AU2016200827A1 - Converter Topologies for AC-to-AC and AC-to-DC Power Transferring through Solid-state Transformer and their Control Methods - Google Patents

Abstract

A series of circuit topologies for AC-to-AC and AC-to-DC power transferring through solid state transformer (SST) and their control methods are invented. By having passive filters and reactive power compensator, such SST can operate at high frequency close to several hundred kilohertz even higher. A new multi-stage AC/DC converter is adopted to interface SST with high voltage AC grid, from which power is supplied or exchanged. A first system based on the developed SST is adopted to transfer power between two AC grids. A second system based on the developed SST is used to charge battery or power DC load or DC microgrid from AC grid. A third system based on the developed SST is used to supply power to multiple loads, including AC grid, DC load, charging battery, and powering DC microgrid. Furthermore bi-directional AC/DC or AC/AC systems based on the developed SST have also been invented. -o - D r D <~r ~n 0 u0-* 0-* 1 D (D ~ (D - 3 co 3 co 3 co D 7- (1 D 7- D 7 = 0C- 0 CD0 0D 0 C0 CD 0 Ln ~ QD Ln QD LIA 3w CD D _0 _ D-0_ rDD 0 r 0 m

Description

Converter Topologies for AC-to-AC and AC-to-DC Power Transferring through Solid-state Transformer and their Control Methods

FIELD OF INVENTION

This invention relates to a new series of medium-frequency transformer based power converters for the applications in the field of power substations, electric vehicle charging, inter-linking of multiple microgrids or grids, and powering DC microgrid or DC load from AC power system.

BACKGROUND

[0001] A solid-state transformer (SST) is broadly defined as power converters which include medium-frequency or high-frequency transformer isolated DC/DC converters. With the evolvement of magnetic materials and switch techniques, it is becoming more and more feasible to operate such converters in the kilohertz to megahertz range in the near future and with high efficiency. The magnetic materials suitable for such applications include nano-crystalline FINEMET, METGLASS, modified Mn-Zn etc. For the application of even higher frequency in the megahertz range, air-cored inductor and air-cored transformer could be adopted. The switches can be Silicon Carbide or Gallium Nitride. This makes SST a feasible and compact forming unit with high efficiency for many applications, such as SST based substation, SST based electric vehicle charger, SST based converters for inter-linking multiple microgrids or grids for mutual support, and for powering DC microgrids or DC loads from AC power system etc.

[0002] Although traditional 50Hz transformer based substations have been in use for many years, now it is possible to build solid-state transformer isolated substations as their alternatives to fulfil power distribution, which have several advantages over traditional substations: a) it provides DC bus link which can power DC load; b) it can be used to link multiple microgrids for mutual support; c) it can flexibly handle reactive power demand etc.

[0003] Currently the main control method for SST is phase-shift based method. Its switching losses cannot be managed with a wide range variation of load levels. Hence its operation at high frequency may have some obstacles. In this invention, a new method is introduced to solve this problem through introducing passive filters’ design. Such passive filters can shape both voltage across and current through the medium-frequency transformer in SST to nearly sinusoidal waveforms. Therefore the core losses in the transformer can be contained. Moreover by having tunable reactive power compensation in SST, nearly zero current switching of inverter in SST can be achieved at different power levels. Furthermore established techniques are to share common DC-link capacitor when powering multiple loads. Such approach is potentially hard to manage power delivering to each load smoothly. In this invention, passive filters are adopted to regulate the power to different loads by producing nearly DC current to the SST. Hence overall effect to the SST is the addition of different levels of DC current and power supply to each load is independent from each other. With such techniques, several new applications listed above become feasible.

BRIEF DESCRIPTION OF THE DRWAING

[0004] Detailed description is described with reference to the accompanying figures.

[0005] FIG. 1 shows AC to AC power transferring through the proposed medium-frequency transformer isolated DC/DC converter as disclosed herein; [0006] FIG. 2 shows AC to DC power transferring through the proposed medium-frequency transformer isolated DC/DC converter as disclosed herein; [0007] FIG. 3 shows AC/DC converter used in the circuits in FIG. 1 and FIG. 2; [0008] FIG. 4 shows DC/DC converter used in FIG. 1 and FIG. 2 which contains passive filter, medium-frequency transformer and reactive power compensator; [0009] FIG. 5 shows circuit used in FIG. 4 for voltage waveform shaping to produce nearly sinusoidal waveform; [0010] FIG. 6 shows circuit used in FIG. 4 for current waveform shaping to produce nearly sinusoidal waveform; [0011] FIG. 7 shows passive filter used in FIG. 4 for producing nearly constant current; [0012] FIG. 8 shows converter circuit topology for multiple-channel power transferring; [0013] FIG. 9 shows circuit used in FIG. 8 for producing multiple power supply channels; [0014] FIG. 10 shows circuit topology for transferring AC power to AC grid or AC load through solid state transformer and multi-level or multi-stage rectifier; [0015] FIG. 11 shows circuit topology for transferring AC power to DC grid or DC load through multi-level or multi-stage rectifier; [0016] FIG. 12 shows DC/DC converter used in the circuits as shown in FIG. 10 and FIG. li; [0017] FIG. 13 shows circuit topology for bi-directional power exchange between two AC systems or between AC systems and DC load, battery or DC microgrid, where some inductors are tunable; [0018] FIG. 14 shows forming components in the circuit topology of FIG. 13; [0019] FIG. 15 shows medium-frequency transformer with passive filter and tunable reactive power compensator for the circuit in FIG. 14; [0020] FIG. 16 shows bi-directional power transferring between AC system and DC system through solid state transformer and multilevel or multistage converters; [0021] FIG. 17 shows bi-directional power transferring between two AC systems through solid state transformer and multilevel or multistage converters; [0022] FIG. 18 shows a cascaded H-bridge multilevel DC/AC converter with LCCL filter; [0023] FIG. 19 shows a LCL filter; [0024] FIG. 20 shows a cascaded H-bridge multilevel DC/AC converter with LC filter; [0025] FIG. 21 shows the control flow of each cell for the cascaded H-bridge multilevel DC/AC converter with LC filter; [0026] FIG. 22 shows the control flow of all cells for H-bridge multilevel DC/AC with LC filter; [0027] FIG. 23 shows a sample microgrid.

