WO2013185847A1 - Method and apparatus for performing power conversion - Google Patents

Method and apparatus for performing power conversion Download PDF

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Publication number
WO2013185847A1
WO2013185847A1 PCT/EP2012/061529 EP2012061529W WO2013185847A1 WO 2013185847 A1 WO2013185847 A1 WO 2013185847A1 EP 2012061529 W EP2012061529 W EP 2012061529W WO 2013185847 A1 WO2013185847 A1 WO 2013185847A1
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WO
WIPO (PCT)
Prior art keywords
input
output
voltage
bus
circuit
Prior art date
Application number
PCT/EP2012/061529
Other languages
French (fr)
Inventor
Petar Grbovic
Original Assignee
Huawei Technologies Co., Ltd.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Huawei Technologies Co., Ltd. filed Critical Huawei Technologies Co., Ltd.
Priority to PCT/EP2012/061529 priority Critical patent/WO2013185847A1/en
Priority to CN201280073496.4A priority patent/CN104350663A/en
Publication of WO2013185847A1 publication Critical patent/WO2013185847A1/en

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Classifications

    • 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
    • H02J9/00—Circuit arrangements for emergency or stand-by power supply, e.g. for emergency lighting
    • H02J9/04—Circuit arrangements for emergency or stand-by power supply, e.g. for emergency lighting in which the distribution system is disconnected from the normal source and connected to a standby source
    • H02J9/06—Circuit arrangements for emergency or stand-by power supply, e.g. for emergency lighting in which the distribution system is disconnected from the normal source and connected to a standby source with automatic change-over, e.g. UPS systems
    • H02J9/062—Circuit arrangements for emergency or stand-by power supply, e.g. for emergency lighting in which the distribution system is disconnected from the normal source and connected to a standby source with automatic change-over, e.g. UPS systems for AC powered loads
    • 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
    • H02M5/00—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases
    • H02M5/40—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into dc
    • H02M5/42—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into dc by static converters
    • H02M5/44—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into dc by static converters using discharge tubes or semiconductor devices to convert the intermediate dc into ac
    • H02M5/453—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into dc by static converters using discharge tubes or semiconductor devices to convert the intermediate dc into ac using devices of a triode or transistor type requiring continuous application of a control signal
    • H02M5/458—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into dc by static converters using discharge tubes or semiconductor devices to convert the intermediate dc into ac using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
    • H02M5/4585—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into dc by static converters using discharge tubes or semiconductor devices to convert the intermediate dc into ac using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only having a rectifier with controlled elements

Definitions

  • the invention relates to a method and apparatus for perform ⁇ ing a power conversion, in particular to perform an AC/DC/AC power conversion, wherein at least one AC input voltage pro ⁇ vided by an AC voltage source is converted into an AC output voltage supplied to a load.
  • an AC-DC-AC power converter can be provided having an intermediate DC circuit with a DC bus voltage, wherein the DC bus circuit connects an input signal conversion circuit and an output signal conversion as depicted in Fig. 1.
  • a conventional uninterruptable power supply comprises a three-phase AC/DC/AC power converter with a constant DC bus voltage as shown in Fig. 1 and a battery DC/DC converter connected to the intermediate DC bus circuit of the AC/DC power converter.
  • Such an uninterruptible power supply system is switchable between a grid mode and a battery mode.
  • the input signal conversion circuit of the AC/DC/AC power converter receives AC phase signals from a three-phase power supply grid to generate an AC output voltage for criti ⁇ cal loads such as a Telecom centre or medical apparatuses in a hospital.
  • the uninterruptible power supply system switches to the battery mode and a DC/DC converter applies a DC voltage to the intermediate DC bus circuit of the AC/DC/AC power con ⁇ verter.
  • the input signal conversion circuit of the AC/DC/AC power converter can be composed of an input rectifier, wherein the input rectifier regulates a DC voltage at certain constant values.
  • the output signal conversion circuit in a conventional AC/DC/AC power converter as shown in Fig. 1 can be an output inverter such as a three-phase PWM inverter.
  • the AC/DC/AC power converter performs a double-AC-DC-AC conver ⁇ sion with a constant DC bus voltage. As can be seen in Fig.
  • a conventional AC/DC power converter comprises an input signal conversion circuit which is con ⁇ nected to the three-phase grid via input inductors Li N i , Li N2 , L IN3 and connected to a three-phase load by means of output inductors L 0UTI , L OUT2 and L 0UT 3 ⁇
  • the intermediate DC circuit comprises two capacitors CI, C2 connected to each other at a common neutral node NN.
  • the common neutral node NN of the in ⁇ termediate DC circuit is connected to a neutral line N con ⁇ necting the three-phase grid with the three-phase load as shown in Fig. 1.
  • the conventional AC/DC/AC power converter as shown in Fig.
  • the invention provides according to a first aspect of the present invention a power converter adapted to convert at least one AC input voltage into an AC output voltage by a power converter comprising the features of claim 1.
  • the power converter is adapted to convert at least one AC input voltage into an AC output voltage, wherein said power converter comprises:
  • an input signal conversion circuit having half-bridge con ⁇ nected switches being connected to each other at a signal in- put node to which said AC input voltage is applied by an AC voltage source,
  • an output signal conversion circuit having half-bridge con ⁇ nected switches being connected to each other at a signal output node from which the AC output voltage is supplied to a load
  • said DC bus circuit having half-bridge connected capacitors connected to each other at a neutral node
  • control circuit adapted to control the switches of said in ⁇ put signal conversion circuit and said output signal conver ⁇ sion circuit such that said bus line voltages of said bus lines follow a voltage signal envelope corresponding to the maximum voltage of the AC input voltage and/or the AC output voltage .
  • the power converter according to the first aspect of the pre ⁇ sent invention has the advantage that it provides minimum switch losses in the signal conversion circuit and therefore a maximum efficiency which can be close to 99%.
  • a further advantage of the power converter according to the first aspect of the present invention is that it can be im- plemented in a UPS system allowing an instantaneous transi ⁇ tion from a grid mode to a battery mode.
  • a further advantage of the power converter according to the first aspect of the present invention is that it can be im- plemented with a minimum size.
  • the input signal conversion circuit has a first and second half-bridge connected input switch being connected to each other at said signal input node being con ⁇ trolled by said control circuit.
  • said first input switch is provided between said signal input node and a positive bus line of said DC bus circuit.
  • the second input switch of said input signal conversion circuit is provided between said signal input node and a negative bus line of said DC bus circuit.
  • said output signal con ⁇ version circuit has a first and second half-bridge connected output switch being connected to each other at said signal output node and being controlled by the control circuit.
  • a first output switch is pro ⁇ vided between said signal output node and a positive bus line of said DC bus circuit.
  • the second output switch is provided between said signal output node and the negative bus line of said DC bus circuit.
  • the DC bus circuit has a first and second half-bridge connected capacitor con ⁇ nected to each other at said neutral node.
  • said first capacitor is pro ⁇ vided between said neutral node and the positive bus line of said DC bus circuit.
  • both ca ⁇ pacitors of said DC bus circuit comprise a diode which is connected in antiparallel direction between the neutral node of said DC bus circuit and the positive or negative bus line of said DC bus circuit.
  • an input inductor is connected between said AC voltage source and the signal input node of said input signal conversion circuit.
  • an output induc- tor is connected between the signal output node of said out ⁇ put signal conversion circuit and said load.
  • control cir ⁇ cuit is adapted to monitor at least the bus line voltages of the bus lines of said intermediate DC bus circuit and the AC input voltage.
  • control cir ⁇ cuit is adapted to monitor at least the bus line voltages at the bus lines of said intermediate DC bus circuit and the AC output voltage.
  • control cir- cuit is adapted to monitor at least the bus line voltages at the bus lines of said intermediate DC bus circuit and the AC input voltage as well as the AC output voltage.
  • control circuit comprises an input controller which is adapted to compare the bus line voltages of the bus lines of said inter ⁇ mediate DC bus circuit with reference bus line voltages to calculate an input duty cycle.
  • input controller of said control circuit further comprises an input bridge modu ⁇ lator to which said calculated input duty cycle is applied and which is adapted to generate switching control signals applied to said first and second input switches of said input signal conversion circuit depending on said input duty cycle.
  • control circuit further comprises an output controller which is adapted to compare the AC output voltage with a reference output voltage to calculate an output duty cycle.
  • the output control ⁇ ler of the control circuit comprises an output bridge modula ⁇ tor to which the calculated output duty cycle is applied and which is adapted to generate switch control signals applied to said first and second switch of said output signal conver ⁇ sion circuit depending on said output duty cycle.
  • the AC input voltage is a single AC phase signal supplied to said power converter by a three-phase power grid.
  • the invention further provides according to a second aspect an uninterruptible power supply system comprising at least one power converter according to the first aspect of the pre ⁇ sent invention as defined in claim 14.
  • the UPS system comprises at least one power converter according to any of the implementations of the power converter according to the first aspect of the present invention wherein for each power converter a corre- sponding DC battery converter with a variable voltage gain is provided, wherein said DC/DC battery converter is connected between a battery and the DC bus circuit of the respective power converter.
  • the invention further provides according to a third aspect a method for converting at least one AC input voltage into an AC output voltage as defined by claim 15.
