THREE PHASE BIDIRECTIONAL AC-DC CONVERTER WITH BIPOLAR VOLTAGE FED RESONANT STAGES

Abstract

A bidirectional AC power converter, having a front-end comprising parallel sets of three switches in series, which connects multi-phase AC to coupling transformer through a first set of tank circuits, for synchronously bidirectionally converting electrical power between the multi-phase AC and a DC potential, and for converting electrical power between the DC potential to a bipolar electrical signal at a switching frequency, controlled such that two of each parallel set of three switches in series are soft-switched and the other switch is semi-soft switched; the coupling transformer being configured to pass the bipolar electrical power at the switching frequency through a second set of the tank circuits to a synchronous converter, which in turn transfers the electrical power to a secondary system at a frequency different from the switching frequency.

Claims

1. An AC power converter, comprising: a front-end comprising parallel sets of at least three switches in series operating at a front-end switching pattern with soft or semi-soft switching exclusively after startup, which connects a multi-phase AC interface to a set of tank circuits having a resonant frequency through a coupling transformer, and producing a first DC potential; a synchronous converter, connecting the set of tank circuits to a second DC potential, the synchronous converters operating at a synchronous converter switching pattern independent of the front-end switching pattern.

2. The AC power converter according to claim 1, wherein the AC power converter is bidirectional, and in a first mode transfers power from the multi-phase AC interface to the DC potential, and is a second mode, transfers power from the DC potential to the multi-phase AC interface.

3. The AC power converter according to claim 1, wherein multi-phase AC interface is a three-phase interface, the front end comprises nine switches arranged as three parallel sets of three switches in series, wherein six switches are soft switched and three switches are semi-soft switched, and the front end is controlled to perform power factor correction.

4. The AC power converter according to claim 3, wherein each of the switches comprises a MOSFET switch, and wherein the multi-phase AC system operates between 50 and 500 VAC at between 30 and 400 Hz.

5. The AC power converter according to claim 1, wherein the set of tank circuits are operated at their resonant frequency of between 2 kHz and 150 kHz.

6. The AC power converter according to claim 1, wherein the front-end switching pattern comprises a frequency >25 kHz.

7. The AC power converter according to claim 1, wherein the synchronous converter comprises a multi-phase interleaved full bridge converter.

8. The AC power converter according to claim 1, wherein the synchronous converter comprises a plurality of single-phase parallel full bridge converters.

9. The AC power converter according to claim 1, wherein each tank circuit comprises at least one capacitor and at least one inductor.

10. The AC power converter according to claim 1, further comprising an automated controller, configured to generate front-end switching pattern, generate the synchronous converter switching pattern, and perform power factor correction.

11. The AC power converter according to claim 10, wherein the automated controller is further configured to sequence a startup of the AC power converter according to different switching parameters than a normal operating sequence.

12. The AC power converter according to claim 1, wherein the second DC potential is connected to a battery system, and the AC power converter is configured in a first operating mode to charge the battery system from the multi-interface, and is configured in a second operating mode to provide power to the AC power interface from the battery system.

13. The AC power converter according to claim 1, wherein the synchronous converter is switched to produce a dynamic waveform distinct from a waveform produced by the front-end.

14. The AC power converter according to claim 1, further comprising an automated controller, configured to control the front end to: initially in a startup mode, charge a capacitor with the first DC potential in a rectifier mode of operation; after charging the capacitor, initiate operation of the tank circuits, by setting the front-end switching pattern to a frequency about double the resonant frequency; and after initiating operation of the tank circuits, reduce a frequency of the front-end switching pattern until a desired output is achieved at the second DC potential.

15. The AC power converter according to claim 1, further comprising an automated controller, configured to selectively control the front end to provide a full power mode of operation wherein the front-end is operated continuously, and a low-power mode of operation wherein the front end is operated intermittently in a burst mode.

16. An AC power conversion method, comprising: providing a multi-phase AC interface and a DC interface; providing a front-end comprising parallel sets of at least three switches in series, which connect the multi-phase AC interface to a set of tank circuits having a resonant frequency through a coupling transformer, and producing a front-end DC potential; charging a capacitor with the front-end DC potential in a rectifier mode of operation by operating the font-end in a startup switching mode; and after charging the capacitor initiating operation of the tank circuits by altering the startup switching mode to an operational switching mode, and synchronously transferring power between the set of tank circuits and a synchronous converter DC potential at the DC potential interface using a synchronous converter.

17. The AC power conversion method according to claim 16, further comprising: in a first mode of operation, receiving AC power at the multiphase AC interface, transferring power from the multiphase AC interface through the tank circuits to the synchronous converter, and delivering DC power through the DC interface; and in a second mode of operation, receiving DC power at the DC interface, converting the DC power to an AC waveform with a synchronous converter, transferring the AC waveform through the tank circuits to the front-end, and delivering multi-phase AC power through the multiphase AC interface.