DETAILED DESCRIPTION

[0028] FIG. 1 shows power transferring between one AC grid 110 and another AC grid or AC load 112 through the proposed converter circuit topology, which includes AC/DC converter 130, Z-filter 140, DC/DC converter with medium-frequency transformer 170, capacitor at DC-link 150, DC/AC inverter 160, passive filter 180 and AC grid or AC load 112. The AC grid 110 and AC grid or AC load 112 can be single-phase or three-phase. Correspondingly connection 122 could be single phase or three phases.

[0029] FIG. 2 shows power transferring from AC grid 110 to DC load, or battery or DC microgrid 190. The circuits between AC grid 110 and capacitor at DC-link 150 are the same as that in FIG. 1. DC power to DC power converter 200 is to manage how much power is to be transferred and also to regulate the voltage across capacitor at DC-link 150 to be around targeted value. It could be a direct DC/DC converter. It could be DC/AC/AC/DC converter or other converters to suit contactless battery charging or other purposes.

[0030] The AC/DC converter 130 in FIG. 1 and FIG. 2 is shown in FIG. 3, which includes passive filter 131, AC/DC rectifier 133, capacitor at DC-link 135 and DC/DC converter 137. The passive filter 131 can be LCL filter as shown in Darning Zhang, and R. Dutta, “Application of Partial Direct- Pole-Placement and Differential Evolution Algorithm to Optimize Controller and LCL Filter Design for Grid-tiedInverter”, AUPEC 2014, Perth, Australia or other filters. The AC/DC converter 130 in both FIG. 1 and FIG. 2 works as a rectifier and can be conventional controllable one in the same structure but working as rectifier as shown in Darning Zhang, and R. Dutta, “Application of Partial Direct-Pole-Placement and Differential Evolution Algorithm to Optimize Controller and LCL Filter Design for Grid-tied Inverter”, AUPEC 2014, Perth, Australia. If the passive filter 131 is LCL filter, then the control method on AC/DC rectifier 133 is the same as the inverter described in Darning Zhang, and R. Dutta, “Application of Partial Direct-Pole-Placement and Differential Evolution Algorithm to Optimize Controller and LCL Filter Design for Grid-tiedInverter”, AUPEC 2014, Perth but with real power reference setting to ensure that power flows from AC grid 110 to AC/DC converter 130 in FIG. 1 and FIG. 2. The circuit topology in FIG. 1 is to achieve the real power transferring from AC grid 110 to AC grid 112 or AC load 112. The level of power transferred is controlled by the AC/DC rectifier 133 by setting proper real power reference and setting reactive power reference to zero. The voltage across capacitor at DC-link 135 is regulated by DC/DC converter 137 as shown in FIG. 3 to be around a pre-set value. Hence the AC/DC rectifier 133 in FIG. 3 mainly controls the power flow without regulating voltage across the capacitor at DC-link 135 in FIG. 3.

[0031] DC/DC converter with medium-frequency transformer 170 in FIG. 1 and FIG. 2 is shown in detail in FIG. 4, which includes single-phase inverter 171, passive filter 172, reactive power compensator 173, sing-phase medium-frequency transformer 174, passive filter 175, single-phase diode rectifier 176 and passive filter 178. The single-phase inverter 171 is controlled by square wave at desired frequency, which could be at low frequency or at a frequency close to MHz range. For one application at a chosen operating frequency, the circuit parameters need be designed specifically to achieve power transferring from AC grid 110 to AC grid 112 or AC load 112 in FIG. 1 or transferring from AC grid 110 to DC load, or battery or DC microgrid 190 in FIG. 2.

[0032] The passive filter 172 used in the circuit 170 in FIG. 4 is given in FIG. 5, where the series inductor 172A is to take most of harmonic voltage drop, multiple shunt series-resonant circuits with inductor and capacitor in series 172B, each of which resonates at each specific harmonic frequency. By having this passive filter 172, at each harmonic frequency, one of the shunt branches’ impedance is zero. Hence almost all of the harmonic voltage at the output of single-phase inverter 171 in FIG. 4 drops across the series inductor 172A. This is to shape the voltage across the single-phase medium-frequency transformer 174 in FIG. 4 to be nearly sinusoidal waveform. The core losses can be contained accordingly. The basic guideline of designing this passive filter 172 is to ensure a) as small fundamental voltage drop across the series inductor 172A as possible without sacrificing its capability to take the harmonic voltage drop and without producing too high harmonic current; b) all the shunt series-resonant circuits should have high enough total impedance at fundamental switching frequency same as that for the singlephase inverter 171 to avoid burdening the single-phase inverter 171 and other input circuit unnecessarily.

[0033] Reactive power compensator 173 in FIG. 4 is in shunt connection and adopted to adjust the reactive power to ensure nearly zero-current switching of the single-phase inverter 171 at different power levels. It can be formed by motorized tunable capacitor or other switchable or changeable capacitors. At each power level, such capacitor is fine-tuned to ensure nearly zero-current switching of single-phase inverter 171, thereby minimizing its switching losses. The shunt reactive power compensator 173 should have enough compensation at rated power level to achieve nearly zero-current switching-off and switching-on for the single-phase inverter 171. When the power is reduced from the rated value, the inductive reactive power drawn by the series inductors at the fundamental switching frequency of the inverter 171 is reduced. Correspondingly the total capacitances of the reactive power compensator 173 are adjusted according to the new operating power level to achieve zero-current switching of the single-phase inverter 171. A look-up table of shunt capacitance of the shunt reactive power compensator 173 against power level can be prepared and used for real time operation at different power levels.

