WO2021218227A1 - 一种模块化电容换相换流器和方法 - Google Patents

一种模块化电容换相换流器和方法 Download PDF

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Publication number
WO2021218227A1
WO2021218227A1 PCT/CN2020/141983 CN2020141983W WO2021218227A1 WO 2021218227 A1 WO2021218227 A1 WO 2021218227A1 CN 2020141983 W CN2020141983 W CN 2020141983W WO 2021218227 A1 WO2021218227 A1 WO 2021218227A1
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WIPO (PCT)
Prior art keywords
phase
converter
capacitor
commutation
thyristor valve
Prior art date
Application number
PCT/CN2020/141983
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English (en)
French (fr)
Inventor
季一鸣
杨一鸣
曾嵘
吴方劼
余占清
杜商安
申笑林
郝致远
王尧玄
王玲
Original Assignee
国家电网有限公司
国网经济技术研究院有限公司
清华大学
国网江苏省电力有限公司经济技术研究院
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Publication of WO2021218227A1 publication Critical patent/WO2021218227A1/zh

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    • 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
    • H02M7/66—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal
    • H02M7/68—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters
    • H02M7/72—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/75—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a thyratron or thyristor type requiring extinguishing means
    • H02M7/757—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a thyratron or thyristor type requiring extinguishing means using semiconductor devices only
    • 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
    • H02M7/02—Conversion of ac power input into dc power output without possibility of reversal
    • H02M7/04—Conversion of ac power input into dc power output without possibility of reversal by static converters
    • H02M7/12—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/145—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a thyratron or thyristor type requiring extinguishing means
    • H02M7/155—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a thyratron or thyristor type requiring extinguishing means using semiconductor devices only
    • H02M7/162—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a thyratron or thyristor type requiring extinguishing means using semiconductor devices only in a bridge configuration
    • 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
    • H02M7/505—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 using devices of a thyratron or thyristor type requiring extinguishing means
    • H02M7/515—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 using devices of a thyratron or thyristor type requiring extinguishing means using semiconductor devices only
    • H02M7/521—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 using devices of a thyratron or thyristor type requiring extinguishing means using semiconductor devices only in a bridge configuration
    • 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
    • H02M1/00—Details of apparatus for conversion
    • H02M1/0067—Converter structures employing plural converter units, other than for parallel operation of the units on a single load
    • H02M1/007—Plural converter units in cascade

Definitions

  • This application relates to a modular capacitor commutated converter and method, and belongs to the technical field of converters.
  • LCC DC transmission technology is currently the most widely used DC transmission technology. It plays an irreplaceable role in the field of long-distance and large-capacity transmission.
  • LCC DC transmission technology uses thyristors as the Converter devices have the following essential defects:
  • the commutation process must be provided by the connected AC system.
  • the strength of the connected AC grid has a significant impact on the operation of the DC system.
  • the LCC DC project cannot supply power to the passive system and it is difficult to operate stably under the weak AC system.
  • the rated firing angle of the LCC converter must generally be controlled at about 15°, and the commutation process requires a larger commutation angle, so a large amount of consumption is required when the LCC converter operates
  • a large number of reactive power compensation equipment must be installed and invested at the same time, usually fixed-capacity AC filters and shunt capacitors directly connected to the high-voltage transmission grid.
  • These reactive power compensation equipment is close to half of the area occupied by the converter station, and the investment is large, which increases the complexity of DC control; the reactive power equipment of fixed capacity needs to be switched on and off continuously during the power change process, and the reactive power changes step by step when switching on and off.
  • the LCC converter When the receiving end is connected to the power grid and the bus voltage amplitude of the converter station drops or the three-phase imbalance occurs, the LCC converter is prone to commutation failure. At this time, the DC current increases significantly, and the active power of the sending and receiving ends is interrupted. , The sending end has excessive reactive power consumption, and the sending end and the receiving end have overvoltage. At present, the receiving ends of multiple LCC DC transmission lines are often fed into a certain load center and coupled with each other. When one DC fails in commutation, it may cause successive commutation failures of multiple DCs, and also when the receiving end AC system fails. It may cause multiple DCs to fail in commutation at the same time, and expand the accident. At this time, the DC is difficult to self-recover, the DC current surges, and serious excess reactive power over voltage of the receiving end system is caused, which threatens the safety and stability of the power grid.
  • the prior art usually starts by improving the topology of the traditional LCC converter, and introduces energy storage elements or new power electronic devices into the traditional topology to improve the commutation performance of the LCC converter.
  • the forced commutated converter is a new type of converter topology that has been studied more.
  • the capacitive commutated converter (Capacitor Commutated Converter, CCC) is the representative. Its basic structure is based on the traditional LCC converter topology.
  • a capacitor is connected in series between the converter transformer and the converter valve of the structure, and the voltage on the capacitor is used to provide auxiliary commutation voltage for the commutation of the converter valve thyristor, so as to advance the commutation time, improve the power factor, and reduce reactive power consumption , And provide support voltage when the AC system fails to reduce the probability of commutation failure.
  • the capacitor charging of the CCC converter is difficult to control. The capacitor voltage increases the insulation level of the converter valve and converter transformer, and the overall harmonic characteristics of the converter are more complicated. Once the commutation failure occurs and the capacitor charging is out of control, the inverter will easily lose its fault self-recovery ability, resulting in continuous commutation failure.
  • This application provides a modular capacitor commutation converter and method, which by introducing a switchable power electronic power device and forming a capacitor module with a DC support capacitor, it is connected to one of the converter converter and converter valve of the LCC DC transmission system At the same time, through the access and removal of multiple capacitor modules, it assists the commutation process of the LCC DC transmission system.
  • This application provides a modular capacitor commutator converter, including: a three-phase six-pulse converter valve, a converter transformer, and a capacitor module; the three-phase six-pulse converter valve is connected in series with the capacitor module, and the capacitor module is arranged in the converter Between the current transformer and the three-phase six-pulse converter valve; the capacitor module includes a capacitor and a converter bridge, the converter bridge includes at least two sub-modules composed of turn-off power electronic devices and diodes in reverse parallel, the converter bridge and The capacitors are connected in parallel.
  • This application also discloses a modular capacitor commutation commutation method, which adopts any one of the above-mentioned modular capacitor commutation converters.
  • the modular capacitor commutation converter operates in the rectification mode, the X-phase commutation is performed.
  • Fig. 1 is a schematic structural diagram of the rectification mode of a modular capacitor commutated converter in an embodiment of the present application (the converter transformer is represented by a star connection);
  • FIG. 2 is a schematic structural diagram of the inverter mode of the modular capacitor commutated converter in an embodiment of the present application (the converter transformer is represented by a star connection);
  • FIG. 3 is a schematic diagram of the structure of a half-bridge capacitor module in an embodiment of the present application.
