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WO1994021021A1 - Convertisseurs resonants pour source de courant a commutation neutre - Google Patents

Convertisseurs resonants pour source de courant a commutation neutre Download PDF

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Publication number
WO1994021021A1
WO1994021021A1 PCT/US1994/002496 US9402496W WO9421021A1 WO 1994021021 A1 WO1994021021 A1 WO 1994021021A1 US 9402496 W US9402496 W US 9402496W WO 9421021 A1 WO9421021 A1 WO 9421021A1
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WO
WIPO (PCT)
Prior art keywords
circuit
auxiliary
transfer
resonant
neutral
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Legal status (The legal status 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 status listed.)
Ceased
Application number
PCT/US1994/002496
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English (en)
Inventor
Gerard Francis Ledwich
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Electric Power Research Institute Inc
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Electric Power Research Institute Inc
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Filing date
Publication date
Application filed by Electric Power Research Institute Inc filed Critical Electric Power Research Institute Inc
Priority to AU63624/94A priority Critical patent/AU6362494A/en
Publication of WO1994021021A1 publication Critical patent/WO1994021021A1/fr
Anticipated expiration legal-status Critical
Ceased legal-status Critical Current

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Classifications

    • 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
    • H02M7/00—Conversion of AC power input into DC power output; Conversion of DC power input into AC power output
    • H02M7/42—Conversion of DC power input into AC power output without possibility of reversal
    • H02M7/44—Conversion of DC power input into AC power output without possibility of reversal by static converters
    • H02M7/48—Conversion of DC power input into AC power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/4826—Conversion of DC power input into AC power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode operating from a resonant DC source, i.e. the DC input voltage varies periodically, e.g. resonant DC-link inverters

