SF₆ Experimental Study on the Variation Characteristics of Closing Prebreakdown Duration during Contact Deterioration of Circuit Breakers

Abstract

The contact morphology change caused by high-current ablation will seriously affect the electric field distribution in the interrupter chamber, which in turn affects the closing prebreakdown arc duration, indicating that the prebreakdown arc duration can be used as one of the indicators to measure the contact ablation state. A circuit breaker simulated ablation test platform was established, and the voltage and current signals, electromagnetic field signals, and vibration signals in the process of circuit breaker closing were measured, and the closing prebreakdown duration was calculated. The results show that under the same size of the ablation current, with the ablation of the contacts, the closing prebreakdown duration shows an overall trend of increasing and then decreasing, and the larger the ablation current is, the larger the change in the closing prebreakdown duration is. At the same time, simulation verification was carried out, and the results show that the electric field distortion on the surface of the static arc contact inside the arc extinguishing chamber is the largest, and the ablation of the contact will further increase the degree of distortion of the electric field inside the arc extinguishing chamber. As the degree of ablation increases, the prebreakdown moment of the circuit breaker closing is advanced, and the prebreakdown duration increases.

1. Introduction

With the rapid development of the national economy, China’s demand for energy, especially the demand for electric energy, continues to increase. China is currently accelerating the construction of large-scale clean energy bases in the Northwest Wilderness, Southwest China, and other large-scale developments of new energy into a large-scale and high proportion of the new stage. In large-scale new energy, a high proportion of the development of the comprehensive benefits are huge, but for the power system, it will bring a series of challenges such as regulation difficulty, difficulty to consume, insufficient outgoing channel, complex operation risk, etc.; how to achieve a stable, safe, and reliable long-distance large-capacity power transmission has become the key [1,2,3].

Circuit breakers play a crucial control and protection role in power grid operation, and in addition to the need to be able to close, carry, and open the working current under normal conditions, they also need to carry, close, and open the current under abnormal working conditions within a specified time [4,5,6]. The SF₆ circuit breaker for a filter group operates more frequently than other substation circuit breakers, and under the high amplitude and high-frequency inrush current generated in the closing process, the contacts and nozzles of the interrupter chamber will be abraded and worn out, and the breaking performance will be seriously reduced [7,8]. In order to ensure the safe and reliable operation of electrical equipment, it is necessary to test and analyze the operating status of power equipment in a timely manner and keep up-to-date with the operation of the equipment. The insulation state of the circuit breaker interrupter chamber directly affects the opening and closing ability of the circuit breaker, in which the key factor affecting the insulation state of the interrupter chamber is the deterioration of the arc contacts. Early detection and treatment of circuit breaker insulation defects can avoid insulation failures and improve the operational reliability of the power grid. For circuit breaker insulation performance assessment, at present, in addition to the voltage test to determine whether it can be put into operation, there is still a lack of early insulation defects detection methods and detection means. Therefore, in order to effectively detect the circuit breaker opening performance and accurately assess its arc contact deterioration state so as to further improve the safety and reliability of grid operation, it is necessary to conduct further in-depth research on SF₆ circuit breaker contact state detection methods.

In the circuit breaker closing process, the dynamic arc contact from the breaking position gradually moves to the static arc contact. When the contact movement to the gap is small enough, the dynamic and static arc contact gap will occur prebreakdown and arc. When the dynamic and static contacts just close the contact, the prebreakdown arc disappears, and the current is transferred to the contacts to maintain the circuit conduction. For circuit breakers in the continuous opening and closing process, a closing inrush current and breaking short circuit current will continue to produce abrasion on the contacts, resulting in contact surface roughness, cracks, and nozzle surface condensation of metal particles and other phenomena, and even make the arc contact deformation, ultimately leading to contact erosion and becoming shorter and more pointed and rougher [9]. The morphological changes caused by contact ablation will seriously affect the electric field distribution within the arc extinguishing chamber, which will change the moment of the occurrence and the end of the prebreakdown and ultimately affect the duration of the closing prebreakdown arc; so, it can be considered as one of the indicators of contact ablation status.

