CN110932538A - Shutdown control method suitable for LCC-MMC hybrid cascade direct-current power transmission system - Google Patents

Shutdown control method suitable for LCC-MMC hybrid cascade direct-current power transmission system Download PDF

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CN110932538A
CN110932538A CN201911243658.4A CN201911243658A CN110932538A CN 110932538 A CN110932538 A CN 110932538A CN 201911243658 A CN201911243658 A CN 201911243658A CN 110932538 A CN110932538 A CN 110932538A
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mmc
sub
voltage
lcc
modules
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蔡晖
祁万春
彭竹弈
赵菲菲
韩杏宁
许偲轩
黄辰成
徐政
严铭
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State Grid Corp of China SGCC
State Grid Jiangsu Electric Power Co Ltd
Economic and Technological Research Institute of State Grid Jiangsu Electric Power Co Ltd
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State Grid Corp of China SGCC
State Grid Jiangsu Electric Power Co Ltd
Economic and Technological Research Institute of State Grid Jiangsu Electric Power Co Ltd
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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
    • H02M1/00—Details of apparatus for conversion
    • H02M1/36—Means for starting or stopping converters
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
    • 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
    • H02M1/32—Means for protecting converters other than automatic disconnection
    • 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
    • H02M1/32—Means for protecting converters other than automatic disconnection
    • H02M1/322—Means for rapidly discharging a capacitor of the converter for protecting electrical components or for preventing electrical shock
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/60—Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]

Abstract

The invention discloses a shutdown control method suitable for an LCC-MMC mixed cascade direct current transmission system, which can avoid overvoltage or overcurrent in the shutdown process, ensure that the voltage and the current are stably and quickly reduced to 0, reduce the sub-module capacitor voltage of an MMC in a mixed cascade structure by adjusting the transformation ratio of a converter transformer, improving the modulation ratio and injecting third harmonic waves into a modulation wave, and feed back a part of energy to an alternating current power grid.

Description

Shutdown control method suitable for LCC-MMC hybrid cascade direct-current power transmission system
Technical Field
The invention belongs to the technical field of direct-current power transmission of a power system, and particularly relates to a shutdown control method suitable for an LCC-MMC hybrid cascade direct-current power transmission system.
Background
According to the technical scheme, a High Voltage Direct Current transmission (LCC-HVDC) technology based on a power grid commutation Converter is mature and widely applied to High Voltage Direct Current transmission occasions such as long distance and large capacity, but the technical scheme has the following defects that ① inverter Stations (LCCs) are easy to cause commutation failure during alternating Current fault and have High requirements on the strength of an alternating Current system, ② consumes a large amount of reactive power during operation, a filter and reactive compensation equipment are required to be arranged, the occupied area is increased, ③ cannot supply power to a passive network, compared with LCCs-HVDC, the Modular Multilevel Converter-based High Voltage Direct Current transmission (MMC-HVDC) technology is favored by Modular design, does not have commutation failure, can realize active power and reactive power control, does not need reactive compensation equipment and can supply power to the passive network, and the like, but has the defect that MMC-HVDC decoupling engineering construction cost is low.
At present, a direct current transmission topological scheme that a rectifying side adopts LCC and an inverting side adopts LCC-MMC mixed cascade converter station is provided in documents, and the scheme has the advantages that ① the rectifying side adopts LCC, fault current can be quickly reduced through forced phase shifting when a line direct current fault occurs, the direct current fault processing capacity is achieved, ② the inverting side high-pressure valve set adopts LCC, the technology is more mature and lower in manufacturing cost compared with MMC technology with the same capacity and the same voltage level, meanwhile, due to the one-way conductivity of LCC thyristors, a fault current path can be blocked when the direct current fault occurs, ③ the inverting side MMC can independently control active power and reactive power, reactive compensation equipment needed by a system is reduced, the system has strong alternating current voltage supporting capacity, meanwhile, the system can still maintain certain power transmission capacity during the phase conversion failure period of the inverting side LCC, and the power shortage of a receiving end system during the alternating current fault is reduced.
The process of requiring the converter station to be taken out of service for maintenance or repair of the dc transmission system is called a shutdown process, in which overvoltage or overcurrent is to be avoided as much as possible and a smooth and rapid drop to 0 of voltage and current is to be ensured. For MMC shutdown, if MMC is directly locked, energy can stay in the sub-module capacitor for a long time and cannot be released, and if the MMC is discharged through the internal resistor, the energy can last for a long time. Therefore, how to quickly release the energy of the sub-modules in the shutdown process of the MMC without causing overvoltage or overcurrent is a problem to be solved in the engineering field.