Definitions

  • the present invention relates to resonant power converters and, more particularly, to a resonant DC link circuit in which zero-current resonant switching takes place with respect to a neutral point to reduce switching losses and voltage stresses.
  • Soft-switching (or resonant) converters are favored devices for static power conversion because they incur far lower switching losses than their hard-switched counterparts. This is because the active devices in soft-switched converters are switched at zero-voltage or zero-current intervals.
  • U.S. Patent No. 4,730,242 to Divan discloses a parallel resonant link converter in which the active switching devices are switched at times of zero-voltage intervals. Switching losses are comparatively small. Hence, commutation may occur at higher frequencies.
  • the link current is bi-directional in soft-switched zero-voltage converters such as Divan '242. Consequently, costly bi ⁇ directional switching devices are required.
  • the typical series-resonant current source inverter comprises an input converter circuit coupled to an output inverter circuit through a DC link.
  • the input converter circuit and output inverter circuit are assembled with uni-directional switching devices such as thyristors, which devices are switched according to a predetermined sequence to accomplish the desired transfer of current.
  • the input converter circuit is switched in accordance with the three phase AC input to maintain a DC level based on the voltage difference between the most positive and most negative phases.
  • the output inverter circuit is switched to synthesize a desired three- phase AC output from the DC level.
  • the input converter circuit may be the mirror image of the output inverter circuit, and only the switching sequences need differ.
  • FIG. 1 illustrates a conventional output inverter circuit for a full line-to-line commutated series-resonant current source inverter such as used in Lipo et al. '511.
  • the inverter circuit comprises a three phase bridge of thyristors 50-55 which takes its input from the resonant DC link circuit (not shown) of the series-resonant current source inverter.
  • the thyristors 50-55 are fired according to a predetermined commutation sequence to supply a synthesized three-phase AC current i ga , i sb , and i sc at the three outputs.
  • One full cycle of the three-phase output current waveform is synthesized by a complete switching cycle of the output inverter circuit.
  • Each complete switching cycle of the output inverter circuit comprises six 60° sectors, and during each 60° switching sector one upper switch 50, 52, and 54 must be gated together with its respective series-connected lower switch 51, 53, and 55.
  • Problems remain in the above-described series-resonant converter circuit, since the switch devices 50-55 must still bear significant voltage and current stresses. The stresses are high because gating of each complementary pair of switch devices 50-55 (each pair comprising one upper switch device 50, 52, and 54 and one lower switch device 51, 53, and 55) incurs over-voltages measured from line-to-line. Such line- to-line voltage stresses compel the use of high-power active switch devices.
  • the finite response time of switches 50-55 results in a wasteful NULL state during each sector during which no current is supplied to the outputs.
  • the NULL state appears in each phase of the three-phase output waveform as an intermittent "dead-time" between pulses.
  • FIG. 2 shows a synthesized output current for an exemplary phase of a conventional soft-switched series-resonant converter. Between pulses, the voltage across the switch devices is insufficient to allow switching. This is caused by the slew time of the resonant capacitance from line-to-line.
  • the switch devices 50-55 sit inactive, and the resulting dead-time (or NULL state) between output pulses is clearly visible in FIG. 2.
  • the dead time can be several times greater than the reverse recovery time of the switches. Consequently, the switch device utilization can be very low.
  • an object of the present invention to provide a neutral forming circuit and suitable commutation sequence for a resonant DC-link current source converter in order to improve device utilization and reduce voltage stresses on the switch devices.
  • a neutral commutated soft-switched current source inverter which incorporates line-to-neutral commutation, and in which each resonant commutation is accomplished using a single switch. This way, resonant over-voltages are incurred on a line-to- neutral basis. Assuming a peak load voltage of V which necessitates an excess link voltage of kV , the power rating of the switches may be reduced to 1/ 3(kV ) .
  • the remaining active switch (which is not gated during commutation) can continue to carry link current, and the device utilization of the converter is improved.
  • the current source converter comprises a neutral-commutated input converter circuit and/or output inverter circuit coupled to a resonant DC link.
  • the converter/inverter circuit includes a full bridge three-phase switching circuit including a plurality of parallely-connected pairs of switch devices, the pairs of switch devices each including a top switch device connected in series to a bottom switch device.
  • the converter/inverter circuit also includes a plurality of resonant inductances each having one end connected between one of the pairs of switch devices and another end for connection to a corresponding phase of a three-phase AC input/load.
