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Vacuum circuit breakers (VCBs) have been extensively employed in switching shunt capacitor banks. However, research on the prestrike characteristics of double-break VCBs in making power frequency voltage remains limited. This study aims to investigate the influence of different closing time differences on the prestrike characteristics of double-break VCBs in making power frequency voltage, and to compare these influences with those of single-break VCBs. Experiments were conducted using vacuum interrupters rated at 24 kV, with contacts made of CuCr40 alloy doped with 1 wt% graphene. Taking the closing time of the high-voltage break as the time zero point, three closing time differences (0 ms, 0.727 ms, and −0.347 ms) were set, and experiments were carried out at six closing phase angles (from 0° to 150° in 30° increments) for each condition. The experimental results demonstrate that when the closing of the high-voltage break lags behind that of the low-voltage break by 0.347 ms, the double-break VCB exhibits optimal prestrike performance, where prestrike is almost entirely suppressed except at the 90° phase angle. Furthermore, the prestrike performance during the closing of the double-break VCB is significantly superior to that of the single-break VCB, characterized by a steeper RDDS curve. These findings provide a theoretical basis for the design of control-switching double-break VCBs.
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1. Introduction
With the increasing proportion of new energy generation in power systems, the application of shunt capacitor banks has become more prevalent to maintain reactive power balance and voltage stability [1].
Vacuum circuit breakers (VCBs) are particularly suitable for switching shunt capacitor banks, owing to their simple structure, low maintenance cost, environmental friendliness, and long electrical lifespan [2]. However, during the switching process, the inherent dispersion in their opening and closing times can lead to inrush currents during closing and restriking during opening, posing serious threats to the safety of capacitors and associated electrical equipment [3,4,5]. Furthermore, during the random closing of capacitor banks, prestrike high-frequency inrush current arcs can cause severe erosion of the vacuum circuit breaker contacts, significantly increasing the probability of post-arc restriking and thereby threatening the safe and stable operation of the power system [6,7,8,9,10,11].
Control-switching of VCBs can effectively mitigate inrush currents during the closing process. Prestrike typically occurs several milliseconds before the mechanical contact of the breaker contacts [12]. Therefore, the target time for mechanical contact must be adjusted according to the prestrike characteristics of the VCB to achieve precise control of the closing operation at specific voltage or current phase angles [13,14]. Additionally, in high-voltage applications, single-break VCBs are limited in terms of breaking capacity and insulation strength, necessitating research into the prestrike characteristics of double-break VCBs [15].
Several methods have been explored in the existing research to mitigate the effects of prestrike in vacuum circuit breakers, primarily including (a) contact material enhancement; (b) contact structure modification; and (c) control-switching. In Ref. [16], the prestrike characteristics of 40.5 kV vacuum interrupters during capacitive making operations was investigated, with a focus on comparing the performance differences between two contact materials: arc-melted CuCr40 and infiltration-prepared CuCr50. The influence of contact materials on the erosion of contact surfaces by inrush currents was studied. The study in [17] compares the prestrike performance of cup-type transverse magnetic field (TMF) and axial magnetic field (AMF) contacts in vacuum interrupters, finding that TMF contacts exhibit a smaller prestrike gap while AMF contacts provide superior arc extinguishing capability at higher inrush currents. In Refs. [18,19], the advantages of capacitive controlled switching were presented. Reduction in the prestrike gaps and the deviation in the prestrike gaps would significantly improve the control accuracy. Although numerous studies have been conducted on the prestrike characteristics of VCB, most have focused on single-break VCBs or been carried out under DC voltage conditions. Research on double-break VCBs under power frequency voltage remains limited. Existing studies often overlook the influence of closing time differences on the prestrike characteristics of VCBs and the resulting dynamic voltage distribution across series breaks. This significantly limits the application of double-break VCBs in AC systems.
Therefore, this study will conducted the following research: First, the actual closing phase angle and breakdown voltage of double-break VCBs under different closing time intervals were studied, with experiments conducted at six closing phase angles (0~150°). Second, a comparative study was performed on the actual closing phase angles of double-break and single-break VCBs under the same six closing phase angle conditions. Finally, the RDDS (Rate of Decaying Dielectric Strength) curves of both double-break and single-break VCBs were obtained, and the experimental patterns were analyzed to provide a basis for precise control-switching. The main innovations of this work include: (i) systematically investigating the prestrike characteristics of double-break VCBs under power frequency AC voltage, in contrast to previous DC-focused studies; (ii) determining the optimal closing time difference under the experimental configuration, where the high-voltage break lags by 0.347 ms; (iii) comparing the prestrike characteristics between single-break and double-break VCBs, revealing that the double-break VCB exhibit a steeper RDDS curve and providing practical engineering guidelines for control-switching applications.