  • FIG. 4 is a schematic structural diagram of a full-bridge capacitor module in an embodiment of the present application.
  • Fig. 5 is the voltage and current waveforms during rectification operation of the modular capacitor commutation converter in an embodiment of the present application
  • Fig. 6 shows the voltage and current waveforms during inverter operation of the modular capacitor commutated converter in an embodiment of the present application.
  • This embodiment provides a modular capacitor commutation converter, as shown in Figure 1 (rectifier mode) and Figure 2 (inverter mode), including: three-phase six-pulse converter valve, converter transformer and capacitor module (That is, the Mutiple Modular Capacitor in Figure 1 and Figure 2); the three-phase six-pulse converter valve is connected in series with the capacitor module, and the capacitor module is arranged between the converter transformer and the three-phase six-pulse converter valve,
  • the converter transformer in Figure 1 and Figure 2 is characterized by its secondary side voltage.
  • the converter transformer can adopt star connection mode or corner connection mode;
  • the capacitor module includes a capacitor and a converter bridge, and the converter bridge includes at least two switches that can be switched off.
  • a sub-module composed of power-off electronic devices and diodes in anti-parallel connection, and the converter bridge is connected in parallel with the capacitor.
  • the capacitor module is introduced, and its voltage provides auxiliary commutation voltage for the commutation of the LCC converter valve thyristor, relaxes the angle limit of the thyristor commutation, improves the degree of freedom of the trigger angle, and enables the commutation to have a natural on-line voltage Before the commutation zero point (rectification) and near the natural commutation cut-off point of the line voltage (inversion), the reactive power consumption is reduced and the active and reactive power characteristics of the LCC DC transmission system are improved.
  • the converter valve in the modular capacitor commutator converter provided by the present application may be a bridge-type N-pulsation converter valve.
  • the bridge-type N-pulsation converter valve includes N bridge arms.
  • three-phase six-pulsation The converter valve will be introduced in detail as an example.
  • the three-phase six-pulse converter valve is a bridge-type three-phase six-pulse converter valve, including six bridge arms, among which, the first bridge arm is connected with the first thyristor valve A1, and the third bridge arm is connected with the third thyristor valve B1, the fifth bridge arm is connected with the fifth thyristor valve C1.
  • the anodes of the first thyristor valve A1, the third thyristor valve B1 and the fifth thyristor valve C1 are all connected to the low-voltage side of the DC output end of the three-phase six-pulse converter valve ( In rectifier mode) or high voltage side (in inverter mode), the cathode of the first thyristor valve A1 is connected to the anode of the second thyristor valve A2 through the second bridge arm, and the cathode of the third thyristor valve B1 is connected to the fourth bridge arm through the fourth bridge.
  • the anode of the thyristor valve B2, the cathode of the fifth thyristor valve C1 are connected through the sixth bridge arm, the anode of the sixth thyristor valve C2, the cathodes of the second thyristor valve A2, the fourth thyristor valve B2 and the sixth thyristor valve C2 are all connected to three High-voltage side (in rectifier mode) or low-voltage side (in inverter mode) of the DC output of phase six-pulse converter valve; when the converter transformer is star-connected, the first thyristor valve A1 and the second thyristor valve A2 are connected through the capacitor module On the secondary side of the A-phase converter transformer, the third thyristor valve B1 and the fourth thyristor valve B2 are connected to the secondary side of the B-phase converter transformer through the capacitor module, and the fifth thyristor valve C1 and the sixth thyristor valve C2 are connected to the C phase through the capacitor module.
  • the first thyristor valve A1 and the second thyristor valve A2 are connected to the connection point of the secondary side of the A-phase converter transformer and the secondary side of the B-phase converter transformer through a capacitor module .
  • the third thyristor valve B1 and the fourth thyristor valve B2 are connected to the connection point of the secondary side of the B-phase converter transformer and the secondary side of the C-phase converter transformer through the capacitor module.
  • the fifth thyristor valve C1 and the sixth thyristor valve C2 are connected through the capacitor
  • the module connects the connection point between the secondary side of the C-phase converter transformer and the secondary side of the A-phase converter transformer.
  • the voltage difference between the high voltage side and the low voltage side of the DC output end of the three-phase six-pulse converter valve is the output DC voltage U dc and the corresponding harmonic voltage.
  • the short-circuit inductance of the A-phase converter transformer itself is used as the commutation inductance of the A-phase converter transformer
  • the short-circuit inductance of the B-phase converter transformer itself is used as the commutation inductance of the B-phase converter transformer.
  • the short-circuit inductance of the C-phase converter transformer itself is used as the commutation inductance of the C-phase converter transformer, which is represented by Lp in Figure 1 and Figure 2.
  • the commutation inductance Lp has the function of energy storage and current limiting.
  • the short-circuit inductance of the A-phase converter transformer itself, the short-circuit inductance of the B-phase converter transformer itself, and the short-circuit inductance of the C-phase converter transformer itself, are the equivalent values of these three short-circuit inductances as the commutation inductance;
  • a smoothing reactor L d is connected to the high-voltage side or low-voltage side of the DC output end of the three-phase six-pulse converter valve.
  • the smoothing reactor L d mainly plays a role of filtering.
  • the capacitor module in this embodiment mainly has two topological structures, namely a half-bridge capacitor module and a full-bridge capacitor module.
  • the half-bridge capacitor module includes: capacitors and converter bridges connected in parallel.
  • the converter bridge includes an upper bridge arm and a lower bridge arm. Both the upper bridge arm and the lower bridge arm are connected with power that can be turned off.
  • a sub-module composed of electronic devices and diodes in anti-parallel connection the anode of the sub-module of the upper bridge arm is connected to the first end of the capacitor, the cathode of the sub-module of the lower bridge arm is connected to the second end of the capacitor, and the cathode of the sub-module of the upper bridge arm is connected to the
  • the positive pole of the submodule of the lower bridge arm is connected and connected to a lead wire a, and the other lead wire b is drawn from the second end of the capacitor.
  • the voltage direction on the capacitor is fixed, as shown in Figure 3, the voltage direction is the voltage direction of the capacitor module.
  • the full-bridge capacitor module includes: capacitors connected in parallel with each other and two converter bridges.
  • Each converter bridge includes an upper bridge arm and a lower bridge arm. Both upper and lower bridge arms are connected with each other.
  • the sub-module composed of power electronic devices and diodes in reverse parallel connection can be turned off.
  • the anodes of the sub-modules of the upper arms of the two converter bridges are connected to the first end of the capacitor, and the sub-modules of the lower arms of the two converter bridges
  • the negative poles are both connected to the second end of the capacitor, the negative poles of the sub-modules of the upper arms of the two converter bridges are respectively connected to the positive poles of the sub-modules corresponding to the lower arms, and the upper of each of the two converter bridges
  • the negative pole of the sub-module of the bridge arm is connected to a lead wire, namely lead wire a and lead wire b.