Many scholars in China have carried out research on the closing prebreakdown characteristics of circuit breakers [10,11,12,13]. Ma Feiyue et al. established an electric–fluid coupling simulation model of the closing prebreakdown process in the interrupter room of a 126 kV SF₆ circuit breaker and investigated the closing prebreakdown characteristics in the process of contact degradation [14]. Peng Jing et al. studied the prebreakdown characteristics of double-break vacuum circuit breakers closing under a DC voltage and measured the prebreakdown opening distance of single-break and double-break vacuum circuit breakers closing under a DC voltage of different voltage levels [15,16]. The prebreakdown moment is greatly affected by the randomness and mechanical dispersion of the discharge. Lin Xin et al. carried out the calculation and experimental study of the prebreakdown characteristics of SF₆ circuit breaker closing and obtained the change rule of the prebreakdown voltage with time or opening distance [17]. Xie Yongjian et al. obtained the relationship between the prebreakdown gap and voltage at different voltages, respectively, and meanwhile studied the influence of closing speed on prebreakdown and comprehensively considered the influence of mechanical dispersion on the closing phase [18]. Qian Kang studied the prebreakdown characteristics of vacuum circuit breakers by conducting DC and industrial frequency AC prebreakdown experiments. After fitting the relationship equation between the breakdown voltage and gap of the vacuum circuit breaker, according to the experimental data, the influence of closing speed on the prebreakdown characteristics was investigated by analyzing the experimental data, and the withstand strengths under the condition of industrial frequency AC voltage and DC condition were compared in the same contact spacing [19].

However, the current study mainly focuses on intact contacts and does not consider the effect of contact ablation degradation on the closing prebreakdown characteristics. After frequent operation, the surface state of the contacts will certainly change greatly, affecting the electric field distribution, which in turn affects the prebreakdown characteristics, a key factor that must be considered for the option closing; no relevant research has been conducted yet. At present, the measurement of contact ablation state is mostly based on dynamic contact resistance; however, at present, the dynamic resistance measurement equipment is inconvenient to carry, and the extraction and processing methods of characteristic parameters have not yet been unified. Therefore, it is very important to propose new evaluation parameters that can assist the judgment of dynamic resistance and the evaluation of the contact ablation state of an SF₆ circuit breaker.

In this paper, a simulated ablation test was carried out for the LW36-126 SF₆ circuit breaker, and the voltage and current signals, electromagnetic field signals, and vibration signals in the process of circuit breaker closing were measured after the completion of each ablation. The closing prebreakdown duration was calculated on the basis of the moments corresponding to the appeal signals, and the phenomenon of the change in the three-phase closing prebreakdown duration with the number of times of ablation is analyzed at the same time. Finally, a simulation study was carried out for the interrupter chamber of this type of circuit breaker, calculating the dynamic change process of the electric field and flow field in the interrupter chamber during the closing process and investigating the influence of different contact ablation states on the closing prebreakdown arc duration, which provides a theoretical basis for the change of the closing prebreakdown duration in the process of circuit breaker ablation.

2. Experimental Setup

2.1. Simulated Ablation Test Rig

The test circuit breaker is an LW36-126 type SF₆ circuit breaker, equipped with a spring-operated mechanism and a rated short-circuit breaking current of 40 kA; it was filled with 8 kg of SF₆ gas with an SF₆ gas pressure of 0.6 MPa.

The simulated ablation test platform, shown in Figure 1, is divided into two parts: the left capacitor charging circuit and the right test circuit. The charging circuit contains a charging transformer T, current limiting resistor R, silicon stack D, and charging switch K, which can achieve control of the capacitor charging voltage and current. On the right side of the test circuit, the inductance L and capacitance C form an LC resonant circuit to provide the frequency short-circuit current. The closing circuit breaker used a ZN-72 vacuum circuit breaker for the input short-circuit current; the test circuit breaker opened the short-circuit current to complete the ablation. The inductance L is 0.15 mH, and the capacitance C is 67.2 mF; the maximum it can provide is a 63 kA test current by changing the capacitance voltage to control the amplitude of the ablation current. A Roche coil was used to measure the circuit current. Two high-voltage probes were connected to the upper and lower outlet plates of the circuit breaker to measure the fracture voltage, and an oscilloscope was utilized to record the arc voltage drop and current during the circuit breaker opening process.

At the beginning of the simulated ablation test, the test circuit breaker was adjusted to be located in the closed position, and the closed circuit breaker was in the open state. The capacitor was charged to the set voltage, and the circuit breaker was closed. The test circuit conduction generates an industrial frequency current, and then the specimen circuit breaker breaks the current to complete the ablation. Due to the inevitable existence of a certain resistance of the test circuit, resulting in LC resonant current attenuation, the third half-wave attenuation amplitude can reach 30%. Therefore, in order to ensure the effect of the current ablation, we need to make the test circuit breaker at the current around the first peak point of the breaking arc and at the second point before the zero point of the completion of the opening. Through the closing circuit breaker and the action time between the test piece circuit breaker to achieve the above conditions, taking into account the vacuum circuit breaker closing action time of 43 ms, the test piece circuit breaker breaking action time of 33 ms, the circuit breaker action signal was set, triggering time sequence shown in Figure 2.

