Overvoltage Simulation Analysis and Suppression of Breaking in a 35 kV Shunt Reactor

Abstract

When a 35 kV distribution network has the problem of insufficient reactive power, the input of a shunt reactor is a common compensation method. Vacuum circuit breakers are widely used in 35 kV distribution networks because of their superior arc extinguishing performance and convenient maintenance. However, in recent years, accidents involving vacuum circuit breakers breaking shunt reactors have occurred more frequently in China, such as high-frequency phase-to-phase short circuits, inter-turn burning losses, bus outlet short circuits, etc., which can cause serious damage and pose a greater threat to the safety of the power system. This paper focuses on the switching overvoltage generated by the vacuum circuit breaker cutting off the shunt reactor. Firstly, the mechanism of overvoltage generation is analyzed theoretically. It is concluded that the equivalent chopping current of the other two phases caused by the continuous reignition of the first open phase is the root cause of the high-amplitude interphase overvoltage. Based on the MODELS custom programming module in EMTP/ATP, according to the process of breaking and reigniting the circuit breaker, this paper uses Fortran language to compile the program and establishes a model of a vacuum circuit breaker, including power frequency current interception, high-frequency current, zero-crossing, breaking, and arc reignition modules. The vacuum circuit breaker is simulated for hundreds of continuous reignitions in milliseconds. Finally, a simulation study on the overvoltage suppression measures of a 35 kV shunt reactor is carried out. The comprehensive comparison of various suppression measures provides a reference for the reasonable selection of actual engineering conditions.

1. Introduction

In recent years, urban environmental construction has paid more attention to the beautification of cities. Therefore, cables are increasingly used in 10 kV~35 kV distribution networks to replace overhead lines, so more reactive power compensation is needed to absorb the charging capacitive reactive power to maintain the stability of the system. When a 35 kV distribution network has the problem of insufficient reactive power, a shunt reactor is generally used to compensate; the switching can be operated in real time according to the voltage monitoring state of the power grid, and the number of shunt reactors can be controlled [1,2]. However, due to the excellent arc extinguishing performance of vacuum circuit breakers, a current interception phenomenon will occur when the reactor is disconnected. According to some accident records, it can be seen that the amplitude of the reignition current and overvoltage is very serious when the shunt reactor is disconnected, which causes great safety hazards to the operation of the power grid. The most common faults are interphase short circuits and inter-turn short circuits [3,4]. Compared with the switching overvoltage during the breaking process, the switching overvoltage during the closing of the shunt reactor is not enough to pose a security threat. Usually, in the process of breaking a shunt reactor, there are many overvoltage accidents caused by repeated reignition between the two poles of the circuit breaker [5]. Therefore, this paper discusses only the breaking overvoltage of shunt reactors.

Since 2005, there have been many accidents when vacuum circuit breakers have been used to break shunt reactors in 35 kV power grids in China [6]. In 2010, many faults occurred continuously in the breaking process of 35 kV shunt reactors in a power supply company in Zhejiang Province. It was found that the main characteristics were damage to the bus side and short circuits of the main transformer outlet [7]. Until 2024, there were 93 sets of 35 kV shunt reactors in operation in the Zhejiang power grid, which resulted in a very serious problem of switching overvoltage caused by breaking 35 kV shunt reactors [8]. The problem of the operating overvoltage of 35 kV shunt reactors is prominent, and when the protection measures commonly used at the 10 kV level are copied, the suppression effect is not obvious [9,10].

According to the mechanism of current interception, the general current interception overvoltage will not produce a high-amplitude phase-to-phase overvoltage. Therefore, it is of great significance that we further understand the mechanism of switching overvoltage generated during the interruption of a reactor by a vacuum circuit breaker, fundamentally understand the phenomenon, and formulate measures or combination schemes to limit the interruption overvoltage of shunt reactors so as to reduce the probability of related accidents, ensure the safety of the power supply, improve the reliability of the power supply, and reduce economic losses. This paper focuses on the 35 kV shunt reactor breaking overvoltage. In view of the current proposed causes of current interception and multiple reignition, the possible causes of the strong shunt reactor removal overvoltage caused by the current interception overvoltage, the reignition overvoltage, and the equivalent current interception overvoltage are considered. Through the simulation of the circuit model with lumped parameters, the three overvoltage generation mechanisms are theoretically analyzed and simulated. The advantages and disadvantages of the various suppression measures are compared and economic and feasible solutions are provided.