  • the voltage direction on the capacitor is fixed, as shown in Figure 4, the voltage direction is the voltage direction of the capacitor module.
  • insulated gate bipolar transistors IGBT
  • IGCT Intergrated Gate Commutated Thyristors
  • This embodiment discloses a modular capacitor commutation commutation method, which is realized by adopting any of the modular capacitor commutation converters in the first embodiment, wherein the X phase and the Y phase are both A phase, B phase and One of the C phases, and the X phase and Y phase are not the same.
  • An embodiment of the present application provides a modular capacitor commutation commutation method, which adopts the modular capacitor commutation converter described in the first embodiment, and the method includes:
  • the X-phase is switched to the Y-phase, and the capacitor modules connected in the corresponding circuits of the X-phase and Y are turned on before the natural commutation zero point P ,
  • the X-phase is switched to the Y-phase, and the X-phase and Y-phase corresponding circuits are switched on before the natural commutation cut-off point Q.
  • the capacitor module, the thyristor valve set to transmit the Y-phase current and the thyristor valve set to transmit the X-phase current commutate around the natural commutation cut-off point Q of the AC line voltage to establish the Y-phase current.
  • the phase change X to Y phase, the line voltage U YX natural commutation zero crossing opening P and X-phase Y corresponds to the capacitor module connected in the circuit, connect the capacitor module in series in the positive direction of the Y phase to increase the voltage on the AC side of the thyristor valve Y2, that is, the voltage at point Y', and connect the capacitor module in series in the opposite direction of X to reduce the thyristor
  • the voltage on the AC side of the valve X2, that is, the voltage at point X' makes the actual commutation voltage relative voltage value U Y'-X' of the thyristor valve Y2 than the corresponding actual AC line voltage U YX relative voltage value on the secondary side of the converter transformer Large, the forward voltage of the thyristor valve Y2 that is about to trigger the conduction is established in advance, so that the thyristor valves Y
  • the forward direction refers to the direction from the lead a to the lead b in FIG. 1, and the reverse refers to the direction from the lead b to the lead a in FIG. 1.
  • change from X-phase to Y-phase then before point P, connect the capacitor module in series in the opposite direction of Y, and connect the capacitor module in series in the positive direction of X-phase.
  • the goal is to establish the thyristor that will trigger the turn-on in advance.
  • the forward voltage of valve Y1 is reversed in advance. It should be noted that the thyristor valves Y1 and Y2 are thyristors set to transmit Y-phase current.
  • the thyristor valve Y1 represents the first thyristor valve A1
  • the thyristor valve Y2 represents The second thyristor valve A2
  • Y'point represents the A'point
  • the thyristor valve X1, X2 is the thyristor set to transmit the X-phase current, for example, when the X-direction is the B-phase, the thyristor valve X1 It means the third thyristor valve B1, the thyristor valve X2 means the fourth thyristor valve B2, and point X'means point B'.
  • the X phase is switched to the Y phase, and the X phase is turned on before the natural commutation cut-off point Q of the online voltage U XY
  • the capacitor module in the circuit corresponding to Y connect the capacitor module in series in the negative direction of the Y phase, reduce the voltage on the AC side of the thyristor valve Y1, that is, the voltage at point Y', and connect the capacitor module in series in the positive direction of the X phase.
  • the voltage on the AC side of the thyristor valve X1 that is, the voltage at point X', so that the actual commutation voltage U X'-Y' of the thyristor valve Y1 is relative to the corresponding actual AC line voltage U XY on the secondary side of the converter transformer
  • the negative voltage of the thyristor valve X1 that is about to be turned off is increased to ensure that the thyristor valve X1 is reliably turned off, and the increased negative voltage after the turn-off is kept higher than the natural commutation cut-off point Q
  • a longer time allows the thyristor valves Y1 and X1 to commutate around the natural commutation cut-off point Q of the AC line voltage to establish a Y-phase current.
  • the modular capacitor commutation converter adopts the following methods to suppress commutation failure: when an abnormal AC voltage is detected but no commutation failure occurs, more capacitor modules are added to the two corresponding circuits to be commutated to increase the reverse Change the range of the cut-off angle; after the commutation failure occurs, close all the capacitor modules in the circuit corresponding to the two-phase voltage of the commutation until the commutation failure is restored.
  • phase A is switched to phase B, advance a certain angle before the natural commutation zero crossing point P, turn on the capacitor modules connected in the corresponding circuits of phase A and B, and trigger the capacitor module to turn off the power electronic devices.
  • the relative voltage value of the commutation voltage B'-A' of the AC side of the thyristor valve B2 is larger than the relative voltage value of the actual AC voltage BA on the secondary side of the converter transformer, which is equivalent to advancing the phase of the AC voltage on the grid side.
  • the forward voltage of the thyristor valve B2 enables the thyristor valves A2 and B2 to commutate before the natural commutation zero-crossing point P of the AC voltage to establish the B-phase current.
  • the trigger capacitor module can be turned off. Power electronic devices, turn off the capacitor module in the B-phase circuit, and keep the A-phase capacitor module in operation, and perform the sequential low-potential commutation process of the six-pulse converter, that is, the low-point C-phase is switched to the A-phase.
  • the change trend of voltage and current in the commutation process is shown in Figure 5.
  • Figure 5 has two phase commutation processes, from phase A to phase B at high potential, and from phase C to phase A at low potential.
  • the horizontal axis in Figure 5 represents time (t), and the dashed line is the converter transformer.
  • the actual secondary-side alternating voltage U Yn, U An in Fig. 1, U Bn like, thin solid line is the actual commutation thyristor valve voltage U Y'n, U A'n in Figure 1, U b'n Etc.
  • the thick solid line is the DC output voltage of the three-phase six-pulse converter valve
  • I a and I b are the commutation currents of the converter valve.
  • the capacitor module When the low-potential commutation process occurs in the rectification state of the six-pulse converter, if the phase C is switched to phase A, the capacitor module must be connected in series in the opposite direction of A, and the capacitor module must be connected in series in the positive direction of phase C.
  • the goal is to establish in advance The forward voltage of the thyristor valve A1 that is about to trigger the conduction, commutates in advance.
  • the relative voltage value of the commutation voltage A'-B' on the AC side of the thyristor valve B1 is larger than the relative voltage value of the actual AC voltage AB on the secondary side of the converter transformer, which is equivalent to delaying the phase of the AC voltage on the grid side.
  • Figure 6 has two phase commutation processes, from phase A to phase B at high potential, and from phase C to phase A at low potential.
  • the horizontal axis in Figure 6 represents time (t), and the dashed line is the converter transformer.