2. Experimental Setups
2.1. Circuit Configuration
The experimental circuit was constructed as shown in Figure 1. The circuit consists of a power frequency source, a transformer, a current-limiting resistor, and the VCB under test, with its key parameters and experimental conditions summarized in Table 1.
The VCB under test is a double-break VCB with a rated voltage of 35 kV. It is composed of two vacuum interrupters connected in series, each with a rated voltage of 24 kV. The contact material is CuCr40 alloy doped with 1 wt% graphene. The voltage across the tested VCB is measured using a voltage divider. The transformer has a primary-to-secondary output voltage ratio of 1:250 (RMS). To ensure the occurrence of prestrike phenomena, the output of the power frequency source is set to 240 V, resulting in an operating voltage of 60 kV for the VCB.
The circuit breaker used was a double-break fast vacuum circuit breaker. A current-limiting resistor was employed to restrict the current in the circuit after closing. The output of the power frequency source was connected to the input of the transformer, which in turn supplied power frequency AC voltage to the double-break vacuum circuit breaker and the resistor. The power frequency source, transformer, and experimental circuit were all grounded. The voltage was measured using a voltage divider to monitor the potential at the high-voltage side of the double-break vacuum circuit breaker. Since the low-voltage side was grounded, the potential measured by the divider could be considered as the voltage across the two breaks.
The closing operation of the VCB is performed using a phase selection closing controller, which enables the adjustment of the closing signal delay time and thus control of the closing phase angle of the VCB. The timing diagram of the phase selection closing control is shown in Figure 2. Since the power frequency voltage has a frequency f of 50 Hz and a period T of 20 ms, the relationship between the delay time and the phase angle under power frequency voltage is as follows: for every 1 ms increase in delay time Td, the voltage phase lags by 20°. A single-phase voltage regulator is used to supply the phase selection closing controller with a standard power frequency voltage as the phase selection reference voltage U0.
The VCB is initially in the open state. The experimental procedure followed these sequential steps: (1) The phase selection controller detected the zero-crossing of the reference voltage U0. (2) After a preset delay time Td, the controller issued a closing command. (3) The VCB, activated by its operating mechanism, closed the two breaks with a predetermined time difference after its inherent closing time tc. (4) A prestrike occurred when the electric field strength exceeded the dielectric strength of the contacts prior to their mechanical touching. (5) Using the rising edge of the closing current signal as the measurement trigger, the transient waveforms were captured by the voltage divider and the Rogowski coil for analysis.
2.2. Circuit Breaker Closing Travel Curve
The closing time difference is one of the primary factors affecting the closing performance of double-break VCB. Experiment I was designed to investigate the influence of different closing time differences on the prestrike characteristics of double-break VCB. The closing time difference, Δtd, is defined as the difference between the closing times of the high-voltage and low-voltage breaks, expressed as:
Δtd = tH − tL(1)
In the experiment, the contact travel was adjusted by varying the length of the linkage between the crank arm and the vacuum interrupter in the double-break VCB, thereby modifying the closing time difference. Three levels of closing time difference were set, 0 ms, 0.5 ms, and −0.5 ms, corresponding to simultaneous closing of the high-voltage break and low-voltage break, with the high-voltage break closing 0.5 ms earlier than the low-voltage break, and the high-voltage break closing 0.5 ms later than the low-voltage break, respectively. However, due to the minimum adjustable length of the linkage, the actual closing time difference deviated from the set values. To determine the actual closing time difference and evaluate the influence of the circuit breaker’s mechanical dispersion on the experimental results, multiple no-load closing operations were performed for each closing time difference setting. The mechanical characteristics corresponding to the three levels of closing time difference were calculated based on the measurement results, as shown in Table 2.