2. Analysis of Phase-to-Phase Overvoltage Caused by Shunt Reactor Breaking

The current proposed mechanism for the breaking overvoltage of 35 kV shunt reactors includes a current-blocking overvoltage, a reburning overvoltage, and an equivalent current-blocking overvoltage. This section uses EMTP/ATP version 6.0 to carry out a preliminary simulation analysis of these three overvoltages. In addition, since these accidents occur frequently in the empty bus condition, the overvoltage of the system is mainly discussed when the system is in the empty bus condition.

The simulation model uses the neutral-point ungrounded system shown in Figure 1 to disconnect the shunt reactor.

2.1. Cut-Off Current Overvoltage

In terms of the causes of the 35 kV shunt reactor switching overvoltage, some literature attributes this to the chopping overvoltage caused by breaking the small current. Because of the strong arc extinguishing ability of vacuum circuit breakers, the contact material selected has less air content and a high corrosion resistance. When the current is close to zero, the amount of metal vapor released is not enough to support the combustion of the arc, and the arc begins to fluctuate and be forced to cross zero, resulting in current interception. As Figure 2 shown, because the voltage and current cannot be mutated, the current interception may cause severe electromagnetic oscillation and the energy of the inductance magnetic field and the electric field energy of the capacitor are exchanged, resulting in current interception overvoltage.

In EMTP/ATP, a simple time-controlled switch (TSWITCH) is used as the vacuum circuit breaker model. The time-controlled switch is set to issue an interruption command at 8 ms, and the current cut-off value is set to 3 A. At this time, the reignition situation is not considered and the arrester is not set. For a 35 kV system, the reference value of the switching overvoltage is 33.1 kV [11]. The simulation results are shown in Figure 3.

It can be obtained from the simulation that when the cut-off value is set to 3 A, the parallel-to-ground overvoltage is 45.3 kV (1.37 p.u.), the parallel-to-phase overvoltage is 77.61 kV (2.34 p.u.), the recovery voltage multiple of the first open phase A phase fracture is 102.9 kV (3.11 p.u.), and the bus-to-ground overvoltage is 57.6 kV (1.74 p.u.).

The peak value of the recovery voltage rise rate of the circuit breaker fracture is about 560 kV/ms by a simple calculation with the differentiator in ATP (TACS: DEVICE59). The recovery voltage rise rate of the circuit breaker fracture is shown in Figure 4. Generally, the rising speed of the recovery voltage of the first open phase fracture is between ~300 and ~600 kV/ms and the insulation strength of the vacuum interrupter is about 20~50 kV/mm. In the optimal case of 50 kV/mm, the breaking speed of the 35 kV vacuum circuit breaker is about 1.6 m/s and the growth rate of the insulation strength at both ends of the vacuum circuit breaker is 80 kV/ms, which is far less than the rising speed of the recovery voltage of the fracture. Therefore, the reignition of the first open phase is unavoidable.

2.2. Restrike Overvoltage

Following is the analysis and simple simulation calculation of the reburning overvoltage. When the time-controlled switch is set to 8 ms, the instruction to cut off the shunt reactor is issued. The arc is extinguished after the phase A current passes zero. After 0.02 ms, the arc is reignited for 0.01 ms and then disconnected. The three-phase current flowing through the vacuum circuit breaker is shown in Figure 5.

The simulation results of the phase A reignition overvoltage are shown in Figure 6.

Now set to different reburning times, the overvoltage size is compared under different reburning times under the empty bus condition. The calculation results are shown in

Table 1. The magnitude of overvoltage at different reignition times.

	Reburning Times	Normal Breaking	1	2	3	4	5
Overvoltage (kV)
The first open phase and anti-side-to-ground voltage		42.7	46.0	49.5	54.8	60.5	66.8
The first open phase bus-side-to-ground voltage		56.5	58.0	58.8	60.7	62.2	64.5
Phase-to-phase overvoltage on the parallel side	A-B	77.6	81.9	86.4	93.3	100.7	108.9
	A-C	77.4	81.7	86.2	93.2	100.5	108.7
	B-C	57.2	57.2	57.2	57.2	57.2	57.2

.