  • the invention accordingly provides in possible first imple- mentation of the method for converting at least one AC input voltage into an AC output voltage according to the third as ⁇ pect of the present invention, a method wherein half-bridge connected switches of an input signal conversion circuit re ⁇ ceiving said AC input voltage at a common signal input node and half-bridge connected switches of an output signal con ⁇ version circuit outputting said AC output voltage at a common signal output node are controlled such that bus line voltages of bus lines of an intermediate DC bus circuit connecting said input signal conversion circuit and said output signal conversion circuit follow a voltage signal envelope corre ⁇ sponding to the maximum voltage of the AC input voltage and/or the AC output voltage.
  • Fig. 1 shows a block diagram of a conventional AC/DC/AC power converter with a constant DC bus voltage
  • Fig. 2 shows a block diagram of a possible implementation of a uninterruptible power supply system according to the second aspect of the present invention comprising a possible imple- mentation of a power converter according to the first aspect of the present invention
  • Fig. 3 shows a signal diagram for illustrating the operation of a possible implementation of a power converter according to the first aspect of the present invention
  • Fig. 4 shows a circuit diagram of a possible implementation of a power converter according to the first aspect of the present invention
  • Fig. 5 shows a block diagram for illustrating a possible implementation for a control circuit as employed in a possible embodiment of the power converter according to the first as ⁇ pect of the present invention
  • Fig. 6 shows a further block diagram of a further possible implementation of a control circuit as employed in a possible embodiment of the power converter according to the first as ⁇ pect of the present invention.
  • Figs. 7a) to 7e) show circuit diagrams of an input bridge equivalent circuit for illustrating the operation of a possi ⁇ ble implementation of a power converter according to the first aspect of the present invention
  • Figs. 8a) to 8e) show signal diagrams for illustrating the operation of a possible implementation of a power converter according to the first aspect of the present invention
  • Figs. 9a) to 9e) show circuit diagrams of output bridge equivalent circuit diagrams of a possible implementation of a power converter according to the first aspect of the present invention
  • Figs. 10a) to lOe) show signal diagrams for illustrating the operation of a possible implementation of a power converter according to the first aspect of the present invention
  • Fig. 11 shows a signal diagram for illustrating the defini- tion of voltage ratings of top and bottom switches as em ⁇ ployed in a possible implementation of the power converter according to the first aspect of the present invention
  • Fig. 12 shows a signal diagram for illustrating the relative inductance versus the input to output voltage ratio provided by a power converter according to the first aspect of the present invention
  • an uninterruptible power supply UPS system according to the first aspect of the present invention comprises at least one power con ⁇ verter 2 according to the first aspect of the present inven- tion.
  • the UPS system can be a single-phase system or as shown in the embodiment of Fig. 2 a three-phase UPS system.
  • the three-phase UPS system comprises for each phase LI, L2, L3 of a three-phase grid 3, a single-phase power converter 2 ac ⁇ cording to the first aspect of the present invention.
  • the UPS system 1 comprises a DC/DC converter 4 con ⁇ nected to a battery 5 as shown in Fig. 2.
  • the AC power converter as shown in the implementation of Fig.
  • the in ⁇ put signal conversion circuit 6 comprises two half-bridge connected switches being connected to each other at a signal input node SIN.
  • This signal input node SIN of the input sig ⁇ nal conversion circuit 6 receives the AC input voltage to be converted, for example via an input inductance Li N as shown in Fig. 2.
  • the output signal conversion circuit 7 can comprise two half-bridge connected switches being con ⁇ nected to each other at a signal output node SON.
  • the voltage at the signal output node SON is applied for example via an output inductance L 0UT as the AC output voltage to the load 9 as shown in Fig. 2.
  • the three-phase UPS system 1 as shown in Fig. 2 comprises three single-phase UPS converters 1-1, 1-2, 1-3 each having an AC/DC/AC power converter 2-i and a DC/DC converter 4-i as illustrated in Fig. 2.
  • the load 9 as shown in Fig. 2 is a three-phase load connected to the output terminals of the three single-phase UPS converters 1-1, 1-2, 1-3.
  • the AC sig ⁇ nal source of the UPS system 1 as shown in Fig. 2 is a three- phase power grid 3 applying the three AC signal voltages or phases LI, L2, L3 to the AC-input terminals AC-input A, AC- input B, AC-input C of the three single-phase UPS converters 1-1, 1-2, 1-3.
  • Each single-phase UPS converter 1-1, 1-2, 1-3 comprises a further DC voltage input connected to the common battery 5 for supplying the DC voltage of the battery 5 to the respective DC/DC converter 4-i within the single-phase UPS converter 1-i.
  • the three-phase power grid 3 and the three-phase load 9 are further connected to each other via a neutral line N being also connected to a neutral node NN within the intermediate DC power circuit 8 of each AC/DC/AC converter 2-i of the three phase UPS system 1.
  • Each intermediate DC bus circuit 8- i connects the respective input signal conversion circuit 6-i with the respective output signal conversion circuit 7-i via two bus lines comprising a positive bus line (plus) and a negative bus line (minus) .
  • the positive bus line (plus) has a positive potential
  • the negative bus line (minus) has a negative potential.
  • the DC bus circuit 8 as shown in Fig. 2 comprises two half-bridge connected capacitors CI, C2 being connected to each other at the neutral node NN.
  • the in ⁇ termediate DC bus circuit 8 further comprises two antiparal- lel diodes Dl, D2 connected in antiparallel direction between the neutral node NN of the DC bus circuit 8 and the positive and respective bus line (plus, minus) of the DC bus circuit 8 as illustrated in Fig. 2.
  • a control circuit 11-i is provided which is adapted to control the switches of the input signal conver- sion circuit 6 and the output signal conversion circuit 7 such that the bus line voltages of the bus lines (plus, mi ⁇ nus) of the DC bus circuit 8 follow the voltage signal enve ⁇ lope corresponding to the maximum voltage of the AC input voltage applied at the AC input of the respective power con- verter 1-i and/or the AC output voltage output the AC output of the respective power converter 1-i.
  • the control circuit 11 accordingly monitors the AC input voltage supplied to the power converter 1-i and/or the AC output voltage output by the AC/DC/AC power converter 1-i.
  • the control voltage can in a possible embodiment monitor the AC input voltage and the AC output voltage at the input and output terminal of the respective single-phase UPS converter 1-i as shown in Fig. 2 or directly the AC input voltage or AC output voltage at the signal input node SIN of the input sig ⁇ nal conversion circuit 6 and at the signal output node SON of the output signal conversion circuit 7.
  • Possible implementa- tions of the control circuit 11 are shown in Figs. 5, 6 de ⁇ scribed later in more detail.
  • the input of the DC/DC battery con- verter 4-i can be connected to the battery 5 via three wires as shown in Fig. 2 or via two wires.
  • the output side of the DC/DC converter 4-i is directly connected to the DC bus lines denoted as plus, minus and neutral.
  • the input signal conversion circuit 6 can be formed by an input rectifier.
  • This input rectifier can be a pulse width modulated (PWM) voltage source converter having the DC side ca ⁇ pacitors and the AC side filter inductors.
  • the DC side ca ⁇ pacitors of the input rectifier are the capacitors CI, C2 of the intermediate DC bus circuit 8.
  • the output signal conversion circuit 7 can be implemented as an output inverter and implemented as a pulse width modulated PWM voltage source converter having DC side capacitors and AC side filter induc ⁇ tors.
  • the DC side capacitors are again the capacitors CI, C2 of the intermediate DC bus circuit 8.
  • the input rectifier and the output inverter can be implemented or composed of active switches such as IGBT or which can comprise parallel freewheeling diodes.
  • the control circuit 11-i is adapted to control the switches of the input signal conversion circuit 6 and the output sig- nal conversion circuit 7 such that the bus line voltages at the bus lines plus, minus of the intermediate DC bus circuit 8 follow a voltage signal envelope corresponding to the maxi ⁇ mum voltage of the AC input voltage and/or the AC output voltage .
  • Fig. 3 illustrates the signal waveforms as provided under the control of the control circuit 11-i.
  • the input signal conver- sion circuit 6 and the output signal conversion circuit 7 are controlled in a way by the control circuit 11 to have a posi ⁇ tive DC bus voltage that follows the positive envelope of the input or output voltage.
  • the input signal conversion circuit 6 and the output signal conversion circuit 7 are further con ⁇ trolled by the control circuit 11 in a way to have a negative DC bus voltage that follows a negative envelope of the AC in ⁇ put voltage or the AC output voltage.
  • the DC bus voltage is following the AC in- put voltage and/or the AC output voltage.
  • Fig. 4 shows a possible implementation of a power converter 2 within a single-phase UPS converter 1 in more detail. As can be seen in Fig. 4 the power converter 2 has a single-phase
  • AC/DC/AC power converter 1 comprising an input signal conversion circuit 6, an output signal conversion circuit 7 and an intermediate DC bus circuit 8 connecting the input signal conversion circuit 6 with the output signal conversion cir- cuit 7.
  • the AC voltage source applies an AC input voltage via the input inductance Li N to the signal input node SIN of the input signal conversion circuit 6.