  • the capacitor module When the low-potential commutation process occurs in the inverter state of the six-pulse converter, such as phase C to phase A, the capacitor module must be connected in series in the positive direction of phase A, and the capacitor module must be connected in series in the opposite direction of C. The goal is to increase The negative voltage of the thyristor valve C2 that is about to be turned off after turning off ensures that C2 is reliably turned off.
  • Modular capacitor commutation converter assists commutation through repeated switching of modular capacitors, expands the trigger angle range of the converter, and can control the commutation current and AC voltage of the converter valve to be close to the same phase, thereby reducing reactive power compensation and improving
  • the power factor greatly reduces the AC filter field of the original LCC DC project.
  • the modular capacitor commutation converter can be switched back to the traditional LCC control mode without switching the capacitor module.
  • more capacitor modules can be added in advance to make The modular capacitor commutation converter emits inductive reactive power.
  • the harmonics in the commutation current and the harmonics of the DC output terminal voltage will be more complicated and can be filtered out with solutions such as active filters. By switching the capacitor module, the range of the turn-off angle is increased, and the probability of commutation failure is reduced.
  • the ⁇ 800kV, 8000MW LCC UHV DC project adopts bipolar, each pole adopts two twelve-pulse converter valves, a total of four six-pulse converters, and the inductive pressure drop of the converter at the sending end is set as 10.5%, the rated firing angle is taken as 15° according to engineering experience, and the ideal no-load DC voltage U dio of each six-pulse converter at the sending end is 233.5kV.
  • the design goal is to transform the LCC converter into a modular capacitor commutated converter, so that the reactive power consumption at the sending end is zero.
  • the traditional LCC converter is triggered at 15°, the commutation angle is about 26° at full power, and its power factor angle can be approximated as:
  • the trigger angle should be advanced by 30.65°, so it should be 30.65°-15° ⁇ 15° before the natural commutation point, that is, the actual AC voltage difference is When there is The forward pressure difference of the two phases is required to provide auxiliary commutation voltage. If a full-bridge capacitor module is used, the voltage provided by the two-phase capacitor module should be the same, that is, the voltage provided by the capacitor module of each phase is:
  • the time for phase A to put in the capacitor module before the commutation trigger In order to maintain the balance of the DC voltage on the capacitor module, the time for phase A to put in the capacitor module before the commutation trigger must be equal to the time for phase B to put the capacitor module into after the commutation, that is, 15° after the commutation, B Phase is still input to the capacitor module, at this time the maximum voltage difference between phase B and phase C is:
  • the cut-off angle of the traditional LCC DC converter is 17°
  • the commutation angle at rated power is generally 20.22°
  • the power factor angle is approximately:
  • the time when the A phase is put into the capacitor module before the commutation trigger must end with the B phase at the end of the commutation After the capacitor modules are put in for the same time, the capacitor modules are put in at 17° before the commutation trigger.
  • the maximum voltage difference between phase A and phase C is:
  • multiple capacitor modules composed of high-power power electronic devices and capacitors that can be turned off are connected in series into the LCC commutation circuit.
  • the voltage of the capacitor modules provides auxiliary commutation voltage for the commutation of the LCC converter valve thyristor, which is relaxed
  • the trigger angle limit of the thyristor commutation improves the freedom of the trigger angle, so that the commutation may occur before the zero-crossing point of the natural commutation of the line voltage (rectification) and near the cut-off point of the natural commutation of the line voltage (inversion), reducing reactive power Consumption improves the active and reactive power characteristics of the LCC DC transmission system.
  • the number of input capacitor modules can be controlled to avoid the excessive insulation level of the capacitor commutator converter.
  • This application can automatically control the number of input capacitor modules according to the monitoring of the AC grid.
  • the capacitor module can be removed to avoid irreversible commutation failure, or the capacitor module can be used to help recover the commutation failure and avoid continuous commutation failure.

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  • Power Engineering (AREA)
  • Inverter Devices (AREA)
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Abstract

本申请属于换流器技术领域，涉及一种模块化电容换相换流器和方法，包括：三相六脉动换流阀、换流变压器和电容模块；三相六脉动换流阀与电容模块串联，且电容模块设置于换流变压器与三相六脉动换流阀之间；电容模块包括电容器和变流桥，变流桥包括至少两个由可关断电力电子器件与二极管反向并联组成的子模块，变流桥与电容器并联。

Description

一种模块化电容换相换流器和方法
本申请要求在2020年4月30日提交中国专利局、申请号为202010362490.5的中国专利申请的优先权，该申请的全部内容通过引用结合在本申请中。
技术领域
本申请是关于一种模块化电容换相换流器和方法，属于换流器技术领域。
背景技术
电网换相换流器(Line Commutated Converter，LCC)直流输电技术是目前应用最多的直流输电技术，在远距离、大容量输电领域中起着不可替代的作用，但由于LCC直流输电技术采用晶闸管作为换流器件，存在着以下本质缺陷：
1、换相过程必须由接入的交流系统提供换相电压，接入的交流网架强度对直流系统运行影响显著，LCC直流工程不能向无源系统供电，在弱交流系统下稳定运行困难。
2、为了建立足够的晶闸管换相正向电压，LCC换流器额定触发角一般必须控制在15°左右，且换相过程需要较大的换相角，因此LCC换流器运行时需要消耗大量无功功率，必须同时装设并投入大量无功补偿设备，一般是固定容量的直接接入高压输电网的交流滤波器和并联电容器。这些无功补偿设备接近换流站占地的一半，投资多，增加了直流控制的复杂性；固定容量的无功设备在功率变化过程中需不断投切，投切时无功阶梯式跃变，造成电压波动；无功设备依靠断路器动作进行投切，断路器动作需要时间较长，无法灵敏的反应，在功率大幅变化如功率反转、换流器故障闭锁时无法及时动作，造成无功过剩和交流系统过电压，这限制了LCC直流工程输电功率变化的速度，大大降低输电灵敏性；电容器直接接入交流输电网，改变了输电网的谐波阻抗特性，容易造成谐波放大。此外，LCC直流工程的这种无功产生机理使其无功功率与有功功 率耦合过密，无法解耦，无法像另一种直流输电型式-电压源型换流器(Voltage Source Converter，VSC)一样灵活的调节无功输出，大大降低了和交流系统的协调性。
3、受端接入电网发生故障造成换流站母线电压幅值下降或三相不平衡时，LCC换流器易发生换相失败，此时直流电流大幅增加，送端、受端有功功率中断，送端无功消耗过剩，送端、受端发生过电压。目前往往多条LCC直流输电线路受端集中馈入某一负荷中心，彼此相互耦合，当一条直流发生换相失败时，可能引发多条直流相继换相失败，当受端交流系统发生故障时亦可能导致多条直流同时换相失败，扩大事故，此时直流难以自恢复，直流电流激增，并造成严重的受端系统无功过剩过电压，对电网的安全稳定性造成威胁。
针对上述LCC直流输电技术的缺陷，现有技术通常从改进传统LCC换流器的拓扑结构入手，在传统拓扑结构中引入储能元件或新型电力电子器件，改善LCC换流器的换相性能。强迫换相换流器就是其中研究较多的一种新型换流器拓扑结构，其中以电容换相换流器(Capacitor Commutated Converter，CCC)为代表，其基本结构是在传统LCC换流器拓扑结构的换流变压器和换流阀之间串接入电容器，用电容器上的电压为换流阀晶闸管的换相提供辅助换相电压，从而将换相时间提前，提高功率因素，减少无功消耗，并在交流系统故障时提供支撑电压，降低换相失败发生的概率。但CCC换流器的电容充电难以控制，电容电压提高了换流阀和换流变的绝缘水平，换流器的整体谐波特性更加复杂。一旦发生换相失败后电容充电失控，换流器易失去故障自恢复能力，导致连续换相失败。
发明内容
