A multi-port hybrid DC circuit breaker for isolating busbar and line faults

Abstract

The present invention relates to a multi-port hybrid DC circuit breaker for realizing busbar and line fault isolation. The topological structure of the multi-port hybrid DC circuit breaker includes: n load transfer switches; n fast disconnecting switches; n residual current switches; n current limiting reactors; 1 main disconnecting switch; n energy discharge branches; n thyristor branches; the n load transfer switches and the fast disconnecting switches constitute the current-carrying branches of n lines; the energy discharge branches are anti-parallel thyristors and energy discharge resistors R c The present invention is scientifically and reasonably designed, integrating n dual-port hybrid DC circuit breakers on n lines into a multi-port hybrid DC circuit breaker, greatly reducing the number of power electronic devices and having good economic performance; in addition, the existence of the energy discharge branch in the multi-port hybrid DC circuit breaker shortens the fault isolation time and reduces the energy discharge demand of the MOV.

Description

Technical Field

[0001] The present invention belongs to the technical field of flexible DC power grid protection and control, and in particular relates to a multi-port hybrid DC circuit breaker for realizing busbar and line fault isolation. Background Art

[0002] With the extensive use of fossil energy, environmental pollution and energy shortage problems have become increasingly serious. To this end, countries around the world have begun to transition from traditional energy to renewable clean energy in their energy strategies. In recent years, renewable clean energy such as wind power and photovoltaics have been integrated into the power grid on a large scale. Flexible DC transmission technology based on modular multilevel converters (MMC) has the advantages of reactive power and active power decoupling control and low harmonic content. It is one of the effective means of large-scale renewable energy consumption and grid connection.

[0003] With the large-scale integration of renewable energy into the power grid, the flexible DC system continues to expand and develop towards a multi-terminal flexible DC system. In a multi-terminal flexible DC system, the DC bus connects multiple DC lines. When a short circuit occurs in the DC line, the dual-port hybrid DC circuit breaker configured on the DC line quickly operates to achieve fault isolation.

[0004] However, since the dual-port hybrid DC circuit breaker has only two ports, one input and one output, it can only achieve fault isolation of one line. Considering that the main breaking branch of the dual-port hybrid DC circuit breaker requires a large number of insulated gate bipolar transistors (IGBT), in the case of a multi-terminal flexible DC system, more dual-port hybrid DC circuit breakers need to be configured on the DC line, resulting in a high economic cost. In addition, in a multi-terminal flexible DC system, DC bus fault is the most serious fault. If the bus fault cannot be isolated in time, it will have a great impact on the system.

[0005] In order to further reduce the manufacturing cost of DC circuit breakers and reliably isolate bus faults, the present invention proposes a multi-port hybrid DC circuit breaker that shares a unidirectional main breaker branch to achieve bus and line fault isolation. Summary of the invention

[0006] The purpose of the present invention is to overcome the shortcomings of the prior art and propose a multi-port hybrid DC circuit breaker for realizing bus and line fault isolation, which greatly reduces the economic cost. By adding an energy discharge branch in the multi-port hybrid DC circuit breaker of the present invention, the energy consumption demand of the lightning arrester is reduced and the fault isolation time is effectively shortened.

[0007] The present invention solves the technical problem by the following technical solutions:

[0008] A multi-port hybrid DC circuit breaker for isolating busbar and line faults, characterized in that the topological structure of the multi-port hybrid DC circuit breaker includes:

[0009] n load transfer switches including n LCSs connected in parallel;

[0010] n fast disconnecting switches, including n UFDs connected in parallel;

[0011] n residual current switches, including n RCBs connected in parallel, for physically isolating the converter station from the fault line after the fault current drops to zero;

[0012] n current limiting reactors, including n parallel connected L dc ;

[0013] 1 main disconnect switch MB, used to achieve initial fault isolation;

[0014] n energy discharge branches;

[0015] n thyristor branches, used to select the flow path of the fault current;

[0016] The n load transfer switches and fast isolating switches constitute the current-carrying branches of the n lines, and are used to conduct the load current;

[0017] The energy discharge branch is composed of a thyristor and an energy discharge resistor R connected in anti-parallel c It is used to discharge the energy in the current limiting reactor.