  • the converter/inverter circuit further includes a neutral forming circuit for connection across the three-phase AC input/load to establish a neutral point bearing a neutral voltage relative to the AC voltage supplied by the AC input or to the AC load.
  • the converter/inverter circuit also includes an auxiliary commutating circuit connected in parallel with the full bridge switching circuit, the auxiliary commutating circuit further comprising a pair of auxiliary switch devices including a top auxiliary switch device and a bottom auxiliary switch device connected together at the neutral point for allowing commutation on a line-to-neutral basis.
  • the invention also includes a method for commutating the above-described input converter and/or output inverter, the method including the steps of turning off an active switch in said switching circuit corresponding to an outgoing phase, turning on one of the auxiliary commutating switches to establish a neutral point, and turning on another successive one of the switches in the switching circuit.
  • FIG. 1 is a schematic diagram of a conventional output inverter circuit for a full line-to-line commutated series- resonant current source inverter such as that shown and described in U.S. Patent No. 4,942,511 issued to Lipo et al.;
  • FIG. 2 illustrates a synthesized output current for an exemplary phase of a conventional soft-switched series- resonant converter;
  • FIG. 3 is a schematic diagram of a neutral-commutated inverter circuit for a series-resonant current source in accordance with one embodiment of the present invention
  • FIG. 4 shows a space vector diagram of a commutation sequence for synthesizing a three-phase sine wave using the inverter circuit of FIG. 3;
  • FIG. 5 illustrates the firing sequence for thyristors 50- 55 of FIG. 3 necessary to generate synthesized three-phase AC output currents i sa , i sb , and i sc ;
  • FIG. 6 is a schematic diagram of an output inverter circuit according to a second embodiment of the invention which is capable of mini-commutation;
  • FIG. 7 is a graph showing the voltage V across capacitor Cr as a function of time during one resonant commutation cycle between bottom switches for the circuits of FIGS. 3 and 6;
  • FIG. 8 shows simulated trace current and voltage outputs for each phase of a synthesized three-phase output waveform from the resonant current source inverter of the present invention as supplied to a resistive load;
  • FIG. 9 shows a space vector diagram of a commutation sequence for synthesizing a three-phase sine wave using a neutral current balancing commutation sequence according to the present invention
  • FIG. 10 is a simulated trace diagram showing the trace of a synthesized three-phase sinusoidal output from the resonant current source inverter of the present invention as supplied to a resistive load using neutral current balancing.
  • series-resonant current source inverters comprise an input converter circuit coupled to an output inverter circuit via a DC link.
  • the input converter circuit and output inverter circuit are assembled with uni-directional switching devices such as thyristors, which devices are switched according to a predetermined sequence to accomplish the desired transfer of current.
  • the input converter circuit is switched in accordance with the three-phase AC input to maintain a DC level based on the voltage difference between the most positive and most negative phases.
  • the output inverter circuit is switched to synthesize a desired three- phase AC output from the DC level.
  • the input converter circuit may be the mirror image of the output inverter circuit, and only the switching sequences need differ. Both circuits have heretofore been switched on a line-to-line basis.
  • the conventional output inverter circuit of FIG. 1 comprises a three-phase bridge of thyristors 50-55 which takes its input from the resonant DC link circuit (not shown) of the series-resonant current source inverter.
  • the thyristors 50-55 are fired according to a predetermined commutation sequence to supply synthesized three-phase AC current i sa , i 8b and i sc , as shown in FIG. 2, at the three outputs.
  • a full cycle of the synthesized three-phase output waveform is generated by a complete switching cycle of the output inverter circuit.
  • Each switching cycle of the output inverter circuit comprises six 60° sectors, hence, the duration of each sector is 1/6 that of a corresponding cycle of the synthesized three-phase output current.
  • the thyristors 50-55 are commutated through a sequence of active switch states in order to distribute link current to the three output phases.
  • one upper switch 50, 52, and 54 must be gated together with its respective series-connected lower switch 51, 53, and 55 during each 60° switching sector.
  • the finite response time of the switches results in a wasteful NULL state during each sector during which no current is supplied to the outputs.
  • FIG. 3 illustrates a neutral-commutated converter circuit for a series-resonant current source in accordance with one embodiment of the present invention.
  • the concept of the invention may be incorporated in either/both the input converter circuit and/or the output inverter circuit of a series-resonant current source converter.
  • the inverter comprises a three-phase bridge 10 which takes its main input from a three-phase supply.
