with the AC grid 116, the converter 110 may include several functional modules that may be interleaved together. [0047] FIGURE 2 is a schematic diagram to the functional modules of the bi-directional converter and inverter power system of FIGURE 1. The circuit diagrams and control schemes are described with respect to FIGURES 3 to 11 and 12 to 17. The bi-directional converter 110 includes a DC input module 210 suitable for receiving a PV panel 112. The DC input module 210 is connected to a MPPT module 212 to extract a maximum amount of power from the PV panel 112. The MPPT module 212 is connected to an energy storage element 214 which is preferably a battery or battery bank in parallel with a capacitor.
[0048] The energy storage element 214 is connected, via a boosting module 216, to a DC output section 218. The DC input module 210 is interleaved or interfaced with the energy storage element 214 and the boosting module 216 so as to form a boost converter module 220 for boosting an input voltage potential to the output section 210 with a high step-up gain ratio and efficient power transfer.
[0049] The DC output section 218 may be connected to DC loads to drive them with the boosted input voltage. Additionally, the DC output section may be connected to a bi-directional DC/AC module 222 suitable for inverting DC power and rectifying AC power as required. The DC/AC module 222 is connected to an AC output section 224 for driving AC loads 120 and to interface with the AC grid 116.
[0050] For an alternate mode of operation, a bi-directional switching arrangement 226 is interfaced with the output section 218 and the boosting module 220 to form a bi-directional module 228 for converting power from the output section 218 to the energy storage element 214. For example, the bi-directional module 228 can step down an AC grid voltage rectified via the bi-directional DC/ AC module 222 to charge the energy storage element. Alternatively, in the case of an external DC power source being directly connected to the DC output section 218, such as via a DC bus, the bi-directional module 228 may charge the energy storage element via the external DC power source.
[0051] FIGURE 3 is a schematic circuit diagram to a bi-directional power converter 310. The converter 310 converts an input DC potential or voltage source 312 on a first input side 314 of the converter to a different potential or voltage at a potential output 318. The input to output voltages 312, 318 may be stepped up or stepped down, that is the bi-directional converter 310 may operate as a boost converter or a buck converter. The converter 310 also stores electrical energy in and discharges energy from an energy storage element 322 which is preferably a battery 324. Alternatively, the energy storage element 322 may also include a high energy capacity capacitor 326 in parallel with the battery. The provision of battery as opposed to the conventional capacitor enables long term high capacity energy storage. The additional capacitor 358 in parallel with the battery 324improves the response time of the energy storage element allowing rapid charging and discharging during a boosting operation of the converter.
[0052] Also, advantageously, the bi-directional converter 310 has three degrees of freedom for control. These are provided by a bi-directional loop switch 356 (S2), a boost converter switch 352 (S1) and varying a coupling between a primary inductor 340 (Lp) and a secondary inductor 348 (Ls). Accordingly, the bi-directionality of the converter 310 allows in the first direction the energy storage element to be charged from a first power source such as a solar panel 112 at a first input side 314 or in the second direction from a second power source 318 derived from the AC grid 120 applied to the second input side 320. Advantageously, the bi directional converter may be switched or re-configured in operation to receive the second power source via the output side 320 alternatively operating as a second input 320.
[0053] In the first mode of operation or the first direction of use of the bi-directional converter 310, the first input voltage 312 is connected across an input node 328 and a reference node 330. An input inductor 332 (L1) connects the input node 328 to a boost node 334 which is connected to a primary node 336 by an input diode 338 (D1). The energy storage element 322 is connected across the primary node 336 and the reference node 330.
[0054] A primary inductor 340 (Lp) is connected between the primary node 336 and a bi directional node 342 which is in turn connected to a secondary node 344 by an output diode 346 (D3). A secondary inductor 348 (LS) is connected between the secondary node 344 and an output node 350. The output voltage 318 is connected across the output node 350 and the reference node 330. The output node 350 and the reference node 330 also correspond to the output voltage potential 318 or output side 320 of the bi-directional converter 310 operating in the first direction or first mode of operation. The reference node 330 may also be referred to as the ground rail 330 when extending to the input side 314 of the bi-directional converter 310.