  • an AC output voltage is output from a signal output node SON of the output signal conversion circuit 7 via an output inductance L 0UT to a load 9 as shown in Fig. 4.
  • the input signal conversion circuit 6 has a first and a second half- bridge connected input switch SW1, SW2 being connected to each other at the signal input node SIN.
  • the first input switch SW1 is provided between said signal input node SIN and a positive bus line plus of the DC bus circuit 8.
  • the second input switch SW2 is provided between the signal input node SIN and a negative bus line minus of the DC bus circuit 8.
  • the intermediate DC bus circuit 8 comprises in the embodiment of Fig. 4 a first capacitor CI and a second capacitor C2.
  • the capacitors CI, C2 are connected to each other at a common neutral node NN as shown in Fig. 4.
  • the neutral node NN is connected to a neutral line connecting the input voltage source and the load 9.
  • the first capacitor CI of the interme ⁇ diate DC bus circuit 8 is provided between the neutral node NN and the positive bus line, (plus) , of the DC bus circuit 8.
  • the second capacitor C2 is provided between the neutral node NN and the negative bus line, (minus) , of the DC bus circuit 8.
  • Both capacitors CI, C2 of the DC bus circuit 8 comprise a diode D C i and D C 2 which is connected in antiparal- lel direction between the neutral node NN of the DC bus cir- cuit 8 and the positive or negative bus line of the DC bus circuit 8 as shown in Fig. 4.
  • antiparallel freewheeling diodes are provided in the input signal conversion circuit 6 .
  • diodes D S i and D S 2 are connected in antiparallel direction between the signal input node SIN and a respective bus line of the DC bus circuit 8.
  • the diode D SW i is connected in paral ⁇ lel to the first input switch SW1 of the input signal conver ⁇ sion circuit 6.
  • the second diode D SW 2 is connected in parallel to the second input switch SW2 of the input signal conversion circuit 6.
  • the output signal conversion circuit 7 also comprises a first and second half-bridge connected output switch SW3, SW4 being connected to each other at the signal output node SON and controlled by said control circuit 11 of the respective
  • the signal output node SON is connected via a coil L ou t to the load 9 connected between nodes 10a, 10b.
  • the first output switch SW3 is provided be- tween the signal output node SON and the positive bus line plus of the intermediate DC bus circuit 8.
  • the second output switch SW4 is provided between the signal output node SON and the negative bus line minus of the DC bus circuit 8.
  • the first and second half-bridge connected output switches SW3, SW4 are also controlled by the control circuit 11.
  • a diode D SW3 is connected in antipar- allel direction as shown in Fig. 4.
  • a diode D sw4 is connected in antiparallel direction.
  • the switches SW1, SW2, SW3, SW4 are electronic switches such as IGBT switches or MOSFETS connected to the control circuit 11, wherein the switching is controlled by the control circuit 11 which supplies control signals SI, S2, S3, S4 to control electrodes of the respective electronic switches SW1, SW2, SW3, SW4.
  • the circuit as shown in Fig. 4 provides a full DC bus voltage envelope tracking as shown in Fig. 3. Waveforms of the DC bus voltage of the AC input voltage and the AC output voltage are illustrated in Fig. 3.
  • the input AC voltage for example, is a sinusoidal voltage:
  • V IN V IN sm&t , (1) wherein Vi N is the voltage magnitude and ⁇ is the angular frequency .
  • the AC/DC/AC power converter 2 as shown in Fig. 4 and the input/output voltages as well as the DC bus voltage have the following relation:
  • V BUS2 V 1N— V BUS ⁇ V BUS2 — V OUT— V BUS ⁇ ⁇ (3)
  • V I N ⁇ V 0UT the input voltage is lower than the output voltage (V I N ⁇ V 0UT ) ⁇
  • V IN V 0UT
  • V IN >V 0UT the input voltage is higher than the output voltage (V IN >V 0UT ) ⁇
  • a simplified equivalent circuit diagram of the input signal conversion circuit 6 can be depicted as shown in Fig. 7a) .
  • the DC bus ca ⁇ pacitors CI, C2 are modelled as voltage sources V B usi and V B us2 as shown in Fig. 7a) .
  • the DC bus voltage can be arbitrary positive voltages. If the input voltage Vi N is positive
  • the first input switch SW1 and the antiparallel di ⁇ ode D S 2 are carrying the input current ii N , while the second input switch SW2 and the antiparallel diode D SW i connected in parallel to the first input switch SW1 are inactive.
  • the first input switch SW1 is closed and the circuit can be described by the following equation:
  • the duty cycle di N can be calculated as follow ⁇ ing :
  • the instantaneous electrical current of the first and second input switches SW1, SW2 of the input signal conversion circuit 6 and its diodes D S i and D S 2 is given by: , sw
  • the second input switch SW2 and the diode D 3W i do not carry any electrical current when the input voltage is positive (0 ⁇ ) .
  • the moving average and RMS electrical current of the first input switch SW1 and the diode D 3W2 is given by:
  • the input current ripple is given by: (12) wherein f sw is a switching frequency.
  • the first input switch SW2 and the diode D S i are carrying the input current iiN, while the first input switch SW1 and the diode D S 2 are inactive .
  • the DC bus voltage can actively be controlled to follow the half-way rectified sinusoidal waveforms:
  • V BUS ( MAX ) is a DC bus voltage magnitude which can be constant .
  • Fig. 8 shows the duty cycle di N of the input signal conver ⁇ sion circuit 6 at the first and second half-bridge connected input switches SW1, SW2.
  • the signal diagram in Fig. 8a) shows a sinusoidal AC input voltage Vi N applied to the input signal conversion circuit 6.
  • Fig 8b) shows the DC bus voltages V B usi / V B us2 over time.
  • Fig. 8d) shows the calculated input duty cy ⁇ cle di .
  • the signal diagram in Fig. 8d) shows the control signal SI output by the control circuit 11 to the first input switch SW1 of the input signal conversion circuit 6.
  • Fig. 8e) shows a signal diagram of a control signal S2 output by the control circuit 11 to the second output switch SW2 of the input signal conversion circuit 6.
  • the current of the switches and diode average and RMS current can be calculated over a fundamental period as follows:
  • the inductance of the input inductor Li N can be calculated for a maximum current ripple given as a design parameter of the circuit:
  • the input bridge losses at the input signal conversion cir- cuit 6 can be calculated as follows:
  • the switch and diode output characteristics can be approxi ⁇ mated by the voltage knee V S wo , V D0 and dynamic resistance r S w , r D . Accordingly, the switching losses of the switches and di ⁇ odes can be calculated as follows: wherein E 0N , E OFF and E Q are switching energies normalised per VA.
  • DC bus capacitors CI, C2 are modelled by two DC bus voltage sources.
  • the con- verter load can be modelled by a voltage source V 0UT ⁇
  • the first output switch SW3 and the diode D sw4 are carrying the load current iouT as shown in Fig. 9b) .
  • the duty cycle of the first output switch SW3 is given by:
  • Fig. 10a shows a sinusoidal AC output voltage applied to the load.
  • Fig. 10b shows the bus voltage V B usi / B us2 ⁇
  • Fig. 10c shows the calcu- lated duty cycle d 0 uT for the output signal conversion circuit 7.
  • the switching control signal S3 shows the switching control signal S3 applied to the first output switch SW3 by the control circuit 11.
  • the signal diagram 10 ⁇ shows a further control signal S4 as ap ⁇ plied to the second output switch SW4 by the control circuit 11.
  • the average current of the switches and the RMS current can be given by:
  • Fig. 11 shows a signal diagram for illustrating the definition of a voltage rating of top and bottom switches in the signal conversion circuit 6 as employed by the AC/DC/AC con ⁇ verter according to the present invention. From equations (16) and (17) it is possible to calculate a total voltage across two switches as:
  • the switches and diodes blocking voltage can be defined as:
  • Fig. 12 shows relative inductance versus the input to output voltage ratio.
  • the relative inductance as shown in Fig. 12 is the ratio of inductance as employed by a conventional power conversion circuit with a constant DC bus voltage to the in ⁇ ductance as employed by the power converter according to the first aspect of the present invention.
  • the inductance as em ⁇ ployed by the conventional power conversion circuit is given by :
  • the relative inductance is dimin ⁇ ished when the input voltage approaches the output voltage, i.e. the ratio between the input and output voltage becomes 1. Accordingly, in the second scenario the input voltage is equal to the output voltage and the relative inductance is low. If the input voltage is lower than the output voltage (V I N ⁇ V 0UT ) the relative inductance increases. At a certain point when the input voltage becomes too low, the UPS system switches from the AC voltage source to the battery 5. On the contrary, if the input voltage is higher than the output voltage (v in > v ou t ) and exceeds a certain limit, the UPS sys ⁇ tem can switch to a fault managing mode to avoid destruction caused by a too high AC input voltage.
  • a switch and diode apparent power can be defined as the prod- uct of a device peak voltage and a peak current.
  • Apparent power of devices as employed by the apparatus according to the present invention is:
  • the power converter according to the first aspect of the present invention can in a possible implementa ⁇ tion require about 50% apparent power per switch compared t conventional converters.
  • Fig. 5 shows a block diagram of a basic control structure of a control circuit 11 of a power converter 2 according to the first aspect of the present invention.
