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. A bidirectional AC power converter, comprising: a nine-switch front-end comprising three parallel sets of three switches in series, which connects a three phase AC system to a set of three tank circuits having a resonant frequency, the nine-switch front-end being configured to: synchronously bidirectionally convert electrical power between the three-phase system and a DC potential on opposite ends of the parallel sets of switches in series, and convert electrical power between the DC potential and the set of three tank circuits operated at their respective resonant frequency; a coupling transformer, configured to bidirectionally couple AC electrical power at the switching frequency for each of the set of three tank circuits; and a synchronous converter, configured to transfer the coupled AC electrical power between the coupling transformer and a secondary system at a switching frequency different from the resonant frequency.

2. The bidirectional AC power converter according to claim 1, wherein each of the three switches in series of the nine-switch front-end comprises a MOSFET switch, and wherein the three phase AC system operates between 50 and 500 VAC at between 30 and 400 Hz.

3. The bidirectional AC power converter according to claim 1, wherein the tank circuits each have a resonant frequency between 2 kHz-150 kHz.

4. The bidirectional AC power converter according to claim 1, wherein the switching frequency is >25 kHz.

5. The bidirectional AC power converter according to claim 1, wherein the synchronous converter comprises a six-switch converter controlled to synchronously convert the AC electrical power at the switching frequency, configured as a three-phase interleaved full bridge converter.

6. The bidirectional AC power converter according to claim 1, wherein the synchronous converter comprises a twelve-switch converter controlled to synchronously convert the AC electrical power at the switching frequency, configured as three single-phase parallel full bridge converter.

7. The bidirectional AC power converter according to claim 1, wherein each tank circuit comprises a capacitor and an inductor, wherein the nine-switch front-end is configured to present a bipolar AC waveform to the tank circuit that has no DC component.

8. The bidirectional AC power converter according to claim 1, further comprising an automated controller, configured to control the nine-switch front-end and the synchronous converter and to perform power factor correction.

9. The bidirectional AC power converter according to claim 8, wherein the automated controller is configured to sequence a startup of the bidirectional AC power converter different from a normal operating sequence.

10. The bidirectional AC power converter according to claim 8, wherein the nine-switch front-end is controlled to balance a phase load on the three phase AC system.

11. The bidirectional AC power converter according to claim 8, wherein a load on the secondary system comprises a battery, and the bidirectional AC power converter is controlled to charge the battery from the three phase AC system in a first mode of operation, and is controlled to power the three phase AC system from the battery in a second mode of operation.

12. The bidirectional AC power converter according to claim 8, wherein the synchronous converter is controlled by the automated controller to produce a dynamic waveform distinct from a waveform of the bidirectionally coupled AC electrical power at the switching frequency.

13. The bidirectional AC power converter according to claim 1, wherein the magnetizing inductance (L.sub.m) of the coupling transformer, and the tank circuit are together configured to maintain zero voltage switching (ZVS) of at least six switches of the nine-switch front end over a range of load conditions comprising a factor of two.

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

15. The bidirectional AC power converter according to claim 1, further comprising an automated controller, configured to selectively control the nine-switch front end in a low-power mode below a threshold portion of rated output, to transfer the coupled AC electrical power through the set of three tank circuits in a burst mode of operation wherein the switches are alternately turned on and off for intervals of several switching cycles.

16. The bidirectional AC power converter according to claim 1, further comprising an automated controller configured to regulate output power of the secondary system by phase shift control of the nine-switch front-end and the synchronous converter.

17. A power converter, comprising: a front-end interfacing with a multi-phase AC system, comprising, for each respective phase, a set of three switches in series; a capacitor in parallel with each of the sets of three switches in series; a resonant tank circuit for each respective phase, connected between two of the set of three switches in series for a respective phase; a synchronous converter, configured to interface with a secondary power system; and a coupling transformer, configured to couple power from the resonant tank circuit for each respective phase to the synchronous converter.

18. The power converter according to claim 17, further comprising an automated control, configured to: control the front-end to synchronously convert electrical power between the multi-phase AC system and a DC potential on the capacitor, and convert the DC potential on the capacitor into a switched frequency which passes through the resonant tank circuits, such that two of the set of three switches are soft-switched and one of set of three switches is semi-soft switched.

19. The power converter according to claim 18, further comprising: an automated controller, configured to control the sets of three switches in series in a startup mode to: charge the capacitor with a desired DC voltage in a rectifier mode of operation; and after charging the capacitor, initiate operation of the resonant tank circuits by switching at a switching frequency of about double a resonant frequency, and subsequently reduce the frequency of operation until a desired output is achieved at the secondary load.

20. A method of power conversion, comprising: providing a power converter, comprising a front-end interfacing with a multi-phase AC system, comprising, for each respective phase, a set of three switches in series, a capacitor in parallel with each of the sets of three switches in series, a resonant tank circuit for each respective phase, connected between two of the set of three switches in series for a respective phase, a synchronous converter, configured to interface with a secondary power system, and a coupling transformer, configured to couple power from the resonant tank circuit for each respective phase to the synchronous converter; and automatically controlling the set of three switches in series and the synchronous converter, to control the front-end to synchronously convert electrical power between the multi-phase AC system and a DC potential on the capacitor, and convert the DC potential on the capacitor into a switched frequency which passes through the resonant tank circuits.

Description

BRIEF DESCRIPTION OF THE FIGURES

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

(2) 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.

(3) 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.

(4) 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.

(5) 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

(6) 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.

(7) 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).

(8) 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.

(9) 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.

(10) 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.

(11) 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).

(12) 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.

(13) The switches are operated according to the following control sequence.

(14) Startup Mode:

(15) 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.

(16) Low Power Mode:

(17) 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).

(18) High Power Mode:

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

(20) Power Transfer Mode:

(21) 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.