[0055] The primary inductor 340 (Lp) is magnetically coupled with the secondary inductor 348 (Ls) to form a transformer. Preferably, each inductor is coiled around a common ferrite or other magnetisable core to form the transformer. The inductance of each inductor and the corresponding properties of the core, if used, in the converter 310 is preferably selected to ensure continuous current mode (CCM) operation under various load conditions. [0056] A first switch, also referred to as a boost switch 352 (S1), is connected between the bi-directional node 342 and the reference node 330 to selectively short the primary inductor 340 across the energy storage element 322 to inductively charge the primary inductor which in turn induces a charge in or induces a voltage potential across the secondary inductor 348.
[0057] A parallel diode 354 (D2) also connects the boost node 334 to the bi-directional node 342 allowing the input inductor 332 (L1) to be selectively shorted across the input node 328 and the ground rail reference node 330 upon pulse width modulation operation of the boosting switch 352 (S1 ) to thereby charge the input inductor 332 (L1 ) via the input voltage 312.
[0058] This first configuration enables the bi-directional converter 310 in the first direction mode to efficiently boost the combined voltage potentials of the energy storage element 322 and the first input voltage / source 312, 314 with a high gain when boost converting or stepping up the voltage potential to the output side 320. Advantageously, excess energy from first input voltage / source 312, 314 can be stored in the energy storage element 322, 324 and, in particular, used to charge the battery 324.
[0059] To stabilise voltage levels and increase the robustness of the bi-directional converter 310 as a boost or buck voltage converter and reduce overall current ripple, respective filter capacitors 358, 360 are respectively connected in parallel with the first input 312, 314 and output / second input 318, 320 of the bi-directional converter 310.
[0060] The operation of uni-directional boost converters with a similar, high voltage gain and coupled inductors but without a battery are described in Li, Y, Soon, LJ., Sathiakumar, S. 2018, ‘Improved quadratic boost converter using cross coupled-inductor J, 2018 IEEE 4th Southern Power Electronics Conference (SPEC), December 2018, pp.1-6 (Li-Soon- Sathiakumar) the contents of which are incorporated herein by reference.
[0061] Pulse width modulation operation of the boost switch 352 (S1) may be used to operate the bi-directional converter 310 to boost or step-up the voltage potential from the first power source of the input side 312, 314 and the energy storage element 322, 324 to drive a load at the output side 320 with a boosted voltage relative to the first input side 312, 314 and the energy storage element 322 such as the battery 324.
[0062] The voltage gain ratio from the first input side 312, 314 to the output side 318, 320 of the bi-directional converter 310 operating in the first direction mode may be defined as: [0063] Where Vout is the output voltage, Vin is the input voltage, N is the turns ratio of the two magnetically coupled inductors 340, 348 (Lp, Ls) and D is the pulse width modulation (PWM) duty cycle of the boosting switch as a percentage of the time it is on per switching period.
[0064] Advantageously, where no input voltage potential source 312 is connected at the first input side 314, the bi-directional converter 310 may be operated as a boost converter to draw power only from the battery 324 to drive a load at the output side 320 with a boosted voltage in comparison to a lower potential or voltage of the battery 324.
[0065] The voltage gain ratio from the battery 324 potential to the output side 318, 320 potential may be defined as:
Figure imgf000011_0001
[0066] Advantageously to the prior art the bi-directional converter 310 may also operate in a second direction or mode from a second power source 318 such as derived from an AC grid 116. In addition, the bi-directional converter 310 may also be operated as a buck converter to provide a step-down voltage potential relative to the second power source 318 at the second input side 320. The further functionality of the bi-directional converter is described in detail below.