[0018] Moreover, the multi-port hybrid DC circuit breaker is installed in a ±500kV three-terminal flexible DC transmission system. When the system is operating normally, LCS1-LCS n 、UFD1-UFD n and RCB1-RCB n The LCS-UFD-RCB is in the closed state, and the other devices are in the open state. Each line transmits power through the LCS-UFD-RCB. When a system failure occurs, the multi-port hybrid DC circuit breaker realizes the working process of fault isolation as follows:

[0019] 1) Line failure

[0020] Assume that at time t0, a ground fault F1 occurs at the P1 port of the multi-port hybrid DC circuit breaker. After the fault detection time Δt0 delay, the protection device detects the fault at time t1. During t0-t1, the current i pj (j=1,2…n) keeps rising, i pj can be solved as:

[0021]

[0022] Assume that in normal operation the current i at port 1 is p1 for i pre1 , then during t0-t1, the current i p1 for:

[0023]

[0024] in:

[0025]

[0026] Where, L ti is the equivalent inductance of the overhead line, L l is the line inductance from port P1 to the fault point;

[0027] After the protection device detects a fault, it immediately turns on MB and thyristor T d1 At the same time, LCS1 is turned off, and the fault current starts to flow to MB. After a Δt1 commutation delay, the fault current flowing through LCS1 drops to zero at t2. At this time, UFD1 is controlled to start opening. After a Δt2 UFD opening delay, UFD1 reaches the rated opening distance at t3 and completes the opening operation. During t1-t3, the equivalent circuit of the system is consistent with the equivalent circuit of the system during t0-t1. During t1-t3, the current i pj (j=1,2…n) still satisfies equations (1)-(3);

[0028] After UFD is successfully opened, at time t4, MB is turned on and thyristor T is turned on. a1 Then, the fault current flows into the MOV, and the MOV begins to discharge the energy stored in the fault current. a1 The conduction of the current limiting reactor L dc1 The energy in the energy dissipation resistor R c After the energy discharge delay of Δt3, the fault current of port P1 drops to zero at t5, and fault isolation is achieved. During t4-t5, the following formula is satisfied:

[0029]

[0030] Where: i p1 (t4) can be obtained by formula (3), U MOV The action voltage of MOV is U dc 1.4-2 times of i Ta1 Flow through thyristor T a1 The current,

[0031] Fault current i at port 1p1 and current flowing through thyristor T a1 The current i Ta1 It can be expressed as:

[0032]

[0033] in:

[0034]

[0035] After the current at port P1 drops to zero, the current limiting reactor L dc1 The energy in the battery is continuously discharged by the energy dissipation resistor R c Discharge, at t6, the current limiting reactor L is completed dc1 The energy discharge process in the thyristor T a1 The current i Ta1 drops to zero, the thyristor T a1 Natural shutdown; during t5-t6, the current flowing through the thyristor T a1 The current i Ta1 for:

[0036]

[0037] where i Ta1 (t5) can be obtained by formula (6);

[0038] At t7, RCB1 is opened to achieve physical isolation between the converter station and the faulty line;

[0039] 2) Busbar failure

[0040] Assume that at time t0, a ground fault F2 occurs at the DC bus, and each port is connected to the RCB j -UFD j -LCS j The fault current is provided to the DC bus. After a delay of Δt0, the protection device detects the fault at time t1. During the period t0-t1, the current i pj (j=1,…,n) is

[0041]

[0042] Among them, i prej is the current of each port before the fault occurs, L ej is the sum of the inductance from each port to the DC bus;

