Active rectifier system with power factor correction

Abstract

A polyphase alternating current (AC) active rectifier system converts polyphase AC from an AC source to direct current (DC) on a DC bus comprising a positive DC bus line and a negative DC bus line with a switching element for each phase configured to direct current from the AC source to the DC bus through four different conducting paths with three electrical potential levels, including a positive DC level on the positive DC bus line, a negative DC level on the negative DC bus line and a neutral DC level between the positive and negative DC level on a neutral DC bus line and suppresses AC common mode electrical potential from the DC bus, comprising: a ground path between the neutral DC bus line and a system ground coupled to a neutral point for the AC source that clamps the level of the neutral DC level to the AC source neutral.

Description

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a prior art active rectifier system of the Vienna rectifier type.

FIG. 2 is a schematic diagram of an active rectifier system with low harmonic distortion according to a possible embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic diagram of a prior art active rectifier system 2 of the Vienna rectifier type. A polyphase AC power source (not shown) typically supplies AC power in three phases A, B and C represented by terminals A, B and C that has a fundamental AC power frequency, typically 400 to 800 Hz, by way of AC input lines 4, 6 and 8. The neutral point of the AC power source represented by terminal N couples to AC system ground by way of AC system ground line 10.

As common with rectifier systems of the switching type, the AC input lines 4, 6 and 8 feed AC current through inputs of inductors 12, 14 and 16 for phases A, B and C, respectively. The rectifier system may include capacitors 18, 20 and 22 coupled between outputs of each of the inductors 12, 14 and 16 and the AC system ground to attenuate high frequency harmonics of the AC power fundamental frequency that the Vienna type rectifier system 2 generates.

Current from the output of each of the inductors 12, 14 and 16 flows through inductor output lines 24, 26 and 28 respectively to respective junctions 30 of respective series connected pairs of input diodes 32 and 34. Respective switching elements 36, such as insulated gate bipolar transistors (IGBTs) connect across each series connected pair of input diodes 32 and 34. A rectifier system switching controller 38 switches the switching elements 36 on and off by way of control lines 40 as hereinafter described.

When a switching element 36 switches off, positive current that flows into the junction 30 for its respective series connected pair of input diodes 32 and 34 passes through its positive input diode 32 and through a respective positive output diode 42 into a positive output line 44 of a DC bus. Likewise, when a switching element 36 switches off, negative current that flows into the junction 30 for its respective series connected pair of input diodes 32 and 34 passes through its negative input diode 34 and through a respective negative output diode 46 into a negative output line 48 of a DC bus. A series connected pair of output capacitors 50 and 52 with a junction 54 connect from the positive output line 44 and the negative output line 48 of the DC bus. The junction 54 represents the neutral point for the DC bus.

A series connected pair of shunt diodes 56 and 58 with a junction 60 connect across each switching element 36. When a switching element 36 switches on, positive current flows through the positive shunt diode 56 and the junction 60 for its respective series connected pair of shunt diodes 56 and 58 and then into a DC bus neutral line 62 that connects to the junction 54 for the output capacitors 50 and 52. Likewise, when a switching element 36 switches off, negative current flows through the negative shunt diode and the junction 60 for its respective series connected pair of shunt diodes 56 and 58 and then into the DC bus neutral line 62.

Consequently, there are four different conduction paths for current through the Vienna type rectifier system 2 for each phase of the power supplied by the AC power source depending on the phase of the current and the state of the switching elements 36. For each phase, current flows to either the positive DC bus 44 or the negative DC bus 48 when its respective switching element 36 switches off or to the DC neutral line 62 when its respective switching element 36 switches on. Therefore, the Vienna type rectifier system 2 also has three different electrical potential levels that correspond to these four different conduction paths.

The Vienna type rectifier operates in current mode control. That is, the switching controller 38 controls the states of the switching elements 36, typically at a high frequency rate that may be as high as approximately 10 to 50 kHz, to control the input phase currents on a real time basis so that they resemble pure sine-waves and thus create minimal harmonic distortion reflected back to the AC power source. Various control strategies are available for this feature, including hysteresis current control and space-vector current derived switching patterns. In any case, the object is to provide an input phase current that replicates the input phase electrical potential shape and phase angle regardless of load, input voltages and so forth.

A serious problem with the Vienna type rectifier system 2 is that the electrical potential on the DC bus neutral line 62 fluctuates because it comprises a common mode potential that is about 25 percent of the peak input potential of the AC source and it has a waveform with a fundamental frequency that is the third harmonic of the AC source frequency, with a generally triangular shape for sinusoidal AC input voltages. As hereinbefore described, this distortion may reflect back to the AC power source and it is too large in magnitude and harmonic content to satisfy aircraft power system emission specifications for high power aeronautical electrical power systems.

