summary
Researchers Zheng Yiming and Xu Hua from the State Grid Zhejiang Electric Power Research Institute, State Grid Zhejiang Electric Power Company, and State Grid Zhejiang Electric Power Company Wenzhou Power Supply Company pointed out in an article published in the 22nd issue of the Journal of Electrical Engineering in 2018 that when disconnecting the parallel reactor of the empty busbar, phenomena such as arc reignition and equivalent current cutting are very likely to occur. The resulting operational overvoltage can cause phase-to-phase short circuits and reactor inter-turn insulation damage, threatening the safety of electrical equipment.
Equipment failures caused by operational overvoltages during the disconnection of 35kV unconnected busbar shunt reactors in 220kV substations occur frequently, jeopardizing the safe and stable operation of the power grid. This paper conducts a systematic analysis of the mechanism, hazards, and suppression strategies for overvoltages generated during the disconnection of shunt reactors. It compares the effects of circuit breaker performance and switching positions on overvoltage suppression, and includes field measurements and PSCAD simulation analysis. This provides solutions and engineering experience for suppressing operational overvoltages of 35kV unconnected busbar shunt reactors.
Vacuum circuit breakers are widely used in power systems due to their advantages such as simple structure, strong arc-extinguishing capability, high reliability, maintenance-free operation, long service life, and suitability for frequent operation. However, when vacuum circuit breakers disconnect 35kV parallel reactors, especially empty busbars, phenomena such as current cut-off, reignition, and equivalent current cut-off can easily occur. The resulting operational overvoltages can cause faults such as phase-to-phase short circuits in reactors, damage to inter-turn insulation in reactors, phase-to-phase short circuits in busbars, and even flashovers on the low-voltage side of the main transformer, seriously threatening the safe operation of the system.
In recent years, there have been multiple incidents of operational overvoltages during the disconnection of 35kV shunt reactors in substations, resulting in damage to switchgear, station service transformers, grounding transformers, and even main transformers. Therefore, comprehensive upgrades and remediation of 35kV unconnected busbar shunt reactors are urgently needed.
Suppressing overvoltage generated during reactor switching in vacuum circuit breakers generally involves two approaches: improving the circuit breaker's structure and installing external voltage limiting devices. Through years of effort by experts both domestically and internationally, numerous methods for limiting this overvoltage have been developed based on these two approaches.
Existing external voltage limiting devices for overvoltage in 35kV shunt reactors mainly involve adding RC absorbers and overvoltage protectors inside the switchgear. However, with the trend towards miniaturization in switchgear design, adding equipment inside the switchgear can easily lead to insufficient insulation distance, ultimately causing flashover. For vacuum circuit breakers, improvements to the circuit breaker contact structure, vacuum interrupter, materials, and circuit breaker opening and closing speed can also limit overvoltage.
Based on previous research, this paper conducts an in-depth and systematic analysis of the mechanism, hazards, and suppression strategies of overvoltage generated during the switching of shunt reactors. It also includes field overvoltage measurements and related PSCAD transient simulation analyses of switching 35kV shunt reactors in a 220kV substation. The paper compares the impact of circuit breaker performance and switching position on overvoltage suppression, providing insights and experience for engineering modifications and operation and maintenance of shunt reactors on empty busbars.
Two types of overvoltages can occur during the disconnection of shunt reactors: current-cutting overvoltages and reignition overvoltages. In most cases, current-cutting overvoltages are relatively low and have a clear upper limit, and are therefore acceptable. The main causes of overvoltages during shunt reactor disconnection are reignition and the non-first-phase equivalent current-cutting caused by reignition. Reignition mainly depends on the transient recovery voltage of the fault and the dielectric insulation recovery characteristics of the fault. If reignition continues to occur, the overvoltage will escalate, potentially harming other equipment in the system.
Re-ignition occurs because the dielectric insulation recovery curve at the break point intersects with the transient recovery voltage at the break point. The suppression strategy for re-ignition of the reactor after disconnection is shown in Figure 1. There are two main approaches to suppressing re-ignition and overvoltage: increasing the dielectric insulation recovery strength and speed, and reducing the transient recovery voltage can both reduce re-ignition and suppress overvoltage. The two approaches are as follows.
Figure 1. Strategy for suppressing the reignition phenomenon of the parallel reactor after removal.
(1) Improve the dielectric insulation recovery strength of the break point and use high-performance circuit breakers. Circuit breakers with faster dielectric insulation recovery speed can be selected to avoid transient recovery voltage as much as possible, or to reduce the number of times the dielectric insulation recovery strength curve intersects with the transient recovery voltage. Theoretically, this approach can reduce the probability of reignition on the one hand, and reduce the duration of reignition on the other hand, so that reignition is more likely to be interrupted by follow current, without forming an equivalent current cut-off in the non-first-opening phase, thus reducing the risk of overvoltage.
(2) Reduce the transient recovery voltage at the break point, i.e., change the system parameters at both ends of the circuit breaker. The transient recovery voltage at the break point is affected by the performance of the circuit breaker itself, and also depends on the system parameters at both ends of the break point. The transient recovery voltage at the break point can be reduced by changing the system parameters at both ends of the break point, so as to avoid the transient recovery voltage curve intersecting with the dielectric recovery characteristic curve as much as possible.
Figure 235kV busbar operation mode and electrical monitoring quantities
in conclusion
1) During the test of disconnecting the parallel reactor by the in-situ vacuum circuit breaker, one equivalent current-cutting phenomenon was recorded. The maximum overvoltage multiple on the bus side was 4.56, and the maximum overvoltage multiple on the reactor side was 5.81, indicating a very serious overvoltage situation. In the simulation, the reignition rate of the in-situ vacuum circuit breaker was as high as 99%, and reignition is highly likely to cause a large overvoltage.
2) When the SF6 circuit breaker in its original position interrupts the parallel reactor, there is still a significant probability of reignition. In the test, the reignition current was either directly interrupted or switched to a freewheeling current interruption, demonstrating good suppression of overvoltage under empty busbar conditions. The surge arresters did not trip. In the simulation, the SF6 circuit breaker in its original position also showed a significant probability of reignition (87%), but the reignition duration was short. In 100% of cases, the reignition current was switched to a freewheeling current interruption or directly interrupted, without any equivalent current cutoff. The probability of an SF6 circuit breaker generating an operating overvoltage sufficient to cause insulation damage when interrupting the parallel reactor is much lower than that of a vacuum circuit breaker.
3) Both actual measurements and simulations show that the SF6 pre-circuit breaker effectively suppresses overvoltage in the case of an empty busbar, with no significant current cut-off or reignition phenomena. This configuration is of reference value for 35kV oil-immersed shunt reactors with four bushings (i.e., without neutral point lead-out conditions).
4) Both field measurements and simulations show that the switching of the neutral point circuit breaker (SF6) to interrupt the parallel reactor has a very significant effect on suppressing overvoltage on both the bus side and the reactor side. No reignition or overvoltage occurred during the test, and the surge arresters did not trip. The interruption process can lead to oscillating overvoltage at the reactor's neutral point. The overvoltage value is many times higher than the overvoltage on the reactor side when the circuit breaker in its original position is interrupted, and the oscillation frequency is also higher. This poses a significant hazard to the inter-turn insulation at the reactor's neutral point, necessitating the installation of a surge arrester. SF6 circuit breakers are recommended.
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