The travel characteristic curves under the three levels of closing time difference were measured using a displacement sensor, as shown in Figure 3. Since the closing time difference was adjusted by varying the length of the linkage between the crank arm and the vacuum interrupter, the total contact travel at the fully open position was not exactly the same for the three settings. The total contact travel mainly affects the circuit breaker’s breaking capability under a short-circuit fault current. However, this experiment focuses on the prestrike characteristics during closing, which generally occur when the contacts are about to touch. Therefore, particular attention is paid to mechanical characteristics such as the closing velocity immediately before contact touch. The closing velocity immediately before contact touch is defined as follows:
(2)
Here, s represents the contact travel from the point at witch the slope of the travel curve becomes stable in the final segment to the instant of contact touch, with units of mm; tg denotes the closing time from the same point to contact touch, with units of ms. According to the measurement results, the double-break VCB used in the experiment exhibits very small mechanical dispersion. The closing velocities immediately before contact touch under the three closing time difference settings of 0.000 ms, 0.727 ms, and −0.347 ms are 3.001 m/s, 2.969 m/s, and 2.816 m/s, respectively, and have almost no impact on the phase selection closing process in this experiment.
In Experiment I, three levels of closing time difference were set: Δtd = 0.000 ms, Δtd = 0.727 ms, and Δtd = −0.347 ms. The variable parameter in this experiment was the closing phase angle, with six settings: 0°, 30°, 60°, 90°, 120°, and 150°.
To further investigate the differences in prestrike characteristics between double-break and single-break VCBs when closing at the same voltage level under power frequency voltage, Experiment II was conducted at the same voltage level. In this experiment, both single-break and double-break VCBs were subjected to power frequency voltage closing tests under different closing phase angles, and their prestrike characteristics during closing were compared. Six closing phase angles were set: 0°, 30°, 60°, 90°, 120°, and 150°.
In Experiment II, the same double-break VCB from Experiment I was used. For the single-break VCB power frequency closing tests, the low-voltage side contact of the double-break VCB was short-circuited to form a single-break VCB.
3. Results
3.1. Influence of Closing Time Differences on the Power Frequency Closing Prestrike Characteristics of Double-Break VCB
Based on experimental setup I, prestrike tests of closing power frequency voltage were conducted on the double-break VCB under different closing time differences. The double-break VCB was regarded as an integrated unit, and the prestrike characteristics under power frequency voltage were investigated as a whole. Figure 4 shows the typical waveform observed during a prestrike event.
By analyzing the experimental waveforms, the triggering time of the closing signal current ti, the moment when the test voltage drops to zero t1, and the test voltage at the instant of voltage drop to zero—which is considered to be the breakdown voltage Ub in this study—can be obtained. The time interval between t1 and the triggering time ti is defined as the closing time tc. The time difference Δt between t1 and the expected closing phase instant t0 is measured to calculate the actual closing phase angle δc. The experimental results are shown in Table 3.
Combining the waveform analysis with the results in Table 2, it can be observed that when Δtd = 0.727 ms, prestrike phenomena rarely occur at expected closing phase angles of 0° and 30°, while they are observed at 60°, 90°, 120°, and 150°. When Δtd = 0.000 ms, prestrike phenomena are rarely observed at the expected closing phase angles of 0°, 30°, and 150°, but are present at 60°, 90°, and 120°. For Δtd = −0.347 ms, prestrike phenomena rarely occur at expected closing phase angles of 0°, 30°, 60°, 120°, and 150°, and only occur at 90°.
The relationship between the average instantaneous voltage value at the moment when the voltage drops to zero and the expected closing phase angle under different closing time differences is shown in Figure 5. When the expected closing phase angles are 0° and 30°, the double-break VCB closes at the expected phase for all three closing time differences, resulting in similar average instantaneous voltage values. At an expected closing phase angle of 90°, corresponding to closing at the voltage peak, prestrike phenomena occur for all three closing time differences. The instantaneous voltage values for Δtd = 0.727 ms and Δtd = 0 ms are similar, while the prestrike instantaneous voltage for Δtd = −0.347 ms is approximately 10% higher than those of the other two cases. Combined with the analysis in Table 2, it can be concluded that the prestrike phenomenon for Δtd = −0.347 ms occurs later than for Δtd = 0.000 ms and Δtd = 0.727 ms, indicating that prestrike is less likely to occur at Δtd = −0.347 ms when closing at 90°. Based on the above analysis, it can be considered that the double-break VCB exhibits the optimal prestrike performance at Δtd = −0.347 ms.