2.2. Simulated Ablation Test Program

Domestic circuit breaker manufacturers have carried out a large number of opening tests on SF₆ circuit breakers and obtained the relationship curve between the permissible number of openings of the circuit breaker N_s and the opening current I_s, as shown in Equation (1).

$$N_s=\left\{\begin{array}{l}{\left(49.5/I_s\right)}^{3.46}\left(I_s<11\mathrm{kA}\right)\hfill \\ {\left(223.5/I_s\right)}^{1.76}\left(I_s\ge 11\mathrm{kA}\right)\hfill \end{array}\right.$$

(1)

where N_s is the actual maximum permissible number of openings and I_s is the actual opening current effective value, kA.

The maximum allowable number of times of ablation under different sizes of current is calculated in accordance with Equation (1), and different sizes of ablation currents are set according to the proportional commutation relationship so that the electrical life of the circuit breaker ends after comprehensive ablation. Each phase of circuit breaker is tested according to the sequence of low current first and high current ablation. The simulated ablation test ablation current and the number of times of ablation are shown in

Table 1. Simulated ablation test ablation current and number of times.

Current/kA	Number of A-Phase Ablations	Number of Ablations in Phases B and C
5	30	/
7.5	/	30
10	/	30
15	/	20
20	/	20
25	20	/
30	19	18

, in which the ablation current and the number of times of ablation of phases B and C are the same.

2.3. Measurement of Closing Prebreakdown Duration

During circuit breaker closing, when the distance between the movable and static arc contacts is sufficiently small, the contact gap will be prebroken to produce an arc, and a current will be generated in the circuit. When the alternating current suddenly passes through the conductor, it will generate a high-frequency electromagnetic field in the surrounding space. Through the circuit voltage and current, the electromagnetic field signals can be judged by contact arc time; that is, the prebreakdown process occurs at that moment.

At the end of the prebreakdown process, the movable and static contacts just close the contact, the prebreakdown arc disappears, and the current is transferred to the contacts to maintain the circuit as on. Dynamic and static contacts contact vibration signals generated by the connecting rod and other parts transferred to the circuit breaker beam and base and other parts. Through the circuit voltage and current, vibration signals can be judged by the contact just close to the moment, that is, the end of the prebreakdown process moment.

The closing prebreakdown duration measurement arrangement is shown in Figure 3, with RF signal sensors, vibration signal sensors, and main circuit voltage and current measurement devices installed in the test circuit. A high-voltage capacitor is used as the power supply, a current-limiting resistor provides circuit protection, a series sampling resistor measures the loop current, and a shunt voltage divider measures the loop voltage. The electromagnetic field signal antenna is placed under the break, capturing the electromagnetic field signal emitted in the process of circuit breaker closing and judging the moment of arc initiation; the vibration sensor is installed on the box of the actuator to measure the vibration signal generated by the contact collision and judge the moment of arc extinguishing. According to the prebreakdown process of the circuit voltage and current changes and the prebreakdown process generated by the electromagnetic field signal changes, the moment of the vibration signal can be judged by the closing prebreakdown duration.

The voltage and current signals recorded by the oscilloscope in the test are shown in Figure 4, the electromagnetic field signals are shown in Figure 5, and the vibration signals and trigger signals are shown in Figure 6. As can be seen from the figure, before the prebreakdown occurs at the moment T₁, the circuit is not on, the voltage is maintained at −66 kV, and the circuit current is zero. After the closing signal is issued, the contacts begin to move. When the contact spacing is small enough, the gap breakdown occurs, and the current appears to increase suddenly; it is at this time that the prebreakdown occurs at the moment of T₁. As the contacts continue to move, the capacitor discharges and the circuit current gradually attenuates. When the contacts collide, the circuit parameters change, resulting in the high-frequency oscillation of the voltage and current; the corresponding moment is the end of the prebreakdown moment T₂. The difference between T₂ and T₁ can be derived from the prebreakdown duration.

