CN102064555B - Chain type STATCOM (Static Synchronous Compensator) chain unit bypass structure with mechanical switch - Google Patents

Abstract

The invention provides a chain-type STATCOM chain link unit bypass structure using a mechanical switch, which is characterized in that a specially designed mechanical switch is used instead of a power electronic device as the bypass circuit of the chain link unit. Small quantity, easy installation and operation, provide reliable protection for the entire chain link unit, realize the redundancy function of the chain link, avoid frequent exit of the chain STATCOM device due to link failure; solve the difficulty in selecting large-capacity switching devices And the control power supply of the switching device is not easy to obtain and other problems.

Description

Chained STATCOM link unit bypass structure with mechanical switch

technical field

The invention belongs to the technical field of dynamic reactive power compensation in electric power systems, and in particular relates to a chained STATCOM chain link unit bypass structure using a mechanical switch.

Background technique

STATCOM-Static Synchronous Compensator (Static Synchronous Compensator) for power transmission system is a new type of static var dynamic compensation device, which is composed of voltage source converters connected in parallel to the system, and the output capacitive or inductive reactive current is continuous , adjustable and independent of the voltage at the point of connection to the system. At present, the main circuit of STATCOM mainly includes three structures, namely multiple structure, multi-level structure and chain structure.

The multiple structure takes the three-phase high-power voltage source converter as the core. The DC voltage on the capacitor is converted to generate a square wave voltage with a phase difference of several degrees. After the electromagnetic coupling of the multiple transformers, a three-phase ladder is generated at the output end. Wave voltage to reduce output harmonics. The disadvantages are: (1) The three phases share a DC voltage support, and phase separation control cannot be performed; (2) The series and parallel connection of turn-off devices is used to increase the voltage and capacity, resulting in voltage and current equalization problems; (3) Multiplexing The transformer has a large loss and occupies a large area.

The multi-level structure is a multi-level structure composed of clamping diodes or clamping capacitors, which can increase the capacity while reducing the series connection, and optimize the harmonic characteristics. Including diode clamp multilevel structure, flying capacitor multilevel structure, basic full bridge cascaded multilevel structure, etc. This structure also has certain disadvantages: (1) It is difficult to realize the capacitor voltage balance control; (2) The switching frequency increases, the switching loss increases, and the efficiency decreases; (3) Multiple independent DC power supplies are required.

The main circuit of the chained STATCOM is a new type of topology. The core part of the main circuit is a voltage source converter with a chained (H-bridge series) structure. Each phase is an independent chain, consisting of N structural The identical basic units are connected in series. Each basic unit is a single-phase bridge voltage source converter capable of outputting three levels. N basic units can be connected in series to obtain a 2N+1 step voltage waveform. As the number of H-bridges increases, the distortion rate of the output voltage is also significantly improved; and as the number of H-bridges increases, the capacity of the device increases accordingly.

The three-phase commutation chain of the chained STATCOM device is composed of multiple chain link units of the same structure connected in series. In order to improve the reliability of the STATCOM device and reduce the frequent switching of the device, according to the enterprise standard Q/GDW241-2008 of the State Grid Corporation, Chained STATCOMs should have at least one redundant link. When less than or equal to the redundant number of link failures, the failed link should be automatically bypassed, and the device will continue to run after the failed link is bypassed; when the redundant number of links fails, the device should stop running. Therefore, a simple, economical and reliable bypass circuit should be designed for each chain link unit to provide protection for the entire chain link unit and realize redundant functions at the same time.

The bypass circuit is used to realize the protection of the entire chain link, and the basic method is an anti-parallel thyristor structure. When the device compensates for reactive power output, the high-frequency, high-dv/dt pulse voltage generated during the H-bridge switching process of the link unit can easily lead to false conduction of the thyristor, and the usual RC absorption circuit cannot solve the problem.

The bypass circuit can also adopt the way of diode rectification and single thyristor bypass. The thyristor is equipped with a resistance-capacitance snubber circuit to prevent excessive dv/dt and excessive reverse recovery voltage from damaging the device. The DC side of the bypass circuit is connected to the DC side of the voltage source converter through a 10kΩ isolation resistor to fix the potential, suppress partial discharge and prevent interference to the thyristor. Chained STATCOM devices have high output voltage and large current. According to the current manufacturing level of thyristors, it is difficult to find square-packaged thyristor modules that meet the requirements of electrical parameters. And other auxiliary equipment, resulting in bulky, inconvenient installation, and not conducive to the compact design of the module; and the thyristor is an uncontrollable device, once it is turned on, it cannot be controlled to turn off, and it will turn off naturally at zero crossing, and cannot maintain the conduction state.