3.2. Comparison of Closing Prestrike Characteristics Between Double-Break and Single-Break VCBs
According to the procedure of Experiment II, one side contact of the double-break VCB was short-circuited to form a single-break VCB. Prestrike tests for closing under power frequency voltage were carried out at different expected closing phase angles with an experimental voltage of 60 kV. Based on the previous research results, when the high-voltage break of the double-break VCB closes with a delay of 0.347 ms relative to the low-voltage break, the prestrike characteristics during power frequency voltage closing are significantly improved. Therefore, the experimental results for Δtd = −0.347 ms were selected for comparison with those of the single-break VCB. The average values of the expected and actual closing phase angles for both Δtd = −0.347 ms and the single-break VCB are presented in Table 4, and the corresponding actual closing phase waveform is shown in Figure 6.
When the expected closing phase angles were 0°, 30°, and 150°, no obvious prestrike phenomena were observed in either the double-break or single-break VCBs. However, at expected closing phase angles of 60° and 120°, significant prestrike phenomena occurred in the single-break VCB, and the dispersion of the actual closing phase was relatively high. At an expected closing phase angle of 90°, prestrike phenomena were observed in both the double-break and single-break VCBs, but the actual closing phase angle of the single-break VCB was 11.24° lower than that of the double-break VCB. Therefore, under the same voltage level and expected closing phase angle, the double-break VCB exhibits superior prestrike performance compared to the single-break VCB, as each break in the double-break configuration is subjected to a lower voltage.
4. Discussion
4.1. Influence of Closing Time Differences
The breakdown voltage of a true vacuum gap and the gap distance generally conform to the relationship U = Kda where U is the breakdown voltage in kV, d is the gap distance in mm, and K and a are constants. In this experiment, the instantaneous voltage value at the moment of voltage drop to zero was regarded as the prestrike voltage Upre. The relationship between the contact gap d and the instantaneous voltage Upre was plotted, and the fitted curves of Upre versus d under different closing time differences are shown in Figure 7. The fitted functions for Δtd = 0.000 ms, Δtd = 0.727 ms, and Δtd = −0.347 ms are U = 10.095d1.83, U = 38.371d0.99, and U = 29.760d1.25, respectively. Furthermore, the results in Figure 7 indicate that at the same prestrike distance, the corresponding prestrike voltage follows the order Upre (Δtd = 0.727 ms) > Upre (Δtd = −0.347 ms) > Upre (Δtd = 0.000 ms). Among the tested conditions, the results for Δtd = 0.727 ms and Δtd = −0.347 ms show relatively minor differences.
In Experiment I, the closing time differences between the high-voltage break and low-voltage break of the double-break VCB were adjusted by changing the length of the linkage between the crank arm and the vacuum interrupter. As a result, the total travel distance at full open position was not exactly the same under the three different closing time differences. As shown in the travel characteristic curves for different closing time differences in Figure 3, the total travel distances satisfy x0.000 ms > x0.727 ms > x−0.347 ms. If the difference in total travel distance is considered to affect the dielectric withstanding capability of the circuit breaker during power-frequency closing, the prestrike characteristics should theoretically follow the order Δtd = 0.000 ms > Δtd = 0.727 ms > Δtd = −0.347 ms. However, the experimental results do not support this assumption, indicating that the difference in total travel distance is not the main reason for the variation in prestrike performance of double-break VCBs under power-frequency closing.
In this study, under power-frequency voltage conditions, the double-break VCB exhibited the best prestrike performance when the close time difference was −0.347 ms. The optimal performance was mainly reflected in a lower probability of prestrike occurring within the phase range of 90–150°. The underlying mechanism is that during the voltage-decreasing phase (90–180°), the delayed closing of the high-voltage break reduces the instantaneous voltage across it at the moment of closure and simultaneously alters the voltage-sharing ratio between the high-voltage and low-voltage breaks, thereby improving the overall prestrike characteristics.
