CN103441489A - Direct-current breaker used for multi-terminal direct current system and control method of direct-current breaker - Google Patents

Abstract

The invention discloses a DC circuit breaker for a multi-terminal DC system and a control method thereof. The circuit breaker includes a lightning arrester, a main switching branch and an auxiliary switching circuit; the lightning arrester is connected in parallel with the main switching branch; Each group of interrupting units includes a DC load switch and a thyristor valve section connected in series. Auxiliary switch circuits with the same structure are arranged in parallel at both ends of each group of current breaking units. Each set of auxiliary switch circuits includes a thyristor-reactance series branch, a diode, a capacitor and a resistor. The diode is connected in parallel at both ends of the interrupting unit, and the thyristor-reactance series branch and the capacitor are connected in parallel at both ends of the diode and grounded through a resistor. In the present invention, the DC current of the main switch branch is broken through the method of injecting reverse current, and the DC load switch in the structure of the DC circuit breaker is used to break the load current.

Description

A DC circuit breaker for a multi-terminal DC system and its control method

technical field

The invention belongs to the technical field of power electronics, and in particular relates to a DC circuit breaker for a multi-terminal DC system and a control method thereof.

Background technique

Multi-terminal DC transmission technology can realize multi-source power supply and multi-drop power receiving. It is a flexible, fast and economical power transmission method to meet the development needs of the electric power industry. The DC circuit breaker is a very important equipment in the multi-terminal DC transmission project. Only by using the DC circuit breaker in the multi-terminal DC system can the characteristics and advantages of the multi-terminal DC system be fully utilized. However, the DC circuit breaker withstands extremely high voltage during the process of breaking the DC current. The energy to be absorbed is particularly large, requiring fast breaking speed, high reliability, and breaking bidirectional current. At present, some scholars are studying its corresponding DC circuit breaker.

Contents of the invention

Aiming at the deficiencies of the prior art, the present invention proposes a DC circuit breaker for a multi-terminal DC system and its control method. The core is to break the DC current by superimposing the reverse current based on the current transfer principle, realizing the bidirectional breaking of the current, which is a DC circuit breaker. The development of the circuit breaker provides a new technical route.

A DC circuit breaker for a multi-terminal DC system provided by the present invention is improved in that it includes a lightning arrester, a main switching branch and an auxiliary switching circuit; the lightning arrester is connected in parallel with the main switching branch;

The branch of the main switch is composed of two sets of interrupting units with the same structure connected in series, which are respectively the first interrupting unit and the second interrupting unit; the first interrupting unit includes a first DC load switch and a first thyristor connected in series Valve section; the first DC load switch includes a parallel AC circuit breaker BRK1 and a first LC series branch; the second current breaking unit includes a second DC load switch and a second thyristor valve section connected in series; the The second DC load switch includes a parallel AC circuit breaker BRK2 and a second LC series branch;

A first auxiliary switch circuit is connected in parallel at both ends of the first current breaking unit; a second auxiliary switch circuit is connected in parallel at both ends of the second current breaking unit;

The first auxiliary switch circuit includes a thyristor T12-reactance L2 series branch, a diode D1, a capacitor C1 and a resistor R1; the diode D1 is connected in parallel at both ends of the first current breaking unit; the thyristor T12-reactance L2 is connected in series The branch and the capacitor C1 are connected in parallel to the two ends of the diode D1, and grounded through the resistor R1;

The second auxiliary switch circuit includes a thyristor T22-reactance L1 series branch, a diode D2, a capacitor C2 and a resistor R2; the diode D2 is connected in parallel at both ends of the second current breaking unit; the thyristor T22-reactance L1 is connected in series The branch and the capacitor C2 are connected in parallel to the two ends of the diode D2 and grounded through the resistor R2.

Wherein, the first LC series branch includes a capacitor C3 and a reactance L3 connected in series; the second LC series branch includes a capacitor C4 and a reactance L4 connected in series.

Wherein, the first thyristor valve section includes a thyristor T1, a thyristor T11, a diode D11 and a resistor R3; the diode D11 is connected in parallel with the thyristor T1 and the thyristor T11 after being connected in series with the resistor R3; the thyristor T1 and the thyristor T11 is an anti-parallel structure; the direction of the diode D11 is opposite to that of the diode D1;

The second thyristor valve section includes a thyristor T2, a thyristor T21, a diode D21 and a resistor R4; the diode D21 is connected in parallel with the thyristor T2 and the thyristor T21 after being connected in series with the resistor R4; the thyristor T2 and the thyristor T21 are Anti-parallel structure; the direction of the diode D21 is opposite to that of the diode D2.