[0043] After the protection device detects a fault, it immediately turns on MB and thyristor T u1 -T un Turn off LCS1-LCS at the same time n , at time t2, flows through LCS1-LCSn The current drops to zero and controls UFD1-UFD n Start opening, UFD1-UFD n At t3, the rated opening distance is reached and the opening operation is completed. During t1-t3, the current i pj (j=1,2…n) still satisfies equations (1) to (3),

[0044] At time t4, MB is turned on and thyristor T is turned on at the same time. b1 -T bn Then the fault current flows into the MOV, and the MOV begins to discharge the energy stored in the fault current. a1 The conduction of the current limiting reactor L dcj is bypassed, so the current limiting reactor L dcj The energy in the energy dissipation resistor R c After Δt3 energy discharge delay, the current i of each port is pj It drops to zero at t5, achieving fault isolation. During t4-t5, the following equation is satisfied:

[0045]

[0046] Among them, i Tbj is the current flowing through the thyristor T bj The current (j=1,…,n),

[0047] The current i of each port pj and current flowing through thyristor T bj The current i Tbj is represented as

[0048]

[0049] in

[0050]

[0051] When the current at each port i pj When the resistor discharges R c Continuously discharge the current limiting reactor L dcj The energy stored in the thyristor flows through the thyristor T at time t6. bj The current i Tbj drops to zero, flowing through the thyristor T bj The current i Tbj for

[0052]

[0053] Among them, i Tbj (t5) can be obtained by formula (10);

[0054] Open RCB at t7 j , port P j Can be physically isolated,

[0055] Based on the above working process, the multi-port hybrid DC circuit breaker has the ability to isolate each line from the DC bus during a DC bus fault, thereby achieving bus fault isolation.

[0056] The advantages and beneficial effects of the present invention are:

[0057] 1. When realizing line fault isolation, the present invention only needs to open the LCS of the line and select the thyristor to discharge the remaining energy through the input of the lightning arrester, thereby realizing line fault isolation; when realizing busbar fault isolation, the LCS of each line and the selected thyristor need to be opened and the lightning arrester is input through the bridge thyristor to discharge the remaining energy, thereby realizing busbar fault isolation.

[0058] 2. The multi-port hybrid DC circuit breaker of the present invention adds an energy discharge branch composed of an anti-parallel thyristor and an energy discharge resistor Rc. The function of this branch is to turn on the corresponding thyristor in the energy discharge branch when the lightning arrester is put into use to discharge the residual energy, so that the lightning arrester bypasses the current limiting reactor when discharging the residual energy, that is, the energy in the current limiting reactor is consumed by the energy discharge resistor.

[0059] 3. The present invention utilizes the idea that all incoming and outgoing lines on the same DC bus share the expensive MB, and integrates n two-port hybrid DC circuit breakers on n lines into a multi-port hybrid DC circuit breaker, which greatly reduces the number of power electronic devices and has good economic performance.

[0060] 4. The multi-port hybrid DC circuit breaker of the present invention is connected to a bridge structure in the MB. A unidirectional MB can achieve fault isolation of the busbar and any line. In addition, the multi-port hybrid DC circuit breaker retains the busbar, which is convenient for the expansion of the DC power grid.

[0061] 5. The presence of the energy discharge branch in the multi-port hybrid DC circuit breaker of the present invention shortens the fault isolation time and reduces the energy consumption demand of the lightning arrester. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] Figure 1 A topological structure diagram of a multi-port hybrid DC circuit breaker of the present invention;

[0063] Figure 2 A diagram of a three-terminal flexible direct current transmission system of the present invention;

[0064] Figure 3A fault current flow path diagram during the period t0-t1 of line fault isolation of the present invention;

[0065] Figure 4 A fault current flow path diagram during the line fault isolation period t1-t3 of the present invention;

[0066] Figure 5 A fault current flow path diagram during the line fault isolation period t4-t5 of the present invention;

[0067] Figure 6 A fault current flow path diagram during the line fault isolation period t5-t6 of the present invention;

[0068] Figure 7 It is a fault current flow path diagram during the bus fault isolation period t0-t1 of the present invention;

[0069] Figure 8 A fault current flow path diagram during the bus fault isolation period t1-t3 of the present invention;

[0070] Fig. 9 It is a fault current flow path diagram during the bus fault isolation period t4-t5 of the present invention;

[0071] Fig.10 This is a fault current flow path diagram during the bus fault isolation period t5-t6 of the present invention. DETAILED DESCRIPTION

[0072] The present invention is further described in detail below through specific examples. The following examples are only illustrative and not restrictive, and the protection scope of the present invention cannot be limited thereto.