At present, in the research at home and abroad, a shutdown control mode and related time sequences of an LCC-HVDC system are proposed in documents. In the MMC-HVDC shutdown control mode, the capacitors of the sub-modules are discharged by connecting bridge arm resistors, a larger bridge arm resistor needs to be additionally connected, the energy of the sub-modules is consumed by the resistors, and the economy is poor; the discharge of the sub-module capacitor is realized through a direct current circuit, but the unidirectional conductivity of the LCC is obviously not suitable for the LCC-MMC mixed direct current transmission system; in addition, a shutdown time sequence and a control strategy of an LCC-MMC hybrid direct-current transmission system (a rectifying side LCC and an inverting side MMC) are researched, but a hybrid sub-module MMC adopted by an inverting side is researched, and at present, no related shutdown control method is researched for the LCC-MMC hybrid cascaded direct-current transmission system.
Disclosure of Invention
In view of the above, the invention provides a shutdown control method suitable for an LCC-MMC hybrid cascaded dc power transmission system, which can avoid overvoltage or overcurrent in a shutdown process, and ensure that voltage and current stably and rapidly drop to 0, can discharge to an inversion side MMC submodule capacitor through a starting resistor, and does not need to additionally access a discharge resistor, and due to the unidirectional conduction characteristic of an inversion side LCC thyristor, a dc line does not need to be isolated by disconnecting an isolation switch in a discharge stage of the inversion side MMC submodule capacitor.
The power transmission system is formed by sequentially connecting a rectification side converter station, a direct current circuit and an inversion side converter station, wherein the inversion side converter station is formed by hybrid cascading LCC (power grid commutation converter) and MMC (modular multilevel converter), the MMC is of a three-phase six-bridge-arm structure, each bridge arm is formed by connecting N sub-modules and an electric reactor in series, each sub-module is a half-bridge sub-module, and N is a natural number larger than 1;
the LCC of the inverter side converter station is controlled by constant direct current voltage, the MMC is controlled by constant direct current voltage and constant reactive power, and the LCC and the MMC are respectively connected to different alternating current systems;
the alternating current side of the rectifier side converter station sequentially passes through the converter transformer T1AC circuit breaker Sac1AC bus L1Connected to a transmitting-end AC system S1(ii) a The alternating current side of the LCC of the inversion side converter station sequentially passes through the converter transformer T2AC circuit breaker Sac2AC bus L2Connected to a receiving ac system S2The AC side of the MMC is sequentially provided with a starting resistor and a converter transformer T3AC circuit breaker Sac3AC bus L3Connected to a receiving ac system S3Said starting resistor having parallel connectionAnd (4) switching.
The shutdown control method comprises the following steps:
(1) after receiving the shutdown signal, the direct current of the power transmission system is first reduced from the setpoint value IdcratedLinearly decreases to a certain value Idcoff；
(2) Converter transformer T2The tap of the converter transformer T is adjusted to the maximum transformation ratio step by step to change the converter transformer T3The tap of (a) is also adjusted to the maximum transformation ratio step by step;
(3) the modulation ratio of the MMC is increased to 1, third harmonic waves are injected into the modulation waves, and meanwhile the reactive power instruction value in MMC control is linearly reduced to 0;
(4) direct current of power transmission system is led from IdcoffLinearly reducing to 0, then forcibly shifting the firing angle of the rectifier side converter station to 120 degrees, and further reducing the active power output by the rectifier side converter station;
(5) after a period of time, the LCC and MMC of the rectifier side converter station and the inverter side converter station are locked, and then the AC circuit breaker S is disconnectedac1、Sac2、Sac3；
(6) Starting a starting resistor to make each submodule capacitor of the MMC enter a discharging state until the capacitor voltage of each submodule is reduced to a certain value UcLThen, all the sub-modules are locked, residual energy is automatically discharged through internal resistors of the sub-modules, and the sub-module capacitor voltage is discharged from UcLAnd drops to 0, so far the shutdown process ends.
Further, in the step (1), the direct current of the power transmission system is reduced, that is, the direct current command value is changed from the rated value I through the rectifier side converter station controllerdcratedLinearly down to IdcoffTo realize the operation;
Idcoff＝λIdcrated
wherein: λ is a current command value scaling factor (about 10%).