  • the control circuit 11 comprises a reference voltage generator 11a for generating a reference DC bus voltage V B usi ⁇ This can be performed in re ⁇ sponse to an output voltage V 0UT ( REF ) to generate a DC bus ref- erence voltage V BUS ( REF ) as shown in Fig. 5.
  • the control cir ⁇ cuit 11 further comprises a second generator for generating a second DC bus reference voltage V B us2 ⁇ REF ) ⁇
  • the second reference voltage generator lib of the control circuit 11 can generate the second DC reference voltage V BUS 2( REF ) depending on an out- put reference voltage V 0 UT ( REF ) as shown in Fig. 5.
  • the control circuit 11 further can comprise a controller 11c which receives the DC bus voltages V BUS i, B us2 from the DC bus circuit 8 of the monitored single-phase power converter 2.
  • the con- troller 11c also receives the reference voltages V B usi ⁇ REF> and V B us2 (REF ) from the generators 11a, lib and the feedback bus signals V B usi and V B us2 to provide an input duty cycle di N for an input bridge modulator lid which generates switching control signals SI, S2 applied to the first and second input switch SW1, SW2 of the input signal conversion circuit 6.
  • the control circuit 11 further comprises a second controller lie which receives a monitored feedback of the output voltage VouT of the monitored single-phase AC/DC/AC converter 2 and an output reference voltage V 0 UT ( REF ) as shown in Fig. 5.
  • the sec ⁇ ond controller lie calculates an output duty cycle d 0 uT which is supplied to an output bridge modulator llf.
  • the output bridge modulator llf generates switching control signals S3, S4 applied the first and second output switch SW3, SW4 of the output signal conversion circuit 7 within the monitored
  • Fig. 6 shows a further implementation of control circuit 11 for a single-phase AC/DC/AC converter 2.
  • the output of the DC bus voltage controller 11c is a refer ⁇ ence for an input current I INRE F applied to an input current controller llg which receives the monitored feedback input current from the respective monitored AC/DC/AC power con ⁇ verter 2.
  • the input current controller llg calculates the in- put duty cycle di n in response to the monitored feedback in ⁇ put current ii N and the reference input current i IN (REF ) re ⁇ ceived from the bus voltage controller 11c.
  • the con ⁇ trol circuit 11 as shown in the implementation of Fig.
  • the method and apparatus for converting an AC input voltage into an AC output voltage according to the present invention provide a high efficiency when compared to conventional power converters and power conversion methods.
  • the efficiency of the apparatus and method according to the present invention can be in a range of 98.5 up to 99% which is significantly higher than the peak efficiency of any conventional power converter or method. Because of the high efficiency the power converter and power conversion method according to the present invention have a low energy consumption.
  • the power converter and method according to the present invention can be used in a wide range of applications.
  • the power converter according to the first aspect of the present inven ⁇ tion can be used in an uninterruptable power supply system comprising the power converter and a DC/DC battery converter.
  • Such a UPS system can be used for example in a data centre, in telecommunication facilities, in industry applications, and for any critical load such as medical devices within a hospital .

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Abstract

The present invention relates to a power converter adapted to convert at least one AC input voltage into an AC output voltage, a power converter (2) comprising: an input signal conversion circuit (6) having half-bridge connected switches (SW1, SW2) being connected to each other at a signal input node, SIN, to which said AC input voltage is applied by an AC voltage source (3); an output signal conversion circuit (7) having half-bridge connected switches (SW3, SW4) being connected to each other at a signal output node, SON, from which the output voltage is supplied to a load (9); an intermediate DC bus circuit (8) connecting said input signal conversion circuit (6) with said output signal conversion circuit (9) via bus lines; a DC bus circuit (8) having capacitors (Ci, C2) connected to each other at a neutral node, NN; and a control circuit (11) adapted to control the switches of said input signal conversion circuit (6) and of said output signal conversion circuit (7) such that said bus line voltages of said bus lines follow a voltage signal envelope corresponding to the maximum voltage of the AC input voltage and/or AC output voltage.

Description

Method and apparatus for performing power conversion
TECHNICAL BACKGROUND The invention relates to a method and apparatus for perform¬ ing a power conversion, in particular to perform an AC/DC/AC power conversion, wherein at least one AC input voltage pro¬ vided by an AC voltage source is converted into an AC output voltage supplied to a load.
In many applications it is necessary to perform a conversion from a first AC input voltage into a second AC input voltage. For example, in an on-line uninterruptable power supply (UPS) system an AC-DC-AC power converter can be provided having an intermediate DC circuit with a DC bus voltage, wherein the DC bus circuit connects an input signal conversion circuit and an output signal conversion as depicted in Fig. 1.
A conventional uninterruptable power supply (UPS) comprises a three-phase AC/DC/AC power converter with a constant DC bus voltage as shown in Fig. 1 and a battery DC/DC converter connected to the intermediate DC bus circuit of the AC/DC power converter. Such an uninterruptible power supply system is switchable between a grid mode and a battery mode. In a grid mode the input signal conversion circuit of the AC/DC/AC power converter receives AC phase signals from a three-phase power supply grid to generate an AC output voltage for criti¬ cal loads such as a Telecom centre or medical apparatuses in a hospital. If there is a failure in the power supply grid, the uninterruptible power supply system (UPS) switches to the battery mode and a DC/DC converter applies a DC voltage to the intermediate DC bus circuit of the AC/DC/AC power con¬ verter. The input signal conversion circuit of the AC/DC/AC power converter can be composed of an input rectifier, wherein the input rectifier regulates a DC voltage at certain constant values. The output signal conversion circuit in a conventional AC/DC/AC power converter as shown in Fig. 1 can be an output inverter such as a three-phase PWM inverter. The AC/DC/AC power converter performs a double-AC-DC-AC conver¬ sion with a constant DC bus voltage. As can be seen in Fig. 1 a conventional AC/DC power converter comprises an input signal conversion circuit which is con¬ nected to the three-phase grid via input inductors LiNi , LiN2 , LIN3 and connected to a three-phase load by means of output inductors L0UTI, LOUT2 and L0UT3 · The intermediate DC circuit comprises two capacitors CI, C2 connected to each other at a common neutral node NN. The common neutral node NN of the in¬ termediate DC circuit is connected to a neutral line N con¬ necting the three-phase grid with the three-phase load as shown in Fig. 1. The conventional AC/DC/AC power converter as shown in Fig. 1 performs a double conversion with a constant DC bus voltage at the bus lines of the intermediate DC cir¬ cuit. This leads to high switching losses within the signal conversion circuits as well as to input and output inductors having a high inductivity. Consequently, the efficiency of a UPS system in the grid mode is relatively low.
Accordingly, it is an object of the present invention to pro¬ vide an apparatus and a method which allows to convert an AC input voltage into an AC output voltage with maximum effi- ciency.
SUMMARY OF THE INVENTION
The invention provides according to a first aspect of the present invention a power converter adapted to convert at least one AC input voltage into an AC output voltage by a power converter comprising the features of claim 1.
According to a first possible implementation of the power converter according to the first aspect of the present inven¬ tion the power converter is adapted to convert at least one AC input voltage into an AC output voltage, wherein said power converter comprises:
an input signal conversion circuit having half-bridge con¬ nected switches being connected to each other at a signal in- put node to which said AC input voltage is applied by an AC voltage source,
an output signal conversion circuit having half-bridge con¬ nected switches being connected to each other at a signal output node from which the AC output voltage is supplied to a load,
an intermediate DC bus circuit connecting said input signal conversion circuit with said output signal conversion circuit via bus lines,
said DC bus circuit having half-bridge connected capacitors connected to each other at a neutral node, and
a control circuit adapted to control the switches of said in¬ put signal conversion circuit and said output signal conver¬ sion circuit such that said bus line voltages of said bus lines follow a voltage signal envelope corresponding to the maximum voltage of the AC input voltage and/or the AC output voltage .
The power converter according to the first aspect of the pre¬ sent invention has the advantage that it provides minimum switch losses in the signal conversion circuit and therefore a maximum efficiency which can be close to 99%.
A further advantage of the power converter according to the first aspect of the present invention is that it can be im- plemented in a UPS system allowing an instantaneous transi¬ tion from a grid mode to a battery mode.
A further advantage of the power converter according to the first aspect of the present invention is that it can be im- plemented with a minimum size. In a possible second implementation of the power converter according to the first implementation of the first aspect of the present invention the input signal conversion circuit has a first and second half-bridge connected input switch being connected to each other at said signal input node being con¬ trolled by said control circuit.
In a further possible third implementation of the second implementation of said power converter according to the first aspect of the present invention said first input switch is provided between said signal input node and a positive bus line of said DC bus circuit.
In a further possible fourth implementation of the second or third implementation of the power converter according to the first aspect of the present invention, the second input switch of said input signal conversion circuit is provided between said signal input node and a negative bus line of said DC bus circuit.
In a further possible fifth implementation of said first to fourth implementation of the power converter according to the first aspect of the present invention said output signal con¬ version circuit has a first and second half-bridge connected output switch being connected to each other at said signal output node and being controlled by the control circuit.