[0073] Figure 2 It is a ±500kV three-terminal flexible DC transmission system, in which the following Figure 1 The multi-port hybrid DC circuit breaker shown.

[0074] The following first presents the topological structure of the multi-port hybrid DC circuit breaker for realizing busbar and line fault isolation of the present invention, and then analyzes the working principles of the multi-port hybrid DC circuit breaker of the present invention for realizing fault isolation when a line fault and a busbar fault occur in a three-terminal flexible DC transmission system. Finally, the performance and economy of the multi-port hybrid DC circuit breaker of the present invention are analyzed.

[0075] 1. Multi-port hybrid DC circuit breaker topology

[0076] The topological structure of the multi-port hybrid DC circuit breaker of the present invention is as follows: Figure 1 The circuit breaker consists of n LCS, n UFD, n RCB, n energy discharge branches, n thyristor branches, n current limiting reactors Ldc and an MB. LSC and UFD constitute the current-carrying branch of each line, which is used to conduct the load current; the thyristor branch is used to select the flow path of the fault current; the energy discharge branch is composed of anti-parallel thyristors and energy discharge resistors R c MB is used to achieve the initial isolation of the fault; RCB is used to achieve the physical isolation between the converter station and the fault line after the fault current drops to zero.

[0077] 2. Working principle of multi-port hybrid DC circuit breaker

[0078] When the system is operating normally, LCS1-LCS n 、UFD1-UFD n and RCB1-RCB n The LCS-UFD-RCB is in the closed state, and the other devices are in the open state. Each line transmits power through the LCS-UFD-RCB.

[0079] When a system fault occurs, the multi-port hybrid DC circuit breaker of the present invention will achieve fault isolation according to a certain operating principle.

[0080] The following analyzes the operating principle of the multi-port hybrid DC circuit breaker of the present invention when a line fault and a bus fault occur in the system. In order to simplify the analysis process, the switching characteristics of the submodule capacitor of the MMC during the fault isolation period are ignored, and the MMC is equivalent to a DC voltage source U dc and equivalent inductance L eqj A series combination.

[0081] 1) Line failure

[0082] Assume that at time t0, a ground fault F1 occurs at the P1 port of the multi-port hybrid DC circuit breaker. After a delay of Δt0 (fault detection time, generally 3ms), the protection device detects the fault at time t1. During t0-t1, the fault current flow path is as follows: Figure 3 During this period, the current i pj (j=1,2…n) keeps rising, i pj can be solved as

[0083]

[0084] Assume that in normal operation the current i at port 1 is p1 for i pre1 , then during t0-t1, the current i p1 for

[0085]

[0086] in

[0087]

[0088] Where, L ti is the equivalent inductance of the overhead line, L l is the line inductance from port P1 to the fault point.

[0089] After the protection device detects a fault, it immediately turns on MB and thyristor T d1 At the same time, LCS1 is turned off. From this point on, the fault current starts to flow to MB. After a delay of Δt1 (commutation time, about tens of microseconds), the fault current flowing through LCS1 drops to zero at time t2. At this time, UFD1 is controlled to start opening. After a delay of Δt2 (UFD opening time, taken as 2ms), UFD1 reaches the rated opening distance at time t3 and completes the opening operation. During t1-t3, the fault current flow path is as follows: Figure 4 As shown. During this period, the equivalent circuit of the system is consistent with the equivalent circuit of the system during t0-t1. Therefore, during t1-t3, the current i pj (j=1,2…n) still satisfies equations (1) to (3).