[0034] Single-phase medium-frequency transformer 174 as shown in FIG. 4 works at the same fundamental frequency as single-phase inverter 171. With the properly designed passive filters, the voltage across its primary and secondary sides is close to sinusoidal waveform.

[0035] Passive filter 175 used in FIG. 4 is shown in FIG. 6 and is adopted to shape the waveform of current through the medium-frequency transformer 174 in FIG. 4. The multiple shunt series-resonant circuits with inductor and capacitor in series are to bypass harmonic currents produced from single-phase diode rectifier 176 as shown in FIG. 4. As at each harmonic frequency, one of the shunt branches’ impedance is zero, that specific harmonic current produced by single-phase diode rectifier 176 in FIG. 4 flows through that shunt branch. By having multiple shunt series-resonant circuits, almost zero harmonic currents flow through the single-phase medium-frequency transformer 174 in FIG. 4, thereby reducing copper losses in it. Certainly the passive filter 175 needs be properly designed to minimize total copper losses in the system in order to ensure overall copper losses are reduced or not change too much to compromise the other advantages in the overall circuit design. The basic guideline of designing the passive filter 175 in FIG. 6 is to ensure a) each branch resonates at each specific harmonic frequency; b) the overall capacitance from all the shunt branches should provide high enough impedance at fundamental frequency of the single-phase inverter 171 to minimize fundamental frequency current flowing through them; c) overall cost for it.

[0036] Single-phase diode rectifier 176 in FIG. 4 is an AC/DC converter formed by four-diode or other equivalents.

[0037] Passive filter 178 in FIG. 4 is shown in FIG. 7, where inductor 178A is to ensure the current flowing through it becomes as nearly as possible a DC current. Other parts in FIG. 7 are a conventional z-filter.

[0038] DC/AC inverter 160 in FIG. 1 can be same as that described in Darning Zhang, and R. Dutta, “Application of Partial Direct- Pole-Placement and Differential Evolution

Algorithm to Optimize Controller and LCL Filter Design for Grid-tied Inverter”, AUPEC 2014, Perth or other inverter topologies. It can be a single-phase or three-phase converter.

[0039] FIG. 8 shows a combination of the circuits modified from FIG. 1 and FIG. 2. It supplies power from AC grid 110 to multiple loads, some of which could be AC grid or AC load 112A and 112B, and others of which could be DC load or battery or DC microgrid 190. To facilitate such application, converter and its ancillary circuits 280 as shown in FIG. 9 is adopted, in which multiple passive filters 178 are used. Other parts in FIG. 9 are the same as those in FIG. 4.

[0040] To suit power supply from high voltage source, circuit topologies shown in FIG. 10 and FIG.ll are adopted. They contain multilevel or multistage AC/DC rectifier with filter 240, which can be conventional multilevel rectifier with filter as described in Hirofumi Akagi, “Classification, Terminology, and Application of the Modular Multilevel Cascade Converter”, IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 26, NO. 11, NOVEMBER 2011, pp.3119-3130 or other multilevel converter topologies.

[0041] Detailed information on converter 260 is given in FIG. 12, which includes DC/DC converter 137, Z-filter 140, and DC/DC converter with medium-frequency transformer 170. The purpose of DC/DC converter 137 in FIG. 12 is to regulate voltage across each Cdc in FIG. 10 or FIG. 11 to be around targeted voltage. The power transferred in the system is regulated by each cell or stage in the multilevel or multistage AC/DC rectifier with filter 240 in FIG. 10 and FIG. 11.

[0042] The power control over each cell or each stage in the multilevel or multistage AC/DC rectifier with filter 240 in FIG. 10 and FIG. 11 is the same as that for AC/DC rectifier 133 in FIG. 3 as described in Hirofumi Akagi, “Classification, Terminology, and Application of the Modular Multilevel Cascade Converter”, IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 26, NO. 11, NOVEMBER 2011, pp.3119-3130 or other papers.

[0043] FIG. 13 shows the bi-directional power transferring either between two AC power systems or between one AC power system and one DC power system through solid-state transformer. In such kind of systems, the bi-directional controllable DC/AC converter with medium-frequency transformer, passive filter and reactive power compensator 560 is shown in FIG. 14, which contains Z-filter 140, tunable inductor 534, bi-directional DC/AC controllable converter 561, medium-frequency transformer with passive filter and reactive power compensator 562, bi-directional controllable AC/DC converter 569, tunable inductors 536A and 536B and Z-filter 140. The medium-frequency transformer with passive filter and reactive power compensator 562 in FIG. 14 is shown in FIG.15, which contains series tunable inductor 563A, multiple shunt series-resonant filters 563B, tunable shunt reactive power compensator 173, single-phase medium-frequency transformer 174, multiple shunt series-resonant filter 564B for different frequencies and series tunable inductor 564A. The inductor 534, inductors 536A and 536B, inductor 563A, and inductor 564A are all tunable ones, whose inductances could be controlled by DC bias current as described in Haifeng Li and Hui Li, “High-frequency Transformer Isolated Bidirectional DC-DC Converter Modules with High Frequency over Wide Load Range for 20kVA Solid-State Transformer”, IEEE Transactions on Power Electronics, Vol. 26, No. 12, December 2011, pp. 3599-3608 and D. Medini and S- Ben-Yaakov, “A current-controlled variable inductor for high frequency resonant power circuits ”, in proc. IEEE Appl. Power Electronics, Conf., Feb., 1994, pp. 219-225 or controlled by other methods.