Further, the tap in the step (2) is located at the converter transformer T2And T3While the valve side winding is fixed by mounting the converter transformer T2And T3Net side tapAnd adjusting to the maximum transformation ratio to reduce the voltage at the direct current outlet of the inversion side converter stations LCC and MMC.
Further, the reactive power command value is linearly reduced to 0 in the step (3), so that no reactive power is exchanged between the MMC and the receiving-end alternating-current system; improving the MMC modulation ratio and injecting third harmonic into the modulation wave for reducing the direct current voltage of the MMC to reduce the direct current voltage to Udcoff；
Figure BDA0002306942270000031
Wherein: u shapeac3Is an AC bus L3Effective value of line voltage of T3maxFor a converter transformer T3The maximum transformation ratio of (2).
Further, after the LCCs and MMCs of the converter station on the rectifying side and the converter station on the inverting side are locked in the step (5), the active power of each converter station is reduced to 0, and the ac circuit breaker S is openedac1、Sac2、Sac3And the alternating current and direct current system is isolated, so that power exchange between the power transmission system and the alternating current system at the transmitting end and the receiving end is not performed any more.
Further, the starting resistor is put into the step (6) to enable each sub-module capacitor of the MMC to enter a discharging state until the voltage of the sub-module capacitor is reduced to a certain value UcLThe expression is as follows:
UcL＝(1+ε)Ucst
wherein: u shapecstTo ensure the minimum turn-on voltage for the IGBT in the sub-module to operate normally, epsilon is a proportionality coefficient (generally, epsilon is 5%).
Further, in the step (6), each sub-module capacitor of the MMC is enabled to enter a discharging state until the sub-module capacitor voltage drops to UcLThe specific process is as follows:
6.1 switching all the submodules of the upper bridge arm of the MMC from a locking state to a bypass state, numbering the submodules of the upper bridge arm of each phase of the MMC, and dividing the submodules into a plurality of groups;
6.2 for the first group of submodules of the A-phase upper bridge arm, firstly, the submodules are switched from the bypass stateSwitching to the input state, opening the parallel switch and consuming energy by using the starting resistor, so that the capacitor voltage of the sub-modules is reduced until the capacitor voltage of the sub-modules is reduced to UcLThen, switching the set of sub-modules from the input state to the bypass state;
6.3 similarly, the second group and the third group of the upper bridge arm of the phase A till the last group of the sub-modules are sequentially processed in the step 6.2 until the capacitor voltages of all the sub-modules of the upper bridge arm of the phase A are reduced to UcL；
6.4, sequentially carrying out the treatment of the steps 6.2-6.3 on the phase B and the phase C in the same way, and waiting until the capacitor voltages of the sub-modules of the upper bridge arm of the three phases are all reduced to UcLThen, switching all the upper bridge arm sub-modules from a bypass state to a locking state;
6.5 according to the steps 6.1-6.4, all the sub-module capacitor voltages of the MMC lower bridge arm are reduced to UcLAnd then, the sub-module capacitor automatically releases residual energy through the internal resistor of the sub-module, and when the voltage of the sub-module capacitor is reduced to 0, the power transmission system is stopped.
Further, in step 6.1, numbering the sub-modules of the upper bridge arm of any phase of the MMC from 1 to N according to the sequence of the capacitance voltage from large to small, selecting k sub-modules as one group in sequence, where the number of the sub-modules in each group except the last group is k, where k satisfies:
Figure BDA0002306942270000041
wherein: floor () is a rounded down function, Uac3Is an AC bus L3Effective value of line voltage of T3maxFor a converter transformer T3The maximum transformation ratio of (2).
Based on the technical scheme, the invention has the following beneficial technical effects:
(1) aiming at an LCC-MMC hybrid cascade direct-current power transmission system, the invention provides a shutdown control method which can avoid overvoltage or overcurrent in the shutdown process and ensure that the voltage and the current stably and rapidly drop to 0.
(2) According to the invention, the sub-module capacitance voltage of the MMC in the mixed cascade structure is reduced by adjusting the transformation ratio of the converter transformer, increasing the modulation ratio m and injecting third harmonic into the modulation wave, and a part of energy can be fed back to the alternating current power grid.