The simulation results show the following:

(1)

There is no obvious change in the overvoltage when the first open phase is reignited, and it can be judged that the overvoltage caused by the reignition is not strong.

(2)

With the increase in the number of reignitions in the first open phase, the reignition current increases continuously and the anti-side-to-ground overvoltage shows an upward trend. The repeated reignition is equivalent to continuously replenishing the energy, bringing about a step-up in the voltage.

(3)

With the increase in the number of reignitions in the first open phase, the phase-to-phase overvoltage between the first open phase A and the other two phases B and C on the anti-side will increase. However, in the simulation calculation, the overvoltage of the latter two phases is small: the B-C phase-to-phase overvoltage multiple is only 1.73 p.u., and with the increase in the number of repeated reignitions, it does not change much.

(4)

The overvoltage of the bus side relative to the ground increases with the increase in the number of reignitions in the first phase.

However, according to the research literature, it is found that the interruption of the shunt reactor will also cause a high overvoltage on the bus side. In addition, the relative ground and phase-to-phase overvoltage of the other two phases (except for the first open phase) is not large, which cannot explain all of the phase-to-phase overvoltage accidents. Therefore, the phenomenon of arc reignition alone is not the root cause of the 35 kV shunt overvoltage accidents.

2.3. Equivalent Cut-Off Overvoltage

It is well-known that the current of each phase differs by 120°. Therefore, after the command circuit breaker is switched off at a certain moment, there must be a phase current passing through the zero point to extinguish the arc. This phase is called the first open phase, and the two phases passing through the zero point are called the second open phase. Figure 7 is a diagram of a recording from a power supply company in a city. It is a typical waveform of the repeated reignition in the first phase leading to the equivalent interception of the latter two phases.

From the above diagram, it can be found that the phase B current of the first open phase crosses zero in 575~576 ms and then reignites continuously for about 3.5 ms. The voltage step-up effect caused by the reburning of the first open phase B phase is very significant in the above figure. As the reburning occurs repeatedly, the oscillation becomes more and more intense. At this time, although the A and C phases have not passed zero, the high-frequency transient current is coupled to the current of the latter two phases A and C through the interphase coupling, so there is also a high-frequency oscillation of the current. At this time, when the high frequency passes through the zero point, the circuit breaker extinguishes the arc in advance, which leads to a situation where the power frequency current of the latter two phases has not passed zero but is forced to break. The magnetic field energy of the A and C two-phase cut-off value will be converted into the electric field energy of the side-to-ground capacitance. This principle is similar to the principle of the previous cut-off overvoltage. One is the power frequency current cut-off, and the other is the high-frequency current cut-off, which can be known from the previous simple simulation calculation and analysis. When the cut-off value is in an extreme range, the overvoltage is very serious.

In summary, the mechanism of the equivalent cut-off overvoltage is that when the first open phase is reignition, the transient current is superimposed on the current of the latter two phases due to the coupling between the three phases. After the reignition occurs, the strength of the fracture voltage increases, the transient oscillation increases continuously, and the high-frequency zero-crossing point of the latter two-phase current occurs. The circuit breaker can break the high-frequency current after continuous processing, so the high-frequency current is cut off and the effect of the overvoltage is similar to that of the circuit breaker. However, for the circuit breaker, the current cut-off is high-frequency, zero-crossing, and arc extinguishing, and no power frequency cut-off occurs. However, due to the open fracture, the current oscillates only between the shunt side capacitance and the reactance inductance for energy exchange, resulting in the same effect as the cut-off overvoltage, which is called the equivalent cut-off overvoltage.

3. Simulation of a 35 kV Shunt Reactor Breaking Overvoltage

The simulation results in the previous section show that the simple cut-off and reignition overvoltage cannot explain the current 35 kV parallel anti-breakover overvoltage accidents. Therefore, this section establishes a vacuum circuit breaker zero-crossing breaking and arc reignition module to simulate the working process of the vacuum circuit breaker. The equivalent cut-off overvoltage is also simulated and calculated.