[0067] A second switch, also referred to as a bi-directional switch 356 (S2 of FIG. 3), is connected across the output diode 346 (D3) between the secondary node 344 and the bi directional node 342. The bi-directional switch 356 (S2) enables a bi-directional loop circuit configuration 342, 356, 344 where a voltage potential as a second power source 318 applied to the previously termed output side 320 now operating as a second input side 320, can be converted in a step-down, buck conversion manner to charge the energy storage element 322, e.g. the battery 324.
[0068] Advantageously, the bi-directional switch 356 operation can be re-configured to operate the magnetically coupled inductors 340, 344 (Lp, Ls) in a buck converter or step-down voltage potential mode to charge the battery. The use of the re-configured operation of the coupled inductors 340, 344 providing a substantial voltage potential step down ratio suitable for optimally charging the battery 624. Examples of the substantial step down ratio are given below with respect to FIGURES 8 to 11.
[0069] The voltage gain or step-down ratio from the second input side 320 to the battery 324 may be defined as:
Figure imgf000012_0001
[0070] This is highly advantageous where the bi-directional converter 310 is connected to a DC bus with a DC output voltage of 340VDC rectified from the AC grid. Typical batteries for use with the bi-directional converter may range from 12V to 40V DC. In order to be charged from a second power source 318 with a 340VDC bus a large step down ratio can be provided.
[0071] The single, uni-directional converters of the prior art cannot provide in the one circuit device a high gain boost converter and then be re-configured in operation to be a buck converter with a large step-down ratio. In addition, an energy storage element 322, 324 may also used advantageously with the bi-directional converter 310 as described herein.
[0072] FIGURE 4 is a schematic of a function and control scheme diagram of a simulation 410 of the bi-directional converter 310 of FIGURE 3 with a single phase inverter 412. The simulation was done using Matlab / Simulink. The inverter 412 is undertaken with the four switches 414, 416, 418, 420 (S1 to S4 of FIG. 4) shown in FIGURE 4. A simulation was conducted to a solar PV array 112 as the first power source 312 at an unregulated nominal voltage of 12V with a battery 324 rated at 40V. The bi-directional converter 310 was operated as a boost converter by controlling boost switch 352 and keeping the bi-directional switch 356 open.
[0073] FIGURE 5 is a schematic of the output side 318, 320 voltage waveform 510 to the boost converter 310 operation of FIGURES 3 and 4. The vertical axis 512 full scale is 450V with increments of 50 V. The horizontal time axis 514 has a full scale of 0.2s in increments of 0.02s. From FIGURE 5 the gain of the bi-directional converter in boost mode is at least 10 relative to the battery and at least 30 to the nominal voltage of the solar PV array 112. The gain may be varied by changing the pulse width modulation (PWM) of the driving signal to the boost switch 352 (S1). The gain may also be advantageously varied by varying the coupling of the primary and secondary inductors 340, 344 (Lp, Ls) as well as the inductance values of the individual inductors. Maximum Power Point Tracking of the bi-directional converter 310 to a PV array 112 is detailed with respect to FIGURE 14.
[0074] FIGURE 6 is a schematic of the 240Vrms 50Flz voltage waveform 610 output of the single phase inverter 412 from the output 510, 320 of the bi-directional converter 310 of FIGURE 5. The vertical axis 612 full scale is 400V with increments of 100V. The horizontal time axis 614 has a full scale of 0.2s in increments of 0.02s. The 240Vrms waveform is suitable for feeding into the mains grid 116. The unique high gain is advantageous for interfacing low voltage PV panels and batteries with the relatively high voltage mains grid.
[0075] FIGURE 7 is a schematic of a function and control scheme diagram for the single phase inverter 412 rectifying the 240Vrms AC to DC suitable for the bi-directional converter 310. The bi-directional converter 310 being operated in the buck converter / step-down mode as detailed with respect to the control scheme of FIGURE 10. The rectification is achieved by operation of the switch pair S1 and S4 of the H-bridge of FIGURE 4 complementary to the switch pair S2 and S3. The switching operation may be performed, for example, by 50kHz pulses with a 50% duty cycle wherein switches S1 and S4 have an inverse duty cycle to S2 and S3.