In a further possible sixth implementation of the fifth implementation of the power converter according to the first aspect of the present invention a first output switch is pro¬ vided between said signal output node and a positive bus line of said DC bus circuit.
In a possible seventh implementation of the fifth or sixth implementation of the power converter according to the first aspect of the present invention, the second output switch is provided between said signal output node and the negative bus line of said DC bus circuit.
In a further possible eighth implementation of the first to seventh implementation of the power converter according to the first aspect of the present invention to each switch of said input conversion circuit or of said output conversion circuit a diode is connected in antiparallel direction be¬ tween the respective signal node of said input conversion circuit and a bus line of said DC bus circuit.
In a further possible ninth implementation of said first to eighth implementation of said power converter according to the first aspect of the present invention the DC bus circuit has a first and second half-bridge connected capacitor con¬ nected to each other at said neutral node.
In a further possible tenth implementation of the ninth implementation of said power converter according to the first aspect of the present invention said first capacitor is pro¬ vided between said neutral node and the positive bus line of said DC bus circuit.
In a further possible eleventh implementation of the ninth or tenth implementation of the power converter according to the first aspect of the present invention said second capacitor is provided between said neutral node and the negative bus line of said DC bus circuit. In a further possible twelfth implementation of any of the ninth to eleventh implementation of the power converter according to the first aspect of the present invention both ca¬ pacitors of said DC bus circuit comprise a diode which is connected in antiparallel direction between the neutral node of said DC bus circuit and the positive or negative bus line of said DC bus circuit. In a further possible thirteenth implementation of the first to twelfth implementation of the power converter according to the first aspect of the present invention an input inductor is connected between said AC voltage source and the signal input node of said input signal conversion circuit.
In a further possible fourteenth implementation of the first to thirteenth implementation of the power converter according to the first aspect of the present invention an output induc- tor is connected between the signal output node of said out¬ put signal conversion circuit and said load.
In a further possible fifteenth implementation of the first to fourteenth implementation of the power converter according to the first aspect of the present invention the control cir¬ cuit is adapted to monitor at least the bus line voltages of the bus lines of said intermediate DC bus circuit and the AC input voltage. In a further possible sixteenth implementation of the first to fourteenth implementation of the power converter according to the first aspect of the present invention the control cir¬ cuit is adapted to monitor at least the bus line voltages at the bus lines of said intermediate DC bus circuit and the AC output voltage.
In a further possible seventeenth implementation of the first to fourteenth implementation of the power converter according to the first aspect of the present invention the control cir- cuit is adapted to monitor at least the bus line voltages at the bus lines of said intermediate DC bus circuit and the AC input voltage as well as the AC output voltage.
In a further possible eighteenth implementation of the first to seventeenth implementation of the power converter according to the first aspect of the present invention the control circuit comprises an input controller which is adapted to compare the bus line voltages of the bus lines of said inter¬ mediate DC bus circuit with reference bus line voltages to calculate an input duty cycle. In a further possible nineteenth implementation of the eighteenth implementation of the power converter according to the first aspect of the present invention the input controller of said control circuit further comprises an input bridge modu¬ lator to which said calculated input duty cycle is applied and which is adapted to generate switching control signals applied to said first and second input switches of said input signal conversion circuit depending on said input duty cycle.
In a further possible twentieth implementation of the first to nineteenth implementation of the power converter according to the first aspect of the present invention said control circuit further comprises an output controller which is adapted to compare the AC output voltage with a reference output voltage to calculate an output duty cycle.
In a further possible twenty-first implementation of the twentieth implementation of the power converter according to the first aspect of the present invention the output control¬ ler of the control circuit comprises an output bridge modula¬ tor to which the calculated output duty cycle is applied and which is adapted to generate switch control signals applied to said first and second switch of said output signal conver¬ sion circuit depending on said output duty cycle. In a further possible twenty-second implementation of any of the first to twenty-first implementation of the power con¬ verter according to the first aspect of the present invention the AC input voltage is a single AC phase signal supplied to said power converter by a three-phase power grid.
The invention further provides according to a second aspect an uninterruptible power supply system comprising at least one power converter according to the first aspect of the pre¬ sent invention as defined in claim 14.
According to a possible first implementation of an uninter- ruptible power supply system according to the second aspect of the present invention the UPS system comprises at least one power converter according to any of the implementations of the power converter according to the first aspect of the present invention wherein for each power converter a corre- sponding DC battery converter with a variable voltage gain is provided, wherein said DC/DC battery converter is connected between a battery and the DC bus circuit of the respective power converter. The invention further provides according to a third aspect a method for converting at least one AC input voltage into an AC output voltage as defined by claim 15.
The invention accordingly provides in possible first imple- mentation of the method for converting at least one AC input voltage into an AC output voltage according to the third as¬ pect of the present invention, a method wherein half-bridge connected switches of an input signal conversion circuit re¬ ceiving said AC input voltage at a common signal input node and half-bridge connected switches of an output signal con¬ version circuit outputting said AC output voltage at a common signal output node are controlled such that bus line voltages of bus lines of an intermediate DC bus circuit connecting said input signal conversion circuit and said output signal conversion circuit follow a voltage signal envelope corre¬ sponding to the maximum voltage of the AC input voltage and/or the AC output voltage.
BRIEF DESCRIPTION OF FIGURES
In the following possible embodiments of an apparatus and a method for converting at least one AC input voltage into an AC output voltage are described with reference to the en¬ closed figures in more detail.
Fig. 1 shows a block diagram of a conventional AC/DC/AC power converter with a constant DC bus voltage;
Fig. 2 shows a block diagram of a possible implementation of a uninterruptible power supply system according to the second aspect of the present invention comprising a possible imple- mentation of a power converter according to the first aspect of the present invention;
Fig. 3 shows a signal diagram for illustrating the operation of a possible implementation of a power converter according to the first aspect of the present invention;
Fig. 4 shows a circuit diagram of a possible implementation of a power converter according to the first aspect of the present invention;
Fig. 5 shows a block diagram for illustrating a possible implementation for a control circuit as employed in a possible embodiment of the power converter according to the first as¬ pect of the present invention;
Fig. 6 shows a further block diagram of a further possible implementation of a control circuit as employed in a possible embodiment of the power converter according to the first as¬ pect of the present invention.
Figs. 7a) to 7e) show circuit diagrams of an input bridge equivalent circuit for illustrating the operation of a possi¬ ble implementation of a power converter according to the first aspect of the present invention; Figs. 8a) to 8e) show signal diagrams for illustrating the operation of a possible implementation of a power converter according to the first aspect of the present invention; Figs. 9a) to 9e) show circuit diagrams of output bridge equivalent circuit diagrams of a possible implementation of a power converter according to the first aspect of the present invention ; Figs. 10a) to lOe) show signal diagrams for illustrating the operation of a possible implementation of a power converter according to the first aspect of the present invention;
Fig. 11 shows a signal diagram for illustrating the defini- tion of voltage ratings of top and bottom switches as em¬ ployed in a possible implementation of the power converter according to the first aspect of the present invention;
Fig. 12 shows a signal diagram for illustrating the relative inductance versus the input to output voltage ratio provided by a power converter according to the first aspect of the present invention;
DETAILED DESCRIPTION OF EMBODIMENTS
As can be seen in the block diagram of Fig. 2 an uninterruptible power supply UPS system according to the first aspect of the present invention comprises at least one power con¬ verter 2 according to the first aspect of the present inven- tion. The UPS system can be a single-phase system or as shown in the embodiment of Fig. 2 a three-phase UPS system. The three-phase UPS system comprises for each phase LI, L2, L3 of a three-phase grid 3, a single-phase power converter 2 ac¬ cording to the first aspect of the present invention. Fur- thermore, the UPS system 1 comprises a DC/DC converter 4 con¬ nected to a battery 5 as shown in Fig. 2. The AC power converter as shown in the implementation of Fig. 2 is formed by an AC/DC/AC converter comprising an input signal conversion circuit 6 and an output signal conversion circuit 7 as well as an intermediate DC bus circuit 8 connecting said input signal conversion circuit 6 and said output signal conversion circuit 7 via bus lines. In a possible implementation the in¬ put signal conversion circuit 6 comprises two half-bridge connected switches being connected to each other at a signal input node SIN. This signal input node SIN of the input sig¬ nal conversion circuit 6 receives the AC input voltage to be converted, for example via an input inductance LiN as shown in Fig. 2. Further, the output signal conversion circuit 7 can comprise two half-bridge connected switches being con¬ nected to each other at a signal output node SON. The voltage at the signal output node SON is applied for example via an output inductance L0UT as the AC output voltage to the load 9 as shown in Fig. 2.
The three-phase UPS system 1 as shown in Fig. 2 comprises three single-phase UPS converters 1-1, 1-2, 1-3 each having an AC/DC/AC power converter 2-i and a DC/DC converter 4-i as illustrated in Fig. 2. The load 9 as shown in Fig. 2 is a three-phase load connected to the output terminals of the three single-phase UPS converters 1-1, 1-2, 1-3. The AC sig¬ nal source of the UPS system 1 as shown in Fig. 2 is a three- phase power grid 3 applying the three AC signal voltages or phases LI, L2, L3 to the AC-input terminals AC-input A, AC- input B, AC-input C of the three single-phase UPS converters 1-1, 1-2, 1-3. Each single-phase UPS converter 1-1, 1-2, 1-3 comprises a further DC voltage input connected to the common battery 5 for supplying the DC voltage of the battery 5 to the respective DC/DC converter 4-i within the single-phase UPS converter 1-i.