The bypass circuit can also adopt a bypass structure composed of two reverse-series IGBT devices. When a fault occurs inside the chain link unit, the H-bridge circuit is blocked and the bypass IGBT is turned on at the same time. In the process, there will be inrush current and voltage. The ability of IGBT devices to withstand the impact is relatively weak, and it is easy to be damaged due to the impact current and voltage. , and the control power supply is not easy to obtain.

Contents of the invention

The present invention adopts a new type of bypass structure-a bypass circuit composed of mechanical switches. Compared with other bypass structures, this circuit has obvious advantages: simple circuit structure, convenient installation and operation, and fewer components. The present invention proposes a chain-type static synchronous compensator STATCOM link unit bypass structure using mechanical switches, which is characterized in that the main circuit of the static synchronous compensator STATCOM adopts a chain structure of H-bridge cascading, and is composed of mechanical switches Chain link unit bypass structure, where

The chain-type STATCOM device three-phase commutation chain is composed of multiple chain-link units with the same structure in series, including chain-type voltage source converters, connecting reactors, main controllers, lightning arresters, current sensing elements and cooling systems;

In the voltage source converter, phase commutation is realized by a turn-off device, and the energy storage element on the DC side is a converter using a capacitor;

The function of the connecting reactor is to filter out the high-order harmonic current generated by the commutation chain, and realize the energy exchange between the chained STATCOM and the system;

The controller is used to control the operation and operation of the chained STATCOM device;

The arrester is used in parallel with both ends of the commutation chain to realize overvoltage protection of the commutation chain;

The current sensing element is connected in series with the commutation chain for current detection;

The cooling system is used to realize the cooling of the converter chain;

The mechanical switch is designed as an AC, indoor, and single-phase operation structure, mainly including an incoming and outgoing line terminal (conductor), a vacuum switching tube, an insulating support, an insulating pull rod, a magnetic operating mechanism, and a controller, etc., according to the control strategy of the device , the mechanical switch performs opening and closing operation under the condition that the device is blocked and has no output current. Therefore, the mechanical switch does not need arc extinguishing capability, and the vacuum switch tube does not need a special arc extinguishing chamber, which greatly reduces the volume of the mechanical switch. , which is convenient for the miniaturization of the chained STATCOM device;

Wherein, the mechanical switch adopts a direct-acting vertical arrangement, so as to reduce the mechanical transmission link to increase the closing speed of the switch. When the chain link unit fails, the mechanical switch can be closed quickly, reducing the blocking time of the device due to the chain link failure. Thereby reducing the impact on the system;

Among them, the mechanical switch is automatic opening and closing, and is equipped with manual opening operation, using magnetic operating mechanism, electromagnetic operation, permanent magnet holding, no mechanical holding and tripping device, the specific operation method is: the control power first passes through The power module charges the capacitor to store enough energy; when a mechanical switch operation is required, the controller sends a pulse to trigger the switching device, the switching device is turned on to form a circuit, and the capacitor discharges to the electromagnetic coil of the magnetic operating mechanism, and the coil is charged and then driven Vacuum tube for opening and closing operation;

Among them, the link unit composed of mechanical switches mainly includes: DC capacitor, H bridge circuit, discharge circuit and bypass circuit;

The DC capacitor acts as a voltage support and serves as an input power source for an energy harvesting power supply;

The H-bridge circuit is the core circuit of the chain link unit, and outputs the compensation voltage according to the controller instruction;

The discharge circuit is mainly used to provide overvoltage protection for the DC bus capacitor, and the DC capacitor is discharged during emergency and normal exit; the circuit is mainly composed of a discharge IGBT (Qd) connected in series with a discharge resistor Rd. When the DC bus voltage exceeds the set threshold, The discharge IGBT is turned on, and the DC bus capacitor is discharged through the discharge resistor;

The bypass circuit provides protection for the entire chain link. When the chain link unit is operating normally, the H bridge circuit is put into operation, and the bypass circuit is out of operation; when a specific fault occurs inside the chain link unit, the signal is reported to the main controller, and the main controller After judgment, the controller issues a command to block the H-bridge circuit, and at the same time the bypass circuit is opened, the output current is transferred to the bypass part, and the function of exiting the faulty link is realized.