Through the characteristic points of the electrical signal in Figure 4, we can accurately judge and calculate the prebreakdown duration. When the characteristic points of the loop current signal are not obvious, and the clutter interference is large, the electromagnetic field signal in Figure 5 can assist in judging the occurrence time of the prebreakdown in Figure 6. The main function of the coil current signal is to ensure the synchronization of other signals. Through the overshoot time of the vibration signal in Figure 6, we can obtain the contact collision time of the contact. However, because the vibration signal is not stable compared with the electrical signal and the variance is large, this paper mainly uses the vibration signal to assist in judging the contact collision time.

3. Closing Prebreakdown Duration Result Analysis

The ablation was carried out according to the ablation current determined in the simulated ablation test program. The measurement of the closing prebreakdown duration was carried out at the end of each ablation, and the average value was taken from five measurements. The variation of the closing prebreakdown duration with the number of ablations is shown in Figure 7, Figure 8 and Figure 9.

Under the same size of ablation current, the closing prebreakdown duration of phase A showed a trend of rising and then falling, in which during the first 30 times of the 5 kA small current ablation test, the closing prebreakdown duration rose less. During the last 39 times of the 25 kA and 30 kA high current ablation tests, the closing prebreakdown duration changed more. The closing prebreakdown duration of phase B showed a trend of rising and then leveling off as the number of times of ablation increased. The closing prebreakdown duration of phase C showed the same rule of change as that of phase B in the first 100 ablation tests, but in the last 18 high-current ablation tests at 30 kA, the closing prebreakdown duration gradually increased. From Figure 7, Figure 8 and Figure 9, it can be seen that there is a certain fluctuation in the closing prebreakdown duration.

The reason for the above phenomenon is that due to the gap breakdown event, there was a great dispersion, which made accurate measurement more difficult. This paper uses the measurement five times to take the average value to reduce the error, but there is still a certain fluctuation. From the size of the error bars in Figure 7, this fluctuation is acceptable, and the results have a certain statistical law. In the early stage of the test, due to the ablation effect of the arc, the contact surface changed from smooth to locally concave and convex, thus distorting the nearby electric field. This resulted in the prebreakdown occurring in advance of the moment, while the dynamic characteristics of the contact were almost unchanged, so the prebreakdown duration gradually increased with the ablation test. At the same time, the larger the ablation current was, the larger the increase in the prebreakdown duration was. As the test was further carried out, the contact surface of the unsmooth state reached a certain degree, and then the ablation of the contact surface only made the contact surface from one form of concavity and convexity to another, which tended to have the same degree of influence on the electric field aberration. The prebreakdown time of the closing gate was gradually stabilized, so the prebreakdown duration of the later part of the test was gradually unchanged. Due to the existence of different sizes of ablation current, the contact morphology change was different. Therefore, in each group of the same current in the ablation test, the prebreakdown duration of the first increased and then decreased and finally tended to stabilize, but in the change of the ablation current size, the prebreakdown duration also began to change. The shortening of contact length due to arc ablation had approximately the same effect on the prebreakdown moment and contact collision moment and did not affect the prebreakdown duration, which also led to the stabilization of the prebreakdown duration in the later part of the test.

4. Simulation Verification

The internal structure of the interrupter chamber of the specimen circuit breaker has axisymmetric characteristics, which mainly include the static main contact, the moving main contact, the static arc contact, the moving arc contact, the pressurized cylinder, etc., as shown in Figure 10. Considering that the simulation calculation of SF₆ involves an electric field and a moving grid, the calculation volume is large and not easy to converge. Therefore, a two-dimensional axisymmetric model was established to reduce the calculation volume, and the simulation model is shown in Figure 11. The contact had a total stroke of 120 mm and an arc contact opening distance of 78 mm and was filled with 0.6 MPa.

The change of electric field distribution in the interrupter room during the closing process is shown in Figure 12a–d. The location of the maximum field strength in the interrupter room always appeared on the surface of the static arc contact, where the electric field distortion was the largest and was also the most likely to produce a flow injection and breakdown location.

The contact moving process breakdown voltage cloud is shown in Figure 13. The cloud focuses on the static and dynamic arc contacts; the more the color of the blue phase indicates that the position of the breakdown voltage is lower and the more likely it is to break down. It can be found that the most easy to breakdown position always appeared in the static arc contact surface, consistent with the previous analysis.