3.1. Modeling of Vacuum Circuit Breaker

This simulation refers to the research results of various research scholars in China and elsewhere. The working process of a vacuum circuit breaker is divided into four modules: circuit breaker breaking module, dielectric dynamic insulation strength module, arc reignition judgment module, and signal start output module, which are programmed as shown in Figure 8.

(1)

Circuit breaker breaking module

Judging that the current passes zero and satisfies the condition of arc extinguishing, the command switch is disconnected. The current zero-crossing judgment module uses the change in the positive and negative symbols of the current value to make this judgment. The calculation step Δt (1 × 10⁻⁸) set in EMTP is much smaller than the period of the power frequency current. Therefore, it can be judged whether the current passes the zero point at this time by judging the positive and negative sign changes of the current at time t and time t − Δt [12]. Firstly, the current signal of the circuit breaker is taken out through the TACS component, and the current signal is delayed by the delayer (TACS: DEVICE53) to output after Δt. If the product of I_t and I_t-Δt (TACS: MULT2) is less than zero, the current zero-crossing point is indicated.

The conditions for judging the interruption of the vacuum circuit breaker also require the high-frequency interruption ability of the vacuum circuit breaker to be set in the program. The ability of the vacuum circuit breaker in relation to the zero-crossing and arc extinguishing of the high-frequency current is related to the change rate of the current near the zero-crossing point to time. Only when the di/dt near the zero-crossing point is less than a certain value can it be successfully broken. After consulting the data, the value of the change rate is generally 50~300 A/us, and 300 A/us is taken in this simulation.

In addition, at the power frequency moment, when the current is about to pass through the first zero-crossing point, the current will fluctuate, and the process of cutting the current off after its oscillation is more complicated. It is difficult to simulate the specific changes in the simulation, and it is of little significance to the accuracy of the simulation. Therefore, the cut-off is directly defined in the simulation as less than the set cut-off value jumps to zero. The cut-off of the vacuum arc is due to the instability of the cathode process [13]. In view of the fact that the current cut-off value of the vacuum circuit breaker can be controlled below 5 A, it is 3 A in this paper.

(2)

Dielectric insulation strength and reignition judgment process

After the breaking of the circuit breaker, the arc gap becomes an insulating medium, and the opening speed and the insulation strength of the vacuum interrupter jointly affect its dynamic dielectric insulation strength. The recovery of dielectric insulation strength is a dynamic process with the increase i the breaking distance of the circuit breaker. In the simulation, it is fitted into a function curve around the breaking time. In this simulation, referring to the dynamic insulation strength curve of the medium in [14], the formula fitted by the test data is selected. The fitting formula for the dynamic insulation strength u_d (kV) and the circuit breaker breaking time t_d (ms) is Formula (1).

$$\left\{\begin{array}{l}{u}_d=23.5\times t_d^{0.846}\left(0<t_d\le 0.44\right)\hfill \\ u_d=36.1\times t_d^{1.440}\left(0.44<t_d\le 1.45\right)\hfill \\ u_d=23.5\times t_d^{0.846}\left(1.45<t_d\le 5\right)\hfill \end{array}\right.$$

(1)

The time between the rigid separation of the 35 kV vacuum circuit breaker contact and the first zero crossing of the current is about 0~3.33 ms, and 1.5 ms is taken in this simulation. The dynamic insulation strength u_d at the current time is calculated and compared with the fracture recovery voltage. If the judgment is less than the latter, the switch is closed to simulate the arc reignition phenomenon.

According to the above elements, the vacuum circuit breaker model constructed in EMTP/ATP is shown in Figure 9. The model is controlled by a MODEL module and an SW_TACS switch. Taking phase A as an example, the MODEL module contains five circuit inputs (the voltage at both ends of the circuit breaker, the current flowing through the circuit breaker, the zero-crossing judgment of the current, and the change rate of the current to time). The current flowing through the circuit breaker (Ia position in Figure 9) is used to determine the power frequency cut-off, and the current zero-crossing judgment (A position in Figure 9) is used to determine the zero-crossing point of the high-frequency current. There are two data outputs (the fracture recovery voltage and the fracture dielectric insulation strength) and one switch control output (controlling the opening and closing of the SW_TACS switch).