The three-phase power grid 3 and the three-phase load 9 are further connected to each other via a neutral line N being also connected to a neutral node NN within the intermediate DC power circuit 8 of each AC/DC/AC converter 2-i of the three phase UPS system 1. Each intermediate DC bus circuit 8- i connects the respective input signal conversion circuit 6-i with the respective output signal conversion circuit 7-i via two bus lines comprising a positive bus line (plus) and a negative bus line (minus) . With respect to the neutral line N the positive bus line (plus) has a positive potential and the negative bus line (minus) has a negative potential. The DC bus circuit 8 as shown in Fig. 2 comprises two half-bridge connected capacitors CI, C2 being connected to each other at the neutral node NN.
In a possible implementation it is also possible that the in¬ termediate DC bus circuit 8 further comprises two antiparal- lel diodes Dl, D2 connected in antiparallel direction between the neutral node NN of the DC bus circuit 8 and the positive and respective bus line (plus, minus) of the DC bus circuit 8 as illustrated in Fig. 2. For each single-phase AC/DC/AC con¬ verter 2-i a control circuit 11-i is provided which is adapted to control the switches of the input signal conver- sion circuit 6 and the output signal conversion circuit 7 such that the bus line voltages of the bus lines (plus, mi¬ nus) of the DC bus circuit 8 follow the voltage signal enve¬ lope corresponding to the maximum voltage of the AC input voltage applied at the AC input of the respective power con- verter 1-i and/or the AC output voltage output the AC output of the respective power converter 1-i.
The control circuit 11 accordingly monitors the AC input voltage supplied to the power converter 1-i and/or the AC output voltage output by the AC/DC/AC power converter 1-i.
The control voltage can in a possible embodiment monitor the AC input voltage and the AC output voltage at the input and output terminal of the respective single-phase UPS converter 1-i as shown in Fig. 2 or directly the AC input voltage or AC output voltage at the signal input node SIN of the input sig¬ nal conversion circuit 6 and at the signal output node SON of the output signal conversion circuit 7. Possible implementa- tions of the control circuit 11 are shown in Figs. 5, 6 de¬ scribed later in more detail.
As can be seen in Fig. 2 the input of the DC/DC battery con- verter 4-i can be connected to the battery 5 via three wires as shown in Fig. 2 or via two wires. The output side of the DC/DC converter 4-i is directly connected to the DC bus lines denoted as plus, minus and neutral. In a possible embodiment of the power converter according to the present invention the input signal conversion circuit 6 can be formed by an input rectifier. This input rectifier can be a pulse width modulated (PWM) voltage source converter having the DC side ca¬ pacitors and the AC side filter inductors. The DC side ca¬ pacitors of the input rectifier are the capacitors CI, C2 of the intermediate DC bus circuit 8. Further, the output signal conversion circuit 7 can be implemented as an output inverter and implemented as a pulse width modulated PWM voltage source converter having DC side capacitors and AC side filter induc¬ tors. The DC side capacitors are again the capacitors CI, C2 of the intermediate DC bus circuit 8. In a possible implemen¬ tation the input rectifier and the output inverter can be implemented or composed of active switches such as IGBT or which can comprise parallel freewheeling diodes. With the power converter 2 according to the first aspect of the present invention it is possible to control the DC bus voltage to follow an envelope of the input or output voltage. The control circuit 11-i is adapted to control the switches of the input signal conversion circuit 6 and the output sig- nal conversion circuit 7 such that the bus line voltages at the bus lines plus, minus of the intermediate DC bus circuit 8 follow a voltage signal envelope corresponding to the maxi¬ mum voltage of the AC input voltage and/or the AC output voltage .
Fig. 3 illustrates the signal waveforms as provided under the control of the control circuit 11-i. The input signal conver- sion circuit 6 and the output signal conversion circuit 7 are controlled in a way by the control circuit 11 to have a posi¬ tive DC bus voltage that follows the positive envelope of the input or output voltage. The input signal conversion circuit 6 and the output signal conversion circuit 7 are further con¬ trolled by the control circuit 11 in a way to have a negative DC bus voltage that follows a negative envelope of the AC in¬ put voltage or the AC output voltage. Under control of the control circuit 11 the DC bus voltage is following the AC in- put voltage and/or the AC output voltage. This leads to a re¬ duction of switching losses at the half-bridge connected switches of the input signal conversion circuit 6 and the output signal conversion circuit 7. Moreover, it leads to a reduction of the size and losses of the input inductors LiN and the output inductors L0UT as compared to a conventional AC/DC power converter.
Fig. 4 shows a possible implementation of a power converter 2 within a single-phase UPS converter 1 in more detail. As can be seen in Fig. 4 the power converter 2 has a single-phase
AC/DC/AC power converter 1 comprising an input signal conversion circuit 6, an output signal conversion circuit 7 and an intermediate DC bus circuit 8 connecting the input signal conversion circuit 6 with the output signal conversion cir- cuit 7. The AC voltage source applies an AC input voltage via the input inductance LiN to the signal input node SIN of the input signal conversion circuit 6. Further, an AC output voltage is output from a signal output node SON of the output signal conversion circuit 7 via an output inductance L0UT to a load 9 as shown in Fig. 4. As can be seen in Fig. 4 the input signal conversion circuit 6 has a first and a second half- bridge connected input switch SW1, SW2 being connected to each other at the signal input node SIN. The first input switch SW1 is provided between said signal input node SIN and a positive bus line plus of the DC bus circuit 8. The second input switch SW2 is provided between the signal input node SIN and a negative bus line minus of the DC bus circuit 8. The intermediate DC bus circuit 8 comprises in the embodiment of Fig. 4 a first capacitor CI and a second capacitor C2. The capacitors CI, C2 are connected to each other at a common neutral node NN as shown in Fig. 4. The neutral node NN is connected to a neutral line connecting the input voltage source and the load 9. The first capacitor CI of the interme¬ diate DC bus circuit 8 is provided between the neutral node NN and the positive bus line, (plus) , of the DC bus circuit 8. The second capacitor C2 is provided between the neutral node NN and the negative bus line, (minus) , of the DC bus circuit 8. Both capacitors CI, C2 of the DC bus circuit 8 comprise a diode DCi and DC2 which is connected in antiparal- lel direction between the neutral node NN of the DC bus cir- cuit 8 and the positive or negative bus line of the DC bus circuit 8 as shown in Fig. 4.
Also in the input signal conversion circuit 6 antiparallel freewheeling diodes are provided. As can be seen in Fig. 4 diodes DS i and DS 2 are connected in antiparallel direction between the signal input node SIN and a respective bus line of the DC bus circuit 8. The diode DSWi is connected in paral¬ lel to the first input switch SW1 of the input signal conver¬ sion circuit 6. The second diode DSW2 is connected in parallel to the second input switch SW2 of the input signal conversion circuit 6.
The output signal conversion circuit 7 also comprises a first and second half-bridge connected output switch SW3, SW4 being connected to each other at the signal output node SON and controlled by said control circuit 11 of the respective
AC/DC/AC power converter 2. The signal output node SON is connected via a coil Lout to the load 9 connected between nodes 10a, 10b. The first output switch SW3 is provided be- tween the signal output node SON and the positive bus line plus of the intermediate DC bus circuit 8. The second output switch SW4 is provided between the signal output node SON and the negative bus line minus of the DC bus circuit 8. The first and second half-bridge connected output switches SW3, SW4 are also controlled by the control circuit 11. To the first output switch SW3 a diode DSW3 is connected in antipar- allel direction as shown in Fig. 4. Further, to the second output switch SW4 a diode Dsw4 is connected in antiparallel direction. The switches SW1, SW2, SW3, SW4 are electronic switches such as IGBT switches or MOSFETS connected to the control circuit 11, wherein the switching is controlled by the control circuit 11 which supplies control signals SI, S2, S3, S4 to control electrodes of the respective electronic switches SW1, SW2, SW3, SW4.
The circuit as shown in Fig. 4 provides a full DC bus voltage envelope tracking as shown in Fig. 3. Waveforms of the DC bus voltage of the AC input voltage and the AC output voltage are illustrated in Fig. 3. The input AC voltage, for example, is a sinusoidal voltage:
VIN =VINsm&t , (1) wherein ViN is the voltage magnitude and ω is the angular frequency .