[0044] For the power transferring from AC grid 114 to AC grid 112 and/or battery, or DC microgrid 190 in FIG. 13, inductor 534 in FIG. 14 and inductor 564A in FIG. 15 should be tuned to a very small or nearly zero value from their respective nominal values while inductor 536A in FIG. 14 and inductor 563A in FIG. 15 should be tuned to their respective nominal values for smooth power transferring. The bi-directional AC/DC controllable converter 561in FIG. 14 is under control mode and works as inverter while bi-directional AC/DC converter 569 in FIG. 14 may work as a rectifier through diodes with control signal of switches set to zero or off state. The bi-directional DC/DC converter 520 in FIG. 13 is to regulate the voltage across capacitor 530 to be around targeted value while the bi-directional DC/DC converter 570 could work in a free-flow mode to allow power directly passing through it or coordinate with the controllable DC/AC converter with filter 580 and other parts to improve the quality of power transferring. The shunt reactive power compensator 173 in FIG. 15 which is formed by tuneable capacitor could be placed either at primary or the secondary side of the singlephase medium-frequency transformer 174 and is adjusted to ensure nearly zero-current switching of AC/DC controllable converter 561in FIG. 14 at different power levels. A look-up table of shunt capacitance of the shunt reactive power compensator 173 against power level can be prepared and used for real time operation at different power levels.

[0045] For the power transferring from AC grid 112 and battery, or DC microgrid 190 to AC grid or AC load 114 in FIG. 13, inductor 534 in FIG. 14 and inductor 564A in FIG. 15 should be tuned to their respective nominal values while inductors 536A in FIG. 14 and inductor 563A in FIG. 15 should be tuned to a very small or nearly zero value from their respective nominal values for smooth power transferring. The bi-directional AC/DC controllable converter 569 is in control mode and works as inverter while bi-directional DC/AC converter 561 works as a rectifier and can naturally commutate through diodes with control signal set to zero or off state. The bi-directional DC/DC converter 570 in FIG. 13 is to regulate the voltage across capacitor 532 to be at targeted value while the bi-directional DC/DC converter 520 could work in a free-flow mode to allow power directly passing through it or coordinate with the controllable AC/DC converter with filter 510 and other parts to improve the quality of power transferring. The shunt reactive power compensator 173 in FIG. 15 is adjusted to ensure nearly zero-current switching of AC/DC controllable converter 569 in FIG. 14 at different power levels. A look-up table of shunt capacitance of the shunt reactive power compensator 173 against power level can be prepared and used for real time operation at different power levels.

[0046] FIG. 16 shows bi-directional power transferring between one high-voltage AC system 114 and one DC system 190 through bi-directional multilevel or multi-stage converters and solid-state transformer. The multilevel or multistage AC/DC converter with filter 240A, which can be conventional multilevel rectifier with filter as described in Hirofiimi Akagi, “Classification, Terminology, and Application of the Modular Multilevel Cascade Converter”, IEEE TRANSACnONS ON POWER ELECTRONICS, VOL. 26, NO. 11, NOVEMBER2011, pp.3119-3130 or other multilevel converter topologies. The method of control over the tunable inductors and reactive power compensation in the bidirectional controllable DC/DC converter with medium frequency transformer, passive filter and reactive power compensator 560 is the same as that for the circuit in FIG. 13 and described in the previous paragraphs.

[0047] FIG. 17 shows bi-directional power transferring between two AC systems through multilevel or multi-stage converters and solid-state transformer, one being 114 and the other being 112. The method of control over the tunable inductors and reactive power compensation in the bi-directional controllable DC/DC converter with medium frequency transformer, passive filter and reactive power compensator 560 is the same as that for the circuit in FIG. 13 and described in the previous paragraphs.

[0048] FIG. 18 shows a H-bridge multilevel DC/AC converter in cascaded connection and with a LCCL filter 620. The converter contains multiple identical H-bridge DC/AC converter cells 610, each of which is composed of four switches and one DC source or equivalent DC source. The equivalent DC source could be formed by a capacitor powered by other source. The LCCL filter 620 is formed by a series inductor Li 621 at the AC side of first cell of H-bridge DC/AC converter cells, followed by a series-connected capacitor Ci 622, then followed by a shunt capacitor C2 624 and then a series inductor L2 623 at the AC grid or AC load side. The LCCL filter 620 can be in other equivalent forms, such as with half of Li 621 being placed at the first cell and with another half of Li 621 being placed at the last cell etc. Such LCCL filter 620 allows the multilevel converter to have drastic power change without producing too much harmonic current. The multilevel DC/AC converter in FIG. 18 can be used in the overall systems as shown in FIG. 10, and FIG. 11 as the multilevel or multistage AC/DC rectifier with filter 240. The multilevel DC/AC converter with the LCCL filter in FIG. 18 can also be used in the overall systems as shown in FIG. 16, and FIG. 17 as the bi-directional multilevel or multistage AC/DC converter with filter 240A.

[0049] Inductor Li 621 and capacitor Ci 622 in LCCL filter 620 in FIG.18 can be combined as an equivalent inductor Lit 625 in FIG. 19. The basic relationship between Lit 625, Li 621, and Ci 622, are given by Equations (1), (2) and (3):

(1) (2) (3) [0050] where kcoeff depends on the targeted harmonic containment in the operation with output of a fraction of rated power. The lower the fraction, the higher the kcoeff· The range is kcoeff >L ωο is the fundamental angular frequency of the AC system connected between terminals X and Y in FIG. 18.