The flow chart for the specific vacuum circuit breaker working programming is shown in Figure 10. I is the current flowing through the circuit breaker, I_mar is the cut-off value, U_d is the recovery strength of the insulating medium, U_f is the recovery voltage of the fracture, di/dt is the change rate of the current to time, and T_s is the minimum arcing time.

When the breaking command of the circuit breaker is issued, it is judged whether the power frequency current is lower than the cut-off value, and, if so, the circuit breaker is broken. After the circuit breaker becomes open, the recovery strength of the insulating medium between the breaks of the vacuum circuit breaker increases with the increase of the opening distance of the break. At the same time, the recovery voltage of the break of the circuit breaker is also enhanced. It is judged whether the recovery strength of the insulating medium is less than the recovery voltage of the break. If it is lower, the circuit breaker is closed to simulate arc reignition. If it is greater, it is continuously judged.

After the arc is reignited, the current is judged to be zero-crossing. This simulation judges whether the current is zero-crossing by comparing the positive and negative symbol changes of the two currents at the time t and the time after a step delay. If I_Δt × I_t-Δt < 0, the current is zero-crossing. Another condition of breaking is the ability of the vacuum circuit breaker to effect zero-crossing arc extinguishing of the high-frequency current, which is related to the change rate of current to time near the zero-crossing point of the current. Only when the di/dt near the zero-crossing point is less than a certain value can the breaking condition be satisfied. In addition, considering the actual situation, the minimum arcing time is set, and the circuit breaker meets the breaking condition only when it is greater than the minimum arcing time. If the above criteria are satisfied, the circuit breaker returns to the breaking state and enters the cycle until the vacuum circuit breaker opening distance is large enough, and the recovery strength of the insulating medium between the breaks is always greater than the recovery voltage, then the vacuum circuit breaker is successfully broken.

3.2. Simulation of Equivalent Interception

When the first open phase continues to rekindle (as shown in Figure 11), the equivalent interception of the latter two phases will occur.

When the latter two phases of phase B and phase C occur at almost the same time, at about 12.5 ms, the current is basically the same and the direction is opposite, as shown in Figure 12a,b, so the polarity of the equivalent current-blocking overvoltage is also opposite.

After several simulations, it is verified that during the breaking process of the 35 kV shunt reactor, when the two-phase current of the post-opening phase is equivalently cut off, the current during the cut-off can be as high as 100 A or more, which leads to a fierce equivalent cut-off overvoltage. By analyzing the continuous reignition and equivalent current interception of the first open phase, it can be found that the cause of the overvoltage during the anti-breakdown is actually the result of the combined effect of the two, which cannot be completely separated.

3.3. Continuous Reburning to Equivalent Interception Simulation

When the two-phase equivalent current is cut off after the continuous reignition of the first open phase, there will be a violent relative ground and phase-to-phase overvoltage on the anti-side. Therefore, the relative ground, phase-to-phase, and inter-turn overvoltage on the anti-side are considered in the simulation. In addition, when the vacuum circuit breaker switches off the shunt reactor, only considering the overvoltage on the shunt reactor side is not enough to meet the protection requirements of all of the equipment in the substation. In many 35 kV shunt reactor removal accidents, strong overvoltages have caused bus-side equipment damage and failures. Therefore, when using the program for simulations, the three-phase ground and phase-to-phase overvoltage on the bus side are also calculated. The voltage waveform of the reactor and the bus side after breaking and resisting is shown in Figure 13.

In the case of normal operation of the bus and the shunt arrester, the relative ground overvoltage of the bus side is about 105.6 kV (3.19 p.u.), and the interphase overvoltage is about 200.5 kV (6.06 p.u.). The relative ground overvoltage of the shunt reactor side is 97.5 kV (2.95 p.u.), and the interphase overvoltage of the shunt reactor side is about 178.8 kV (5.40 p.u.). According to the simulation results, it can be seen that when the arc is repeatedly reignited, the vacuum circuit breaker breaks repeatedly, the energy is exchanged quickly, and a strong impact is generated on the bus side. The phase-to-phase overvoltage on the bus side can reach 6.06 p.u., which is the root cause of bus side overvoltage faults such as damage.