Wherein, in the first auxiliary switch circuit, the anode of the thyristor T12 is connected to the cathode of the diode D1, and its cathode is connected to one end of the reactance L2; one end of the capacitor C1 is connected to the anode of the diode D1 ; The other end of the capacitor C1 is connected to the other end of the reactance L2; the reactance L2 is grounded through the resistor R1;

In the second auxiliary switch circuit, the anode of the thyristor T22 is connected to the cathode of the diode D2, and its cathode is connected to one end of the reactance L1; one end of the capacitor C2 is connected to the anode of the diode D2; The other end of the capacitor C2 is connected to the other end of the reactance L1; the reactance L1 is grounded through the resistor R2.

Wherein, the number of the diode D1 and the thyristor T12 in the first auxiliary switch circuit is more than two;

The number of the diode D2 and the thyristor T22 in the second auxiliary switch circuit is more than two.

Wherein, the cathode of the diode D1 in the first auxiliary switch circuit is connected to the diode D2 in the second auxiliary switch circuit, and the anode is connected to the current-limiting reactor in the line.

The present invention provides a method for controlling a DC circuit breaker for a multi-terminal DC system based on another purpose. The improvement is that the method includes the following steps:

(1) Connect the DC circuit breaker to the line, and keep the thyristor T1, thyristor T2, thyristor T11, thyristor T21, thyristor T12 and thyristor T22 in the locked state, the AC circuit breaker BRK1 and AC circuit breaker BRK2 in the open state, and the capacitor is charged to the rated voltage;

(2) Turn on the DC circuit breaker;

(3) When a fault occurs, determine the type of current in the line, whether it is fault current or DC load current;

(4) According to different current types, DC circuit breakers are turned off in different ways.

Among them, the opening process of the DC circuit breaker in step (2) includes:

1) When turned on, close the AC circuit breaker BRK2 in the second breaking unit;

2) triggering and turning on the thyristor T21 of the second current breaking unit;

3) Close the AC circuit breaker BRK1 in the first breaker unit;

4) Turn on the thyristor T11 in the first current breaking unit.

Among them, step (4) performs shutdown control on the DC circuit breaker according to the current type, including:

For fault current:

① Trigger and conduct the thyristor T22 in the second auxiliary switch circuit;

② Turn on the AC circuit breaker BRK2 in the second circuit breaker unit;

③The arrester acts and absorbs energy;

④ Disconnect the AC circuit breaker BRK1 in the first circuit breaker unit;

For DC load current:

i) disconnecting the AC circuit breaker BRK2 in the second circuit breaking unit;

ii) The arrester operates and absorbs energy.

Among them, the opening process of the DC circuit breaker in step (2) includes:

1> When it is turned on, close the AC circuit breaker BRK1 in the first breaking unit;

2> triggering and turning on the thyristor T1 of the second current interruption unit;

3>Close the AC circuit breaker BRK2 in the second breaking unit;

4> Turn on the thyristor T2 in the second current breaking unit.

Among them, step (4) performs shutdown control on the DC circuit breaker according to the current type, including:

For fault current:

a) triggering and turning on the thyristor T12 in the first auxiliary switch circuit;

b) Opening the AC breaker BRK1 in the first breaker unit;

c) The arrester acts and absorbs energy;

d) disconnecting the AC circuit breaker BRK2 in the second circuit breaker unit;

For DC load current:

1) disconnecting the AC circuit breaker BRK1 in the first circuit breaker unit;

II) The arrester operates and absorbs energy.

Compared with the prior art, the beneficial effects of the present invention are:

The invention combines the ideas of forced commutation and current transfer breaking, and realizes the breaking of direct current in a large-capacity system.

The present invention adopts the semi-controlled device thyristor to form the DC circuit breaker topology, uses the semi-controlled device to replace the expensive full-controlled device, has low on-state loss, low manufacturing cost and heat dissipation requirements, and mature technology.

The invention can be applied to high-voltage and high-current occasions, has simple and compact topological structure, simple control and high expandability.