  • Three- phase bridge 10 includes six zero-current turn-off switches 51-55 arranged in series-connected pairs. The series connection of each pair of turn-off switches 50 & 51, 52 & 53, and 54 & 55 is connected to a corresponding input main through one of inductors 70-72.
  • Inductors 70-72 present a small inductance for soft commutation.
  • an auxiliary commutating circuit 20 is connected in parallel across the three-phase bridge 10.
  • Auxiliary commutating circuit 20 further comprises two zero-current turn-off switches Tal and Ta2 arranged in a series-connected pair.
  • a star-connected filter capacitor bank including capacitors 60-62 is also connected across the input main to form a neutral, and the series connection of the turn-off switches Tal and Ta2 is connected to the neutral via an auxiliary capacitor Cr and auxiliary inductor Lrl.
  • the voltage and current ratings for the bridge switches 50-55 are comparable to conventional hard-switched converters, and the voltage rating for the auxiliary and transfer assist switches Trl-Tr4 is the same. However, the current rating for the latter switches Trl-Tr4 may be much smaller since they carry current only during the resonant commutation pulses.
  • the inductors Lrl and Lr2 are sized in accordance with the rate of change of current permitted by the main switches 50- 55, and the resonant capacitor Cr is chosen to give the desired resonant frequency.
  • the invention also comprises a novel commutation sequence for the above-described inverter.
  • the upper switches 50, 52, and 54 are commutated from the highest voltage phase through to the lowest voltage phase, while the lower switches 51, 53, and 55 are commutated from the lowest voltage phase through to the highest voltage phase.
  • FIG. 4 shows a space vector diagram of a commutation sequence for synthesizing a three-phase sine wave according to the present invention. Each 360° cycle is broken into six 60o sectors, and the sequence alternates between commutation of the upper switches 50, 52, and 54 for 60° to commutation between lower switches 51, 53, and 55 for the next 60°. This way, either a lower switch 51, 53, or 55 remains on, or an upper switch 50, 52, or 54 remains on throughout each of the six sectors.
  • the upper switch Tal of the auxiliary commutation circuit 20 is used to assist the upper switches 50, 52, and 54 in commutating from the lowest voltage phase back to the highest phase.
  • the lower switch Ta2 of the auxiliary commutation circuit 20 is used to assist the lower switches 51, 53, and 55 in commutating from the highest voltage phase back to the lowest.
  • the voltage across the auxiliary capacitor Cr is initially higher by a factor kV than any incoming phase voltage V so that the appropriate upper switch can be successfully commutated "on.”
  • Switch Tal is then gated “on,” and the link current is diverted through switch Tal so that the conducting upper switch can be successfully commutated “off.”
  • the voltage across the auxiliary capacitor Cr then drops below the lowest voltage phase at the end of each cycle by -kV , thereby charging capacitor Cr in preparation for commutation back to the highest voltage phase.
  • Capacitor Cr continues to charge until its voltage exceeds the incoming lowest voltage phase by factor -kV p .
  • auxiliary commutation circuit 20 Use of the auxiliary commutation circuit 20 is substantially the same with reqard to commutation of the lower switches from the highest voltage phase back to the lowest with only polarities being reversed.
  • the present invention facilitates current flow even during the NULL states. This is because the auxiliary commutating circuit 20 is gated during the NULL state to maintain a path through the auxiliary commutation circuit 20 to the neutral.
  • the interval during each NULL state in which the auxiliary commutating circuit 20 is gated will be termed the "resonant reset condition," and during the resonant reset condition, some power is still transferred to the load despite the existence of the NULL state.
  • the NULL state AN occurs when switch 53 is turned “off” and switch 5" has yet to be gated “on” (capacitor Cr is charging) During this NULL state AN, the lower switch Ta2 of the auxiliary commutation circuit 20 is gated to initiate the resonant reset condition as noted above. In the resonant reset condition AN, current flows into phase "a" from the neutral.
  • aA + bB + cC y AB + x AC + q AN + (1 - x - y - q) AA (1)
  • q is the fraction of the switch cycle spent in the resonant speed state AN
  • AA is a NULL OUTPUT state which can be entered by turning both of switches 53 and 55 are "off" to thereby generate zero output current.
  • Equating the coefficients in the two sides yields the following:
  • the duration of the resonant reset condition AN is the time it takes for the link current flowing into the auxiliary capacitor CR to cause the capacitor voltage to ramp between the peak negative and positive commutating levels. Since the peak commutating levels are constant, the duration of the resonant reset condition becomes known. Consequently, for an assigned time in the remaining states, the sector time is known and the fraction q can be determined. The fraction of the total cycle to spend in states AC and AB is given by x and y respectively in equations (5) and (6) .
  • phase switching can be ordered so that there are two natural commutations before a resonant transfer is required.