[0076] FIGURES 8 and 9 are respective voltage waveforms 810, 910 of the 240Vrms mains and the rectified 340VDC. The vertical axis 612 of FIGURE 8 is as for FIGURE 6. The vertical axis 912 for FIGURE 9 has a full scale of 500V with 100V increments. The common horizontal time axis 914 has a full scale of 0.5 s in increments of 0.05s. An initial voltage spike 916 from switching and inrush current to the inductors may be reduced by a soft starting procedure.
[0077] FIGURE 10 is a schematic of a function and control scheme diagram of the bi directional converter 310 of FIGURE 3 in the buck converter or step-down mode to charge the battery 324. The bi-directional switch 356 is kept closed for the second mode of operation of the bi-directional converter. The control scheme used is advantageously making use of both inputs 312, 320, respectively: the DC first input source 312 from for example a solar PV array 112 and the second input source 318 derived the utility grid 116.
[0078] In such a configuration, the charging current to the battery 324 will be the sum of current from the input sources 312, 318. The switching duty cycle of bi-directional switch 356 can be selectively operated to vary the current charging the battery from input source 318. Advantageously, during such an operation, charging from input source 312 can operate independently in parallel. This is provided in part by input diode 338 and parallel diode 354 which block reversing current from input source 318 ensuring each charging operation can work independently.
[0079] FIGURE 11 is a schematic diagram of the voltage waveform 1110 across the battery 324 during charging from the two power sources 312, 318 of the first and second input sides 314, 320. The vertical axis 1112 has a full scale of 80V with increments of 10V. The horizontal time axis 1114 as a full scale of 0.2s with increments of 0.02s. The bi-directional converter 310 is able to hold the terminals of a battery at a voltage just above the nominal battery voltage to charge the battery from both inputs simultaneously. From FIGURES 9 and 11 a substantial step-down ratio of approximately 9 is achieved and maintained. It will be readily appreciated that bi-directional converter 310 can also charge the battery 324 in the absence of either input source 312, 318.
[0080] FIGURE 12 is a schematic circuit diagram of another bi-directional converter 1210 with another inverter 1211. The converter 1210 includes all the functionality of the bi directional converter 310 described with respect to FIGURES 3 to 11. The bidirectional converter 1210 and inverter 1211 have additional features to interface a PV panel 1212, 112 panel to the AC grid 116. The features of converter 1210 which correspond with those of the above bi-directional converter 310 of FIGURES 3 to 11 are correspondingly named. The PV Panel 1212 is connected across the input node 1228 and the reference node 1230 as a DC power source 312 to provide a DC input voltage. A further switch, referred to as an input switch 1216 (Q3) is connected between the boost node 1234 and the reference node 1230. Another additional switch referred to as a parallel switch 1218 (Q4) is connected from the boost node 1234 to the parallel diode 1254 (D1 in FIG. 12) for selectively disconnecting the parallel diode 1254 (D1) from the boost node 1234.
[0081] FIGURE 13 is a schematic of a function and control scheme diagram of the bi directional converter 1210 and inverter 1211 of FIGURE 12. FIGURE 13 also provides further details to the MPPT control of power transfer from the PV panel 1212 to the battery 1224 and bi-directional converter 1210.
[0082] The inventors constructed a prototype of the circuit of FIGURES 12 and 13. The circuit component values were as follows:
• L1 , 1232: 3000 mH
• Lp, 1240: 300 mH
• Ls, 1248: 2700 mH • C2, 1260: 470 m
• Lf, 1227: 5 mH
• Cf, 1262: 200 m
• D1 , D2, D3 and D4, 1254, 1238, 1246, 1253: MBR201 OOCT
• Switches Q1 , Q2, Q3, Q4, S1 , S2, S3 and S4: 2SK4207
• Battery 1224: Spirit (SPT)12-7.2 by SunStonePower (www.sunstonepower.com)
• Solar panel 1212: Powertech ZM9094
It will be readily appreciated that the type and rating of the circuit components may be selected by a person skilled in the art to provide a bi-directional converter 1210 operating to at least 1 kW. For example, MOSFET/IGBT switching devices may be used together with a control scheme developed with the MyRio processor kit by National Instruments.