[0051] By combining Li 621 with Ci 622 as one equivalent inductor Lit 625, the LCCL filter 620 in FIG. 18 can be reduced to an equivalent LCL filter as shown in FIG. 19. Other forms of passive filter equivalent to LCCL filter 620 can be adopted as well in FIG. 18.

[0052] Then the values of Lit 625, C2 624 and L2 623 in FIG. 19 can be chosen at rated power by using the method described in Darning Zhang, and R. Dutta, “Application of Partial Direct-Pole-Placement and Differential Evolution Algorithm to Optimize

Controller and LCL Filter Design for Grid-tied Inverter”, AUPEC 2014, Perth, Australia, pp.1-6 or other methods to meet total harmonic current distortion requirement at the rated power of the converter. Then Li 621 and Ci 622 can be determined from Equations (2) and (3) respectively for a chosen kcoeff· [0053] By treating LCCL filter 620 in FIG. 18 as an equivalent LCL filter as shown in FIG. 19, and by using voltage between terminals X and Y divided by the number of cells or stages and total real and reactive power references divided by the number of cells or stages, the same method can be used to produce gating signals for each cell as that described in Darning Zhang, and R. Dutta, “Application of Partial Direct-Pole-Placement and Differential Evolution Algorithm to Optimize Controller and LCL Filter Design for Grid-tiedInverter”, AUPEC 2014, Perth, Australia, pp.1-6.

[0054] The control signals for different cells could be different. By treating LCCL filter in FIG. 18 as an equivalent LCL filter as shown in FIG. 19, and by using voltage between terminals X and Y in FIG. 18 and total real and reactive power references for the converter in FIG. 18, the gating signals for different cells are produced by combining phase shift pulse-width modulation or level shift pulse-width modulation with the method shown in Darning Zhang, and R. Dutta, “Application of Partial Z)/Vec/-Polc-Placcmcnt and Differential Evolution Algorithm to Optimize Controller and LCL Filter Design for Grid-tiedInverter”, AUPEC 2014, Perth, Australia, pp.1-6.

[0055] Fig. 20 shows a H-bridge multilevel DC/AC converter in cascaded connection and with a LC filter 630. Such series LC filter 630 has good effect in reducing harmonic current generation as described in Darning Zhang and Phu Le, “Application of Modified all-phase FFT to Extract Signals in Microgrid for Condition Monitoring and Control Purposes”, AUPEC September 2015, Wollongong, Australia, pp. 1-5. The overall effect of the LC filter 630 is inductive to achieve harmonic filtering purpose and equivalent to a total inductor LT. The relationship between L 631, C 632 and Lp is given by Equation (4). At fundamental frequency, the LC filter 630 is equivalent to Lp while at harmonic frequencies, it is approximately equivalent to the inductor L 631. To allow drastic change of the fundamental-frequency current flowing through the LC filter 630 from several percent of rated power through 100% rated power without exceeding the specified total harmonic current distortion (THDi), typically 5%, the inductor L 631 and capacitor C 632 needs properly chosen as described herein. LT is determined according to THDi requirement, e.g. 5% at the rated power using the method given in Karman Jalili and

Steffen Bernet, “Design of LCL filters of Active-Front-End Two-Level Voltage-Source Converters”, IEEE Transactions on Industrial Electronics, Vol. 56, No. 5, May 2009, pp. 1674-1689 and other open sources. Then L 631, and C 632 can be determined by Equations (5) and (6) with pCOeff >1, where pcoeff depends on the fraction of rated power at which the THDi requirement is kept below 5% and the lower the fraction, the higher the

Pcoeff·

(4) (5) (6) [0056] The control signals for different cells in FIG. 20 can be identical. The control flow for each cell based on proportional resonant controller is shown in FIG. 21. The reference current i*(t) used in Step SI 660 in FIG. 21 is calculated by Equation (7) with Prcr and Qref equal to given total real power reference and total reactive power reference divided by the number of cells or stages respectively, Vrms equal to the rms value of the voltage between terminals X and Y in FIG. 20 divided by the number of cells or stages in FIG. 20, θι being the initial angle of the reference current and being determined by Equation (8) with θγ being the initial angle of the voltage between terminals X and Y in FIG. 20, and &amp;>o being the fundamental angular frequency of the system at the AC side between terminals X and Y in FIG. 20.

[0057] The control flow based on proportional resonant controller for the circuit in FIG. 20 is given in FIG. 21, from which the open-loop transfer function can be determined and is given by Equation (9) and the corresponding closed-loop transfer function is given by Equation (10), where R denotes small resistance of the inductor L 631, Gpr(s) denotes proportional resonant controller and is given by Equation (11) with two control parameters Kp and K„ which can be determined using conventional controller design method based on the closed-loop transfer function in Equation (10).

[0058] The output from Step S3 in FIG. 21 is compared with sawtooth produced with reference to the DC input of each cell in FIG. 21 to produce gating signals. Such gating signals are applied to all the cells or stages in the circuit in FIG. 20. (7)

(8) (9) (10)

(ID

[0059] Alternatively one may use total real-power and reactive-power references for the converter in FIG. 20 as Pref and Qref, and use rms value of the voltage between terminals X and Y in FIG. 20 as Vrms to produce reference current i*(t) in Equation (7). Then the control flow as given in FIG. 22 is used to produce the total reference voltage and shown in Step S3. To produce gating signal, a sawtooth is generated with reference to total DC voltage of all the cells, which is equal to DC voltage of each cell multiplied by the number of cells with the assumption that each cell has the same DC input. The total reference voltage is compared with the sawtooth to produce gating signals, identically applied for all the cells or stages.