The invention uses a fast isolating switch to realize bidirectional DC current breaking in multiple working conditions, and the breaking process is fast and arc-free.

The invention utilizes the auxiliary switch circuit to realize the soft opening of the DC circuit breaker, and reduces the stress borne by the power device during the opening process.

The invention takes the lead in applying the DC load switch to the DC circuit breaker, which can directly complete the DC current cut-off and conversion under the load current working condition, enhance the service life of the DC circuit breaker, and increase its reliability.

The invention belongs to a hybrid high-voltage direct current circuit breaker, which combines the advantages of a mechanical direct current circuit breaker and a solid state direct current circuit breaker, is relatively less difficult to develop, and is suitable for various multi-terminal direct current systems.

The invention uses the circuit to charge the capacitor in the auxiliary switch circuit without additional charging equipment, thereby reducing the difficulty of electrical isolation between equipment, reducing the occupied area and cost, and being easy to realize engineering.

Description of drawings

Fig. 1 is a structural diagram of a DC circuit breaker provided by the present invention.

Fig. 2 is a schematic diagram of the waveform of the breaking fault current provided by the present invention.

Fig. 3 is a schematic diagram of the waveform of the breaking load current provided by the present invention.

Detailed ways

The specific implementation manners of the present invention will be further described in detail below in conjunction with the accompanying drawings.

A DC circuit breaker for a multi-terminal DC system proposed in this embodiment is connected to the power grid through current-limiting reactors connected in series at both ends. Its structure is shown in Figure 1, including a lightning arrester, a main switch branch and an auxiliary switch circuit; The arrester is connected in parallel with the main switch branch.

In the figure, the branch of the main switch is composed of two sets of first current breaking units and second current breaking units with the same structure connected in series. The first breaking unit includes the first DC load switch and the first thyristor valve section connected in series; the first DC load switch includes the high-voltage SF ₆ circuit breaker BRK1 connected in parallel and the first LC series composed of capacitor C3 and reactance L3 connected in series Branch circuit; the first thyristor valve section includes thyristor T1, thyristor T11, diode D11 and resistor R3; diode D11 and resistor R3 are connected in parallel with thyristor T1 and thyristor T11 respectively; thyristor T1 and thyristor T11 are in anti-parallel structure; diode D11 and Diode D1 is in the opposite direction. The second breaking unit includes the second DC load switch and the second thyristor valve section connected in series; the second DC load switch includes the high-voltage SF ₆ circuit breaker BRK2 connected in parallel and the second LC series branch composed of capacitor C4 and reactance L4 connected in series ; The second thyristor valve section includes thyristor T2, thyristor T21, diode D21 and resistor R4; diode D21 and resistor R4 are connected in parallel with thyristor T2 and thyristor T21 respectively; thyristor T2 and thyristor T21 are in anti-parallel structure; diode D21 and diode D2 in the opposite direction.

In this embodiment, a first auxiliary switch circuit is connected in parallel at both ends of the first current breaking unit, and a second auxiliary switching circuit is connected in parallel at both ends of the second current breaking unit.

As shown in Figure 1, the first auxiliary switch circuit includes a thyristor T12-reactance L2 series branch, a diode D1, a capacitor C1 and a resistor R1; the diode D1 is connected in parallel at both ends of the first current breaking unit; the thyristor T12-reactance L2 series branch The capacitor C1 is connected in parallel with the two ends of the diode D1, and is connected to the ground through the resistor R1.

The second auxiliary switch circuit includes a thyristor T22-reactance L1 series branch, a diode D2, a capacitor C2 and a resistor R2; the diode D2 is connected in parallel at both ends of the second interrupting unit; the thyristor T22-reactance L1 series branch The capacitor C2 is connected to the two ends of the diode D2 in parallel with the ground, and is connected to the ground through the resistor R2.

In the first auxiliary switch circuit, the diode D1 includes at least two diodes in series in the same direction (only one is marked in the figure), which are connected in parallel at both ends of the first current breaking unit, and the direction is the same as that of the diode D11 in the first thyristor valve section. in the opposite direction. The thyristor T12 is also composed of at least two thyristors connected in series, its anode is connected to the anode of the thyristor T1 in the first current breaking unit, the cathode of the thyristor T12 is connected to the reactance L2; the other end of the reactance L2 is connected to one end of the capacitor C1, and the capacitor C1 The other end is connected to the first DC conversion switch; the resistance R1 is grounded between the reactance L2 and the capacitor C1, the R1 plays a role of current limiting, and the reactance L2 plays a role of suppressing short-circuit current.

