Gaobo Wu

Study on Static and Dynamic Voltage Distribution Characteristics and Voltage Sharing Design of a 126-kV Modular Triple-Break Vacuum Circuit Breaker

The static and dynamic voltage distribution characteristics and voltage sharing design of a 126-kV modular triple-break vacuum circuit breaker (VCB) was discussed in this paper. The finite-element method and power frequency tests were used to calculate and verify the static voltage distribution ratios and distributed capacitance parameters of multi-break VCBs, respectively. A model combining the post arc current model and equivalent capacitance model was proposed for simulations of dynamic transient recovery voltage (TRV) distribution characteristics of the triple-break VCB. The simulation results indicated that besides the stray capacitances, the TRV sharing design should take the influence of residual charge (RC) into account, and the influence of RC on the TRV distribution is relevant to the differences of RC parameters among the series interrupters. Therefore, the appropriate value of grading capacitors should meet the requirement of the worst case. Based on the investigation of this paper, a 126 kV U-shaped triple-break VCB prototype with 1000 pF grading capacitors has been produced, and the successful large current breaking tests indicate the good breaking capacity and suitable voltage sharing design of the triple-break VCB.

Voltage distribution characteristics of multiple-break vacuum circuit breakers

The static voltage distributions and capacitance matrixes of two triple-break vacuum circuit breakers with different structure arrangements were calculated with the finite element method(FEM). Then the voltage distributions of two arrangements under ac voltages were measured. Experiment results verify the accuracy of the numerical calculations. The results show that the vertical arrangement has a better voltage distribution than the U-shaped arrangement. And the capacitance matrixes are useful to choose suitable grading capacitors. The voltage distribution characteristics after high current interruption was preliminary analyzed. The ohmic impedance of the gap during post-arc current period was calculated using the “Continuous Transition Model” to evaluate its influences on the voltage distribution. The results indicate that the voltage distribution may be determined by the ohmic impedances in the first few microseconds after current zero, especially after arcing conditions followed by large post-arc currents.

Influence of arcing conditions on post-arc dielectric strength recovery of VCB—Simulation based on an improved CTM

For vacuum circuit breaker (VCB), the contact surface temperature is still high after high current arcing, evaporation of metal vapor continues with considerable rates. The metal vapor and residual charge at current zero, i.e., the arc memory which depends on arcing conditions, determines the post-arc dielectric strength recovery of VCB. Considering the well-known continuous transition model (CTM) proposed by Andrews and Varey only shows the effect of residual charge on dielectric strength during sheath expanding period, thus, a submodel including metal vapor density and breakdown criteria corresponding to Paschen limit is added to the existing CTM. Moreover, it has taken into account the charge exchange between ions and vapor atoms. Its accuracy is verified by comparing simulation result with test and the other simulation data. Then based on the improved CTM, influence of arcing conditions including arcing time t0, interrupted current amplitude ip and ramp di/dt on the post-arc dielectric strength recovery of VCB are simulated and analyzed.

Study on transient recovery voltage distribution mechanism and grading capacitor of double-break vacuum circuit breaker

To keep the uniform distribution of the transient recovery voltage (TRV) across each vacuum interrupter (VI) is very important for the breaking capability of double-break vacuum circuit breakers (VCBs). According to the transient equivalent circuit of double-break VCBs, a mathematical model describing the TRV distribution characteristics was established; it revealed that the bias of TRV distribution was caused by the stray capacitance and the imbalanced post-arc plasma characteristics in each VI. The validity of the TRV distribution model was proved by simulation results based on a vacuum arc model. The experimental results under different values of grading capacitors demonstrated their effectiveness in improving the even degree of the TRV distribution in double-break VCBs. However, too large grading capacitor significantly magnified the amplitude and duration of the reignition current, which was detrimental to the successful breaking of double-break VCBs. It is advisable for this reason to limit the value of grading capacitors to those ranges which can guarantee sufficiently improve the TRV distribution.

Static voltage distribution of 126 kV modular multi-break vacuum circuit breaker prototype

In order to analyze the potential distributions of U-shaped arrangement 126 kV triple-break vacuum circuit breaker prototype, formed with fiber-controlled vacuum circuit breaker modules, and electric field distributions interior vacuum interrupters, three dimensional finite element models were established. The potential distributions and the electric field distributions interior vacuum interrupters of four models, including single pole prototype without supporting structure, single pole prototype with supporting structure, double poles prototype with supporting structure, three phases double poles prototype with supporting structure were analyzed with the established finite element models. The calculated results indicate that, under U-shaped arrangement, the static voltage distributions of the triple-break vacuum circuit breaker are non-uniform. The static voltage distribution of the high voltage break is more than 65%, and suitable voltage grading measures are needed. The supporting structure can be not installed in the circuit breaker in series prototype test.

Study on reignition characteristics of 126 kV vacuum circuit breaker with triple breaks

Starting from a transient equivalent model of a triple-break 126 kV vacuum circuit breaker (VCB) containing a dielectric reignition criterion, the reignition current waveform of the vacuum interrupter (VI) which took over the higher part of the transient recovery voltage (TRV) was simulated. A series damped resistor was added in the grading capacitor branch to extinguish the reignition current as early as possible. The results show that the effect of the damped resistor is optimal when its value is threshold between oscillatory and non-oscillatory decay. Excessive TRV which appeared across other un-reignition VIs and power frequency recovery voltage (PFRV) which appeared across the reignition VI were simulated. A metal oxide arrester (MOA) was installed in parallel with each VI to ensure breaking successfully by limiting the excessive TRV and PFRV. It is shown that the voltage limiting effect is preferable when a MOA with the reference voltage of 50 kV is in parallel with a grading capacitor of 1000 pF, also the thermal limit can be satisfied. Research results are expected to provide a basis for choosing proper dynamic voltage sharing measures for a triple-break 126 kV VCB.

Self-voltage sharing effects of prebreakdown current flowing through vacuum interrupters in series

To provide some theoretical and experimental bases for the development of multiple-break vacuum circuit breakers (VCB) in series, power frequency voltage breakdown tests of single and triple-break vacuum interrupter (VI) in series with a short gap length were carried out. After deriving from the test results, a phenomenon was found that the voltage distribution of each break was improved to a certain extent as the total voltage rising, and then the conclusion was proved by experimental results of power frequency voltage distribution in double-break vacuum interrupters in series. The power frequency prebreakdown current was introduced to analyze the above test phenomenon, and a self-voltage sharing effect of prebreakdown current flowing through vacuum interrupters in series was proposed. In fact, the success interruption of vacuum circuit breakers depends on transient recovery voltage (TRV). Therefore, for multiple-break vacuum circuit breakers in series, it is necessary to take some reasonable measures of voltage sharing such as shunt grading capacitors, rather than relying solely on the self-voltage sharing effects.