4. Suppression Measures for Breaking the Overvoltage of the 35 kV Shunt Reactor

From the above theoretical and simulation analysis, it can be seen that the strong switching overvoltage of the shunt reactor is mainly caused by the repeated reignition of the first open phase of the vacuum circuit breaker and the equivalent current interception of the latter two phases. Therefore, this section mainly studies the switching overvoltage suppression measures of the 35 kV shunt reactor, including whether to take the system outgoing line, the switching position of the circuit breaker, and some common ways to limit the overvoltage, such as installing the combined arrester and the resistance–capacitance absorber, etc., and simulates and analyzes their protection effects on the relative ground and interphase overvoltage of the bus side and the shunt reactor side.

4.1. Increase System Outgoing Lines

Due to the small capacitance of the bus side to the ground, the continuous multiple breakdowns of the circuit breaker will have a strong impact on the bus side. Increasing the system outgoing line is equivalent to increasing the bus-side ground capacitance, which can reduce the bus-side overvoltage. The effect of increasing the system outlet on breaking and anti-overvoltage suppression is shown in

Table 2. The influence of the bus-side system to ground capacitance on overvoltage.

Bus-Side System to Ground Capacitance		Empty Busbar	2	4
Bus-side overvoltage (kV)	Correspondingly	105.6	103.6	98.6
	Interphase	200.5	174.2	153.5
Parallel anti-side overvoltage (kV)	Correspondingly	97.5	95.4	91.3
	Interphase	178.8	178.7	173.0
Bus-side system to ground capacitance		6	8	10
Bus-side overvoltage (kV)	Correspondingly	97.9	96.4	93.8
	Interphase	157.4	156.6	147.5
Parallel anti-side overvoltage (kV)	Correspondingly	94.5	96.0	97.3
	Interphase	170.2	177.2	170.4

.

It can be seen from the simulation that when the capacitance of the bus-side system to the ground increases to a large enough value, the bus-side overvoltage relative to the ground will generally not cause harm again. This condition is very easy to achieve when the belt line is running, so the most simple and economical treatment method is to increase the system outgoing line. However, it can also be seen from the simulation calculation that increasing the outgoing line of the system can only reduce the risk of overvoltage at the bus side relative to the ground and has little effect on the overvoltage at the shunt side, and the overvoltage at the shunt side is only limited by the arrester within a certain range.

In practice, it is impossible to change power grid planning, etc., and it may be difficult to configure the 35 kV line. It is impossible to suppress and resist the breaking overvoltage solely from the control system. Therefore, it is necessary to study and find other technical measures to control the overvoltage.

4.2. Change the Circuit Breaker Switching Position

Now, the circuit breaker is placed on the local side of the terminal shunt reactor for switching to reduce the side-to-ground capacitance, as shown in Figure 14a. When the cable of the connecting line is long, the capacitance value of the shunt side is large, or the resistance–capacitance absorber is installed, the oscillation frequency generated by the shunt side and the reactance-forming circuit will be reduced after breaking, and the amplitude of the recovery voltage of the circuit breaker fracture can also be limited. The voltage drop is low and the number of reignitions is small, which is also beneficial to overvoltage protection, as shown in Figure 14b.

This section compares the influence of the conventional circuit breaker configuration method, pre-switching on the spot according to schematic diagram one, and switching on the neutral point after the spot according to schematic diagram two on the overvoltage. The simulation results are shown in

Table 3. Overvoltage magnitude under different switching configurations.

Circuit Breaker Position	Bus-Side Overvoltage (kV)		Parallel Anti-Side Overvoltage (kV)
	Correspondingly	Interphase	Correspondingly	Interphase	Turn to Turn
Routine	105.6	200.5	97.5	178.8	114.0
Schematic Figure 14a	51.9	78.1	79.6	115.6	85.2
Schematic Figure 14b	39.8	60.7	56.6	87.2	73.7

.