[0083] The bi-directional converter 1210 of FIGURES 12 and 13 can step up the voltage of the PV Panel 1212 for MPPT and also invert the DC output voltage with the inverter 1211 to output an AC voltage. The inverter 1211 being a H-bridge 1219 connected across the output node 1250 and reference node 1230 of the bi-directional converter 1210. Additionally, power from the 240V AC grid 116 may be rectified to 340V DC by the inverter / rectifier 1211 and stepped down by the bi-directional converter 1210 to be stored in the battery 1224.
[0084] FIGURE 14 is a table of the charging current of the battery 1224 and the power drawn from the unregulated PV panel 1212 with increasing light intensity on the PV panel 1212. The circuit of FIGURES 12 and 13 in MPPT operation was used to provide the results in FIGURE 14. The circuit was tested to the MPPT operation by the parallel switch 1218 (Q4 in FIG. 12), the boost switch 1252 (Q1 in FIG. 12) and the bi-directional switch 1256 (Q2 in FIG. 12) being kept open to isolate the battery 1224 from the AC source 116. The input switch 1216 (Q3 in FIG. 12) is then operated to track a Maximum Power Point of the PV panel 1212 and to charge the battery 1224.
[0085] The table of FIGURE 14 shows: in the first column the battery charging current 1410, in the second column the voltage potential 1412 across the PV panel output, in the third column the power draw 1414 from the PV panel 1212 and in the fourth column the duty cycle 1416 of the input switch 1216 (Q3). The results of the FIGURE 14 table show the duty cycle 1416 of the input switch 1216 (Q3) being controlled automatically to correspond with the increased insolation intensity onto the PV panel 1212. A maximum power point for a light intensity 1418 is obtained at approximately 3.1W at a Q3, 1216 switch duty cycle of approximately 45%. [0086] Moreover, with the converter 1210, a PV panel operating at a maximum power point can be used to charge a battery with the present invention whilst also charging the battery via power from the AC source 116. In such a configuration, variations of the switching duty cycle of the bi-directional switch 1256 do not substantially impact the operation of PV panel charging. The charging current to the battery 1224 will be the sum of current from the PV panel 1212 and from the rectified, stepped down AC grid 116. The switching duty cycle of the input switch 1216 (Q3) is operated to drawn maximum power from the PV panel. The switching duty cycle of bi directional switch 1256 (Q2) can be selectively operated to vary the current charging the battery from the AC grid 116 to supplement the charging current from the PV panel. Charging from the PV panel 121 and the AC grid can operate independently in parallel. This is provided in part by input diode 1238 and parallel diode 1254 which block reversing current from the AC grid as well as by parallel switch 1218 (Q4) which selectively isolates the PV panel.
[0087] The inverter / rectifier 1211 has a H-bridge 1219 switching arrangement of two sets of series switches in parallel across the output node 1250 and reference node 1230 of the bi directional converter 1210 as shown in FIGURE 12. That is, a first set 1221 of series switches S1 , 1264 and S2, 1266 is in parallel to a second set 1223 of series switches S3, 1268, and S4, 1270. Between switches S1 and S2 the active terminal 1225 of the AC mains grid 116 is connected via a filter inductor 1227 (Lf). Between switches S3 and S4 the neutral terminal 1229 of the AC mains grid 116 is connected. An AC filter capacitor 1262 (Cf) is connected across the active 1225 and neutral 1229 terminals to the AC mains grid 116. The filter inductor 1227 (Lf) and AC filter capacitor 1262 (Cf) form a filter to achieve a smooth AC waveform when inverting a DC voltage to supply or feed into the AC grid 116. Preferably, the filter 1227, 1262 is configured to filter out frequencies other than 50Hz to 60Hz.