The beneficial effects of the present invention are:

Avoid frequent device exits;

The bypass circuit has a simple structure and is easy to install and operate;

The switch has no arc extinguishing device and is small in size;

The switch closing speed is fast, reducing the impact on the system;

The small number of circuit elements facilitates compactness of the device.

Description of drawings

The present invention will be further described below in conjunction with the accompanying drawings.

Figure 1 is a chain structure STATCOM and output waveform diagram.

Fig. 2 is a schematic diagram of a three-phase commutation chain and a single link.

Fig. 3 is a bypass circuit diagram of an anti-parallel thyristor.

Figure 4 is a rectifier bridge thyristor bypass circuit diagram.

Fig. 5 is a bypass circuit diagram of the reverse series IGBT.

Figure 6 is a mechanical switch bypass circuit diagram.

Fig. 7 is a main circuit diagram inside the chain link unit.

Fig. 8 is a schematic structural diagram of a mechanical switch.

Fig. 9 is a flow chart of bypass structure control.

Detailed ways

The structure and output waveform of the chained STATCOM are shown in Figure 1. It is a chained STATCOM device composed of H bridges in series. In addition to the general characteristics of multilevel converters, this device also has the following characteristics:

(1) There is no need for clamping diodes or clamping capacitors, and for the same number of levels, the minimum number of components is required, which is easy to package;

(2) It is easy to achieve a very high AC output voltage, fundamentally eliminating the need for multiple transformers, and has a good application prospect in large-scale FACTS equipment;

(3) Based on the composition method of cascading low-voltage and small-capacity converters, the technology is mature, avoiding the series connection of devices, easy to modularize, and easy to realize compact structural design.

The output voltage of the commutation chain is the sum of the output voltages of N H-bridges:

v _s ＝v _k1 +v _k2 +…+v _kN

where N is the number of cascaded H-bridges for each phase.

The chained STATCOM is connected in parallel to the power system. Through the collection and detection of the system voltage and current, the target compensation amount is calculated, and then the corresponding reactive power is output by controlling the amplitude and phase angle of the output voltage of the commutation chain.

The schematic diagram of the three-phase commutation chain and a single chain link is shown in Figure 2. The commutation chain is composed of three-phase delta-connected cascaded H-bridge voltage source converters. The devices are connected to the system in parallel by connecting reactors. The amplitude and phase angle of the output voltage are used to realize the exchange of active power and reactive power between the chained STATCOM and the system. In order to improve the reliability of chained STATCOM devices and reduce the impact of chain link unit failure on the system, each chain link unit must be equipped with a simple, reliable and economical bypass circuit to provide protection for the entire chain link unit.

The bypass circuit requires rapid operation and high flow capacity, so it is generally necessary to choose a bypass method based on power electronic devices. The simplest way is the anti-parallel thyristor structure, as shown in Figure 3, the thyristors in this structure are easy to be misconnected and damaged; it is also possible to use two IGBTs in reverse series, as shown in Figure 5, in this way, high voltage should be selected , Large-capacity IGBT devices, the cost is high; it can also be connected to the thyristor through diode rectification, as shown in Figure 4, the thyristor is equipped with a resistance-capacity buffer absorption circuit to prevent excessive dv/dt and excessive reverse recovery voltage damage to the device. The DC side of the bypass circuit is connected to the DC side of the voltage source converter through a 10kΩ isolation resistor to fix the potential, suppress partial discharge and prevent interference to the thyristor. Chained STATCOM devices have high output voltage and large current. According to the current manufacturing level of thyristors, it is difficult to find square-packaged thyristor modules that meet the requirements of electrical parameters. And other auxiliary equipment, resulting in bulky, inconvenient installation, and not conducive to the compact design of the module; and the thyristor is an uncontrollable device, once it is turned on, it cannot be controlled to turn off, and it will turn off naturally at zero crossing, and cannot maintain the conduction state.