(3) In the sub-module capacitor discharging stage, the starting resistor and the △ winding structure on the converter transformer valve side form a discharging loop, no discharging resistor needs to be additionally connected, and the discharging principle is simple, reliable and effective.
(4) Due to the unidirectional conduction characteristic of the LCC thyristor on the inversion side, the direct-current line is not required to be isolated in a mode of disconnecting the isolating switch in the capacitor discharge stage of the MMC submodule on the inversion side.
Drawings
Fig. 1 is a schematic structural diagram of an LCC-MMC hybrid cascaded direct-current power transmission system.
Fig. 2 is a schematic diagram of a twelve-pulse bridge type commutation unit structure.
Fig. 3 is a schematic structural diagram of a half-bridge sub-module MMC.
Fig. 4 is a schematic diagram of a shutdown control flow of the LCC-MMC hybrid cascaded dc power transmission system.
Fig. 5 is an equivalent circuit diagram of inverter side MMC submodule capacitor fast discharge.
Fig. 6(a) is a simulation graph of the inversion side LCC dc voltage and the inversion side MMC dc voltage in the inversion side MMC submodule capacitor discharging process.
Fig. 6(b) is a simulation graph of the system direct current in the inverter-side MMC submodule capacitor discharging process.
Fig. 6(c) is a simulation graph of the commutation side LCC trigger angle during the inverter side MMC submodule capacitor discharging process.
Fig. 7(a) is a simulation curve diagram of the capacitance and voltage of the upper bridge arm of the a phase in the process of discharging the capacitance of the MMC submodule on the inversion side.
Fig. 7(B) is a simulation curve diagram of the capacitance and voltage of the upper bridge arm of the B phase in the process of discharging the capacitance of the MMC submodule on the inversion side.
Fig. 7(C) is a simulation curve diagram of the capacitance and voltage of the upper bridge arm of the C phase in the process of discharging the capacitance of the MMC submodule on the inversion side.
Fig. 7(d) is a simulation curve diagram of a phase a lower bridge arm capacitance voltage in the inverter side MMC submodule capacitance discharging process.
Fig. 7(e) is a simulation curve diagram of the capacitance and voltage of the lower bridge arm in the phase B during the capacitance discharging process of the inverter-side MMC submodule.
Fig. 7(f) is a simulation curve diagram of the capacitance voltage of the C-phase lower bridge arm in the process of discharging the capacitance of the inverter-side MMC submodule.
Detailed Description
In order to more specifically describe the present invention, the following detailed description is provided for the technical solution of the present invention with reference to the accompanying drawings and the specific embodiments.
As shown in fig. 1, the LCC-MMC hybrid cascade dc transmission system includes a rectification-side converter station connected to a sending-end ac system, a smoothing reactor at an outlet of the rectification side, a dc line, a smoothing reactor at an outlet of the inversion side, and an inversion-side converter station connected to a receiving-end ac system. The rectifying side converter station comprises an AC circuit breaker S connected to an AC systemac1Converter transformer T1A grid commutation converter (LCC) and a bus Us1An alternating current filter is arranged on the base; the inversion side converter station comprises a high-pressure valve bank and a low-pressure valve bank, wherein the high-pressure valve bank comprises an alternating current system S2Connected AC circuit breaker Sac2Converter transformer T2A grid commutation converter (LCC) and a bus Us2An alternating current filter is arranged on the base; the low-pressure valve set comprises an AC system S3Connected AC circuit breaker Sac3Converter transformer T3Starting resistor RstModular Multilevel Converter (MMC). The inversion side LCC and the MMC adopt a cascade mode, wherein the anode of the direct current side of the high-voltage valve group LCC is connected with a direct current circuit through a smoothing reactor, and the cathode of the direct current side of the high-voltage valve group LCC is connected with the MMC; the direct current side positive pole of low pressure valves MMC links to each other with contravariant side LCC through smoothing reactor, negative pole ground connection.
The LCC on the rectifying side adopts fixed direct current control, the LCC on the inverting side adopts fixed direct current control, the additional later-prepared turn-off angle control is used for reducing the risk of phase change failure occurrence when alternating current voltage drops, the MMC on the inverting side adopts fixed direct current voltage and fixed reactive power control, and the LCC and the MMC on the inverting side are respectively connected into different alternating current systems.