The AC output voltage is also a purely sinusoidal voltage: v0UT =V0UTs at , (2) wherein V0UT is the output voltage magnitude and ω is the an¬ gular frequency and t is the time. The AC/DC/AC power converter 2 as shown in Fig. 4 and the input/output voltages as well as the DC bus voltage have the following relation:
VBUS2 — V1N— VBUS\ VBUS2 — V OUT— V BUS\ · (3) There are three different scenarios possible. In a first sce¬ nario the input voltage is lower than the output voltage (VIN<V0UT) · In a second scenario the input voltage is equal to the output voltage (VIN=V0UT) and in the third scenario the input voltage is higher than the output voltage (VIN>V0UT) ·
If in the first scenario the input voltage is lower than the output voltage, a simplified equivalent circuit diagram of the input signal conversion circuit 6 can be depicted as shown in Fig. 7a) . In the equivalent circuit the DC bus ca¬ pacitors CI, C2 are modelled as voltage sources VBusi and VBus2 as shown in Fig. 7a) . The DC bus voltage can be arbitrary positive voltages. If the input voltage ViN is positive
(0<G)t<n) the first input switch SW1 and the antiparallel di¬ ode DS 2 are carrying the input current iiN, while the second input switch SW2 and the antiparallel diode DSWi connected in parallel to the first input switch SW1 are inactive. As can be seen in Fig. 7b) the first input switch SW1 is closed and the circuit can be described by the following equation:
T ^JiL = v
N dt + v 0<t<d T (4)
BUS\ for u ^ 1 ^ U1N1 sw wherein diN is the duty cycle of the first input switch SW1 and TS is a switching period.
When the first input switch SW1 is opened and the second in¬ put switch SW2 of the input signal conversion circuit 6 is closed, the diode DS 2 is carrying the inductor current iiN, as shown in Fig. 8c) . = v VIN _v VBUS2 f„nr U1N1TSW <t 1<T SW (5)
Figure imgf000018_0001
Accordingly, the duty cycle diN can be calculated as follow¬ ing :
Figure imgf000019_0001
The instantaneous electrical current of the first and second input switches SW1, SW2 of the input signal conversion circuit 6 and its diodes DS i and DS 2 is given by:
Figure imgf000019_0002
, sw
iD2(t)= 0 kTsw <t<{k+dIN)T, sw
^2( =½( for ik+ d T^ Kt ik + lfc (8) sw
The second input switch SW2 and the diode D3Wi do not carry any electrical current when the input voltage is positive (0<ωτ<π) .
Figure imgf000019_0003
The moving average and RMS electrical current of the first input switch SW1 and the diode D3W2 is given by:
Figure imgf000019_0004
Figure imgf000019_0005
Accordingly, the input current ripple is given by: (12)
Figure imgf000020_0001
wherein fsw is a switching frequency.
When the input voltage is negative (π<ωτ< 2π) , the first input switch SW2 and the diode DS i are carrying the input current iiN, while the first input switch SW1 and the diode DS 2 are inactive .
In the normal operation mode of the AC/DC/AC converter the input voltage ViN and the input current are given by:
2Pn
(t) = I]N sin t =——sin t (13)
V IN
vIN{t) = VIN s (Qt (14)
OUT sin cot (15)
The DC bus voltage can actively be controlled to follow the half-way rectified sinusoidal waveforms:
Figure imgf000020_0002
lVtV) = -V y BUS(MAX) sinmtj'i°r { ' wherein VBUS(MAX) is a DC bus voltage magnitude which can be constant .
When the input voltage is positive (0<ωτ<π) and substituting equation (14) and equation (16) into equation (6), the input duty cycle is given by:
Figure imgf000020_0003
When the input voltage is negative (π<ωτ<2π) , the duty cycle is complementary to the duty cycle given by equation (18) . Thus, the duty cycle over a fundamental period is given by: dm (t) = - (19)
Figure imgf000021_0001
Fig. 8 shows the duty cycle diN of the input signal conver¬ sion circuit 6 at the first and second half-bridge connected input switches SW1, SW2. The signal diagram in Fig. 8a) shows a sinusoidal AC input voltage ViN applied to the input signal conversion circuit 6. Fig 8b) shows the DC bus voltages VBusi/ VBus2 over time. Fig. 8d) shows the calculated input duty cy¬ cle di . Further, the signal diagram in Fig. 8d) shows the control signal SI output by the control circuit 11 to the first input switch SW1 of the input signal conversion circuit 6. Fig. 8e) shows a signal diagram of a control signal S2 output by the control circuit 11 to the second output switch SW2 of the input signal conversion circuit 6.
The current of the switches and diode average and RMS current can be calculated over a fundamental period as follows:
Figure imgf000021_0002
and
2 P I,N
Dl(A V) lD2(A V)
π V B, US (MAX)
Figure imgf000021_0003
wherein PIN is the input power of the input signal The input inductor current ripple is a function of time: jsinot
Figure imgf000022_0001
Accordingly, a maximum current ripple can be calculated from equation (22) as follows:
Figure imgf000022_0002
The inductance of the input inductor LiN can be calculated for a maximum current ripple given as a design parameter of the circuit:
Figure imgf000022_0003
The input bridge losses at the input signal conversion cir- cuit 6 can be calculated as follows:
1 P SWl(A V) = 1 P SW2(A V) VsWo w(AV) + rSwI SW(RMS)
Figure imgf000022_0004
and
1 P Dl(AV) = 1 P Dl(AV) VD D( ) + RD^D(RMS)
Figure imgf000022_0005
The switch and diode output characteristics can be approxi¬ mated by the voltage knee VSwo , VD0 and dynamic resistance rSw , rD. Accordingly, the switching losses of the switches and di¬ odes can be calculated as follows:
Figure imgf000023_0001
wherein E0N, EOFF and EQ are switching energies normalised per VA.
Similar to the input signal conversion circuit 6 also to sig¬ nal output conversion circuit 7 can be depicted by an equiva¬ lent circuit diagram as shown in Fig. 9. DC bus capacitors CI, C2 are modelled by two DC bus voltage sources. The con- verter load can be modelled by a voltage source V0UT ·
If the output voltage V0UT is positive, the first output switch SW3 and the diode Dsw4 are carrying the load current iouT as shown in Fig. 9b) . The duty cycle of the first output switch SW3 is given by:
Figure imgf000023_0002
If the DC bus voltage is controlled to be equal to the output voltage VBus (MAX)
Figure imgf000023_0003
the duty cycle becomes zero dOuT(t)=0. Therefore, the first output switch SW3 is opened while the second output switch SW4 is closed and the output voltage is positive .
When the output voltage V0UT is negative, the first output switch SW3 is carrying the load current ίουτ , while the second output switch SW4 is opened as shown in Figs. 9c), 9d) . The duty cycle can be calculated over a fundamental period as
Figure imgf000024_0001
As can be seen from equation (30) the switches SW3, SW4 of the output signal conversion circuit commutate at a fundamen¬ tal frequency as illustrated in Fig. 10. Fig. 10a) shows a sinusoidal AC output voltage applied to the load. Fig. 10b) shows the bus voltage VBusi/ Bus2 · Fig. 10c) shows the calcu- lated duty cycle d0uT for the output signal conversion circuit 7. Fig. lOd) shows the switching control signal S3 applied to the first output switch SW3 by the control circuit 11. The signal diagram 10± shows a further control signal S4 as ap¬ plied to the second output switch SW4 by the control circuit 11. The switching frequency f=l:t can be, for example, 50Hz.
The average current of the switches and the RMS current can be given by:
2 P IN
SW3(AV) SW4(AV)
π V, OUT
(31)
P IN
SW3(RMS) = i SW4(RMS) V ' OUT
Accordingly, the conduction losses are given by:
1 P SW3(AV) = 1 P SWA(AV) = V ' SW0 iLSW(AV) +† 'r SW1 I S2W(RMS)
. w
Figure imgf000024_0002
In a possible implementation it is possible to neglect the switching losses since the switches commutate at the funda¬ mental frequency and at a zero current zero voltage condi¬ tion. Fig. 11 shows a signal diagram for illustrating the definition of a voltage rating of top and bottom switches in the signal conversion circuit 6 as employed by the AC/DC/AC con¬ verter according to the present invention. From equations (16) and (17) it is possible to calculate a total voltage across two switches as:
VsWl + VsW2 = VBUSl + VBUS2 = '^BUS(MAX) \S'^n- {(i)^ _ (33)
Accordingly, the switches and diodes blocking voltage can be defined as:
^SW ^D(max) — ^BUS(MAX ) _ (34)
Fig. 12 shows relative inductance versus the input to output voltage ratio. The relative inductance as shown in Fig. 12 is the ratio of inductance as employed by a conventional power conversion circuit with a constant DC bus voltage to the in¬ ductance as employed by the power converter according to the first aspect of the present invention. The inductance as em¬ ployed by the conventional power conversion circuit is given by :
Figure imgf000025_0001
Consequently, the relative inductance is given by:
Figure imgf000025_0002
As can be seen in Fig. 12 the relative inductance is dimin¬ ished when the input voltage approaches the output voltage, i.e. the ratio between the input and output voltage becomes 1. Accordingly, in the second scenario the input voltage is equal to the output voltage and the relative inductance is low. If the input voltage is lower than the output voltage (VIN<V0UT) the relative inductance increases. At a certain point when the input voltage becomes too low, the UPS system switches from the AC voltage source to the battery 5. On the contrary, if the input voltage is higher than the output voltage (vin > vout ) and exceeds a certain limit, the UPS sys¬ tem can switch to a fault managing mode to avoid destruction caused by a too high AC input voltage.