For the calculation results of the breakdown voltage in the arc interrupter, the prebreakdown voltage curve is shown in Figure 14. The contact prebreakdown voltage decreased with the approach of the contact. The 126 kV circuit breaker selected in this paper actually worked in a system with a phase voltage of 66 kV, under which the contact broke down at 29.0946 ms. The contact arc was extinguished at 30 ms, so the duration of the non-ablative contact prebreakdown arc was 0.9054 ms. It is in good agreement with the phase B test result of 0.9349 ms.

Studies have shown that in the circuit breaker, in the process of opening and closing the current, the contact due to arc ablation, on the one hand, will lead to a loss of material at the end of the static contact and become pointed. This will lead to the contact being in the process of breaking and closing the contact stroke becomes shorter; on the other hand, the surface of the contact becomes uneven.

In order to simplify the modeling, this model sets a wave texture on the contact surface, which consists of semicircles with a radius of 0.1 mm, to simulate the phenomenon of the surface of the end of the contact becoming uneven under ablation. According to the deterioration process, while considering the increase in the contact ablation angle and the unevenness of the contact surface, the simulation model of the contact is set up with five ablation degrees from light to heavy, as shown in Figure 15. In the first two models, the unevenness of the surface started to appear, but it still did not lead to the reduction of stroke. In the last three models, the range of unevenness of the surface and the reduction of stroke gradually increased.

A comparison of the field strength distribution near the contact at 28 ms for the very slightly ablated and no ablation cases is shown in Figure 16. The contact ablation led to a change in the location of the maximum field strength found, and the maximum field strength occurred at the slight ablation; its value increased from 506.8 V/m to 724.6 V/m, an increase of 42.9%, where it was more prone to flow injection discharge and breakdown.

The calculation results of the prebreakdown characteristic curve of the circuit breaker under different ablation states are shown in Figure 17. The breakdown voltage did not decrease monotonically with the shortening of contact distance, and a small recovery of breakdown voltage occurred during the closing movement, which was reflected in the fluctuating decline of the breakdown voltage curve. Overall, as the degree of corrosion increased, the breakdown voltage curve shifted downward, and the curve was at the bottom when the stroke was reduced by 10 mm. The breakdown voltage curve for the slightly ablated case was lower than that for the case of 5 mm stroke reduction before 25 ms, and the relationship between the two curves was reversed after 25 ms. The prebreakdown arc duration was calculated by extracting the moments when the voltage was 66 kV, and as shown in Figure 18, the prebreakdown arc duration generally showed an increasing trend as the degree of ablation increased. The prebreakdown arc duration under very slight ablation was not much different from that without ablation, and then the prebreakdown arc duration rose with the increase in the ablation degree, reaching 1.766 ms when the stroke was reduced by 10 mm. The prebreakdown arc duration decreased when the stroke was reduced by 15 mm compared with that when the stroke was reduced by 10 mm, which indicates that the ablation degree of the contact for the aberration effect of the electric field may be a great value This indicates that there may be an extreme value point for the contact ablation on the electric field aberration, beyond which a further increase in the contact ablation degree will improve the electric field distribution to some extent.

5. Conclusions

In this paper, based on the circuit breaker simulation ablation test for the measurement of the closing prebreakdown duration, the phenomenon of the change of three-phase closing prebreakdown duration with the number of ablations was analyzed. At the same time, the simulation was verified, which showed the theoretical feasibility of reflecting the insulation state of the contacts through the measurement of prebreakdown time. The conclusions are as follows:

(1)

The experimental study shows that with the ablation of the contact, the duration of the prebreakdown at closing showed an overall increasing trend. However, under different test conditions, the prebreakdown duration in the final stage of ablation may increase, decrease, or fluctuate unchanged. The larger the ablative current is, the larger the change of prebreakdown duration is;

(2)

Under the action of each group of ablative currents of the same size, with the conduct of the ablative test, the duration of the closing pre-breakdown first increased and then decreased and gradually became stable. However, when the ablation current was changed, the pre-breakdown duration also began to change;

(3)

The simulation study showed that the maximum field strength in the arc extinguishing chamber occurred on the surface of the static arc contact, where the electric field distortion was the largest and breakdown was most likely. The ablation of the contact further increased the distortion of the electric field in the arc extinguishing chamber, resulting in the breakdown voltage dropping. With the increase in ablation degree, the pre-breakdown time of the circuit breaker was advanced, and the duration of the pre-breakdown arc increased. The ablation degree of contacts may have a maximum value point for the distortion effect of the electric field, beyond which the further increase in the ablation degree of contacts will improve the distribution of the electric field to a certain extent, resulting in a decrease in the duration of closing prebreakdown.