As shown in fig. 2, the twelve-pulse bridge type converter unit is composed of two thyristors and a six-pulse bridge, and can convert three-phase alternating current into direct current. The rectification side LCC adopts a double-twelve-pulse structure, namely, two twelve-pulse bridge type converter units are cascaded to form the rectification side LCC, and the inversion side LCC adopts a single-twelve-pulse structure.
As shown in fig. 3, the inverter-side converter station adopts an MMC three-phase six-leg structure, and each leg comprises N sub-modules and a series reactor L0The upper and lower bridge arms in the same phase form a phase unit. Wherein u isvj(j ═ a, b, c) is the converter valve side j alternating voltage, ivjFor j alternating currents, u, on the valve side of the converterpjAnd unjOutput voltage i for submodules of j-phase upper and lower bridge armspjAnd injBridge arm currents of j-phase upper and lower bridge arms, IdcIs a direct current, UdcTo output a dc voltage.
Each sub-module unit is of a half-bridge sub-module (HBSM) structure, and one half-bridge sub-module is formed by 2 IGBT (insulated gate bipolar transistor) tubes G1、G2Antiparallel diode D1、D2Output capacitor C, internal resistor RCAnd (4) forming. Wherein u issmFor sub-module output voltage uCFor sub-module capacitor voltage, ismFor sub-module current, internal resistance RCConnected in parallel across the capacitor C.
When G is1Conducting G2Turning off the submodule to be in an input state; when G is1Off G2Conducting, and enabling the sub-modules to be in a bypass state; when G is1、G2All are turned off, and the sub-module is in a locking state.
As shown in fig. 4, the shutdown control method of the LCC-MMC hybrid cascaded dc power transmission system specifically includes the following steps:
(1) after receiving the shutdown signal, regulating the DC command value of the constant DC controller of the LCC on the rectifying side to make the DC current follow the rated value IdcratedLinearly decreasing to another value Idcoff，IdcratedAnd IdcoffSatisfies the following conditions:
Idcoff＝λIdcrated
wherein: λ is a current command value proportionality coefficient, and is generally 0.1. The reduction of the direct current enables the LCC output active power at the rectifying side to be reduced, the overvoltage phenomenon caused by surplus energy of a direct current circuit in subsequent operation is prevented, and preparation is made for a subsequent shutdown step.
(2) Inverter side LCC step-by-step adjustment converter transformer T2The MMC on the inversion side adjusts the converter transformer T step by step from the tap to the maximum transformation ratio3Tap to maximum transformation ratio. Generally, the taps are located on the net side of the converter transformer, while the valve side windings are fixed; the tap is a mechanical switch, the variation of each gear is small, the action process is slow, and overvoltage or overcurrent caused by sudden change of alternating voltage can not occur; the voltage at direct current outlets of the LCC and the MMC at the inversion side can be reduced by adjusting the network side tap of the converter transformer to the position of the maximum transformation ratio.
(3) And the MMC reactive power instruction value of the inversion side is linearly reduced to 0, so that no reactive power is exchanged between the MMC alternating current and direct current systems. Meanwhile, the modulation ratio is increased to 1, and third harmonic waves are injected into the modulation waves to reduce the direct-current voltage.
The modulation ratio is defined as:
Figure BDA0002306942270000081
wherein: u shapepmIs the amplitude of the side phase voltage of the AC valve, UdcIs a dc voltage.
Under the condition of unchanging alternating voltage, the direct-current voltage U can be reduced to a certain extent by improving the modulation ratiodc(ii) a The third harmonic injection is to realize the reduction of the effective value of the alternating voltage by superposing a third harmonic component in the modulation wave, thereby indirectly reducing the direct-current voltage of the inverter-side MMC.
The DC voltage is finally reduced to UdcoffThe expression is as follows:
Figure BDA0002306942270000082
wherein: u shapeac3For inverter side MMCEffective value of the mains side line voltage, T3maxThe maximum transformation ratio of the MMC converter transformer on the inversion side is obtained.
(4) The LCC on the rectifying side drives the direct current from I through a constant direct current controllerdcoffDown to 0 and then forced phase shift to 120 deg., further reducing the active power at the output of the rectifying side.
(5) After a period of time, the rectifying side LCC, the inversion side LCC and the inversion side MMC are locked, so that no active power is exchanged between the AC-DC system, the active power is reduced to 0, and then the AC circuit breaker S of the rectifying side and the inversion side is disconnectedac1、Sac2、Sac3And the AC-DC system is isolated, so that power exchange is not performed between the AC-DC systems of the converter station.