18. The AC power conversion method according to claim 16, wherein multi-phase AC interface is a three-phase interface, the front end comprises nine switches arranged as three parallel sets of three switches in series, wherein six switches are soft switched and three switches are semi-soft switched, and the front end is controlled to perform power factor correction.

19. The AC power conversion method according to claim 15, wherein the startup mode comprises: charge a capacitor with the front-end DC potential in a rectifier mode of operation; after charging the capacitor, initiating operation of the tank circuits, by setting a front-end switching pattern to a frequency about double the resonant frequency; and after initiating operation of the tank circuits, reducing a frequency of the front-end switching pattern until a desired output is achieved at the DC potential interface.

20. A power converter, comprising: a front-end interfacing with a multi-phase AC system, comprising, for each respective phase, a set of at least three switches in series, each of the switches being soft switched or semi-soft switched in an operation mode; a capacitor in parallel with the sets of three switches in series, storing a front-end DC potential; a resonant tank circuit for each respective phase, connected between two of the set of three switches in series for a respective phase; and a synchronous converter, configured to interface with a secondary power system; and wherein the front-end is DC isolated from the synchronous converter.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0098] FIG. 1 shows a prior art (conventional) three phase bidirectional front end, with three-phase interleaved full bridge output side converters.

[0099] FIG. 2 shows a prior art (conventional) three phase bidirectional front end, with triple single phase parallel full bridge output side converters for the respective phases.

[0100] FIG. 3 shows a three-phase integrated bidirectional nine-switch front end, with three phase interleaved full bridge output side converters, according to the present invention.

[0101] FIG. 4 shows a three-phase integrated bidirectional nine-switch front end with triple single phase parallel full bridge output side converters, according to the present invention.

[0102] FIG. 5 shows a variant of the circuit according to FIG. 4, with dual nine-switch front ends and differential input connection at input side of resonant tank, and triple single phase parallel full bridge output side converters.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0103] FIG. 3 and FIG. 4 show the two alternate topologies. In FIG. 3, the front-end side converter has three legs. Each leg has three switches connected as top, middle and bottom. The midpoints of top and middle switches of the converter are connected to the three-phase grid, with an interfacing inductor in each leg.

[0104] The midpoints of middle and bottom switches of the converter are connected to the primary side of three High Frequency (HF) Transformers through an L-C resonant link to provide galvanic isolation. The output side converter has three legs with two switches in each leg. The one terminal of secondary side of the three HF transformers are connected to each leg of the output side converter through another L-C resonant link. The other terminal of the secondary side of the three HF transformers are connected to the adjacent leg of the output side converter (i.e. to pints ‘b’, ‘c’ and ‘a’ respectively).

[0105] An electrolytic DC capacitor is connected at the DC link of the nine-switch front end converter. Another DC capacitor is connected in parallel to the battery at the output side converter to filter out the DC ripple.

[0106] In FIG. 4, the 9-switch front end converter is same as in FIG. 3. However, the output side converter constitutes three separate sub-converters connected in parallel (each converter with two legs having two switches in each leg). The two terminals of secondary side of the three HF transformers are connected to each leg of the sub-converters through an L-C resonant link. A DC electrolytic capacitor is connected across output of each sub-converter to filter out the DC ripple.

[0107] FIG. 5 differs in that each input phase is duplicated, and the nine-switch front end is correspondingly duplicated. The midpoints of middle and bottom switches of the legs corresponding to each phase are respectively connected to a nano-crystalline core based, multi-phase transformer through an L-C resonant link. The output side is same as in FIG. 4.

[0108] With use of Silicon Carbide (SiC) MOSFETs rated for 1200 V or above, this converter is practically realizable with high switching frequency operation (>75 kHz).

[0109] To maintain zero voltage switching (ZVS) across different loading conditions, the design of magnetizing inductance (L.sub.m) of the high frequency transformer, L-C resonant tank design and switch selection are important. Therefore, an optimized value of magnetizing inductance (L.sub.m) of high frequency transformer and L-C resonant tank design are provided for the range of loads. A desired switch (SiC MOSFET) based on the figure of merit (FOM) is selected to have ZVS across different output power levels for both the front end and output side converters.

[0110] The switches are operated according to the following control sequence.

[0111] Startup Mode:

[0112] Charge the DC link capacitor with a desired DC voltage using the nine-switch front-end in a rectifier mode of operation. Thereafter, the L-C resonant converter is turned on with a high frequency (i.e., around two times the resonant frequency of the converter) and the frequency is reduced until the converter stabilizes to a constant desired output DC voltage.

[0113] Low Power Mode:

[0114] At below 20% loads, the L-C resonant converter would be operated in burst mode of operation (the switches will be turned on at certain interval of few switching cycles).

[0115] High Power Mode:

[0116] At 20%-100% loads, the L-C resonant converter would be operated with normal (cycle-continuous) switching operation.

[0117] Power Transfer Mode:

[0118] Power transfer during both charging and discharging modes for different output power schemes is regulated through a phase shift control of the bridges between the nine-switch front-end and the output side converters.