[0090] After UFD is successfully opened, at time t4, MB is turned on and thyristor T is turned on. a1 Then, the fault current flows into the MOV, and the MOV begins to discharge the energy stored in the fault current. a1 The conduction of the current limiting reactor L dc1 The energy in the energy dissipation resistor R c After the delay of Δt3 (energy discharge delay), the fault current at port P1 drops to zero at t5, achieving fault isolation. During t4-t5, the fault current flow path is as follows: Figure 5 During this period, the following equation is satisfied

[0091]

[0092] where i p1 (t4) can be obtained by formula (3), U MOV is the action voltage of MOV (usually U dc 1.4-2 times of Ta1 Flow through thyristor T a1 of current.

[0093] Fault current i at port 1 p1 and current flowing through thyristor T a1 The current i Ta1 It can be expressed as

[0094]

[0095] in

[0096]

[0097] After the current at port P1 drops to zero, the current limiting reactor L dc1 The energy in the battery is continuously discharged by the energy dissipation resistor R c At t6, the current limiting reactor L is completed. dc1 The energy discharge process in the thyristor. At this time, the current flowing through the thyristor T a1 The current i Ta1 drops to zero, the thyristor T a1 During t5-t6, the fault current flow path is as follows: Figure 6 As shown. Flowing through thyristor T a1 The current i Ta1 for

[0098]

[0099] Among them, i Ta1 (t5) can be obtained by formula (6).

[0100] Then, at t7, RCB1 is opened to achieve physical isolation of the converter station and the faulty line.

[0101] From the above analysis, it can be seen that the operating principle of the multi-port hybrid DC circuit breaker of the present invention during line fault will not affect the power transmission of the normal line. In addition, the multi-port hybrid DC circuit breaker of the present invention can use the above operating principle to achieve isolation of any fault line.

[0102] 2) Busbar failure

[0103] Assume that at time t0, a ground fault F2 occurs at the DC bus. Each port is connected to the RCB j -UFD j -LCS j The fault current is supplied to the DC bus. After a delay of Δt0, the protection device detects the fault at time t1. During t0-t1, the fault current flow path is as follows: Figure 7 The current i of each port is pj (j=1,…,n) is

[0104]

[0105] Among them, i prej is the current of each port before the fault occurs, L ej is the sum of the inductances from each port to the DC bus.

[0106] After the protection device detects a fault, it immediately turns on MB and thyristor Tu1 -T un Turn off LCS1-LCS at the same time n At time t2, the current flowing through LCS1-LCS n The current drops to zero and controls UFD1-UFD n Start opening the gate. UFD1-UFD n At t3, the rated opening distance is reached and the opening operation is completed. During t1-t3, the fault current flow path is as follows: Figure 8 During this period, the current i pj (j=1,2…n) still satisfies equations (1) to (3).

[0107] At time t4, MB is turned on and thyristor T is turned on at the same time. b1 -T bn Then the fault current flows into the MOV, and the MOV begins to discharge the energy stored in the fault current. a1 The conduction of the current limiting reactor L dcj is bypassed. Therefore, the current limiting reactor L dcj The energy in the energy dissipation resistor R c After Δt3 delay (energy discharge delay), the current i pj At t5, it drops to zero, achieving fault isolation. During t4-t5, the fault current flow path is as follows: Fig. 9 During this period, the following equation is satisfied

[0108]

[0109] Among them, i Tbj is the current flowing through the thyristor T bj current (j=1,…,n).

[0110] The current i of each port pj and current flowing through thyristor T bj The current i Tbj is represented as

[0111]

[0112] in

[0113]

[0114] When the current at each port i pj When the resistor discharges R c Continuously discharge the current limiting reactor L dcj The energy stored in the thyristor T at time t6 bj The current i TbjDuring this period, the fault current flows along the path Fig.10 As shown. Flowing through thyristor T bj The current i Tbj for

[0115]

[0116] Among them, i Tbj (t5) can be obtained by formula (10).