[0060] One may also use phase-shift pulse-width-modulation or level shift pulse-width-modulation to produce a group of sawtooth waveforms with the number equal to two times the cells in the circuit in FIG. 20. Then gating signals for different cells are produced by comparing the sawtooth waveforms with the reference voltage produced in Step S3 in FIG. 22.

[0061] FIG. 23 shows a sample microgrid, where there is one grid-forming generator 700, three grid-supporting generators 720, 721, and 722, two grid-feeding generators 740 and 741, nine loads 710 through 718, one energy storage 750 and two reactive power compensators 760 and 761. All the components are inter-linked by the three-phase connection network 730.

[0062] In a microgrid system, there must be at least one grid-forming generator which provides reference frequency and/or reference voltage for the system while there could be more than one grid-feeding generators and grid-supporting generators. The grid-feeding generators are usually interfaced with renewable energy generation system and pump in maximum power into the microgrid. The grid-supporting generators take role to facilitate the grid-forming generator to stabilize system in terms of voltage and/or frequency.

[0063] One may use three-identical multilevel converters as shown in FIG. 18 to form a three-phase DC/AC converter circuit either in Δ or Y connection. Such three-phase circuits could be used as grid-tied inverters for grid-forming, grid-supporting and gridfeeding generators in a microgrid, such as in the sample microgrid in FIG. 23, operating either in grid-connected or islanded modes as described in Darning Zhang and Eliathamby Ambikairajah, “De-coupled PQ Control for Operation of Islanded Microgrid", 28-30 September, AUPEC 2015, Wollongong, Australia, pp. 1-6.

[0064] One may also use three-identical multilevel converters as shown in FIG. 20 to form a three-phase DC/AC converter circuit either in Δ or Y connection. Such three-phase circuits could be used as grid-tied inverters for grid-forming, grid-supporting and gridfeeding generators in a microgrid, such as in the sample microgrid in FIG. 23, operating either in grid-connected or islanded modes.

Claims (30)