Under power-frequency voltage, when the high-voltage break closes with a delay, the low-voltage break closes first and generates an arc. Since the high-voltage break remains open at this stage, most of the system voltage is distributed across it, resulting in a lower voltage across the low-voltage break and consequently a lower arc energy. Due to the reduced arc energy and shorter arc duration, the probability of surface ablation and micro-protrusion formation on the breaks decreases, enhancing the dielectric recovery capability and reducing the prestrike probability. Subsequently, when the high-voltage break closes during the voltage-decreasing phase (90–180°), it does so under a lower voltage, directly reducing its prestrike probability and energy. This strategy also ensures that the closing process of the high-voltage break is not affected by a prestrike occurring on the low-voltage side, thereby preventing simultaneous prestrike events at both breaks.
It is worth noting that the conclusion in Ref. [20] indicates that the prestrike characteristics of both breaks were optimal when the high-voltage break closed 0.2 ms earlier than the low-voltage break. This is the opposite of the conclusion of this paper. The possible reason lies in the fact that, under DC voltage conditions, the voltage remains constant, and the voltage-sharing ratio between the high-voltage break and the low-voltage break becomes the determining factor for the prestrike behavior. Since the high-voltage break bears a higher voltage proportion, it is more likely to experience a prestrike during the early stage of operation, while the low-voltage break, with a lower voltage proportion, tends to experience prestrike in the later stage. When a prestrike occurs, the entire voltage is transferred across the opposite break. If the high-voltage break closes first, the prestrike takes place earlier, when the contact gap of the low-voltage break is still large, resulting in a relatively minor impact. Conversely, if the low-voltage break closes first, the high-voltage break has a smaller gap at the moment of prestrike, leading to a more pronounced effect.
4.2. Comparison of RDDS Curves Between Double-Break and Single-Break VCBs
To further investigate the influence of the number of contacts on the prestrike characteristics of VCBs, the double-break VCB with a pronounced prestrike phenomenon at Δtd = 0.000 ms was selected for study. The prestrike voltage Upre and the prestrike contact gap d for the double-break and single-break VCBs are given by Equations (3) and (4), respectively.
Upre_Double = 63.203d0.25 (3)
Upre_Single = 43.507d0.66(4)
The time difference between the occurrence of prestrike and the instant of initial contact closure is defined as the prestrike-to-contact time difference td. Near the instant of contact closure, the prestrike gap d and td satisfy Equation (5).
d = vc td(5)
The closing velocities for the double-break and single-break VCBs are vc_double = 2.969 m/s and vc_single = 1.749 m/s, respectively.
Upre_Double = 82.83td0.25(6)
Upre_Single = 62.78td0.66(7)
Since prestrike occurs prior to the expected closing phase, td is defined as a negative value in the description of the RDDS curve to indicate that prestrike occurs before contact closure. The RDDS curves for the double-break and single-break VCBs are shown in Figure 8. It can be observed that, under power frequency AC voltage, the RDDS curve for the double-break VCB is significantly steeper and nearly vertical near the instant of contact closure. This phenomenon can be explained by field emission theory. In the double-break structure, the total voltage is distributed across two series gaps, meaning that each gap is subjected to a lower electric field intensity under the same voltage level compared to a single-break vacuum circuit breaker. This suppressed electric field effectively inhibits electron emission from micro-protrusions on the contact surface, thereby delaying the breakdown process. As a result, the dielectric strength remains high until the contacts are in very close proximity, eventually leading to a sharp, nearly vertical drop in the RDDS curve just before mechanical contact occurs.
The steeper RDDS curve helps relax the requirement for closing speed consistency. Since the dielectric strength drops abruptly only within an extremely short period before contact touching, variations in closing speed result in a significantly narrower time window during which prestrike may occur at unintended phase angles. This indicates a lower probability of the dielectric strength curve intersecting with the applied voltage before the intended closing phase angle, making prestrike events less likely. This characteristic facilitates more precise control-switching in double-break VCBs compared to their single-break counterparts.
4.3. Calculation Method for Expected Closing Phase and Corresponding Speed Requirements in Control-Switching
Based on the above research on the prestrike characteristics of double-break VCBs during closing under power frequency voltage, recommendations are made for the selection of the expected phase point and the corresponding speed dispersion requirements for control-switching.