To sum up, the present invention adopts a bypass circuit composed of mechanical switches, as shown in Figure 6, compared with the above bypass methods, this circuit has a simple structure, fewer components and lower cost. At present, the available mechanical switches mainly include isolating switches, load switches, and circuit breakers. The isolating switch is mainly used to isolate the power supply, forming an obvious disconnection point to ensure the safety of maintenance. There is no special arc extinguishing device, and it can only switch on and off a small current. It is generally operated manually; the load switch is a function between the isolating switch and the circuit breaker. The electrical appliances between the circuit breakers have a simple arc extinguishing device, which can switch on and off a certain load current and overload current, but the volume is large; the circuit breaker has a special arc extinguishing device, which can switch on and off the load current and short-circuit current, and has electromagnetic The operating mechanism can automatically control opening and closing, but the action time is long and the volume is large, which is not conducive to the compact design of the chain link unit. Therefore, it is necessary to design a mechanical switch with simple structure, convenient installation, small size and single-phase operation as the bypass circuit of the chain link unit.

The mechanical switch is designed as an AC, indoor, single-phase operation structure. Its structural diagram is shown in Figure 8. It mainly consists of the incoming line terminal (using a conductor) 1, the outgoing line terminal (using a conductor) 2, the vacuum switch tube 3, It is composed of insulating support 4, insulating pull rod 5, magnetic force operating mechanism 6, and control circuit 7. The inlet and outlet terminals 1, 2 and the vacuum switch tube 3 constitute the main circuit part of the switch. The magnetic operating mechanism 6 and the control circuit 7 are used as the control circuit and auxiliary circuit of the mechanical switch to control the charging and discharging of the coil, thereby controlling the distribution of the mechanical switch. Closing operation. According to the control strategy of the device, the mechanical switch performs opening and closing operations under the condition that the device is locked and has no output current. Therefore, the mechanical switch does not need arc extinguishing capability, and the vacuum switch tube does not need a special arc extinguishing chamber, which greatly reduces The volume of the mechanical switch is reduced, which is convenient for the miniaturization of the chained STATCOM device.

The mechanical switch adopts direct-acting vertical arrangement, which can minimize the mechanical transmission link and improve the closing speed of the switch. When the chain link unit fails, the mechanical switch can be closed quickly, reducing the blocking time of the device due to the chain link failure, thereby reducing the impact on the system.

The mechanical switch is automatic opening and closing, and is equipped with manual opening operation. It adopts magnetic operating mechanism, electromagnetic operation, permanent magnet holding, no mechanical holding and tripping device. The specific operation method is: control the power supply to the capacitor through the power module Charge to make it store enough energy; when the mechanical switch operation is required, the main controller sends a closing signal to the unit controller of each chain link unit, and the unit controller then sends the control signal to the bypass circuit in the chain link unit The circuit sends a closing signal, the switch closing contact of the control circuit is closed, the closing circuit is turned on, the capacitor discharges to the electromagnetic coil of the magnetic operating mechanism, and the coil is charged to drive the vacuum tube to perform closing operation;

The magnetic operating mechanism is composed of a permanent magnet 8, a moving iron core 9 and a closing coil 10, and the working position of the magnetic operating mechanism is the opening position during normal operation. When closing, the closing coil passes current, and the magnetic field generated increases the magnetic field strength of the magnetic circuit below, and the magnetic field lines passing through the air gap below increase, and the suction force generated on the lower end of the moving iron core increases, and acts on the contact of the vacuum switch tube through the main shaft and the pull rod On, make the contact of the vacuum switch tube change from the opening position to the closing position. When the moving iron core reaches the final position, the closing coil no longer has current through the control circuit. The control loop adopts prior art known to those skilled in the art.

The internal main circuit of the bypass circuit link composed of mechanical switches is shown in Figure 7, which mainly includes DC capacitors, H-bridge circuits, discharge circuits, bypass circuits, energy harvesting power supplies, drive circuits, and unit controllers. The H-bridge circuit, DC capacitor, discharge circuit and energy-taking power supply are connected in parallel. The H-bridge circuit includes two bridge arms connected in parallel. The midpoint of the two bridge arms is used as the AC output end of the chain link unit. A bypass circuit is connected in parallel, and the energy harvesting power supply provides the obtained control power to the unit controller and the drive circuit, and the unit controller controls the H bridge circuit, DC capacitor, discharge circuit, bypass circuit and energy harvesting power supply respectively, and drives The circuit drives the IGBT device in the H bridge circuit and the IGBT device in the discharge circuit. The H-bridge circuit includes two bridge arms connected in parallel, each bridge arm is composed of upper and lower power electronic components connected in series, and each power electronic component is formed by parallel connection of high-power IGBT devices and diodes according to the reverse current flow direction, The upper ends of the two bridge arms are connected to the positive pole of the DC capacitor C, and the lower ends of the two bridge arms are connected to the negative pole of the DC capacitor C. The midpoint of the two bridge arms is used as the AC output end of the link, and the AC output ends are connected in parallel. After that, it is connected end-to-end with the AC end of the adjacent link unit. In the figure, U is the output terminal of the left bridge arm of the adjacent link unit, and V is the output terminal of the right bridge arm of the adjacent link unit; the discharge circuit includes IGBT The device Qd, the discharge resistor Rd, the diode and two resistors R1 and R2, the IGBT device is connected in series with the discharge resistor and then connected in parallel with the two serially connected resistors, and the diode is connected in parallel at both ends of the discharge resistor. The unit controller adopts a single-chip microcomputer.