  • a natural commutation can occur whenever the voltage difference between the outgoing and incoming phase is negative. Since switching occurs from the highest phase voltage to the lowest, the voltage difference between the outgoing and incoming phase is negative for two out of three transitions.
  • the phase switching within each sector can be ordered so that there are two natural commutations before a resonant transfer is required.
  • the phase switching within each sector depends on that of the previous sector.
  • switching normally starts and finishes in one of the the NULL OUTPUT states (AA, BB, or CC) .
  • the NULL OUTPUT states are not equivalent since it could require a simultaneous top and bottom resonant transfer which is not possible with the current configuration.
  • the commutation sequence according to the present invention provides a solution by always ending a sector in the active state which is common to the next sector.
  • state AC is common to sectors 1 and 2.
  • One pattern of modulation for the output inverter circuit over six sectors of a complete 360° switching cycle would thus be:
  • FIG. 6 illustrates a second embodiment of the invention which is capable of mini-commutation.
  • Reverse thyristors Ta5 and Ta6 must be provided across Tal and Ta2 switches Tal and Ta2 to allow controlled reverse conduction during resonance around the load voltage.
  • switches Tal and Ta2 could be bi-directional.
  • the operation of the embodiment of FIG. 6 is the same as that of FIG. 3 and, in addition, the upper or lower transfer assist switch Tal or Ta2 corresponding to the switches (upper or lower) being commutated is turned “on” to initiate commutation "off" of the outgoing phase. Once the current through Cr has risen sufficiently, the incoming phase is fired. The capacitor CR will resonate around this new voltage to approximately its starting voltage.
  • the invention may additionally include a transfer-assist circuit 30 as shown in FIGS. 3 and 6, the circuit 30 including two parallely-connected bi-polar transfer-assist switches Ta3 and Ta4 connected through a second inductance Lr2 across the capacitance Cr.
  • the transfer assist circuit 30 may be used to enhance the transfer speed, and this is done simply by turning “on” a transfer assist switch Ta3 or Ta4 (depending on top or bottom commutation) after the reverse recovery time Tq for the commutated switch 50-55.
  • the additional current diverted through the transfer assist circuit 30 speeds the transfer.
  • the active transfer-assist switch Ta3 or Ta4 turns “off” naturally when the voltage reverses since the current therethrough reaches zero.
  • the use of transfer assist circuit 30 can halve the time it takes for the link current Id to ramp the capacitor Cr voltage to the level where the next incoming phase can be commutated "on” to about 2.5 Tq.
  • a resonant transfer will then take 50uS. If the resonant transfer time is to be less than 10% of the PWM switch period, the effective switching frequency will be close to 2kHz. A slight reduction in frequency will result from the occasional need to complete an additional resonant transfer when the phase voltage difference is too low for natural commutation.
  • the operation of the transfer assist circuit 30 is better seen with reference to FIG. 7, which illustrates the voltage V across capacitor Cr as a function of time during one resonant commutation cycle between bottom switches. The voltage V starts at one of the phase voltages.
  • the auxiliary commutation circuit 20 is turned on at 14.965ms, and Cr begins to charge.
  • FIG. 8 shows simulated trace current outputs IL(1) ,
  • trace voltage output V(4) shows the voltage across capacitance Cr.
  • the commutation sequence was devoted a fixed time to the active and NULL states, while the time in the resonant reset state depended on the need for additional resonant or mini- commutations.
  • natural commutation was unreliable and resonant or mini- commutations were used. However, this was an infrequent event, and there were normally two natural commutations and one resonant commutation in each sector.
  • the circuit was capable of satisfactory commutation between a phase to neutral voltage of 500V to another phase at -500V, i.e, a peak line- to-line voltage of 1000V using switches rated at 1300V peak (neglecting safety factors) .
  • the commutation sequence of the present invention has been described with reference to the simplest sequence for sector 1, namely, state AB to AC via the NULL OUTPUT state AA.
  • state AB state AB to AC via the NULL OUTPUT state AA.
  • the resonant reset state AN will appear for a period.
  • current flows into phase A out of the neutral point via auxiliary capacitor Cr. If the same sequence were used continually, the voltage of the neutral point would gradually fall until resonant commutations are no longer possible.
  • the commutation sequence may be slightly modified to balance the currents to the neutral during resonant commutations, thereby eliminating this problem. To accomplish balancing of the neutral currents, every neutral current flow must be balanced with a reverse flow.
  • NULL OUTPUT state AA is of long duration, for the time can be used to commutate for a period to state NA and back to AA.
  • long states AC wherein state NC can be inserted
  • long states AB wherein state NB can be inserted.
  • FIG. 9 shows a space vector diagram of a neutral current balancing commutation sequence for synthesizing a three-phase sine wave according to the present invention.