In the second auxiliary switch circuit, the diode D2 includes at least two diodes in series in the same direction (only one is marked in the figure), which are connected in parallel at both ends of the second current breaking unit, and the direction is the same as that of the diode D21 in the second thyristor valve section. in the opposite direction. The thyristor T22 is also composed of at least two thyristors connected in series, its anode is connected to the second DC switch in the second current breaking unit, the cathode of the thyristor T22 is connected to the reactance L1; the other end of the reactance L1 is connected to one end of the capacitor C2, The other end of the capacitor C2 is connected to the anode of the thyristor T2 in the second current breaking unit; the resistor R2 is grounded between the reactance L1 and the capacitor C2, and the R2 acts as a current limiter, and the reactance L1 acts as a short-circuit current suppressor.

In this embodiment, the cathode of the diode D1 in the first auxiliary switch circuit is connected to the diode D2 in the second auxiliary switch circuit, and the anode is connected to the current-limiting reactor in the line.

Correspondingly, this embodiment proposes a control method for a DC circuit breaker for a multi-terminal DC system. Since the structure of this circuit breaker is symmetrical, the breaking mechanism of bidirectional current flow is exactly the same. Only the forward direction is taken for principle analysis, and the reverse direction can be compared. The current direction _Idc shown in Figure 1 is the positive direction of the current. The opening and breaking process of the DC circuit breaker under different working conditions is described:

Working condition 1: breaking fault current

Opening process:

The capacitors C1 and C2 are charged through the auxiliary switch circuits C1-R and C2-R through the circuit and kept in a constant charged state, and the voltage is stable at the rated voltage level of the system. Close the SF6 circuit breaker BRK2, at this time the system DC current flows through the D1-BRK2-D21-R2 branch and then triggers the conduction of T21. Since D21 is a series of multiple diodes, the impedance is much larger than the isolation valve branch, so the current is quickly transferred to T21 Then close the SF6 circuit breaker BRK1 and turn on T11. Since D11 is also a series of multiple diodes, the impedance is much larger than that of the T11 branch, and the current is quickly transferred to the BRK1-T11-BRK2-T21 branch, and the current will flow through the circuit breaker. After stabilization and the voltages of capacitors C1 and C2 are stable, the system enters steady-state operation. At this point, the DC circuit breaker opening process ends, and the voltage direction of capacitors C1 and C2 is positive up and negative down, which is the same as the direction shown in the figure.

Breaking process:

When the DC circuit breaker is required to break the fault current of the line, T22 is triggered and turned on. Due to the effect of the capacitor voltage, the capacitor C2 quickly generates a pulse current opposite to the forward current of the line in the circuit formed by T21-BRK2-T22-inductor L2. Make the current in the thyristor T21 cross zero, because D2 is turned on to provide reverse voltage to T21, the thyristor T21 is turned off, and the SF6 circuit breaker BRK2 is opened in the state of no current; when the SF6 circuit breaker BRK2 recovers the blocking ability, the current is completely transferred Go to the BRK1-T11-T22-L2-C2 branch, and charge the capacitor C2 at the same time, so that the voltage of the capacitor C2 changes from positive at the top and negative at the bottom to positive at the bottom, which is opposite to the direction shown in the figure. When the voltage of the BRK1-T11-T22-L2-C2 branch reaches the MOV operating voltage of the arrester, the MOV of the arrester operates, and the current is transferred to the branch where the MOV of the arrester is located. When the charging current of the capacitor C2 is zero, the current is completely transferred to the MOV branch. The arrester MOV absorbs the energy stored in the line and the current-limiting inductance, the thyristor T22 is turned off, and the SF6 circuit breaker BRK1 is opened. When the current in the arrester gradually decreases to 0, the arrester returns to the blocking state, and the current flowing through the circuit breaker is completely broken. , so far the shutdown process ends.