From Figure 5, it can be seen that as the expected closing phase angle increases from 0° to 150°, the average instantaneous voltage at the zero-crossing first increases and then decreases, reaching a peak at 90°. According to Figure 8, the RDDS curve of the double-break VCB is nearly vertical near the instant of contact closure, with a slope k >> 1. For capacitor bank switching, neglecting interference factors such as prestrike, the ideal closing instant is at the voltage zero-crossing. However, in practical phase selection control, inherent controller delays and manual operation cannot guarantee that every closing operation occurs exactly at the voltage zero-crossing. According to Ref. [21], the preferred rated window for phase selection closing is 2 ms, i.e., closing within ±1 ms of the zero-crossing, which corresponds to 0° ± 18° under power frequency AC voltage. Therefore, to avoid prestrike phenomena during closing and to ensure the safe and stable operation of power system equipment, the expected closing phase point for control-switching of double-break VCBs should be selected within −18° to 18° of the power frequency voltage.
Considering the potential impact of mechanical dispersion of the operating mechanism on the dispersion of closing speed, which directly affects the accuracy of phase selection closing, this uncertainty not only reduces the reliability of phase selection control but may also cause prestrike events at higher actual voltages when the expected closing phase is large. Therefore, to achieve precise phase control, the error between the actual prestrike phase and the expected closing phase should not exceed ±10%. In practical applications, strict control of the mechanical characteristics of the operating mechanism is required, and specific requirements for the closing speed range affected by mechanical dispersion must be proposed for double-break VCBs. Ignoring whether prestrike occurs during the voltage rising or falling stage and considering only the amplitude of the prestrike voltage, taking an expected closing phase of −30° as an example, the schematic RDDS curve for the critical closing speed of the double-break VCB is shown in Figure 9.
When the expected closing phase is −30°, the dielectric strength U, closing speed vc, and prestrike-to-contact time difference td can be expressed as in Equation (8):
UDouble = 63.203 × [vc × (td − ta)]0.25(8)
where ta is the time corresponding to precise phase control, and for an expected closing phase of −30°, ta = −T/12. To ensure that the error between the actual prestrike phase and the expected closing phase does not exceed ±10%, it is required that no prestrike occurs at ωtd = −33°; at td = −11T/120, the dielectric strength U must be greater than the actual applied voltage. Under the experimental conditions of this chapter, the closing speed vc, considering the effect of mechanical dispersion of the operating mechanism, must not be less than 1.355 m/s.Based on the above research, the double-break vacuum circuit breaker demonstrates the following distinct advantages: (i) reduced probability of prestrike; (ii) a steeper RDDS curve enabling more precise phase control; and (iii) lower voltage stress per break, which contributes to extended contact lifespan. However, its drawbacks include (i) a more complex mechanical structure; (ii) requirements for precise control-switching operations; and (iii) higher manufacturing costs.
5. Conclusions
This study investigates the prestrike characteristics of double-break vacuum circuit breakers (VCBs) during closing operations under power frequency voltage. The main findings are summarized as follows: Optimal Closing Sequence: The closing time difference (Δtd) between the series breaks is a critical parameter affecting prestrike performance. Under the experimental conditions of this study, optimal performance was achieved when the high-voltage break closed 0.347 ms after the low-voltage break (Δtd = −0.347 ms). Under this configuration, prestrike occurred only at the 90° phase angle (voltage peak) and exhibited the lowest voltage dispersion. This finding provides clear guidance for optimizing the control strategy of double-break VCBs. Superiority of the Double-Break Configuration: Under identical voltage levels and expected closing phases, the double-break VCB demonstrated significantly superior prestrike performance compared to the single-break VCB. This superiority is evidenced by (i) a reduced probability of prestrike occurrence across various phase angles, and (ii) a steeper RDDS curve near the instant of contact closure. This characteristic renders the double-break VCB less sensitive to timing variations and facilitates more precise control-switching. Practical Engineering Guidance: To achieving precise control-switching in practical applications, this study provides the following specific recommendations: (i) the target phase selection point should be set within 0° ± 18° to avoid prestrike during closing; (ii) to ensure the actual prestrike phase error remains within ±10% of the expected value, the closing speed must be maintained above a calculated threshold, accounting for mechanical dispersion (e.g., a required speed > 1.355 m/s for a −30° closing target under the present experimental conditions).