By comprehensively comparing the conventional position switching and the switching according to the first or second position of the schematic diagram, it can be found that the protection of the anti-side relative to the ground is better in the second configuration of the schematic diagram.

4.3. Install the Resistance-Capacitance Absorber

Increasing the shunt capacitance value of the reactor circuit can also reduce the overvoltage. The resistance–capacitance absorber has a simple structure (as shown in Figure 15) and is one of the more direct measures to suppress overvoltage. The principle of the resistance–capacitance absorber is to add capacitance and resistance to change the parameters of the system to reduce the oscillation of the high-frequency oscillation circuit and suppress the overvoltage oscillation and high-frequency current, so as to achieve the effect of suppressing the overvoltage.

Combined with the previous practical operational experience, the common parameters of R₁ + C₁ are 0.01 μF + 100 Ω, 0.1 μF + 100 Ω, and 0.2 μF + 50 Ω. Three kinds of resistance–capacitance absorbers with different capacities are installed on the shunt side and the bus side for comparison. The simulation results are shown in

Table 4. Overvoltage under different resistance–capacitance absorber configurations.

Resistance–Capacitance Absorber Configuration	Installation Site	Bus-Side Overvoltage (kV)
		Correspondingly	Interphase
No configuration	/	105.6	200.5
0.05 μF + 100 Ω	busbar side	43.5	69.6
	And anti-side	88.1	165.3
0.1 μF + 100 Ω	busbar side	41.7	70.8
	And anti-side	89.1	164.8
0.2 μF + 50 Ω	busbar side	37.3	62.8
	And anti-side	89.4	168.1
Resistance–capacitance absorber configuration	Installation site	Parallel anti-side overvoltage (kV)
		Correspondingly	Correspondingly
No configuration	/	97.5	97.5
0.05 μF + 100 Ω	busbar side	77.8	77.8
	And anti-side	88.7	88.7
0.1 μF + 100 Ω	busbar side	77.7	77.7
	And anti-side	87.7	87.7
0.2 μF + 50 Ω	busbar side	77.9	77.9
	And anti-side	89.3	89.3

.

From the above simulation results, it can be concluded that when the resistance–capacitance absorber is configured on the bus side, the overvoltage risk on the bus side can be effectively eliminated. At the same time, the simulation also shows that the resistance–capacitance absorber is installed on the shunt side. The overvoltage on the bus side and the shunt side is reduced to a certain extent, but it still causes a relatively strong overvoltage, especially interphase overvoltage, and the protection effect is not good.

4.4. Install Combined Arrester

The measures commonly used to limit the overvoltage of zinc oxide arrester (MOA) are composed of non-linear valves in series inside the porcelain sleeve. The main material of the valves is zinc oxide and some other oxides, which are called zinc oxide arresters. The main structures are four-star arresters and six-phase arresters, and the structure diagram is shown in Figure 16. When the rated voltage is working, the current flowing through the valve plate is small, which is equivalent to an insulator. When the voltage added to it exceeds the fixed value, the resistance of the valve plate is reduced and the valve plate is equivalent to being turned on. The residual voltage will limit the voltage to the withstand voltage of the protection device, thus protecting the device, and the process is reversible. After the voltage is restored, the valve plate will return to the original high-resistance state.

The simulation calculation does not set the arrester, the four star arrester, the six phase arrester, the bus side relative to the ground, the phase- to-phase and shunt side relative to the ground, or the phase-to-phase overvoltage size. The simulation results are shown in

Table 5. Comparison of the overvoltage before and after the installation of each arrester.

Arrester Configuration	Installation Site	Bus-Side Overvoltage (kV)
		Correspondingly	Interphase
No configuration	/	207.7	232.4
Three-star shaped lightning arrester	busbar side	114.4	219.0
	And anti-side	165.5	224.6
	Bus side + parallel resistance side	105.6	200.5
Four star lightning arrester	busbar side	112.0	140.3
	And anti-side	126.1	178.8
	Bus side + parallel resistance side	85.1	121.9
Six-phase lightning arrester	busbar side	106.7	140.0
	And anti-side	122.1	161.7
	Bus side + parallel resistance side	89.0	114.5
Arrester configuration	Installation site	Parallel anti-side overvoltage (kV)
		Correspondingly	Correspondingly
No configuration	/	165.9	165.9
Three-star shaped lightning arrester	busbar side	143.5	143.5
	And anti-side	106.9	106.9
	Bus side + parallel resistance side	97.5	97.5
Four-star lightning arrester	busbar side	135.6	135.6
	And anti-side	90.5	90.5
	Bus side + parallel resistance side	86.5	86.5
Six-phase lightning arrester	busbar side	129.3	129.3
	And anti-side	83.7	83.7
	Bus side + parallel resistance side	81.8	81.8

.

