A Hybrid DC Circuit Breaker with Fault Current Suppression Ability and Its Working Method

Abstract

A hybrid DC circuit breaker with the ability to suppress fault current is composed of three parts: an NCP module, an EAP module, and an FP module. Its working method includes five stages: before the SM is closed, after the SM is closed, the NCP module exits operation, the EAP module exits operation, and different operation modes are selected. It can adopt different control strategies for different types of faults to achieve the line protection function; it has a simple structure, is easy to implement, and can, for transient faults, mainly absorb and suppress the fault current through the energy absorption branch and the freewheeling circuit to avoid overcurrent damage to the line; for permanent faults, the energy absorption branch disconnects after short-time operation to completely isolate the converter side from the fault side.

Description

Technical Field

[0001] The present invention relates to the field of DC transmission line protection, and particularly to a hybrid DC circuit breaker with the ability to suppress fault current.

Background Art

[0002] DC transmission has advantages in terms of low loss, high controllability, and reactive power compensation. However, compared with AC lines, the DC transmission system lacks a natural zero-crossing of the current, making it difficult to cut off faults. It cannot be achieved by a mechanical circuit breaker, so certain DC short-circuit fault protection measures are required. DC short-circuit fault protection measures are mainly divided into two types. One is to improve the topology of the converter, and the other is to add a DC circuit breaker to the line.

[0003] Compared with improving the topology of the converter, the DC circuit breaker requires fewer power electronic devices and less energy loss. Therefore, the DC circuit breaker is often used as a protection measure for DC short-circuit faults. DC circuit breakers are mainly divided into three categories: mechanical circuit breakers, solid-state circuit breakers, and hybrid circuit breakers. Since there is no zero-crossing of the current in DC short-circuit faults, when using a mechanical circuit breaker, an additional resonant circuit needs to be configured to generate a zero-crossing of the current to achieve fault removal, but the fault current removal time is long. The solid-state circuit breaker is composed of pure semiconductor devices. Although it can quickly cut off the fault current, it requires a large number of series devices, which will increase the power loss in the normal operating state. The hybrid DC circuit breaker combines the advantages of mechanical and solid-state circuit breakers, reducing the fault removal time while reducing the switching loss.

[0004] DC short-circuit faults usually include two types. Among them, the single-pole short-circuit fault is the most common DC-side fault type, and its fault characteristics are related to the grounding point of the system. After one pole fails, the voltage of the faulty pole drops to 0, and the voltage of the non-faulty pole becomes twice the original value. During the whole process of fault occurrence and elimination, the positive and negative pole currents of the system always remain stable. Therefore, the single-pole short-circuit fault on the DC side has a relatively small impact on the system operation. As long as a DC-side line with a higher withstand voltage value is selected, the stable operation of the system under short-term fault conditions can be ensured. Secondly, the bipolar short-circuit fault is one of the most serious DC-side faults. When the bipolar short-circuit fault occurs, the positive and negative poles of the transmission line bus are short-circuited, and the DC bus voltage drops rapidly to 0. The short-circuit fault results in a very small loop impedance, which will cause serious overcurrent phenomena in the DC bus and AC bus.

Summary of the Invention

[0005] The object of the present invention is to provide a hybrid DC circuit breaker with the ability to suppress fault current, which can overcome the serious overcurrent phenomenon caused by bipolar short-circuit faults, as well as the problem that general DC circuit breakers do not have the function of fault current limiting and need to be additionally equipped with current limiters. It is a hybrid DC circuit breaker for suppressing overcurrent caused by bipolar short-circuit faults.

[0006] The technical solution of the present invention: A hybrid DC circuit breaker with the ability to suppress fault current, which is characterized in that it includes three parts: an NCP (Nominal Current Path) module, an EAP (Energy Absorption Path) module, and an FP (Freewheeling Path) module; wherein, the EAP module and the NCP module are in a parallel connection relationship, one end is connected to the current outflow end of the DC bus, and the other end is connected to the FP module and the same DC bus (equivalent to being connected in series to the DC bus at the current outflow end); the other end of the FP module is connected to the other DC bus, that is, the DC bus inflow end, equivalent to being connected in parallel to the two DC buses.

[0007] The NCP module is composed of a load connection switch LCS (Load Communication Switch) and a mechanical switch K; one end of the switch K is connected to the DC bus outflow end, and the other end is connected to the LCS, and is connected in series with the connection switch LCS; the other end of the load connection switch LCS is connected to the FP module and the same DC bus.