It is important to note that the quantitative results obtained in this study are specific to the experimental setup employed. However, the research methodology used to investigate the influence of closing time differences on the prestrike characteristics of double-break VCBs holds general significance. Prestrike characteristics are also influenced by various other factors, such as the voltage level, current level, contact structure, and contact material. Based on these findings, future research will focus on the impact of closing time difference closing under the following conditions: (i) different contact materials and structures; (ii) high-current breaking scenarios; and (iii) higher or lower voltage levels.
Conceptualization, S.W. and X.Y.; methodology, F.M.; software, Y.N.; validation, Z.G., H.S. and M.X.; data curation, S.W.; writing—original draft preparation, S.W.; writing—review and editing, X.Y.; visualization, Y.N.; supervision, X.Y.; project administration, X.Y.; funding acquisition, X.Y. All authors have read and agreed to the published version of the manuscript.
Data are contained within the article.
Author Yuqian Niu was employed by the State Grid Zhejiang Electric Power Co., Ltd., Hangzhou Power Supply Company. Author Feiyue Ma was employed by the State Grid of Ningxia Electric Power Co., Ltd., Electrical Power Research Institute. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
The following abbreviations are used in this manuscript:
| VCB | Vacuum Circuit Breaker |
| RDDS | Rate of Decaying Dielectric Strength |
Footnotes
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Figure 1 Double-break vacuum circuit breaker prestrike experimental circuit: (a) schematic diagram, (b) photograph of experimental setup.
Figure 2 Control-switching timing.
Figure 3 Closing travel curves at different closing time differences.
Figure 4 Typical waveform of prestrike test.
Figure 5 Relationship between the mean instantaneous voltage and the expected closing phase angle.
Figure 6 Relationship between the expected closing phase and the actual closing phase.
Figure 7 Relationship between prestrike distance and pre-breakdown voltage.
Figure 8 RDDS curves of double-break and single-break vacuum circuit breakers.
Figure 9 RDDS curve at an expected closing phase of −30°.
Key experimental parameters and environmental conditions.
| Experimental Conditions | Parameters | ||
|---|---|---|---|
| Ambient Temperature: | 23 ± 2 °C | ||
| Relative Humidity: | 45% ± 5% | ||
| Experimental equipment | Parameters | ||
| VCB | Rated voltage: | Rated current: | Rated frequency: |
| Number of Operations: | Type: | Manufacturer: | |
| Vacuum Interrupter | Rated voltage: | Contact material: | Contact Structure: |
| Power frequency source | Output range: 0~400 V | ||
| Resistor | 8 kΩ | ||
| Transformer | Transformation ratio: 1:250 | ||
| Voltage divider | Transformation ratio: 12,500:1 | ||
| Rogowski coil | Transformation ratio: 3 V:1000 A | ||
Mechanical characteristics under different closing time differences.
| Closing Time Difference Setting/ms | Actual Closing Time Difference/ms | No-Load Closing Time/ms | Average Mechanical Dispersion/ms |
|---|---|---|---|
| 0 | 0.000 | 14.5 | 0.001 |
| 0.5 | 0.727 | 13.2 | 0.085 |
| −0.5 | −0.347 | 14.3 | 0.141 |
Experiment I—actual closing phase angle.
| Group | Parameters | 0° | 30° | 60° | 90° | 120° | 150° |
|---|---|---|---|---|---|---|---|
| Δtd = 0.727 ms | Angle/° | 0 | 29.37 | 50.69 | 71.42 | 103.68 | 107.28 |
| Time/ms | 14.44 | 14.10 | 13.78 | 13.36 | 13.48 | 13.70 | |
| Δtd = 0.000 ms | Angle/° | 0 | 27.61 | 48.71 | 67.32 | 106.94 | 136.17 |
| Time/ms | 15.34 | 15.08 | 14.52 | 13.88 | 14.66 | 15.11 | |
| Δtd = −0.347 ms | Angle/° | 0 | 27.05 | 56.02 | 80.35 | 116.64 | 150.19 |
| Time/ms | 14.78 | 14.68 | 14.66 | 14.62 | 14.87 | 15.10 |
Experiment II—actual closing phase angle.
| Group | 0° | 30° | 60° | 90° | 120° | 150° |
|---|---|---|---|---|---|---|
| Double-break (Δtd = −0.347 ms) | 0.00 | 27.05 | 56.02 | 80.35 | 116.64 | 150.19 |
| Single-break | −1.44 | 24.24 | 44.86 | 69.11 | 100.08 | 148.84 |
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