(6) Inversion side MMC input starting resistor RstEntering a sub-module capacitor voltage discharge state, the sub-module voltage needs to be discharged to UcLThe expression is as follows:
UcL＝(1+ε)Ucst
wherein: u shapecstThe minimum trigger voltage for ensuring the normal work of the IGBT can be ensured by enough energy of the sub-module capacitor; in order to compensate the energy dissipated in the sub-module resistor and the stray resistor of the discharge loop, the sub-module capacitor discharge lower limit voltage UcLNeeds to be slightly higher than the minimum trigger voltage U of the submodulecstIn general,. epsilon.5% is used.
Discharging the sub-module capacitor voltage to UcLThe specific process is as follows:
① all sub-modules of the ABC three-phase upper bridge arm of each phase in the MMC on the inversion side are switched to the bypass state from the locking state.
②, numbering the upper bridge arm submodules of each phase from 1 to N according to the sequence of the capacitor voltage from large to small, selecting k submodules as one group in sequence, wherein the number of each group of submodules except the last group is k.
③ for each group of submodules of the A-phase upper bridge arm, switching the submodules from the bypass state to the input state, and starting the resistor R through the AC sidestAnd energy is consumed, so that the capacitor voltage of the sub-modules in the group is reduced. Waiting until the capacitor voltage of the set of sub-modulesDown to UcLThereafter, the set of sub-modules is switched from the on state to the bypass state.
④ the second, third and last sub-modules of the A-phase upper bridge arm are processed in step ③ until the capacitor voltage of all sub-modules of the A-phase upper bridge arm is reduced to UcLAnd switches from the throw-in state to the bypass state.
Fig. 5 shows an equivalent circuit when a three-phase lower bridge arm is locked, a B-phase upper bridge arm and a C-phase upper bridge arm are bypassed, and a certain group of submodules of an a-phase upper bridge arm are put into operation. As can be seen from the figure, the capacitor of the sub-module group may form a discharge loop (loop 1) by the B-phase upper bridge arm, the B-phase starting resistor, the converter transformer and the a-phase starting resistor, or may form a discharge loop (loop 2) by the C-phase upper bridge arm, the C-phase starting resistor, the converter transformer and the a-phase starting resistor. Meanwhile, in order to ensure quick discharge and ensure that the discharged submodule groups do not charge the lower bridge arm as much as possible, the total capacitance voltage of the input submodule groups cannot exceed the minimum total capacitance voltage of all submodules of each phase of the lower bridge arm, and the number k of each group of submodule groups can be calculated according to the constraint condition to meet the following requirements:
Figure BDA0002306942270000091
in the formula: floor (x) is a floor rounding function, representing the largest integer smaller than variable x.
⑤ the B phase and the C phase are processed in sequence in steps ③ to ④ in the same manner.
⑥ the capacitor voltage of the upper bridge arm submodules of equal to three phases is reduced to UcLAnd then, switching all the upper bridge arm submodules from the bypass state to the locking state.
⑦, all the sub-modules of the ABC three-phase lower bridge arm in each phase in the MMC at the inversion side are switched from a locking state to a bypass state, and similarly, the capacitor voltage of the ABC three-phase sub-modules of the lower bridge arm is subjected to the discharging processing of steps ② to ⑥ in sequence.
All sub-module capacitor voltages of the MMC at the inversion side are reduced to UcLAnd then, the sub-module capacitor automatically discharges residual energy through the internal resistor. Due to internal parallel connectionThe resistance is large, so the free discharge process of the sub-module capacitor can last for a long time, and the system shutdown process is finished when the voltage of the sub-module capacitor drops to 0.
In order to further verify the effectiveness of the shutdown control method, a simulation model is built in power system electromagnetic transient simulation software PSCAD according to the LCC-MMC hybrid cascade direct-current power transmission system shown in FIG. 1, and system parameters are shown in Table 1.