Working condition 2: breaking steady DC load current

Opening process:

The capacitors C1 and C2 are charged through the auxiliary switch circuits C1-R and C2-R through the circuit and kept in a constant charged state, and the voltage is stable at the rated voltage level of the system. Close the SF6 circuit breaker BRK2, at this time the system DC current flows through the D1-BRK2-D21-R2 branch and then triggers the conduction of T21. Since D21 is a series of multiple diodes, the impedance is much larger than the isolation valve branch, so the current is quickly transferred to T21 Then close the SF6 circuit breaker BRK1 and turn on T11. Since D11 is also a series of multiple diodes, the impedance is much larger than that of the T11 branch, and the current is quickly transferred to the BRK1-T11-BRK2-T21 branch, and the current will flow through the circuit breaker. After stabilization and the voltages of capacitors C1 and C2 are stable, the system enters steady-state operation. At this point, the DC circuit breaker opening process ends, and the voltage direction of capacitors C1 and C2 is positive up and negative down, which is the same as the direction shown in the figure.

Breaking process:

When it is necessary to break the steady-state DC load current of the system, the load current can be cut off directly through the action of the DC conversion switch. Open the SF6 circuit breaker BRK2, and its parallel LC branch will superimpose self-excited oscillation current to the SF6 circuit breaker BRK2 branch, so that the current of the SF6 circuit breaker BRK2 branch appears zero-crossing and opens, and then the system current begins to flow to its parallel LC The capacitor of the branch is charged. When the capacitor voltage reaches the MOV operating voltage of the arrester, the MOV operates, and all the current is transferred to the branch where the MOV is located. The MOV absorbs the energy stored in the line and the current-limiting inductor, and the current in the arrester gradually decreases to 0. The lightning arrester returns to the blocking state, so far the coincident current flowing through the circuit breaker is completely broken, and the shutdown process ends.

As shown in Figure 2, it is a schematic diagram of the waveform of the breaking fault current. Through the control of the DC circuit breaker proposed in this embodiment, it can be seen from the figure that after 1.4 ms the main branch DC current is broken, the current is finally transferred to the arrester for energy dissipation.

As shown in Figure 3, it is a schematic diagram of the waveform of the breaking load current. Through the control of the DC circuit breaker proposed in this embodiment, it can be seen from the figure that after 1.3 ms the main branch DC current is broken, the current is finally transferred to the arrester for energy dissipation.

Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to the above embodiments, those of ordinary skill in the art should understand that: the present invention can still be Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention shall be covered by the scope of the claims of the present invention.

Claims (11)