[0008] The load connection switch LCS is composed of two NPN-type triodes with reverse-parallel diodes. The collector of the first triode is connected to the current outflow end of the mechanical switch K, and the emitter is connected to the emitter of the second triode; the collector of the second diode is connected to the current outflow end of the EAP module, that is, the two triodes form a reverse series relationship.

[0009] The EAP module is composed of a thyristor T1, a large fault current suppression resistor R3, a parallel energy absorption capacitor C1, and a freewheeling resistor R1; the cathode of the thyristor T1 is connected in series with the large fault current suppression resistor R3 and then connected in series with the structure of the parallel energy absorption capacitor C1 and the freewheeling resistor R1 connected in parallel; the anode of the thyristor T1 receives the DC bus current; the other end of the parallel energy absorption capacitor C1 and the freewheeling resistor R1 connected in parallel is connected to the other DC bus and the FP module.

[0010] The FP module is composed of a pair of anti-parallel diodes T2 and T3 that control the current flow direction, a current suppression resistor R4, an energy absorption capacitor C2, and a freewheeling resistor R2. The diodes T2 and T3 are connected in an anti-parallel manner to control the current flow direction. After being anti-parallel connected, they are in series with the current suppression resistor R4, and then in series with the energy absorption capacitor C2 and the freewheeling resistor R2 that are connected in parallel with each other.

[0011] A working method of a hybrid DC circuit breaker with the ability to suppress fault current, characterized in that it includes four operating modes, namely before the SM (Sub-module) is closed, after the SM is closed, when the NCP module exits operation, and when the EAP module exits operation. Assume that a bipolar short-circuit fault occurs in the DC transmission line at time t0, the SM is blocked at time t1, the breaker branch is disconnected at time t2, and the absorption branch is put into operation at the same time. The fault current absorption branch exits operation at time t3. Specifically, it includes the following steps:

[0012] (1) Before the SM is closed, the time period is t0 - t1:

[0013] After the fault occurs at time t0, the DC-side current of the system rises rapidly, but has not reached twice the current value yet, and the bipolar short-circuit fault has not been recognized, so the sub-module blocking cannot be triggered. At this time, the system still operates in the normal working mode, and the current flows through the breaker branch module, the bipolar short-circuit fault module, and finally returns to the MMC (Modular Multilevel Converter) module.

[0014] (2) After the SM is closed, the time period is t1 - t2:

[0015] When it is detected at time t1 that the DC-side current reaches twice the current value during normal operation, the blocking signal of all sub-modules is issued. After the sub-modules are blocked, the MMC module is equivalent to an uncontrollable rectifier bridge, and the branch conduction situation remains the same as that from t0 to t1.

[0016] (3) When the NCP module exits operation, the time period is t2 - t3

[0017] While judging the fault type, the system conducts the work of suppressing the fault current. At time t2, the NCP module exits operation, the EAP module is put into operation, and the FP module is also put into operation at the same time. The large fault current suppression resistor R3 on the EAP module suppresses the fault current, and the energy absorption capacitor C1 is connected in parallel to absorb the fault current. When the parallel energy absorption capacitor C1 absorbs enough current, it presents an open-circuit state. At this time, the current is transmitted through the freewheeling resistor R1, and the diode T2 on the FP module is controlled to conduct; the current i fwCharge the energy absorption capacitor C2 through the diode T2, and at the same time, use the current suppression resistor R4 to suppress the fault current in the branch. The current i L is transmitted through the line to the fault point; within this time interval, the system can provide short-term power support for the load, and no further control is required for short-term faults;

[0018] (4) The EAP module exits operation. The EAP module exits operation during the time period t3 - t4

[0019] At time t3, if it is determined that the fault is a permanent fault, the EAP module exits operation, and the FP module continues to be put into use to achieve complete isolation between the MMC side and the fault side. At this time, the electric charge stored in the parallel energy absorption capacitor C1 is consumed through the freewheeling resistor R1, and the current slowly decays to 0; the diode T3 on the FP module conducts, and the energy absorption capacitor C2 discharges to the fault side, and the current gradually decays to 0 through the branch resistor;

[0020] (5) Select different operating modes according to two types of faults: instantaneous faults and permanent faults; among them, instantaneous faults mainly achieve the suppression of fault current, that is, the above (1)-(3); permanent faults mainly achieve complete isolation between the fault side and the non-fault side, that is, the above (1)-(4).