TABLE 1
Figure BDA0002306942270000101
Fig. 6(a) to 6(c) show response curves of dc voltage, dc current, and firing angle. The direct current transmission system receives a shutdown signal when t is 1.0s, and the direct current gradually drops to 0.625kA from 6.25 kA; when t is 2.0s, the LCC at the inversion side reduces the direct-current voltage by adjusting the tap joint of the converter transformer to the maximum transformation ratio, and the MMC at the inversion side reduces the direct-current voltage by adjusting the tap joint of the converter transformer, improving the modulation ratio and injecting third harmonic waves into the modulation waves; when t is 3.0s, the direct current voltage of the MMC on the inverting side is reduced to 280kV, and the LCC on the rectifying side is adjusted to reduce the direct current to 0; when t is 3.2s, the rectification side LCC is forced to shift the phase, and the direct current voltage of the inversion side LCC is further reduced; when t is 3.5s, all converter stations are locked, so that power exchange does not occur before the converter station alternating current and direct current systems; when t is 4.0s, the direct current voltage of the inversion side LCC is already reduced to 0, and the alternating current circuit breakers of all the converter stations are disconnected; switching on the starting resistor R when t is 4.5sstAnd entering an inversion side MMC submodule discharging stage.
The sub-module discharge process is shown in fig. 7(a) -7 (f). Calculating according to a formula to obtain the number k of each group of discharging sub-modules to be 37, and dividing each bridge arm of each phase into 3 groups to discharge because the number N of the sub-modules to be 100; from the figure, the ABC three phases of the upper bridge arm and the ABC three phases of the lower bridge arm are sequentially discharged, the voltage of the sub-module is discharged from 2.8kV to 1.05kV, then the capacitor of the sub-module automatically discharges residual energy through the internal resistor, and the system is stopped when the voltage of the capacitor of the sub-module is reduced to 0.
The embodiments described above are presented to enable a person having ordinary skill in the art to make and use the invention. It will be readily apparent to those skilled in the art that various modifications to the above-described embodiments may be made, and the generic principles defined herein may be applied to other embodiments without the use of inventive faculty. Therefore, the present invention is not limited to the above embodiments, and those skilled in the art should make improvements and modifications to the present invention based on the disclosure of the present invention within the protection scope of the present invention.

Claims (8)

1. The outage control method is suitable for an LCC-MMC hybrid cascade direct-current power transmission system, the power transmission system is formed by sequentially connecting a rectification side converter station, a direct-current line and an inversion side converter station, the inversion side converter station is formed by LCC and MMC hybrid cascade, the MMC is of a three-phase six-bridge-arm structure, each bridge arm is formed by connecting N sub-modules and an electric reactor in series, the sub-modules adopt half-bridge sub-modules, and N is a natural number larger than 1;
the LCC of the inverter side converter station is controlled by constant direct current voltage, the MMC is controlled by constant direct current voltage and constant reactive power, and the LCC and the MMC are respectively connected to different alternating current systems;
the alternating current side of the rectifier side converter station sequentially passes through the converter transformer T1AC circuit breaker Sac1AC bus L1Connected to a transmitting-end AC system S1(ii) a The alternating current side of the LCC of the inversion side converter station sequentially passes through the converter transformer T2AC circuit breaker Sac2AC bus L2Connected to a receiving ac system S2The AC side of the MMC is sequentially provided with a starting resistor and a converter transformer T3AC circuit breaker Sac3AC bus L3Connected to a receiving ac system S3The starting resistor is provided with a parallel switch;
the shutdown control method is characterized by comprising the following steps:
(1) after receiving the shutdown signal, the direct current of the power transmission system is first reduced from the setpoint value IdcratedLinearly decreases to a certain value Idcoff；
(2) Converter transformer T2The tap of the converter transformer T is adjusted to the maximum transformation ratio step by step to change the converter transformer T3The tap of (a) is also adjusted to the maximum transformation ratio step by step;
(3) the modulation ratio of the MMC is increased to 1, third harmonic waves are injected into the modulation waves, and meanwhile the reactive power instruction value in MMC control is linearly reduced to 0;
(4) direct current of power transmission system is led from IdcoffLinearly reducing to 0, then forcibly shifting the firing angle of the rectifier side converter station to 120 degrees, and further reducing the active power output by the rectifier side converter station;
(5) after a period of time, the LCC and MMC of the rectifier side converter station and the inverter side converter station are locked, and then the AC circuit breaker S is disconnectedac1、Sac2、Sac3；
(6) Starting a starting resistor to make each submodule capacitor of the MMC enter a discharging state until the capacitor voltage of each submodule is reduced to a certain value UcLThen, all the sub-modules are locked, residual energy is automatically discharged through internal resistors of the sub-modules, and the sub-module capacitor voltage is discharged from UcLAnd drops to 0, so far the shutdown process ends.