1. What is claimed is:

   1. A circuit, comprising in sequence a single-phase controllable DC/AC inverter; a first passive filter, comprising in sequence a series inductor; and a plurality of shunt branches, each of which consists of a series inductor; and a series capacitor for resonance with the series inductor in the shunt branch at each harmonic frequency with the fundamental frequency being the same as that for the single-phase controllable DC/AC inverter; and a tuneable shunt reactive power compensator; a single-phase medium-frequency or high-frequency transformer, which could also be placed preceding or in front of the tuneable shunt reactive power compensator instead of being placed after it; a second passive filter, comprising a plurality of shunt branches, each of which consists of a series inductor; and a series capacitor for resonance with the series inductor in the shunt branch at each harmonic frequency with the fundamental frequency being the same as that for the DC/AC inverter; and a single-phase diode rectifier.
2. 2. A circuit system, transferring power from one AC grid to another AC grid or AC load, comprising in sequence a first AC grid; a first LCL filter or a first L filter or a first LC filter or a first LCCL filter at the side of the first AC grid; an AC/DC rectifier either single-phase or three-phase; a first capacitor at DC-link; a DC/DC converter; a first Z-filter; the circuit in Claim 1; a series inductor; a second Z-filter; a second capacitor at DC-link; a DC/AC inverter either single-phase or three-phase; a second LCL filter or a second L filter or a second LC filter or a second LCCL filter; and a second AC grid or an AC load.
3. 3. A circuit system, transferring power from an AC grid to a DC load or battery or DC microgrid, comprising in sequence an AC grid; a LCL filter or a L filter or a LC filter or a LCCL filter at the side of AC grid; an AC/DC rectifier either single-phase or three-phase; a first capacitor at DC-link; a DC/DC converter; a first Z-filter; the circuit in Claim 1; a series inductor; a second Z-filter; a capacitor at DC-link; a DC power to DC power converter; a DC load, or battery, or DC microgrid.
4. 4. A circuit, combined from Claim 2 and Claim 3 to suit power supply to multiple loads’ power demand from an AC grid, consisting of an AC grid; a LCL filter or a L filter or a LC filter or a LCCL filter; an AC/DC rectifier either single-phase or three-phase; a capacitor at DC-link; a DC/DC converter; a Z-filter A; the circuit in Claim 1; a plurality of parallel branches connected at the output of the circuit of Claim 1, each of which comprises a series inductor; and a Z-filter B; each output of Z-filter B being connected either with a DC system, comprising in sequence a capacitor at DC-link; a DC power to DC power converter; a DC load or battery or DC microgrid; or with an AC system, comprising in sequence a capacitor at DC-link; a DC/AC inverter; a LCL filter or a L filter or a LC filter or a LCCL filter; and an AC grid or AC load.
5. 5. A circuit, transferring AC power from one AC grid at high voltage to another AC grid or AC load at low voltage through a multilevel or multistage rectifier, the circuit comprising in sequence, a first AC grid; a LCL filter or a L filter or a LC filter or a LCCL filter; a multilevel or multistage rectifier with multiple outputs, each of whose output is connected with in sequence a DC link capacitor; a DC/DC converter; a Z-filter A; the circuit in Claim 1; a series inductor; and a Z-filter B; the outputs of each Z-filter B being connected in parallel with a DC/AC converter, consisting of in sequence a capacitor of DC-link; a DC/AC inverter; a LCL filter or a L filter or a LC filter or a LCCL filter; and an AC grid or AC load.
6. 6. A circuit, transferring AC power from an AC grid at high voltage to a DC load, battery or DC microgrid through multilevel or multistage rectifier, comprising in sequence, a LCL filter or a L filter or a LC filter or a LCCL filter; a multilevel or multistage AC/DC rectifier with multiple DC outputs, each of whose output is connected with in sequence a DC link capacitor; a DC/DC converter; a Z-filter A; the circuit in Claim 1; a series inductor; and a Z-filter B; the outputs of each Z-filter B being connected in parallel with a DC/DC converter, consisting of in sequence a capacitor of DC-link; a DC power to DC power converter; a DC load, or battery, or DC microgrid.
7. 7. A method, using passive filters for the system in Claims 2,3,4,5, and 6 to shape current through and voltage across the medium-frequency transformer to be as close as possible to sinusoidal waveform, the method comprising using the series inductor after the diode rectifier in the circuits in Claims 2,3,4,5, and 6 to shape the current as close as possible to a DC current; using the second passive filter in Claim 1 to filter the harmonic currents; using the first passive filter in Claim 1 to filter harmonic voltage to shape the voltage across the medium-frequency transformer as close as possible to sinusoidal waveform.
8. 8. A method of reactive power compensation, being for the system in Claims 2,3,4,5, and 6 to achieve nearly zero-current switching in the square wave single-phase inverter at different power levels, the method comprising using the shunt tuneable capacitors as reactive power compensator as described in the circuit in Claim 1; building up a look-up table with suitable capacitance values against power levels through the circuit in Claim 1 to achieve nearly zero-current switching of the singlephase inverter at each power level; using the look-up table to choose a suitable capacitance at each power level to achieve nearly zero-current switching of the single-phase DC/AC inverter in the real-time operation.
9. 9. A method, being for the coordination between real power control for the AC/DC rectifier and voltage control for the DC/DC converter after the capacitor at DC-link connected with the AC/DC rectifier in the circuits in Claims 2,3,4,5, and 6, the method comprising real power being controlled by the AC/DC rectifier which is not involved with the regulation of voltage across the capacitor at DC-link; voltage across the capacitor at DC-link being solely regulated by the DC/DC converter.
10. 10. A circuit, comprising in sequence a first bi-directional DC/AC controllable converter; a series tuneable inductor B; a first plurality of shunt branches, each of which consists of a series inductor; and a series capacitor for resonance with the series inductor in the shunt branch at each harmonic frequency with the fundamental frequency being the same as that for the DC/AC inverter; and a tuneable shunt reactive power compensator; a single-phase medium-frequency or high-frequency transformer, which could also be placed in front of the tuneable shunt reactive power compensator instead of being placed after it; a second plurality of shunt branches, each of which consists of a series inductor; and a series capacitor for resonance with the series inductor in the shunt branch at each harmonic frequency with the fundamental frequency being the same as that for the DC/AC inverter; and a series tuneable inductor C; a second bi-directional controllable AC/DC converter.
11. 11. A circuit, for achieving bi-directional power transferring between two AC systems or between one AC system and one DC system, comprising in sequence, a first AC grid or AC load; a LCL filter or a L filter or a LC filter or a LCCL filter; a bi-directional controllable AC/DC converter; a DC-link capacitor; a bi-directional DC/DC converter; a Z-filter A; a series tuneable inductor A; the circuit in Claim 10; a single or a plurality of parallel branches connected at the output of the circuit of Claim 10, each of which comprises in sequence a series tuneable inductor D; and a Z-filter B; each output of the Z-filter B being connected with one or a plurality of DC systems, each of which comprises in sequence a capacitor at DC-link; a bi-directional DC power to DC power converter; a DC load or battery or DC microgrid; and/or with one or a plurality of AC systems, each of which comprise in sequence a capacitor at DC-link; a DC/AC inverter; a LCL filter or a L filter or a LC filter or a LCCL filter; and a second AC grid or AC load.
12. 12. A circuit, for achieving bi-directional power transferring between one AC system and one DC system through a multilevel or multistage AC/DC converter with multiple outputs, each of whose output is connected with in sequence a DC link capacitor; a bi-directional DC/DC converter; a Z-filter A; a series tuneable inductor A; the circuit in Claim 10; a series tuneable inductor D; and a Z-filter B; the output of each of the Z-filter B being connected in parallel with a DC/DC converter, consisting of in sequence a DC-link capacitor; a bidirectional DC power to DC power converter; and a DC load, or battery, or DC microgrid.