本申请提供了一种模块化电容换相换流器和方法，其通过引入可关断电力电子功率器件并与直流支撑电容器构成电容模块，接入LCC直流输电系统换流变与换流阀之间，通过多个电容模块的接入和切除，辅助LCC直流输电系统的 换相过程。
本申请提供了一种模块化电容换相换流器，包括：三相六脉动换流阀和换流变压器和电容模块；三相六脉动换流阀与电容模块串联，且电容模块设置于换流变压器与三相六脉动换流阀之间；电容模块包括电容器和变流桥，变流桥包括至少两个由可关断电力电子器件与二极管反向并联组成的子模块，变流桥与电容器并联。
本申请还公开了一种模块化电容换相换流方法，采用上述任一项的模块化电容换相换流器，当模块化电容换相换流器运行在整流模式时，由X相换到Y相,在自然换相过零点P前开启X相和Y相对应电路中接入的电容模块，设置为传输Y相电流的晶闸管阀和设置为传输X相电流的晶闸管阀在交流线电压的自然换相过零点P前进行换相，建立Y相电流；当模块化电容换相换流器运行在逆变模式时，由X相换到Y相,在自然换相截止点Q前开启X相和Y相对应电路中接入的电容模块，设置为传输Y相电流的晶闸管阀和设置为传输X相电流的晶闸管阀在交流线电压的自然换相截止点Q点附近进行换相，建立Y相电流；其中，X相和Y相均为A相、B相和C相中的一相，且X相和Y相不相同。
附图说明
图1是本申请一实施例中模块化电容换相换流器的整流模式的结构示意图(换流变压器用星接表示)；
图2是本申请一实施例中模块化电容换相换流器的逆变模式的结构示意图(换流变压器用星接表示)；
图3是本申请一实施例中半桥电容模块的结构示意图；
图4是本申请一实施例中全桥电容模块的结构示意图；
图5是本申请一实施例中模块化电容换相换流器整流运行时电压电流波形；
图6是本申请一实施例中模块化电容换相换流器逆变运行时电压电流波形。
具体实施方式
为了使本领域技术人员更好的理解本申请的技术方向，通过具体实施例对本申请进行详细的描绘。然而应当理解，具体实施方式的提供仅为了更好地理解本申请，它们不应该理解成对本申请的限制。在本申请的描述中，需要理解的是，所用到的术语仅仅是用于描述的目的，而不能理解为指示或暗示相对重要性。
实施例一
本实施例提供了一种模块化电容换相换流器，如图1(整流模式)和图2(逆变模式)所示，包括：三相六脉动换流阀、换流变压器和电容模块(即图1及图2中的多模块电容器(Mutiple Modular Capacitor))；三相六脉动换流阀与电容模块串联，且电容模块设置于换流变压器与三相六脉动换流阀之间，图1和图2中的换流变压器用其二次侧电压表征，换流变压器可以采取星接模式或角接模式；电容模块包括电容器和变流桥，变流桥包括至少两个由可关断电力电子器件与二极管反向并联组成的子模块，变流桥与电容器并联。
本实施例中引入电容模块，其电压为LCC换流阀晶闸管的换相提供了辅助换相电压，放宽了晶闸管换相的角度限制，提高了触发角的自由度，使换相能够在线电压自然换相过零点之前(整流)和线电压自然换相截止点附近(逆变)进行，减少了无功消耗，改善了LCC直流输电系统的有功和无功特性。
其中，本申请提供的模块化电容换相换流器中的换流阀可以为桥式N脉动换流阀，桥式N脉动换流阀包括N个桥臂，本实施例以三相六脉动换流阀为例进行详细介绍。三相六脉动换流阀为桥式三相六脉动换流阀，包括六个桥臂，其中，第一桥臂上连接有第一晶闸管阀A1，第三桥臂上连接有第三晶闸管阀B1，第五桥臂上连接有第五晶闸管阀C1，第一晶闸管阀A1、第三晶闸管阀B1和第五晶闸管阀C1的阳极均连接三相六脉动换流阀的直流输出端的低压侧(整流模式时)或高压侧(逆变模式时)，第一晶闸管阀A1的阴极通过第二桥臂连接第 二晶闸管阀A2的阳极，第三晶闸管阀B1的阴极通过第四桥臂连接第四晶闸管阀B2的阳极，第五晶闸管阀C1的阴极通过第六桥臂连接、第六晶闸管阀C2的阳极，第二晶闸管阀A2、第四晶闸管阀B2和第六晶闸管阀C2的阴极均连接三相六脉动换流阀的直流输出端的高压侧(整流模式时)或低压侧(逆变模式时)；在换流变压器星接时，第一晶闸管阀A1和第二晶闸管阀A2通过电容模块连接A相换流变压器二次侧，第三晶闸管阀B1、第四晶闸管阀B2通过电容模块连接B相换流变压器二次侧，第五晶闸管阀C1、第六晶闸管阀C2通过电容模块连接C相换流变压器二次侧，在换流变压器角接时，第一晶闸管阀A1和第二晶闸管阀A2通过电容模块连接A相换流变压器二次侧和B相换流变压器二次侧的连接点，第三晶闸管阀B1、第四晶闸管阀B2通过电容模块连接B相换流变压器二次侧和C相换流变压器二次侧的连接点，第五晶闸管阀C1、第六晶闸管阀C2通过电容模块连接C相换流变压器二次侧和A相换流变压器二次侧的连接点。三相六脉动换流阀的直流输出端的高压侧和低压侧的电压差为输出的直流电压U dc和对应的谐波电压。
在换流变压器星接时，将A相换流变压器本身的短路电感作为A相换流变压器的换相电感，将B相换流变压器本身的短路电感作为作为B相换流变压器的换相电感，将C相换流变压器本身的短路电感作为C相换流变压器的换相电感，在图1和图2中以Lp表示，换相电感Lp具有储能、限流的作用，在换流变压器角接时，将A相换流变压器本身的短路电感、B相换流变压器本身的短路电感和C相换流变压器本身的短路电感，这三个短路电感的等效值，作为换相电感；三相六脉动换流阀的直流输出端高压侧或低压侧连接有平波电抗器L d平波电抗器L d主要起到滤波的作用。
本实施例中的电容模块主要为两种拓扑结构，分别为半桥电容模块和全桥电容模块。
半桥电容模块，如图3所示，包括：相互并联的电容器和变流桥，变流桥包括一个上桥臂和一个下桥臂，上桥臂和下桥臂均连接有可关断电力电子器件 和二极管反向并联构成的子模块，上桥臂的子模块的正极连接电容器第一端，下桥臂的子模块的负极连接电容器的第二端，上桥臂的子模块的负极与下桥臂的子模块的正极相连并连接一条引出线a，另一条引出线b从电容器的第二端引出。电容器上的电压方向固定，如图3所示，电压方向即为电容模块的电压方向。
全桥电容模块，如图4所示，包括：相互并联的电容器和两个变流桥，每一个变流桥包括一个上桥臂和一个下桥臂，上桥臂和下桥臂均连接有可关断电力电子器件和二极管反向并联构成的子模块，两个变流桥的上桥臂的子模块的正极均连接电容器第一端，两个变流桥的下桥臂的子模块的负极均连接电容器的第二端，两个变流桥的上桥臂的子模块的负极分别连接对应下桥臂的子模块的正极，并且两个变流桥中的每一个变流桥的上桥臂的子模块的负极连接一条引出线，即引出线a和引出线b。电容器上的电压方向固定，如图4所示，电压方向即为电容模块的电压方向。
目前大功率电力电子技术发展迅速，各种大功率开关器件性能不断提高，特别是具备可关断能力的电力电子器件，如绝缘栅双极型晶体管(Insulated Gate Bipolar Transistor，IGBT)或集成门极换流晶闸管(Intergrated Gate Commutated Thyristors，IGCT)等，将其反向与二极管并联组成子模块，子模块构成变流桥，并与电容器并联，构成电容模块。可以方便的通过该电力电子器件的通断，控制电容模块与该电容模块对应的电容换相换流器串联或断开。
实施例二
本实施例公开了一种模块化电容换相换流方法，采用实施例一中的任一种模块化电容换相换流器实现，其中，X相和Y相均为A相、B相和C相中的一相，且X相和Y相不相同。
本申请实施例提供的一种模块化电容换相换流方法，采用实施例一所述的模块化电容换相换流器，该方法包括：
在所述模块化电容换相换流器运行在整流模式时的情况下，由X相换到Y相，在自然换相过零点P前开启X相和Y相对应电路中接入的电容模块，设置为传输Y相电流的晶闸管阀和设置为传输X相电流的晶闸管阀在交流线电压的自然换相过零点P前进行换相，建立Y相电流；
当在所述模块化电容换相换流器运行在逆变模式时的情况下，由X相换到Y相,在自然换相截止点Q前开启X相和Y相对应电路中接入的电容模块，设置为传输Y相电流的晶闸管阀和设置为传输X相电流的晶闸管阀在交流线电压的自然换相截止点Q点附近进行换相，建立Y相电流。