The DC capacitor acts as a voltage support and acts as an input power source for the energy harvesting power supply.

The H-bridge circuit is the core circuit of the link unit, which outputs the compensation voltage according to the instructions of the controller.

The discharge circuit is mainly used to provide overvoltage protection for the DC bus capacitor, and discharge the DC capacitor during emergency and normal exit; the circuit is mainly composed of a discharge IGBT (Qd) connected in series with a discharge resistor Rd. When the DC bus voltage exceeds the set threshold, the discharge IGBT Conduction, discharge the DC bus capacitance through the discharge resistor; clamp resistors R1 and R2 are also installed in the chain link, which can be used as auxiliary discharge resistors to fully discharge the DC capacitor when the discharge circuit is out of work.

The unit controller is responsible for accepting and executing the commands issued by the main controller, uploading the internal state of the power module to the main controller, and controlling, monitoring and protecting the power module.

The driving circuit is used to drive and trigger the IGBT device, and the circuit is in a relatively high changing electric field environment, which has high requirements on the anti-interference ability of the signal. The signal is connected to the interface board through optical fiber. The optical fiber connection can ensure the electrical isolation between the interface board and the drive circuit, and can solve the interference problem in the signal transmission process.

The bypass circuit provides protection for the entire chain link. When the chain link unit is operating normally, the H bridge circuit is put into operation, and the bypass circuit is out of operation; when a specific fault occurs inside the chain link unit, the signal is reported to the main controller, and the main controller After judgment, an order is issued to block the H-bridge circuit, and at the same time, the bypass circuit is opened, and the output current is transferred to the bypass part to realize the function of exiting the faulty link.

When the chain link unit is working normally, the bypass circuit is out of operation, and the mechanical switch is in a standby state; when a specific fault occurs inside the chain link unit, and only the faulty link can be exited through bypass redundancy, the device can still work normally Then consider bypassing the faulty link and switch to the bypass circuit. Switching conditions include: DC voltage is too high, DC undervoltage, discharge resistor overheating, discharge IGBT turn-on failure, discharge IGBT turn-off failure, H-bridge IGBT temperature is too high, 2-way DC energy-taking power supply failure, discharge IGBT turn-on times exceed the limit wait. Its bypass structure control process is shown in Figure 9.

The specific control methods are:

1) When the unit controller detects a fault in the chain link unit of the device, it blocks all IGBT trigger pulses of the faulty chain link H-bridge unit, and reports the fault type of the chain link unit to the main controller;

2) After the main controller receives the reported fault signal, it issues a control command to block the IGBT trigger pulses of all three-phase link units;

3) The master controller judges whether the number of bypass links of each phase is less than the number of redundant links;

4) If the requirements are not met, the main circuit breaker and the chain link unit are discharged, and the device is out of operation;

5) If the requirements are met, close the mechanical switch of the faulty link, open the bypass circuit, read the position signal of the mechanical switch, and start the wave recording;

6) After the bypass is successful, adjust the control command, unlock the trigger pulses of all three-phase links, re-send the PWM signal, start the commutation chain, put the device back into operation, and complete the bypass switching operation.

Since the mechanical switch is held by a permanent magnet, once the bypass is successful, even if the control power supply is powered off, the mechanical switch will not be disconnected.

In some cases, although it is a link unit failure, the entire STATCOM unit will be out of service. These failures include:

1) All 3 DC energy harvesting power supplies are faulty;

2) H-bridge IGBT short-circuit fault;

3) H-bridge IGBT drive power undervoltage fault;

4) Link communication failure;

5) Bypass failure;

6) The DC voltage has an upward trend in the bypass state;

7) The number of restarts of the unit controller exceeds the limit.