[0117] Then, at t7, RCB is turned on. j , port P j Can be physically isolated.

[0118] Based on the above working principle, it is obvious that the multi-port hybrid DC circuit breaker of the present invention has the ability to isolate each line from the DC bus during a DC bus fault, thereby achieving bus fault isolation.

[0119] 3. Performance and economic analysis of multi-port hybrid DC circuit breaker

[0120] 1) Performance analysis

[0121] The newly added energy discharge branch of the multi-port hybrid DC circuit breaker of the present invention enables the MOV to bypass the current limiting reactor when discharging energy, thereby accelerating fault isolation and shortening the time required for fault isolation. In addition, the energy discharge resistor R c Most of the energy stored in the current limiting reactor is released, thereby reducing the energy discharge requirement of the MOV. Therefore, the multi-port hybrid DC circuit breaker for realizing busbar and line fault isolation proposed by the present invention shows good performance in terms of fault isolation time and MOV energy discharge requirement.

[0122] 2) Economic analysis

[0123] When the DC bus connects n lines, each line needs to be equipped with a two-port hybrid DC circuit breaker. The two-port hybrid DC circuit breaker requires n UFDs, n LCSs, n RCBs, n MBs and n MOVs to achieve fault isolation of any line. The port hybrid DC circuit breaker of the present invention shares one MB, which requires n UFDs, n LCSs, n RCBs, n thyristor branches, one MB and one MOV. In addition, in order to speed up the fault isolation process and reduce the MOV energy discharge demand, n energy discharge branches are also required. Because there are a large number of expensive IGBTs in the MB. When the DC bus connects more lines, the multi-port hybrid DC circuit breaker of the present invention requires fewer IGBTs than the two-port hybrid DC circuit breaker. Therefore, the multi-port hybrid DC circuit breaker of the present invention has better economic performance.

[0124] Although the embodiments and drawings of the present invention are disclosed for illustrative purposes, those skilled in the art will appreciate that various substitutions, changes and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. Therefore, the scope of the present invention is not limited to the contents disclosed in the embodiments and drawings.

Claims

1. A multi-port hybrid DC circuit breaker for isolating busbar and line faults, characterized in that: The topological structure of the multi-port hybrid DC circuit breaker includes: n load transfer switches including n LCSs connected in parallel; n fast disconnecting switches, including n UFDs connected in parallel; n residual current switches, including n RCBs connected in parallel, for physically isolating the converter station from the fault line after the fault current drops to zero; n current limiting reactors, including n parallel connected L dc ; 1 main disconnect switch MB, used to achieve initial fault isolation; n energy discharge branches; n thyristor branches, used to select the flow path of the fault current; The n load transfer switches and fast isolating switches constitute the current-carrying branches of the n lines, and are used to conduct the load current; The energy discharge branch is composed of a thyristor and an energy discharge resistor R connected in anti-parallel c It is used to discharge the energy in the current limiting reactor.