当模块化电容换相换流器运行在整流模式时，如图1所示，在高电位处，由X相换到Y相，在线电压U Y-X的自然换相过零点P前开启X相和Y相对应电路中接入的电容模块，在Y相正向串联接入电容模块，提高晶闸管阀Y2交流侧的电压，即Y’点的电压，在X相反向串联接入电容模块，降低晶闸管阀X2交流侧的电压，即X’点的电压，使得晶闸管阀Y2实际的换相电压相对电压值U Y’-X’比对应的换流变压器二次侧实际交流线电压U Y-X相对电压值大，提前建立即将触发导通的晶闸管阀Y2的正向电压，使得晶闸管阀Y2和X2在交流线电压的自然换相过零点P前进行换相，建立Y相电流。正向是指图1中引线a到引线b的方向，反向是指图1中引线b到引线a的方向。在低电位处，由X相换到Y相,则在P点前，在Y相反向串联接入电容模块，在X相正向串联接入电容模块，目标是提前建立即将触发导通的晶闸管阀Y1的正向电压，提前换相。需要说明的是，晶闸管阀Y1，Y2为设置为传输Y相电流的晶闸管，示例性的，在Y向为A相的情况下，晶闸管阀Y1就表示第一晶闸管阀A1，晶闸管阀Y2就表示第二晶闸管阀A2，Y’点就表示A’点；同理，晶闸管阀X1,X2为设置为传输X相电流的晶闸管，示例性的，在X向为B相的情况下，晶闸管阀X1就表示第三晶闸管阀B1，晶闸管阀X2就表示第四晶闸管阀B2，X’点就表示B’点。
当模块化电容换相换流器运行在逆变模式时，如图2所示，在高电位处，由X相换到Y相，在线电压U X-Y的自然换相截止点Q前开启X相和Y相对应电路中接入的电容模块，在Y相负向串联接入电容模块，降低晶闸管阀Y1交流侧的电压，即Y’点的电压，在X相正向串联接入电容模块，提高晶闸管阀X1交流侧的电压，即X’点的电压，使得晶闸管阀Y1实际的换相电压U X’-Y’相对电压值比对应的换流变压器二次侧实际交流线电压U X-Y相对电压值大，将即将关断的晶闸管阀X1在关断后的负向电压增加，保证晶闸管阀X1可靠关断，并且该增加的关断后的负向电压保持了比自然换相截止点Q更长的时间，使得晶闸管阀Y1和X1在交流线电压的自然换相截止点Q点附近进行换相，建立Y相电流。在低电位处，由X相换到Y相，则在Q点前，在Y相正向串联接入电容模块，在X相负向串联接入电容模块，目标是增加即将关断的晶闸管阀X2的在关断后的负向电压，保证X2可靠关断。
模块化电容换相换流器采用如下方式抑制换相失败：在检测到交流电压异常而未发生换相失败时，在即将换相的两相对应的电路中加入更多电容模块，以增加逆变关断角的范围；在换相失败发生后，关闭换相的两相电压对应的电路中所有的电容模块，直到换相失败恢复。
为了更清楚的说明模块化电容换相换流器的工作过程，以从A相换到B相的过程进行说明。
当模块化电容换相换流器运行在整流模式时，如图5所示，在六脉动换流器高电位换相过程发生前，如图1中的A相和B相将要换相，由A相换到B相，在自然换相过零点P前提前一定的角度，开启A相和B相对应电路中接入的电容模块,，触发电容模块中可关断电力电子器件，在B相正向串联接入电容模块，提高晶闸管阀B2交流侧的电压即B’点的电压，在A相反向串联接入电容模块，降低晶闸管阀A2交流侧的电压即A’点的电压，由此，晶闸管阀B2交流侧换相电压B’-A’相对电压值比换流变压器二次侧实际交流电压B-A相对电压值大，等 效于提前网侧交流电压的相位，提前建立即将触发导通的晶闸管阀B2的正向电压,使得晶闸管阀A2和B2可以在交流电压的自然换相过零点P点之前进行换相，建立B相电流，换相过程结束后，触发电容模块中可关断电力电子器件，关闭B相电路中的电容模块，A相电容模块保持投入，进行顺序的六脉动换流器的低电位换相过程，即低点位C相换到A相。在换相过程中电压和电流的变化趋势如图5所示。其中，图5有两次换相过程，在高电位从A相换到B相，和在低电位从C相换到A相，图5中横轴表示时间(t)，虚线为换流变压器二次侧实际交流电压U Yn，如图1中的U An，U Bn等，细实线为晶闸管阀实际换相电压U Y’n，如图1中的U A’n，U B’n等，粗实线为三相六脉动换流阀的直流输出端电压，I a和I b是换流阀换相电流。在六脉动换流器整流状态低电位换相过程发生时，如C相换到A相，须在A相反向串联接入电容模块，在C相正向串联接入电容模块，目标是提前建立即将触发导通的晶闸管阀A1的正向电压，提前换相。
当模块化电容换相换流器运行在逆变模式时，如图6所示，在六脉动换流器高电位换相过程发生前，如图1中的A相和B相将要换相，由A相换到B相，在自然换相截止点Q前提前一定的角度，开启A相和B相对应电路中接入的电容模块,触发电容模块中可关断电力电子器件，在B相反向串联接入电容模块，降低晶闸管阀B1交流侧的电压即B’点的电压，在A相正向串联接入电容模块，提高晶闸管阀A1交流侧的电压即A’点的电压，由此，晶闸管阀B1交流侧换相电压A’-B’相对电压值比换流变压器二次侧实际交流电压A-B相对电压值大，等效于延迟网侧交流电压的相位，将即将换相关断的晶闸管阀A1在关断后的负向电压增加，并该增加的关断后的负向电压相对自然换相截止点Q保持了更长的时间，保证A1可靠关断，使得晶闸管阀A1和B1可以在交流电压的自然换相截止点Q点附近进行换相，建立B相电流。换相过程结束后，触发电容模块中可关断电力电子器件，关闭B相电路上的电容模块，A相电容模块保持投入，进行顺序的六脉动换流器低电位换相过程，即低电位C相换到A相。在换相过程中电压和电流的变化趋势如图6所示。其中,图6有两次换相过程，在高电位 从A相换到B相，和在低电位从C相换到A相，图6中横轴表示时间(t)，虚线为换流变压器二次侧实际交流电压U Yn，如图2中的U An，U Bn等，细实线为晶闸管阀实际换相电压U Y’n，如图2中的U A’n，U B’n等，粗实线为三相六脉动换流阀的直流输出端电压，I a和I b是换流阀换相电流。在六脉动换流器逆变状态低电位换相过程发生时，如C相换到A相，须在A相正向串联接入电容模块，在C相反向串联接入电容模块，目标是增加即将关断的晶闸管阀C2在关断后的负向电压，保证C2可靠关断。
模块化电容换相换流器通过模块化电容器的反复投切辅助换相，扩大换流器的触发角度范围，可以控制换流阀换相电流和交流电压接近同相，从而减少无功补偿，提高功率因数，大大减少原LCC直流工程的交流滤波器场。当交流系统需要吸收无功时，该模块化电容换相换流器可以切换回传统LCC的控制方式，不投切电容模块，当交流系统需要无功支撑时，可提前多投入电容模块，使模块化电容换相换流器发出感性无功。换相电流中的谐波和直流输出端电压的谐波会更加复杂，可用有源滤波器等方案进行滤除。通过投切电容模块，增加关断角的范围，降低换相失败的概率。
实施例三
本实施例以应用于典型±800kV，8000MW LCC特高压直流工程送端的一个具体案例为例，来具体说明实施例一和实施例二中的技术方案。
已知±800kV，8000MW的LCC特高压直流工程采用双极，每极采用两个十二脉动换流阀，共四个六脉动换流器的结构，送端换流变的感性压降设为10.5％，额定触发角按照工程经验取为15°，则送端每个六脉动换流器的理想空载直流电压U dio为233.5kV。
换流变压器二次侧交流电压线电压
Figure PCTCN2020141983-appb-000001
为172.9kV。
设计目标为将LCC换流器改造为模块化电容换相换流器，使得送端无功消 耗为零。
对于整流模式，传统的LCC换流器，在15°时触发，在满功率时换相角度约为26°，其功率因数角度可近似为：
acos((cos(15°)+cos(15°+26°))/2)＝30.65°，
投入换相电容模块，应使得触发角度提前30.65°，因此应该在自然换相点之前的30.65°-15°≈15°时，即交流电压实际压差为
Figure PCTCN2020141983-appb-000002
时，有
Figure PCTCN2020141983-appb-000003
的正向压差，因此需要两相的电容模块共提供辅助换相电压