13. 13. A circuit, for achieving bi-directional power transferring between two AC systems through a multilevel or multistage AC/DC converter with multiple DC outputs, each of whose output is connected with in sequence a DC link capacitor; a bi-directional DC/DC converter; a Z-filter A; a series tuneable inductor A; the circuit in Claim 10; a series tuneable inductor D; and a Z-filter B; the output of each of the Z-filter B being connected in parallel with a DC/AC converter, consisting of in sequence a bidirectional DC/DC converter; a DC-fink capacitor; a bi-directional controllable DC/AC converter; a LCL filter or a L filter or a LC filter or a LCCL filter; and an AC grid or AC load.
14. 14. A method, using passive filters for the system in Claims 11,12 and 13 to shape current through and voltage across the medium-frequency transformer to be as close as possible to sinusoidal waveform, the method comprising in case one when the power flows from first element to last element in the circuits in Claims 11,12 and 13 tuning the series tuneable inductor A and the series tuneable inductor C in the circuits in Claims 11,12 and 13 to a very small or nearly zero value from their respective nominal values; tuning the series tuneable inductor B and the series tuneable inductor D in the circuits in Claims 11,12 and 13 to their respective nominal values; in case two when the power flows from last element to first element in the circuits in Claims 11,12 and 13 tuning the series tuneable inductor A and the series tuneable inductor C in the circuits in Claims 11,12 and 13 to their respective nominal values; tuning the series tuneable inductor B and series tuneable inductor D in the circuit in Claims 11,12 and 13 to a very small or nearly zero value from their respective nominal values.
15. 15. A method of reactive power compensation, being for the systems in Claims 11,12 and 13 to achieve nearly zero-current switching in the square wave single-phase inverter at different power levels, the method comprising using shunt tuneable capacitors as reactive power compensator as described in the circuit in Claims 11,12 and 13; building up a look-up table with suitable capacitance values against power levels through the circuits in Claims 11,12 and 13 to achieve zero-current switching for bidirectional DC/AC controllable converter in Claim 10 working as a single-phase inverter; using the look-up table to choose a suitable capacitance at each power level to achieve nearly zero-current switching in the real-time operation for the bi-directional DC/AC controllable converter in Claim 10 working as a single-phase inverter.
16. 16. A method, being for the coordination between real power control for the bi-directional AC/DC controllable converter working as an AC/DC rectifier and voltage control for the DC/DC converter after the capacitor at DC-link connected with the bi-directional AC/DC controllable converter working as an AC/DC rectifier in the circuits in Claims 11,12 and 13, the method comprising real power being controlled by the AC/DC rectifier which is not involved with the regulation of voltage across the capacitor at DC-link; voltage across the capacitor at DC-link being solely regulated by the DC/DC converter.
17. 17. A multilevel converter circuit, consisting of a plurality of identical H-bridge DC/AC converter cells in cascaded connection; and a LCCL filter, each cell consisting of a DC source or an equivalent DC source or an equivalent DC load; a H-bridge converter with an AC side and a DC side; and the LCCL filter consisting of a first series connected inductor at the AC side of the first cell of H-bridge DC/AC converter cells in cascaded connection; a series capacitor positioned after the series inductor; a shunt capacitor positioned after the series capacitor and between the first and last cells; a second series inductor positioned after the shunt capacitor.
18. 18. A three-phase circuit, consisting of three identical circuits, each of which is the same as the circuit in Claim 17.
19. 19. A n-phase circuit, consisting of n identical circuits, each of which is the same as the circuit in Claim 17, where n is greater than 3.
20. 20. A microgrid system, wherein a grid-forming generator and/or each of a plurality of grid-supporting generators and /or each of a plurality of grid-feeding generators consist of the three-phase circuit in Claim 18.
21. 21. A control method for the three-phase circuit in Claim 18, and Claim 20, wherein identical control signals are applied to different cells or stages in each phase.
22. 22. A microgrid system, wherein a grid-forming generator is formed by a three-phase circuit, each phase constituting a multilevel converter circuit, consisting of a plurality of identical H-bridge DC/AC converter cells in cascaded connection; and a LC filter, each cell consisting of a DC source or an equivalent DC source; a H-bridge converter with an AC side and a DC side; and the LC filter consisting of a series-connected inductor at the AC side of the first cell of the H-bridge DC/AC converter cells in cascaded connection; a series-connected capacitor positioned after the series inductor.
23. 23. A microgrid system, wherein each of a plurality of grid-supporting generators is formed by a three-phase circuit, each phase constituting a multilevel converter circuit, consisting of a plurality of identical H-bridge DC/AC converter cells in cascaded connection; and a LC filter, each cell consisting of a DC source or an equivalent DC source; a H-bridge converter with an AC side and a DC side; and the LC filter consisting of a series-connected inductor at the AC side of the first cell of the H-bridge DC/AC converter cells in cascaded connection; and a series-connected capacitor positioned after the series inductor.
24. 24. A microgrid system, wherein each of a plurality of grid-feeding generators is formed by a three-phase circuit, each phase constituting a multilevel converter circuit, consisting of a plurality of identical H-bridge DC/AC converter cells in cascaded connection; and a LC filter, each cell consisting of a DC source or an equivalent DC source; a H-bridge converter with an AC side and a DC side; and the LC filter consisting of a series-connected inductor at the AC side of the first cell of the H-bridge DC/AC converter cells in cascaded connection; and a series-connected capacitor positioned after the series inductor.
25. 25. A multilevel converter circuit, consisting of a plurality of identical H-bridge DC/AC converter cells in cascaded connection; and a LC filter, each cell consisting of a DC source or an equivalent DC source or an equivalent DC load; a H-bridge converter with an AC side and a DC side; and the LC filter consisting of a series-connected inductor at the AC side of the first cell of the H-bridge DC/AC converter cells in cascaded connection; and a series-connected capacitor positioned after the series inductor.
26. 26. A three-phase circuit, consisting of three identical circuits, each of which is the same as the circuit in Claim 25.
27. 27. A n-phase circuit, consisting of n identical circuits, each of which is the same as the circuit in Claim 25, where n is greater than 3.
28. 28. A control method for the three-phase circuit in Claim 22, Claim 23, Claim 24 and Claim 26, wherein identical gating signals are applied to different cells or stages in each phase.
29. 29. A method, wherein a passive filter is used between the first and last cells of a multilevel DC/AC converter which has a DC side and an AC side, is connected with an AC system after the passive filter and works with a rated power level; the passive filter is placed at the AC side of the DC/AC converter and includes a part formed by one series inductor or one equivalent series inductor in series with one capacitor or one equivalent capacitor at the AC side of the DC/AC converter; the capacitance of the capacitor partly cancels the inductance of the inductor, leaving overall effect of the inductor and capacitor in series as an equivalent inductive element; the inductance of the inductor and capacitance of the capacitor are chosen in such a way that the equivalent inductive element as a part of the passive filter is sufficient to contain harmonic current into the AC system within limit, say 5%, at a fraction of the rated power; the fraction of the rated power could be 6% or any other value falling in the range of 0% and 100%.
30. 30. A microgrid, wherein the method in Claim 29 is adopted to design a passive filter working with DC/AC inverters used as a part of grid-forming, grid-supporting, and grid-feeding generators.