The conclusions obtained from the calculation results are as follows:

(1)

Not only can the four-star arrester and the six-phase arrester suppress the relative ground overvoltage in the same way as the three-star arrester, but they also have a good effect on the phase-to-phase overvoltage protection.

(2)

The installation of conventional arresters, four-star arresters, and six-phase arresters on the bus side mainly protects the bus-side overvoltage, which has a certain impact on the overvoltage of the shunt side but lacks protection; installation on the load side is also the main protection for the anti-side overvoltage, which has a certain impact on the bus side overvoltage, but the protection is insufficient.

(3)

The installation of arresters on the bus side alone cannot limit the overvoltage on the shunt side. Similarly, the installation of arresters on the shunt side alone cannot protect the overvoltage on the bus side, so the arresters on both sides are indispensable.

A comprehensive comparison of the main overvoltage suppression measures of the 35 kV shunt reactor breaking overvoltage is shown in

Table 6. Comprehensive comparison of the main overvoltage suppression measures.

Comparison of Measure	Main Protective Effects	Protect the Shortcomings	Capitalized Cost	Other Deficiencies
Increase system outgoing lines	busbar side	And the anti-side cannot be protected	Nil	The line configuration cannot be arbitrarily changed.
Change the switching position	Bus side + parallel resistance side	Transient frequency increases	General	Not applicable to all shunt reactors
resistance-capacitance absorber	busbar side	And the anti-lateral protection effect is poor	General	Harmonics may be generated There are no current standards
Four star lightning arrester	Parallel anti-side/bus side	Protect only one side	Lower	It has a partial pressure effect and insufficient reliability.
Six-phase lightning arrester	Parallel anti-side/bus side	Protect only one side	General	Fewer running data

.

There are many factors that affect the 35 kV breaking overvoltage. It is difficult to solve the overvoltage problem on the other side by using unilateral treatment measures. Therefore, a reasonable selection should be made in combination with the actual situation of the application.

5. Conclusions

In this paper, the mechanism of the breaking overvoltage of a 35 kV shunt reactor is simulated and analyzed, and the measures to suppress the breaking overvoltage of 35 kV shunt reactor are simulated and compared. The conclusions are as follows:

(1)

Because the root cause of the reignition of the first open phase is that the growth rate of the insulation strength at both ends of the vacuum circuit breaker is much smaller than the rising speed of the recovery voltage of the fracture, so the reignition is inevitable, the simple reignition overvoltage is not strong, and it is not enough to cause the accident to endanger safety. The root cause is the equivalent chopping overvoltage of the latter two phases caused by the repeated reignition of the first open phase.

(2)

The circuit breaker pre-switching or neutral-point switching is better for breaking and anti-overvoltage protection, but two circuit breakers need to be installed.

(3)

The resistance–capacitance absorber installed on the bus side can effectively eliminate the risk of overvoltage on the bus side, but it cannot prevent the occurrence of breaking overvoltage on the shunt side. Resistance–capacitance absorbers are installed on the shunt reactor side, and the overvoltage on the bus side and shunt reactor side is reduced to a certain extent, but it will still cause relatively strong overvoltage, especially phase-to-phase overvoltage.

(4)

The four-star arrester has obvious protection for relative ground and interphase overvoltage, but it has structural shortcomings, a high failure rate, and insufficient reliability. The effect of the installation of a six-phase arrester in a 35 kV system to suppress the breaking overvoltage of the shunt reactor is similar to that of the four-star arrester, but the reliability of the six-phase arrester is higher than that of the four-star arrester, so its comprehensive performance is better.