1. A DC circuit breaker for a multi-terminal DC system, characterized in that it comprises a lightning arrester, a main switch branch and an auxiliary switch circuit; the lightning arrester is connected in parallel with the main switch branch; The branch of the main switch is composed of two sets of interrupting units with the same structure connected in series, which are respectively the first interrupting unit and the second interrupting unit; the first interrupting unit includes a first DC load switch and a first thyristor connected in series Valve section; the first DC load switch includes a parallel AC circuit breaker BRK1 and a first LC series branch; the second current breaking unit includes a second DC load switch and a second thyristor valve section connected in series; the The second DC load switch includes a parallel AC circuit breaker BRK2 and a second LC series branch; A first auxiliary switch circuit is connected in parallel at both ends of the first current breaking unit; a second auxiliary switch circuit is connected in parallel at both ends of the second current breaking unit; The first auxiliary switch circuit includes a thyristor T12-reactance L2 series branch, a diode D1, a capacitor C1 and a resistor R1; the diode D1 is connected in parallel at both ends of the first current breaking unit; the thyristor T12-reactance L2 is connected in series The branch and the capacitor C1 are connected in parallel to the two ends of the diode D1, and grounded through the resistor R1; The second auxiliary switch circuit includes a thyristor T22-reactance L1 series branch, a diode D2, a capacitor C2 and a resistor R2; the diode D2 is connected in parallel at both ends of the second current breaking unit; the thyristor T22-reactance L1 is connected in series The branch and the capacitor C2 are connected in parallel to the two ends of the diode D2 and grounded through the resistor R2. 2. The DC circuit breaker according to claim 1, wherein the first LC series branch comprises a capacitor C3 and a reactance L3 connected in series; The second LC series branch includes a capacitor C4 and a reactance L4 connected in series. 3. The DC circuit breaker according to claim 1, wherein the first thyristor valve section comprises a thyristor T1, a thyristor T11, a diode D11 and a resistor R3; and the diode D11 is connected in series with the resistor R3 respectively to the The thyristor T1 and the thyristor T11 are connected in parallel; the thyristor T1 and the thyristor T11 are in an anti-parallel structure; the direction of the diode D11 is opposite to that of the diode D1; The second thyristor valve section includes a thyristor T2, a thyristor T21, a diode D21 and a resistor R4; the diode D21 is connected in parallel with the thyristor T2 and the thyristor T21 after being connected in series with the resistor R4; the thyristor T2 and the thyristor T21 are Anti-parallel structure; the direction of the diode D21 is opposite to that of the diode D2. 4. The DC circuit breaker according to claim 1, characterized in that, in the first auxiliary switch circuit, the anode of the thyristor T12 is connected to the cathode of the diode D1, and its cathode is connected to one end of the reactance L2 connected; one end of the capacitor C1 is connected to the anode of the diode D1; the other end of the capacitor C1 is connected to the other end of the reactance L2; the reactance L2 is grounded through the resistor R1; In the second auxiliary switch circuit, the anode of the thyristor T22 is connected to the cathode of the diode D2, and its cathode is connected to one end of the reactance L1; one end of the capacitor C2 is connected to the anode of the diode D2; The other end of the capacitor C2 is connected to the other end of the reactance L1; the reactance L1 is grounded through the resistor R2. 5. The DC circuit breaker according to claim 1, wherein the number of the diode D1 and the thyristor T12 in the first auxiliary switch circuit is more than two; The number of the diode D2 and the thyristor T22 in the second auxiliary switch circuit is more than two. 6. The DC circuit breaker according to any one of claims 1-5, wherein the diode D1 in the first auxiliary switch circuit is connected to the cathode of the diode D2 in the second auxiliary switch circuit, and the anode is connected to the diode D2 in the second auxiliary switch circuit. Current limiting reactor connection in the line. 7. A method for controlling a DC circuit breaker for a multi-terminal DC system, characterized in that the method comprises the following steps: (1) Connect the DC circuit breaker to the line, and keep the thyristor T1, thyristor T2, thyristor T11, thyristor T21, thyristor T12 and thyristor T22 in the locked state, the AC circuit breaker BRK1 and AC circuit breaker BRK2 in the open state, and the capacitor is charged to the rated voltage; (2) Turn on the DC circuit breaker; (3) When a fault occurs, determine the type of current in the line, whether it is fault current or DC load current; (4) According to different current types, DC circuit breakers are turned off in different ways. 8. The control method according to claim 7, characterized in that the process of opening the DC circuit breaker in step (2) includes: 1) When turned on, close the AC circuit breaker BRK2 in the second breaking unit; 2) triggering and turning on the thyristor T21 of the second current breaking unit; 3) Close the AC circuit breaker BRK1 in the first breaker unit; 4) Turn on the thyristor T11 in the first current breaking unit. 9. The control method according to claim 8, characterized in that step (4) performs shutdown control on the DC circuit breaker according to the current type, including: For fault current: ① Trigger and conduct the thyristor T22 in the second auxiliary switch circuit; ② Turn on the AC circuit breaker BRK2 in the second circuit breaker unit; ③The arrester acts and absorbs energy; ④ Disconnect the AC circuit breaker BRK1 in the first circuit breaker unit; For DC load current: i) disconnecting the AC circuit breaker BRK2 in the second circuit breaking unit; ii) The arrester operates and absorbs energy. 10. The control method according to claim 7, characterized in that the process of opening the DC circuit breaker in step (2) includes: 1> When it is turned on, close the AC circuit breaker BRK1 in the first breaking unit; 2> triggering and turning on the thyristor T1 of the second current interruption unit; 3>Close the AC circuit breaker BRK2 in the second breaking unit; 4> triggering and turning on the thyristor T2 in the second current breaking unit. 11. The control method according to claim 10, characterized in that step (4) performs shutdown control on the DC circuit breaker according to the current type, including: For fault current: a) triggering and turning on the thyristor T12 in the first auxiliary switch circuit; b) Opening the AC circuit breaker BRK1 in the first circuit breaking unit; c) The arrester acts and absorbs energy; d) disconnecting the AC circuit breaker BRK2 in the second circuit breaking unit; For DC load current: 1) disconnecting the AC circuit breaker BRK1 in the first circuit breaker unit; II) The arrester operates and absorbs energy.

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