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Inverter Devices (AREA)

Abstract

Convertisseur résonant pour source de courant à liaison c.c. qui comporte un circuit (60-62) de formation de neutre et un circuit de commutation auxiliaire (20) pour effectuer une commutation phase-neutre, ce qui permet d'améliorer l'utilisation du dispositif de commutation (50-55) et de réduire les contraintes de tension. La présente invention comporte également un circuit (30) d'assistance de transfert pour accélérer des transferts résonants qui emploient la commutation neutre.
PCT/US1994/002496 1993-03-10 1994-03-08 Convertisseurs resonants pour source de courant a commutation neutre Ceased WO1994021021A1 (fr)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AU63624/94A AU6362494A (en) 1993-03-10 1994-03-08 Neutral commutated soft switched current source inverters

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US2913893A 1993-03-10 1993-03-10
US08/029,138 1993-03-10

Publications (1)

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WO1994021021A1 true WO1994021021A1 (fr) 1994-09-15

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Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2012116708A2 (fr) 2011-02-28 2012-09-07 Tallinn University Of Technology Procédé de génération d'une irruption de courant pour des onduleurs dotés d'une z-source, d'une quasi-z-source et d'une trans-z-source aux ondes sinusoïdales modifiées
CN102723851A (zh) * 2011-03-29 2012-10-10 艾默生网络能源系统北美公司 一种桥臂电路

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4730242A (en) * 1986-09-25 1988-03-08 Wisconsin Alumni Research Foundation Static power conversion and apparatus having essentially zero switching losses
US4757435A (en) * 1986-03-19 1988-07-12 Westinghouse Electric Corp. Static-controlled current-source AC/DC power converter and DC/AC power converter, and protection system embodying the same
US4847747A (en) * 1988-09-26 1989-07-11 Westinghouse Electric Corp. Commutation circuit for load-commutated inverter induction motor drives
US4884182A (en) * 1987-04-22 1989-11-28 Hitachi, Ltd. Current source power converting apparatus with overvoltage protection

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4757435A (en) * 1986-03-19 1988-07-12 Westinghouse Electric Corp. Static-controlled current-source AC/DC power converter and DC/AC power converter, and protection system embodying the same
US4730242A (en) * 1986-09-25 1988-03-08 Wisconsin Alumni Research Foundation Static power conversion and apparatus having essentially zero switching losses
US4884182A (en) * 1987-04-22 1989-11-28 Hitachi, Ltd. Current source power converting apparatus with overvoltage protection
US4847747A (en) * 1988-09-26 1989-07-11 Westinghouse Electric Corp. Commutation circuit for load-commutated inverter induction motor drives

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2012116708A2 (fr) 2011-02-28 2012-09-07 Tallinn University Of Technology Procédé de génération d'une irruption de courant pour des onduleurs dotés d'une z-source, d'une quasi-z-source et d'une trans-z-source aux ondes sinusoïdales modifiées
US9214876B2 (en) 2011-02-28 2015-12-15 Tallinn University Of Technology Method of shoot-through generation for modified sine wave Z-source, quasi-Z-source and trans-Z-source inverters
CN102723851A (zh) * 2011-03-29 2012-10-10 艾默生网络能源系统北美公司 一种桥臂电路
CN102723851B (zh) * 2011-03-29 2015-08-26 艾默生网络能源系统北美公司 一种桥臂电路

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