[0088] The bi-directional converter 1210 and inverter 1211 circuit of FIGURES 12 and 13 may also be configured to charge the battery 1224 from the AC mains grid source 116. Part of the bi-directional converter 1210 to the battery 1224 is used together with the filter capacitor 1260 (C2) and the H-bridge 1219 of the inverter / rectifier 1211. In use the switches S1 , 1264 and S4, 1270 of the H-bridge 1219 are turned ON and OFF together and S2, 1266 and S3, 1268 are switched complementary to S1 , 1264 and S4, 1270. The switching control signals are at a frequency of 50kHz with a 50% duty cycle.
[0089] In another configuration or mode of operation the bi-directional converter 1210 and inverter 1211 can boost convert the voltage potential from the battery 1224 then invert it to AC to apply to a resistive load with a low pass filter, instead of the AC mains grid. FIGURES 15 to 17 are to results where there is no DC input such as a PV panel or otherwise to the bi directional converter 1210. The bi-directional converter 1210 and inverter 1211 of FIGURES 12 and 13 was tested using boost switch Q1 , 1252 with inverter switches, S1 , S2, S3 and S4, 1262-1270.
[0090] FIGURE 15 is a schematic diagram of the voltage waveform 1510 of the switching signal and duty cycle of boost switch Q1 , 1252. The vertical voltage scale is in divisions of 2v and the horizontal time scale is in divisions of 10 ps.
[0091] FIGURE 16 is a schematic diagram of the voltage waveform 1610 of the potential across the output of the bi-directional converter at filter capacitor C2, 1260. The vertical voltage scale is in divisions of 20v and the horizontal time scale is in divisions of 10ms.
[0092] FIGURE 17 is a schematic diagram of the voltage waveform 1710 of the AC voltage potential across the load of the inverter 1211. The vertical voltage scale is in divisions of 20v and the horizontal time scale is in divisions of 10ms.
[0093] FIGURES 18(a) and 18(b) are schematic diagrams of the waveforms of the current flowing from the solar panel via the input diode D2 (1238) and the current flowing from the AC side via the primary inductor Lp (1240) to the battery 1224 for bi-directional converter of FIGURES 12 or 13. In FIGURE 18(a) the bi-directional switch Q2 (1256) has a duty cycle of approximately 15%. In FIGURE 18(b) the bi-directional switch Q2 duty cycle is approximately 20%. The charging current of the battery 1224 is the sum of the currents through D2 1238 and Lp 1240. Each current varies differently during operations. It was found that the D2 current was a larger proportion of the battery charging current. Moreover, by increasing the duty cycle of Q2, the current of Lp can be increased. It was also found that by changing the duty cycle of Q2, there is almost no influence on the operation of the solar panel charging. This is because diode D1 and D2 can block reversing current from the AC side to the DC side and ensure each charging operation can work independently. In practical applications, a user can adjust how much energy is charging the battery by changing the duty cycle of the bi-directional switch Q2.
[0094] In this case the nominal voltage of the battery is a lead-acid battery having a nominal voltage of between 12 and 14 volts. Thus, with a duty cycle of 50%, referring to Figure 16, the bi-directional converter is capable of boosting the battery voltage, via the magnetically coupled inductors, with a gain ratio of 3 resulting in the DC output voltage waveform 1610 of approximately 40 volts. Furthermore, the bi-directional converter can transform the DC voltage to an AC voltage as displayed in Figure 17. The AC output voltage waveform 1710 is a relatively smooth sinewave with only minor switching losses during inversion.