2. The multi-port hybrid DC circuit breaker for isolating busbar and line faults according to claim 1, characterized in that: The multi-port hybrid DC circuit breaker is installed in a ±500kV three-terminal flexible DC transmission system. When the system is operating normally, LCS1-LCS n 、UFD1-UFD n and RCB1-RCB n The LCS-UFD-RCB is in the closed state, and the other devices are in the open state. Each line transmits power through the LCS-UFD-RCB. When a system failure occurs, the working process of the multi-port hybrid DC circuit breaker to achieve fault isolation is as follows: 1) Line failure Assume that at time t0, a ground fault F1 occurs at the P1 port of the multi-port hybrid DC circuit breaker. After the fault detection time Δt0 delay, the protection device detects the fault at time t1. During t0-t1, the current i pj (j=1,2…n) keeps rising, i pj can be solved as: Assume that in normal operation the current i at port 1 is p1 for i pre1 , then during t0-t1, the current i p1 for: in: Where, L ti is the equivalent inductance of the overhead line, L l is the line inductance from port P1 to the fault point; After the protection device detects a fault, it immediately turns on MB and thyristor T d1 At the same time, LCS1 is turned off, and the fault current starts to flow to MB. After a Δt1 commutation delay, the fault current flowing through LCS1 drops to zero at t2. At this time, UFD1 is controlled to start opening. After a Δt2 UFD opening delay, UFD1 reaches the rated opening distance at t3 and completes the opening operation. During t1-t3, the equivalent circuit of the system is consistent with the equivalent circuit of the system during t0-t1. During t1-t3, the current i pj (j=1,2…n) still satisfies equations (1)-(3); After UFD is successfully opened, at time t4, MB is turned on and thyristor T is turned on. a1 Then, the fault current flows into the MOV, and the MOV begins to discharge the energy stored in the fault current. a1 The conduction of the current limiting reactor L dc1 The energy in the energy dissipation resistor R c After the energy discharge delay of Δt3, the fault current of port P1 drops to zero at t5, and fault isolation is achieved. During t4-t5, the following formula is satisfied: Where: i p1 (t4) can be obtained by formula (3), U MOV The action voltage of MOV is U dc 1.4-2 times of i Ta1 Flow through thyristor T a1 The current, Fault current i at port 1 p1 and current flowing through thyristor T a1 The current i Ta1 It can be expressed as: in: After the current at port P1 drops to zero, the current limiting reactor L dc1 The energy in the battery is continuously discharged by the energy dissipation resistor R c Discharge, at t6, the current limiting reactor L is completed dc1 The energy discharge process in the thyristor T a1 The current i Ta1 drops to zero, the thyristor T a1 Natural shutdown; during t5-t6, the current flowing through the thyristor T a1 The current i Ta1 for: where i Ta1 (t5) can be obtained by formula (6); At t7, RCB1 is opened to achieve physical isolation between the converter station and the faulty line; 2) Busbar failure Assume that at time t0, a ground fault F2 occurs at the DC bus, and each port is connected to the RCB j -UFD j -LCS j The fault current is provided to the DC bus. After a delay of Δt0, the protection device detects the fault at time t1. During the period t0-t1, the current i pj (j=1,…,n) is Among them, i prej is the current of each port before the fault occurs, L ej is the sum of the inductance from each port to the DC bus; After the protection device detects a fault, it immediately turns on MB and thyristor T u1 -T un Turn off LCS1-LCS at the same time n , at time t2, flows through LCS1-LCS n The current drops to zero and controls UFD1-UFD n Start opening, UFD1-UFD n At t3, the rated opening distance is reached and the opening operation is completed. During t1-t3, the current i pj (j=1,2…n) still satisfies equations (1) to (3), At time t4, MB is turned on and thyristor T is turned on at the same time. b1 -T bn Then the fault current flows into the MOV, and the MOV begins to discharge the energy stored in the fault current. a1 The conduction of the current limiting reactor L dcj is bypassed, so the current limiting reactor L dcj The energy in the energy dissipation resistor R c After Δt3 energy discharge delay, the current i of each port is pj It drops to zero at t5, achieving fault isolation. During t4-t5, the following equation is satisfied: Among them, i Tbj is the current flowing through the thyristor T bj The current (j=1,…,n), The current i of each port pj and current flowing through thyristor T bj The current i Tbj is represented as in When the current at each port i pj When the resistor discharges R c Continuously discharge the current limiting reactor L dcj The energy stored in the thyristor flows through the thyristor T at time t6. bj The current i Tbj drops to zero, flowing through the thyristor T bj The current i Tbj for Among them, i Tbj (t5) can be obtained by formula (10); Open RCB at t7 j , port P j Can be physically isolated, Based on the above working process, the multi-port hybrid DC circuit breaker has the ability to isolate each line from the DC bus during a DC bus fault, thereby achieving bus fault isolation.

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