Figure PCTCN2020141983-appb-000004
如果采用全桥电容模块，则两相电容模块提供的电压应该一致，即每一相的电容模块需要提供的电压为：
Figure PCTCN2020141983-appb-000005
为了保证足够的正向电压时间积分，在换相前的15°即自然换相点前的15°+15°＝30°就应投入电容模块。换相在自然换相点后的-30°+15°+26°≈11°时结束。为了维持电容模块上的直流电压平衡，A相在换相触发前投入电容模块的时间，必须与B相在换相结束后投入电容模块的时间相等，即在换相结束后的15°，B相依然投入电容模块，此时B相和C相的最大电压差为：
Figure PCTCN2020141983-appb-000006
同样，对于逆变模式，传统LCC直流换流器的关断角度为17°，额定功率时换相角一般为20.22°，功率因数角度近似为：
acos((cos(17°)+cos(17°+20.22°))/2)＝28.8°。
改造后投入电容模块，使得触发角度延迟28.8°，即在自然截止点之后的28.8°-17°＝11.8°时，由电容模块提供
Figure PCTCN2020141983-appb-000007
的压差，且一直持续投 入电容模块直到压差为零。因此需要每相全桥电容模块提供的电压为：
Figure PCTCN2020141983-appb-000008
此时触发换相在自然截止点前-11.8°+20.22°＝8.42°时开始，为了维持电容上的电压，A相在换相触发前投入电容模块的时间，必须与B相在换相结束后投入电容模块的时间相等，则在换相触发前的17°投入电容模块，此时A相和C相的最大电压差为：
Figure PCTCN2020141983-appb-000009
本申请由于采取以上技术方案，其具有以下特点：
1、本申请采用可关断大功率电力电子器件和电容器构成的多个电容模块串联入LCC换相电路，电容模块的电压为LCC换流阀晶闸管的换相提供了辅助换相电压，放宽了晶闸管换相的触发角度限制，提高了触发角的自由度，使换相可能在线电压自然换相过零点之前(整流)和线电压自然换相截止点附近(逆变)发生，减少了无功消耗，改善了LCC直流输电系统的有功和无功特性，可以通过控制电容模块投入的数量，避免电容换相换流器绝缘水平过高的情况。
2、本申请可根据交流电网监测情况自动控制投入电容模块的数目，在交流电压偏低或不平衡时多投入电容模块进行平衡，延长允许换相的时间范围，降低发生换相失败的概率，而在换相失败发生后，可切除电容模块，避免换相失败不可恢复，或者投入电容模块，帮助恢复换相失败，避免连续换相失败。

Claims (10)

  1. 一种模块化电容换相换流器，包括：三相六脉动换流阀、换流变压器和电容模块；所述三相六脉动换流阀与所述电容模块串联，且所述电容模块设置于换流变压器与所述三相六脉动换流阀之间；所述电容模块包括电容器和变流桥，所述变流桥包括至少两个由可关断电力电子器件与二极管反向并联组成的子模块，所述变流桥与所述电容器并联。
  2. 如权利要求1所述的换流器，其中，所述三相六脉动换流阀包括六个桥臂，其中，第一桥臂连接有第一晶闸管阀(A1)，第三桥臂上连接有第三晶闸管阀(B1)，第五桥臂上连接有第五晶闸管阀(C1)，所述第一晶闸管阀(A1)、第三晶闸管阀(B1)和第五晶闸管阀(C1)的阳极均连接所述三相六脉动换流阀的直流输出端的低压侧或高压侧，所述第一晶闸管阀(A1)的阴极通过第二桥臂连接第二晶闸管阀(A2)的阳极、第三晶闸管阀(B1)的阴极通过第四桥臂连接第四晶闸管阀(B2)的阳极、且第五晶闸管阀(C1)的阴极通过第六桥臂连接第六晶闸管阀(C2)的阳极，所述第二晶闸管阀(A2)、第四晶闸管阀(B2)和第六晶闸管阀(C2)的阴极均连接所述三相六脉动换流阀的直流输出端的高压侧或低压侧，在换流变压器采用星接模式的情况下，第一晶闸管阀(A1)和第二晶闸管阀(A2)通过所述电容模块连接A相换流变压器二次侧，第三晶闸管阀(B1)、第四晶闸管阀(B2)通过所述电容模块连接B相换流变压器二次侧，第五晶闸管阀(C1)、第六晶闸管阀(C2)通过所述电容模块连接C相换流变压器二次侧。
  3. 如权利要求2所述的换流器，其中，将A相换流变压器本身的短路电感作为A相换流变压器的换相电感，将B相换流变压器本身的短路电感作为B相换流变压器的换相电感，将C相换流变压器本身的短路电感，作为C相换流变压器的换相电感。
  4. 如权利要求1-3任一项所述的换流器，其中，所述电容模块为半桥电容模块或全桥电容模块。
  5. 如权利要求4所述的换流器，其中，所述半桥电容模块包括：相互并联 的电容器和变流桥，所述变流桥包括一个上桥臂和一个下桥臂，所述上桥臂和下桥臂均连接有所述可关断电力电子器件和二极管反向并联构成的子模块，上桥臂的所述子模块的正极连接电容器第一端，下桥臂的所述子模块的负极连接电容器的第二端，上桥臂的子模块的负极与下桥臂的子模块的正极相连并连接一条引出线，另一条引出线从所述电容器的第二端引出。
  6. 如权利要求4所述的换流器，其中，所述全桥电容模块包括：相互并联的电容器和两个变流桥，每一个变流桥包括一个上桥臂和一个下桥臂，所述上桥臂和下桥臂均连接有所述可关断电力电子器件和二极管反向并联构成的子模块，两个变流桥的上桥臂的所述子模块的正极均连接电容器第一端，两个变流桥的下桥臂的所述子模块的负极均连接电容器的第二端，两个变流桥的上桥臂的子模块的负极分别连接对应下桥臂的所述子模块的正极，并且所述两个变流桥中的每一个变流桥的上桥臂的所述子模块的负极连接一条引出线。
  7. 如权利要求1-6任一项所述的换流器，其中，所述电容器的电压方向固定，所述电压方向为所述电容模块的电压方向。
  8. 一种模块化电容换相换流方法，采用如权利要求1-7任一项所述的模块化电容换相换流器，包括：
    在所述模块化电容换相换流器运行在整流模式的情况下，由X相换到Y相，在自然换相过零点P前开启X相和Y相对应电路中接入的电容模块，设置为传输Y相电流的晶闸管阀和设置为传输X相电流的晶闸管阀在交流线电压的自然换相过零点P前进行换相，建立Y相电流；
    在所述模块化电容换相换流器运行在逆变模式的情况下，由X相换到Y相,在自然换相截止点Q前开启X相和Y相对应电路中接入的电容模块，设置为传输Y相电流的晶闸管阀和设置为传输X相电流的晶闸管阀在交流线电压的自然换相截止点Q点附近进行换相，建立Y相电流；
    其中，X相和Y相均为A相、B相和C相中的一相，且X相和Y相不相同。
  9. 如权利要求8所述的方法，其中，
    在所述模块化电容换相换流器运行在整流模式的情况下，在高电位处换相时，在X相换相到Y相前，在Y相正向串联接入电容模块，在X相负向串联接入电容模块，以提前建立即将触发导通的设置为传输Y相电流的晶闸管阀的正向电压；在低电位处换相时，在X相换相到Y相前，在Y相负向串联接入电容模块，在X相正向串联接入电容模块，以提前建立即将触发导通的设置为传输Y相电流的晶闸管阀的正向电压；
    在所述模块化电容换相换流器运行在逆变模式的情况下，在高电位处换相时，在X相换相到Y相前，在Y相负向串联接入电容模块，在X相正向串联接入电容模块，以保证设置为传输X相电流的晶闸管阀的可靠关断；在低电位处换相时，在X相换相到Y相前，在Y相正向串联接入电容模块，在X相负向串联接入电容模块，以保证设置为传输X相电流的晶闸管阀的可靠关断。
  10. 如权利要求8或9所述的方法，其中，所述模块化电容换相换流器采用如下方式抑制换相失败：
    在检测到交流电压异常而未发生换相失败的情况下，在即将换相的两相对应的电路中加入更多电容模块，以增加逆变角的范围；
    在换相失败发生后，关闭换相的两相电压对应的电路中所有的电容模块，直到换相失败恢复。
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