A switch and diode apparent power can be defined as the prod- uct of a device peak voltage and a peak current. Apparent power of devices as employed by the apparatus according to the present invention is:
P(SW) = VBUS(MAXj2-^- (37)
In contrast, the apparent power of devices in conventional converters is given by:
P(SW) = 2 V 2- (38)
Accordingly, the power converter according to the first aspect of the present invention can in a possible implementa¬ tion require about 50% apparent power per switch compared t conventional converters.
Fig. 5 shows a block diagram of a basic control structure of a control circuit 11 of a power converter 2 according to the first aspect of the present invention. The control circuit 11 comprises a reference voltage generator 11a for generating a reference DC bus voltage VBusi · This can be performed in re¬ sponse to an output voltage V0UT(REF) to generate a DC bus ref- erence voltage VBUS(REF) as shown in Fig. 5. The control cir¬ cuit 11 further comprises a second generator for generating a second DC bus reference voltage VBus2 <REF) · The second reference voltage generator lib of the control circuit 11 can generate the second DC reference voltage VBUS2(REF) depending on an out- put reference voltage V0UT ( REF ) as shown in Fig. 5. The control circuit 11 further can comprise a controller 11c which receives the DC bus voltages VBUSi, Bus2 from the DC bus circuit 8 of the monitored single-phase power converter 2. The con- troller 11c also receives the reference voltages VBusi <REF> and VBus2 (REF ) from the generators 11a, lib and the feedback bus signals VBusi and VBus2 to provide an input duty cycle diN for an input bridge modulator lid which generates switching control signals SI, S2 applied to the first and second input switch SW1, SW2 of the input signal conversion circuit 6.
The control circuit 11 further comprises a second controller lie which receives a monitored feedback of the output voltage VouT of the monitored single-phase AC/DC/AC converter 2 and an output reference voltage V0UT ( REF ) as shown in Fig. 5. The sec¬ ond controller lie calculates an output duty cycle d0uT which is supplied to an output bridge modulator llf. The output bridge modulator llf generates switching control signals S3, S4 applied the first and second output switch SW3, SW4 of the output signal conversion circuit 7 within the monitored
AC/DC/AC converter 2.
Fig. 6 shows a further implementation of control circuit 11 for a single-phase AC/DC/AC converter 2. In this embodiment the output of the DC bus voltage controller 11c is a refer¬ ence for an input current I INRE F applied to an input current controller llg which receives the monitored feedback input current from the respective monitored AC/DC/AC power con¬ verter 2. The input current controller llg calculates the in- put duty cycle din in response to the monitored feedback in¬ put current iiN and the reference input current i IN (REF ) re¬ ceived from the bus voltage controller 11c. Further, the con¬ trol circuit 11 as shown in the implementation of Fig. 6 comprises an output current controller llh which calculates the output duty cycle d0uT supplied to the output bridge modulator llf in response to the output reference current ϊου ( REF ) out- put by the output voltage controller lie and the monitored output current out of the respective AC/DC/AC converter 2.
The method and apparatus for converting an AC input voltage into an AC output voltage according to the present invention provide a high efficiency when compared to conventional power converters and power conversion methods. The efficiency of the apparatus and method according to the present invention can be in a range of 98.5 up to 99% which is significantly higher than the peak efficiency of any conventional power converter or method. Because of the high efficiency the power converter and power conversion method according to the present invention have a low energy consumption. The power converter and method according to the present invention can be used in a wide range of applications. For example, the power converter according to the first aspect of the present inven¬ tion can be used in an uninterruptable power supply system comprising the power converter and a DC/DC battery converter. Such a UPS system can be used for example in a data centre, in telecommunication facilities, in industry applications, and for any critical load such as medical devices within a hospital .

Claims

Claims :
A power converter adapted to convert at least one AC in¬ put voltage into an AC output voltage,
a power converter (2) comprising:
an input signal conversion circuit (6) having half-bridge connected switches (SW1, SW2) being connected to each other at a signal input node, SIN, to which said AC input voltage is applied by an AC voltage source (3) ;
an output signal conversion circuit (7) having half-bridge connected switches (SW3, SW4) being connected to each other at a signal output node, SON, from which the output voltage is supplied to a load (9);
an intermediate DC bus circuit (8) connecting said input signal conversion circuit (6) with said output signal conversion circuit (9) via bus lines;
a DC bus circuit (8) having capacitors ( Ci , C2 ) connected to each other at a neutral node, NN; and
a control circuit (11) adapted to control the switches of said input signal conversion circuit (6) and of said out¬ put signal conversion circuit (7) such that said bus line voltages of said bus lines follow a voltage signal enve¬ lope corresponding to the maximum voltage of the AC input voltage and/or AC output voltage.
The power converter according to claim 1,
wherein said input signal conversion circuit (6) has a first and a second half-bridge connected input switch (SW1, SW2) being connected to each other at said signal input node, SIN, and being controlled by said control circuit (11),
wherein said first input switch (SW1) is provided between said signal input node, SIN, and a positive bus line (plus) of said DC bus circuit (8);
wherein said second input switch (SW2) is provided be¬ tween said signal input node, SIN, and a negative bus line (minus) of said DC bus circuit (8) . The power converter (2) according to claim 1 or 2, wherein said output signal conversion circuit (7) has a first and second half-bridge connected output switch (SW3, SW4) being connected to each other at said signal output node, SON, and being controlled by said control circuit (11),
wherein said first output switch (SW3) is provided be¬ tween said signal output node, SON, and the positive bus line (plus) of said DC bus circuit (8),
wherein said second output switch (SW4) is provided be¬ tween said signal output node, SON, and a negative bus line (minus) of the DC bus circuit (8) .
The power converter according to one of the preceding claims 1 to 3,
wherein to each switch of said input conversion circuit (6) or of said output conversion circuit (7) a diode is connected in antiparallel direction between the respec¬ tive signal node of said conversion circuit and a bus line of said DC bus circuit (8) .
The power converter according to one of the preceding claims 1 to 4,
wherein the DC bus circuit (8) has a first and second half-bridge connected capacitor (CI, C2) connected to each other at said neutral node, NN,
wherein said first capacitor (CI) is provided between said neutral node, NN, and the positive bus line (plus) of said DC bus circuit (8),
wherein said second capacitor (C2) is provided between said neutral node, NN, and the negative bus line (minus) of said DC bus circuit (8) .
The power converter according to one of the preceding claims 1 to 5, wherein said neutral node, NN, of said DC bus circuit (8) is connected to said AC voltage source (3) and said load (9) .
The power converter according to claim 5,
wherein to both capacitors (1, 2) of said DC bus circuit (8) a diode is connected in antiparallel direction be¬ tween the neutral node, NN, of said DC bus circuit (8) and the positive or negative bus line (plus, minus) of said DC bus circuit (8) .
The power converter according to one of the preceding claims 1 to 7,
wherein an input inductor (LI) is connected between said AC voltage source (3) and the signal input node, SIN, of said input signal conversion circuit (6) .
The power converter according to one of the preceding claims 1 to 8,
wherein an output inductor (L2) is connected between the signal output node, SON, of said output signal conversion circuit (7) and said load (9) .
The power converter according to one of the preceding claims 1 to 9,
wherein said control circuit (11) is adapted to monitor at least the bus line voltages at the bus lines (plus, minus) of said intermediate DC bus circuit (8) and the AC input voltage and/or the AC output voltage.
The power converter according to one of the preceding claims 1 to 10,
wherein said control circuit (11) comprises an input con¬ troller (llC) which is adapted to compare the bus line voltages of the bus lines of said intermediate DC bus circuit (8) with reference bus line voltages to calculate an input duty cycle, d±n, applied to an input bridge modulator (lid) which is adapted to generate switching control signals (SI, S2) applied to said first and second input switches (SW1, SW2) of said input signal conversion circuit ( 6) .
12. The power converter according to one of the preceding
claims 1 to 11,
wherein said control circuit (11) comprises an output controller (lie) which is adapted to compare the AC out- put voltage with a reference output voltage to calculate an output duty cycle, dout / applied to an output bridge modulator (llf) which is adapted to generate switch con¬ trol signals (SW3, SW4) applied to said first and second output switch of said output signal conversion circuit (7).
13. The power converter according to one of the preceding
claims 1 to 12,
wherein said AC input voltage is a signal phase supplied to said power converter (2) by a three-phase power grid
(3) .
14. An uninterruptible power supply, UPS, system comprising at least one power converter (2) according to one of the preceding claims 1 to 13 and for each power converter (2) a corresponding DC/DC battery converter (4) with a variable voltage gain,
wherein said DC/DC battery converter (4) is connected between a battery (5) and the DC bus circuit (8) of the re- spective power converter (2) .
15. A method for converting at least one AC input voltage
into an AC output voltage,
wherein half-bridge connected switches of an input signal conversion circuit (6) receiving said AC input voltage at a common signal input node and half-bridge connected switches of an output signal conversion circuit (7) out- putting said AC output voltage at a common signal output node are controlled such that bus line voltages of bus lines of an intermediate DC bus circuit (8) connecting said input signal conversion circuit (6) and said output signal conversion circuit (7) follow a voltage signal en¬ velope corresponding to the maximum voltage of the AC in¬ put voltage and/or the AC output voltage.
PCT/EP2012/061529 2012-06-15 2012-06-15 Method and apparatus for performing power conversion WO2013185847A1 (en)

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