[0021] Advantages of the present invention: A hybrid DC circuit breaker with the ability to suppress fault current is proposed, which can effectively suppress the overcurrent phenomenon caused by bipolar short-circuit faults, and can adopt different control strategies according to the actual fault type to achieve line protection. For instantaneous faults, it can absorb and suppress the fault current to ensure that the amplitude of the DC line transmission current is maintained within the range that the line can withstand, thereby avoiding damage to the line due to overheating caused by large current; for permanent faults, it can achieve complete isolation between the converter side and the fault side.

Description of the Drawings

[0022] Figure 1 is a schematic diagram of the topology structure of the DC power transmission system in a hybrid DC circuit breaker with the ability to suppress fault current involved in the present invention.

[0023] Figure 2 is a schematic diagram of the topology structure of a hybrid DC circuit breaker with the ability to suppress fault current involved in the present invention

[0024] Figure 3 is a schematic diagram of the operating principle of a hybrid DC circuit breaker with the ability to suppress fault current involved in the present invention (where, Figure 3-a is from t0 - t1: the stage before the SMs are blocked, Figure 3-b is from t1 - t2: the stage after the SMs are blocked; Figure 3-c is from t2 - t3: the stage after the NCP module exits the operation, Figure 3-dFor t3 - t4: The stage after the EAP module exits the operation).

Specific implementation manners

[0025] Embodiment: A hybrid DC circuit breaker with the ability to suppress fault current, as Figure 2 shown, which is characterized in that it includes three parts: an NCP module, an EAP module, and an F module; among them, the EAP module and the NCP module are in a parallel connection relationship, one end is connected to the current outflow end of the DC bus, and the other end is connected to the FP module and the same DC bus, equivalent to being connected in series to the DC bus at the current outflow end; the other end of the FP module is connected to another DC bus, that is, the DC bus inflow end, equivalent to being connected in parallel to the two DC buses.

[0026] The NCP module is composed of a load connection switch LCS and a mechanical switch K; one end of the switch K is connected to the DC bus outflow end, and the other end is connected to the LCS, and is connected in series with the connection switch LCS; the other end of the load connection switch LCS is connected to the FP module and the same DC bus, as Figure 2 shown.

[0027] The load connection switch LCS is composed of two NPN - type transistors with reverse - parallel diodes. The collector of the first transistor is connected to the current outflow end of the mechanical switch K, and the emitter is connected to the emitter of the second transistor; the collector of the second diode is connected to the current outflow end of the EAP module, that is, the two transistors form a reverse - series relationship, as Figure 2 shown.

[0028] The EAP module is composed of a thyristor T1, a large fault - current - suppressing resistor R3, a parallel energy - absorbing capacitor C1, and a free - wheeling resistor R1; the cathode of the thyristor T1 is connected in series with the large fault - current - suppressing resistor R3 and then connected in series with the structure of the parallel energy - absorbing capacitor C1 and the free - wheeling resistor R1 connected in parallel; the anode of the thyristor T1 receives the DC bus current; the other ends of the parallel energy - absorbing capacitor C1 and the free - wheeling resistor R1 connected in parallel are connected to another DC bus and the FP module.

[0029] The FP module is composed of a pair of anti - parallel diodes T2 and T3 that control the current flow direction, a current - suppressing resistor R4, an energy - absorbing capacitor C2, and a free - wheeling resistor R2; the diodes T2 and T3 are connected in an anti - parallel connection manner to control the current flow direction. After being anti - parallel, they are connected in series with the current - suppressing resistor R4, and then connected in series with the energy - absorbing capacitor C2 and the free - wheeling resistor R2 connected in parallel, as Figure 2 shown.

[0030] To enable those skilled in the art to better understand the solution of this application, the following will further elaborate on this application in conjunction with the accompanying drawings.

[0031] The present invention provides a hybrid DC circuit breaker with the ability to suppress fault current, including:

[0032] As Figure 1 shown, the most basic two-terminal DC power transmission structure is adopted, and modular multilevel converter (MMC) is used as the converter for AC-DC conversion; on the left side of the power transmission system is module MMC1, and on the right side is module MMC2. The DC side clamping resistor is grounded, and the resistance value of the clamping resistor is R d . Module MMC1 and module MMC2 are connected by a DC power transmission line, and the resistance of the power transmission line is R line , and the inductance is L line . Under normal operating conditions, module MMC1 adopts DC bus voltage control, and module MMC2 adopts output power control.