It is mainly considered that these types of faults may cause IGBT damage, or the chain link unit has completely lost monitoring, or cannot enter the bypass state, so it is necessary to put the STATCOM device into use after the cause is found out.

Claims (4)

1. a chain static synchronous compensator STATCOM bypass structure of chain link unit that adopts mechanical switch, is characterized in that, STATCOM STATCOM main circuit adopts the chain structure of H bridge cascade, and forms bypass structure of chain link unit by mechanical switch, wherein

Chain type STATCOM device comprises change of current chain, linked reactor, master controller, lightning arrester, current sensing and cooling system, and wherein, chain type STATCOM device change of current chain is that the link units by a plurality of same structures is in series;

Described link units, realizes commutation by turn-off device, and DC side energy-storage travelling wave tube is for adopting the converter of capacitor;

The effect of described linked reactor is to leach the higher harmonic current that change of current chain produces, and realizes the energy exchange between chain type STATCOM and system;

Described controller is used for controlling operation and the operation of chain type STATCOM device;

Described lightning arrester and the use in parallel of change of current chain two ends, for realizing the overvoltage protection of change of current chain;

Described current sensing is series at change of current chain, for current detecting;

Described cooling system is for realizing the cooling of change of current chain;

Described mechanical switch is designed to exchange, the structure of indoor, single-phase operation, mainly comprise turnover line end, vacuum switch tube, insulating supporting, insulated tension pole, magnetic actuator and control loop etc., according to the control strategy of device, mechanical switch is to carry out breaking-closing operating under the condition of device locking no-output electric current, therefore, mechanical switch does not need arc extinguishing ability, vacuum switch tube can not need special arc control device, greatly reduced the volume of mechanical switch, be convenient to chain type STATCOM device and realize densification.

2. structure as claimed in claim 1, is characterized in that

Described mechanical switch adopts Direct Action Type to be arranged vertically, thereby reduce mechanical transfer link to improve the closing speed of switch, when link units breaks down, mechanical switch can quick-make, reduce device because of chain link fault blocking time, thereby reduce the impact on system.

3. structure as claimed in claim 2, is characterized in that

Described mechanical switch is automatic divide-shut brake, and disposes manual brake separating operation, adopts magnetic actuator, electromagnetism actuating, permanent magnetism keeps, and machinery-free keeps and trip gear, concrete operation method is: control power supply and first by power module, to capacitor, charge, make it store enough energy; When needs carry out mechanical switch operation, controller sends switching signal, the switch combined floodgate junction closure of control loop, and closing circuit conducting, capacitor is just to the solenoid electric discharge of magnetic actuator, and the charged rear drive vacuum tube of coil carries out closing operation;

Described magnetic actuator consists of permanent magnet, moving iron core and closing coil, and during normal operation, the service position of magnetic actuator is open position; During combined floodgate, closing coil passes through electric current, the magnetic field producing has increased the magnetic field intensity of magnetic circuit below, by the magnetic line of force of air gap below, increase, the suction that lower end moving unshakable in one's determination is produced increases, by main shaft, pull bar, act on vacuum switch tube contact, make vacuum switch tube contact become closing position from open position; When moving arrival unshakable in one's determination final position, by control loop, make closing coil no longer include electric current and pass through.

4. structure as claimed in claim 1, is characterized in that

The link units consisting of mechanical switch mainly comprises: DC capacitor, H bridge circuit, discharge circuit, bypass circuit, getting can power supply, drive circuit and cell controller;

Described DC capacitor plays voltage support effect, and as the input power of getting energy power supply;

Described H bridge circuit is the core circuit of link units, according to controller instruction output bucking voltage;

Described discharge circuit is mainly used in as dc-link capacitance provides overvoltage protection, DC capacitor electric discharge while promptly and normally exiting; Main circuit is in series by discharge IGBT and discharge resistance, and after DC bus-bar voltage surpasses the threshold value of adjusting, discharge IGBT conducting, discharges to dc-link capacitance by discharge resistance;

Described bypass circuit provides protection for whole chain link, and when link units is normally moved, H bridge circuit is devoted oneself to work, and bypass circuit is out of service; When the inner generation of link units specific fault, signal reporting is to master controller, and master controller is given an order through judgement, blocks H bridge circuit, and bypass circuit is open-minded simultaneously, and output current is transferred to bypass segment, realizes fault chain link exit function.

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