FIG. 2 is a schematic diagram of an active rectifier system 64 with low harmonic distortion according to a possible embodiment of the invention. Because of the current mode control on the input to the Vienna type rectifier system 2 described in connection with FIG. 1, it may operate with the DC neutral line 62 clamped to the AC source neutral when the output centre point connects to AC system ground with a grounding line 66 coupled to the junction 54 as shown in FIG. 2. If the switching controller 32 is of the current hysteresis type, it may accommodate this modification without change. If the switching controller 32 is of the space vector type, it may accommodate this modification with as little as some minor pulse width table changes to the space-vector switching algorithms, as shall be appreciated by those skilled in the art of software development.

Clamping the DC bus neutral line to the AC system ground effectively drops the undesired common mode electrical potential cross the boost converter and therefore it requires no additional filter inductors. This removes the common mode electrical potential from the output of the DC bus and any connected DC loads. Accordingly, any connected DC loads do not have to add filtering to absorb this common mode electrical potential.

The described embodiment of the invention is only an illustrative implementation of the invention wherein changes and substitutions of the various parts and arrangement thereof are within the scope of the invention as set forth in the attached claims.

Claims

1. A polyphase alternating current (AC) active rectifier system for converting polyphase AC from an AC source to direct current (DC) on a DC bus comprising a positive DC bus line and a negative DC bus line with a switching element for each phase configured to direct current from the AC source to the DC bus through four different conducting paths with three electrical potential levels, including a positive DC level on the positive DC bus line, a negative DC level on the negative DC bus line and a neutral DC level between the positive and negative DC level on a neutral DC bus line that suppresses AC common mode electrical potential from the DC bus, comprising: a ground path between the neutral DC bus line and a system ground coupled to a neutral point for the AC source that clamps the level of the neutral DC bus to the AC source neutral.

2. The rectifier system of claim 1, wherein the polyphase AC comprises three phase AC.

3. The rectifier system of claim 1, wherein a pair of series connected output capacitors connect across the positive and negative DC bus lines and their junction connects to the DC neutral line.

4. The rectifier system of claim 1, further comprising an input inductor for each phase, wherein each switching element selectively couples its respective input inductor to the DC neutral line.

5. The rectifier system of claim 1, further comprising a switching controller for controlling each switching element.

6. The rectifier system of claim 5, wherein the switching controller switches each switching element at a high frequency rate.

7. The rectifier system of claim 5, wherein the switching controller is of the current hysteresis type.

8. The rectifier system of claim 5, wherein the switching controller is of the space-vector current type.

9. A three phase alternating current (AC) active rectifier system for converting three phase AC from an AC source to direct current (DC) on a DC bus comprising a positive DC bus line and a negative DC bus line with a switching element for each phase configured to direct current from the AC source to the DC bus through four different conducting paths with three electrical potential levels, including a positive DC level on the positive DC bus line, a negative DC level on the negative DC bus line and a neutral DC level between the positive and negative DC level on a neutral DC bus line that suppresses AC common mode electrical potential from the DC bus, comprising: a pair of series connected output capacitors connect across the positive and negative DC bus lines and their junction connects to the DC neutral line;an input inductor for each phase, wherein each switching element selectively couples its respective input inductor to the DC neutral bus line; anda ground path between the neutral DC bus line and a system ground coupled to a neutral point for the AC source that clamps the level of the neutral DC bus to the AC source neutral.

10. The rectifier system of claim 9, further comprising a switching controller for controlling each switching element.

11. The rectifier system of claim 10, wherein the switching controller switches each switching element at a high frequency rate.

12. The rectifier system of claim 10, wherein the switching controller is of the current hysteresis type.

13. The rectifier system of claim 10, wherein the switching controller is of the space-vector current type.

14. A three phase alternating current (AC) active rectifier system for converting three phase AC from an AC source to direct current (DC) on a DC bus comprising a positive DC bus line and a negative DC bus line with a switching element for each phase configured to direct current from the AC source to the DC bus through four different conducting paths with three electrical potential levels, including a positive DC level on the positive DC bus line, a negative DC level on the negative DC bus line and a neutral DC level between the positive and negative DC level on a neutral DC bus line that suppresses AC common mode electrical potential from the DC bus, comprising: a pair of series connected output capacitors connect across the positive and negative DC bus lines and their junction connects to the DC neutral line;an input inductor for each phase, wherein each switching element selectively couples its respective input inductor to the DC neutral bus line;a switching controller for controlling each switching element; anda ground path between the neutral DC bus line and a system ground coupled to a neutral point for the AC source that clamps the level of the neutral DC bus to the AC source neutral.

15. The rectifier system of claim 14, wherein the switching controller switches each switching element at a high frequency rate.

16. The rectifier system of claim 14, wherein the switching controller is of the current hysteresis type.

17. The rectifier system of claim 14, wherein the switching controller is of the space-vector current type.