2. The shutdown control method according to claim 1, characterized in that: in the step (1), the direct current of the power transmission system is reduced, namely, the direct current instruction value is changed from the rated value I through the rectifier side converter station controllerdcratedLinearly down to IdcoffTo realize the operation;
Idcoff＝λIdcrated
wherein: λ is a current command value proportionality coefficient.
3. The shutdown control method according to claim 1, characterized in that: the tap in the step (2) is positioned at the converter transformer T2And T3While the valve side winding is fixed by mounting the converter transformer T2And T3And adjusting the network side tap to the maximum transformation ratio to reduce the voltage at the direct current outlets of the inverter side converter stations LCC and MMC.
4. The shutdown control method according to claim 1, characterized in that: linearly reducing the reactive power instruction value to 0 in the step (3) so as to ensure that no reactive power is exchanged between the MMC and the receiving end alternating current system any more; improving the MMC modulation ratio and injecting third harmonic into the modulation wave for reducing the direct current voltage of the MMC to reduce the direct current voltage to Udcoff；
Figure FDA0002306942260000021
Wherein: u shapeac3Is an AC bus L3Effective value of line voltage of T3maxFor a converter transformer T3The maximum transformation ratio of (2).
5. The shutdown control method according to claim 1, characterized in that: after the LCC and MMC of the converter station at the rectifying side and the converter station at the inverting side are locked in the step (5), the active power of each converter station is reduced to 0, and the alternating current breaker S is disconnectedac1、Sac2、Sac3And the alternating current and direct current system is isolated, so that power exchange between the power transmission system and the alternating current system at the transmitting end and the receiving end is not performed any more.
6. The shutdown control method according to claim 1, characterized in that: and (6) putting a starting resistor to enable each sub-module capacitor of the MMC to enter a discharging state until the voltage of the sub-module capacitor is reduced to a certain value UcLThe expression is as follows:
UcL＝(1+ε)Ucst
wherein: u shapecstIn order to ensure the minimum conducting voltage for normal work of the IGBT in the submodule, epsilon is a proportionality coefficient.
7. The shutdown control method according to claim 1, characterized in that: and (6) enabling each sub-module capacitor of the MMC to enter a discharging state until the sub-module capacitor voltage is reduced to UcLTool (A)The process is as follows:
6.1 switching all the submodules of the upper bridge arm of the MMC from a locking state to a bypass state, numbering the submodules of the upper bridge arm of each phase of the MMC, and dividing the submodules into a plurality of groups;
6.2 for the first group of sub-modules of the A-phase upper bridge arm, firstly, the sub-modules are switched from a bypass state to an input state, the parallel switch is opened, energy consumption is carried out by using the starting resistor, the capacitor voltage of the sub-modules is reduced, and when the capacitor voltage of the sub-modules is reduced to UcLThen, switching the set of sub-modules from the input state to the bypass state;
6.3 similarly, the second group and the third group of the upper bridge arm of the phase A till the last group of the sub-modules are sequentially processed in the step 6.2 until the capacitor voltages of all the sub-modules of the upper bridge arm of the phase A are reduced to UcL；
6.4, sequentially carrying out the treatment of the steps 6.2-6.3 on the phase B and the phase C in the same way, and waiting until the capacitor voltages of the sub-modules of the upper bridge arm of the three phases are all reduced to UcLThen, switching all the upper bridge arm sub-modules from a bypass state to a locking state;
6.5 according to the steps 6.1-6.4, all the sub-module capacitor voltages of the MMC lower bridge arm are reduced to UcLAnd then, the sub-module capacitor automatically releases residual energy through the internal resistor of the sub-module, and when the voltage of the sub-module capacitor is reduced to 0, the power transmission system is stopped.
8. The shutdown control method according to claim 7, characterized in that: in the step 6.1, numbering the submodules of the bridge arm on any phase of the MMC according to the sequence of the capacitance voltage from large to small by 1-N, sequentially selecting k submodules as one group, wherein the number of each group of submodules except the last group is k, and k satisfies the following conditions:
Figure FDA0002306942260000031
wherein: floor () is a rounded down function, Uac3Is an AC bus L3Effective value of line voltage of T3maxFor a converter transformer T3The maximum transformation ratio of (2).
CN201911243658.4A 2019-12-06 2019-12-06 Shutdown control method suitable for LCC-MMC hybrid cascade direct-current power transmission system Pending CN110932538A (en)

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