[0033] The topological structure of the hybrid DC circuit breaker is as Figure 2 shown.

[0034] The DC circuit breaker mainly includes three parts: NCP module, EAP module, and FC module. Since the mechanical switch has a relatively slow action time, the NCP module includes a load connection switch LCS and a mechanical switch K. LCS realizes the function of rapid isolation, and the mechanical switch K mainly realizes the function of reliable turn-off. EAP mainly includes thyristor T1, a large fault current suppression resistor R3, a parallel energy absorption capacitor C1, and a freewheeling resistor R1. FP mainly includes a pair of antiparallel diodes T2 and T3 for controlling the current flow direction, a current suppression resistor R4, and an energy absorption capacitor C2 and its freewheeling resistor R2.

[0035] A hybrid DC circuit breaker with the ability to suppress fault current is characterized in that the topological operation principle of the DC circuit breaker is as shown in Figure 3, and it includes before SM closure, after SM closure, NCP out of operation, and EAP out of operation; among them, it is assumed that a bipolar short-circuit fault occurs in the DC power transmission line at time t0, the sub-module is blocked at time t1, the breaker branch is disconnected at time t2, and the absorption branch is put into operation at the same time, and the fault current absorption branch exits operation at time t3: The specific working method includes the following content:

[0036] As Figure 3-a shown, t0 - t1: After the fault occurs at time t0, the DC side current of the system rises rapidly. Before reaching 2 times the current value, the bipolar short-circuit fault has not been identified, and the sub-module blocking cannot be triggered. The system still operates in the normal working mode. The current flows through the breaker branch, the bipolar short-circuit fault, and finally returns to the MMC.

[0037] As Figure 3-bAs shown, from t1 to t2: When the DC-side current is detected to reach twice the normal operating value at time t1, a sub-module locking signal is generated. After the sub-module is locked, the MMC is equivalent to an uncontrolled rectifier bridge, and the branch conduction situation remains the same as that from t0 to t1.

[0038] As Figure 3-c shown, from t2 to t3: While judging the fault type, the system conducts fault current suppression work. At time t2, the NCP stops operating, the EAP starts operating, and at the same time the FP also starts operating. The large resistor R3 on the EAP suppresses the fault current, and the capacitor C1 absorbs the fault current. When the capacitor C1 absorbs enough current, it presents an open-circuit state, and at this time the current is transmitted through R1. By controlling the conduction of T2 on the FP, the current i fw charges the capacitor C2 through T2, and at the same time uses the resistor R4 to suppress the fault current of the branch. The current i L is transmitted through the line to the fault point. During this time interval, the system can provide short-term power support for the load, and no further control is required for short-term faults.

[0039] As Figure 3-d shown, from t3 to t4: At time t3, after it is judged that the fault is a permanent fault in combination with other system algorithms, the EAP stops operating, and the FP continues to be used, realizing complete isolation between the MMC side and the fault side. The electric charge stored in the capacitor C1 is consumed through R1, and the current slowly decays to 0. T3 on the FP conducts, and the capacitor C2 discharges to the fault side, and the current gradually decays to 0 through the branch resistor.

[0040] Step 4: Select different operating modes according to the two types of instantaneous faults and permanent faults. Instantaneous faults mainly achieve the suppression of fault current, and permanent faults mainly achieve complete isolation between the fault side and the non-fault side.

[0041] For an instantaneous short-circuit fault, after the fault is removed, the EAP stops operating, the NCP is turned off, and the system operates normally.

[0042] For a permanent fault, the EAP is disconnected, the EP continues to be put into use, the converter station is completely isolated from the fault point, and the capacitor of the freewheeling branch continues to supply power to the fault point, and the current quickly decays to 0.

[0043] In summary, the present invention can quickly suppress the fault current and effectively isolate DC faults. Different switching control methods are provided for instantaneous faults and permanent faults to ensure the safety of the system under different types of faults. At the same time, it can achieve rapid isolation of faults and reduce the damage of fault current to DC transmission lines.