[0095] The converter advantageously includes three degrees of freedom for control, namely the boosting switching, bi-directional switching and coupling between inductors. These controllable degrees of freedom allow the converter to direct power bi-directionally across a wide variety of voltage gains and also facilitates MPPT of a PV panel and controlled battery charging from both the input and output sides.
[0096] In the two bi-directional converters 310, 1210 examples of Figures 3, 4, 12 and 13 the particular addition of switches for selectively connecting and disconnecting key components provides a wide variety of bi-directional boost and buck functionality / operational modes with the same inductor and diode circuit components. This achieves high gain bi directional power flow and efficient use of renewable energy from the PV panel all with minimal components.
[0097] It will be further appreciated that the invention provides a high gain and high step- down ratio bi-directional power converter-inverter AC-DC power flow system with a minimum of components. The minimum of components provides substantial advantages to a compact, versatile solution for interfacing a PV panel and battery with each other in island mode applications and / or additionally interfacing with the grid. The minimum of components also provides significant cost and fabrication savings, essential in suburban, household and remote area power provision.
[0098] It will also be readily appreciated that component values, control schemes and circuit integration described herein may be varied by a person skilled in the art to provide a DC-AC power converter-inverter system with at least a 1 kW capacity.
[0099] Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms.
INTERPRETATION
[00100] Reference throughout this specification to “one embodiment”, “some embodiments” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment”, “in some embodiments” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments.
[00101] As used herein, unless otherwise specified the use of the ordinal adjectives "first", "second", "third", etc., to describe a common object, merely indicate that different instances of like objects are being referred to, and are not intended to imply that the objects so described must be in a given sequence, either temporally, spatially, in ranking, or in any other manner.
[00102] In the claims below and the description herein, any one of the terms comprising, comprised of or which comprises is an open term that means including at least the elements/features that follow, but not excluding others. Thus, the term comprising, when used in the claims, should not be interpreted as being limitative to the means or elements or steps listed thereafter. For example, the scope of the expression a device comprising A and B should not be limited to devices consisting only of elements A and B. Any one of the terms including or which includes or that includes as used herein is also an open term that also means including at least the elements/features that follow the term, but not excluding others. Thus, including is synonymous with and means comprising.
[00103] As used herein, the term “exemplary” is used in the sense of providing examples, as opposed to indicating quality. That is, an “exemplary embodiment” is an embodiment provided as an example, as opposed to necessarily being an embodiment of exemplary quality.
[00104] It should be appreciated that in the above description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, FIG., or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the Detailed Description are hereby expressly incorporated into this Detailed Description, with each claim standing on its own as a separate embodiment of this invention. [00105] Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those skilled in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination.
[00106] Furthermore, some of the embodiments are described herein as a method or combination of elements of a method that can be implemented by a processor of a computer system or by other means of carrying out the function. Thus, a processor with the necessary instructions for carrying out such a method or element of a method forms a means for carrying out the method or element of a method. Furthermore, an element described herein of an apparatus embodiment is an example of a means for carrying out the function performed by the element for the purpose of carrying out the invention.
[00107] In the description provided herein, numerous specific details are set forth. Flowever, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.
[00108] Similarly, it is to be noticed that the term coupled, when used in the claims, should not be interpreted as being limited to direct connections only. The terms "coupled" and "connected," along with their derivatives, may be used. It should be understood that these terms are not intended as synonyms for each other. Thus, the scope of the expression a device A coupled to a device B should not be limited to devices or systems wherein an output of device A is directly connected to an input of device B. It means that there exists a path between an output of A and an input of B which may be a path including other devices or means. "Coupled" may mean that two or more elements are either in direct physical or electrical contact, or that two or more elements are not in direct contact with each other but yet still co-operate or interact with each other.
[00109] Thus, while there has been described what are believed to be the preferred embodiments of the invention, those skilled in the art will recognize that other and further modifications may be made thereto without departing from the spirit of the invention, and it is intended to claim all such changes and modifications as falling within the scope of the invention. For example, any formulas given above are merely representative of procedures that may be used. Functionality may be added or deleted from the block diagrams and operations may be interchanged among functional blocks. Steps may be added or deleted to methods described within the scope of the present invention.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS
1 . A bi-directional power converter for interfacing an energy storage element between a DC input side and a DC output side, the converter including: a reference node and an input node for receiving an input voltage potential; a boost node, a primary node, a bi-directional node, a secondary node, and an output node for interfacing with an output voltage potential; an input inductor connected between the input node and the boost node; an input diode connected between the boost node and the primary node; a parallel diode connected between the boost node and the bi-directional node; the energy storage element connected between the primary node and the reference node; a primary inductor connected between the primary node and the bi-directional node; an output diode connected between the bi-directional node and the secondary node; a secondary inductor connected between the secondary node and the output node; a boost switch connected between the bi-directional node and the reference node; and a bi-directional switch connected between the secondary node and the bi-directional node; wherein the primary inductor is magnetically coupled with the secondary inductor; wherein the boost switch is operable to selectively short the input inductor across the input voltage and to selectively short the primary inductor across the energy storage element, to boost the input voltage potential to the output node; and wherein the bi-directional switch is operable to selectively bypass the output diode and to alternatively buck the output voltage from the output node to the energy storage element.
2. The bi-directional power converter according to claim 1 , wherein the energy storage element includes at least one of a battery and a capacitor.
3. The bi-directional power converter according to claim 2, wherein the energy storage element is a battery with a capacitor connected in parallel.
4. The bi-directional power converter according to any one of claims 1 to 3, further including an input switch connected between the boost node and the reference node, and a parallel switch for selectively disconnecting the parallel diode from the input node, wherein the input switch is operable to track a maximum power point of a photovoltaic panel across the input node and the reference node.
5. The bi-directional power converter according to any one of claims 1 to 4, wherein varying the coupling between the primary inductor and the secondary conductor varies at least one of the boost to the input voltage potential and the buck to a voltage potential applied to the output node.
6. The bi-directional power converter according to any one of claims 1 to 5, further including an H-bridge connected between the output node and the reference node to interface the DC output side with an AC grid by selectively inverting the DC output voltage potential or to rectifying an AC grid voltage potential.
7. The bi-directional power converter according to any one of claims 1 to 6, further including filter capacitors between the input node and the reference node and between the output node and reference to node to respectively filter the input voltage and output voltage.
8. A bi-directional power flow system for interfacing an energy storage module with a direct current (DC) input module and a DC output module, the system including: the DC input module for receives a DC input voltage potential; the energy storage module is coupled to the DC input module; the DC output module for interfaces with a DC output voltage potential; a converter module for interfacing the energy storage module to the DC output module, the converter module including a pair of magnetically coupled inductors and a first switching arrangement; and a bi-directional module interfaced with the DC output module, the converter module and the energy storage module, the bi-directional module including a second switching arrangement; wherein the first switching arrangement is operable to step the DC input voltage potential and the energy storage module voltage potential up to the DC output voltage potential at the DC output module; and wherein the second switching arrangement is operable to step an applied DC voltage potential at the DC output module down to the voltage potential of the energy storage module.
9. The bi-directional power flow system of claim 8, wherein the DC input module includes a third switching arrangement operable to track a maximum power point of a source of the DC voltage potential.
10. The bi-directional power flow system of claim 9, wherein the first switching arrangement is interfaced with the third switching arrangement.
11 . The bi-directional power flow system of any one of claims 8 to 10, further including a DC-AC (alternating current) inverter and rectifier module, wherein the DC-AC inverter and rectifier module is coupled with the DC output module.
PCT/AU2021/050260 2020-03-30 2021-03-22 A bi-directional ac-dc power converter – inverter WO2021195692A1 (en)

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CN114221546A (en) * 2021-12-14 2022-03-22 国网福建省电力有限公司宁德供电公司 Interleaved parallel high-gain Boost direct-current converter, control method and control device
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