Claims

1. A hybrid DC circuit breaker with the ability to suppress fault current, characterized in that It includes three parts: the NCP module, the EAP module, and the FP module; among them, the EAP module and the NCP module are connected in parallel, with one end connected to the current outflow end of the DC bus and the other end connected to the FP module and the same DC bus; the other end of the FP module is connected to another DC bus, that is, the DC bus inflow end; The NCP module is composed of a load connection switch LCS and a mechanical switch K; one end of the switch K is connected to the DC bus outflow end, and the other end is connected to the LCS, and is connected in series with the connection switch LCS; the other end of the load connection switch LCS is connected to the FP module and the same DC bus; The EAP module is composed of a thyristor T1, a large fault current suppression resistor R3, a parallel energy absorption capacitor C1, and a freewheeling resistor R1; the cathode of the thyristor T1 is connected in series with the large fault current suppression resistor R3 and then connected in series with the structure of the parallel energy absorption capacitor C1 and the freewheeling resistor R1 connected in parallel; the anode of the thyristor T1 receives the DC bus current; the other end of the parallel energy absorption capacitor C1 and the freewheeling resistor R1 connected in parallel is connected to another DC bus and the FP module; The FP module is composed of a pair of anti-parallel diodes T2 and T3 that control the current flow direction, a current suppression resistor R4, an energy absorption capacitor C2, and a freewheeling resistor R2; the diodes T2 and T3 are connected in an anti-parallel connection mode that controls the current flow direction. After being anti-parallel connected, they are connected in series with the current suppression resistor R4 and then connected in series with the energy absorption capacitor C2 and the freewheeling resistor R2 connected in parallel.

2. The hybrid DC circuit breaker with the ability to suppress fault current according to claim 1, characterized in that The load connection switch LCS is composed of two NPN transistors with anti-parallel diodes. The collector of the first transistor is connected to the current outflow end of the mechanical switch K, and the emitter is connected to the emitter of the second transistor; the collector of the second diode is connected to the current outflow end of the EAP module, that is, the two transistors are connected in a reverse series relationship.

3. The working method of the hybrid DC circuit breaker with fault current suppression ability according to claim 1, characterized in that It includes four operating modes, namely before SM closing, after SM closing, NCP module out of operation, and EAP module out of operation. Assume that a bipolar short-circuit fault occurs in the DC transmission line at time t0, the SM is blocked at time t1, the breaker branch is disconnected at time t2, and the absorption branch is put into operation at the same time. The fault current absorption branch exits operation at time t3; Specifically, it includes the following steps: (1) Before SM closing, the time period is t0 - t1: After the fault occurs at time t0, the DC side current of the system rises rapidly, but has not reached twice the current value. The bipolar short-circuit fault has not been identified and the sub-module blocking cannot be triggered. At this time, the system still operates in the normal mode, and the current flows through the breaker branch module, the bipolar short-circuit fault module, and finally returns to the MMC module; (2) After SM closing, the time period is t1 - t2: When it is detected at time t1 that the DC side current reaches twice the normal operating current, lock signals for all sub-modules are issued; after the sub-modules are blocked, the MMC module is equivalent to an uncontrolled rectifier bridge, and the branch conduction situation remains the same as that in t0 - t1; (3) The NCP module exits the operation, and the time period is t2 - t3 While judging the fault type, the system performs the work of suppressing the fault current; at time t2, the NCP module exits the operation, the EAP module is put into operation, and at the same time the FP module is also put into operation; the large fault current suppression resistor R3 on the EAP module suppresses the fault current, and at the same time the parallel energy absorption capacitor C1 absorbs the fault current. When the parallel energy absorption capacitor C1 absorbs enough current, it presents an open circuit state. At this time, the current is transmitted through the freewheeling resistor R1, and the diode T2 on the FP module is controlled to conduct; the current i fw charges the energy absorption capacitor C2 through the diode T2, and at the same time uses the current suppression resistor R4 to suppress the fault current of the branch, and the current i L is transmitted to the fault point through the line; within this time interval, the system can provide short-term power support for the load, and no further control is required for short-term faults; (4) The EAP module exits the operation. The EAP module exits the operation, and the time period is t3 - t4 At the moment of t3, if it is determined that the fault is a permanent fault, the EAP module exits the operation, and the FP module continues to be put into use to achieve complete isolation between the MMC side and the fault side. At this time, the electric quantity stored in the parallel energy absorption capacitor C1 is consumed through the freewheeling resistor R1, and the current slowly decays to 0; the diode T3 on the FP module conducts, and the energy absorption capacitor C2 discharges to the fault side, and the current gradually decays to 0 through the branch resistor; (5) Different operation modes are selected according to two types of instantaneous faults and permanent faults; among them, the instantaneous fault mainly realizes the suppression of the fault current, that is, the above (1)-(3); the permanent fault mainly realizes the complete isolation between the fault side and the non-fault side, that is, the above (1)-(4).

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