Daochun Huang

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.

A novel approach to estimate temperature of conductor in cable joint

The power cable has been widely used for decades. As an important accessory of power cable, the cable joint is the weak link because of site installation conditions. Partial discharge and insulation aging in it cause temperature rise, which may lead to explosion of the cable joint in serious conditions. There are many researches about temperature rise of the cable and cable joints. By measuring the surface temperature and conductor current, some scholars in Tokyo proposed a way to estimate conductor temperature in cable joint with a 2-D thermal circuit. A method to estimate soil thermal parameters and assess cable ratings is introduced, in which the location of hot spots, cable installation data, load and DTS-recorded temperature should be the given information. Considering the influence of moisture content on thermal resistivity and specific heat of the cable surroundings, an algorithm for the estimation of the time-dependent temperature evolution of power cables is presented. Just as partial discharge is an feature of defect in the cable joint, temperature rise can also reflect defect in it. So, the estimation of transient temperature is of great meaning. In this paper, the authors propose a novel approach to estimate transient temperature of conductor in cable joint for buried cables.

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.

Mechanical fault diagnosis of high voltage outdoor disconnector based on motor current signal analysis

High voltage disconnector is one of the most widely used switchgear equipment in power system. Mechanical faults caused by improper installation and corrosion of metal components severely affect the normal operation of disconnector. In this paper, various mechanical faults of outdoor disconnector from actual field are displayed and discussed. To diagnose the mechanical fault of motor-driven disconnector, the stator current of driving motor is detected by an acquisition system consist of Hall current sensor, N1 data acquisition card and Lab VIEW software, while the relation equation between motor current and output torque of operating mechanism is derived. The stator current waveforms during switch-on and off process of disconnector are analyzed. The proposed method based on motor current signal analysis has been applied to a 220 kV pantograph disconnector by comparing the characteristics such as peak value and peak time of motor current signals in normal operation situation with the condition of jammed transmission mechanism, that is, simulated mechanical fault. The results indicate that there is a quadratic function relationship between the motor current and the operating torque. The stator current waveform of driving motor may distort when mechanical faults occur on the disconnector. The proposed method of this research work can effectively detect mechanical faults of disconnector thus to improve the reliability of power equipment.

Review of numerical simulation methods used in interrupting process of vacuum switch arc

The interruption of vacuum switches is a complex physical process and numerical simulation in close coordination with appropriate key experiments is a feasible way to study it in depth. Numerical simulation helps to explain the experimental results and predict the breaking performance of vacuum switches. Vacuum arc plasma is not in local thermal equilibrium, and magnetic fields and effects of contacts play key roles in the interrupting process, consequently, numerical simulation of the interruption process of vacuum switches is still immature in contrast to SF6 switches. However, it has been developed rapidly in recent years as the vacuum arc theory consummates unceasingly. The application status of the magneto-hydrodynamic (MHD) model, the particle in cell-Monte Carlo collisions (PIC-MCC) simulation, the fluid-particle hybrid model, the anode thermal model and the analytical model in the numerical simulation of the interruption process of vacuum switch arc are summarized in this paper. Furthermore, some problems needed to be further discussed or possible research directions have been put forward.

Influence of fault parameters on breaking performance of vacuum circuit breaker

In order to analyze the influence of power system fault parameters on the breaking performance of vacuum circuit breaker (VCB), a black box model of VCB based on the diffuse vacuum arc interruption has been established in this paper. Taking the current zero as the demarcation point, arc voltage is modeled during the arcing period and post arc current (PAC) is modeled based on the theory of Langmuir probes during the post arc period. Based on this model, the equivalent discount factor of the breaking capability of VCB caused by the direct current (DC) component of short circuit current is evaluated using different methods. In addition, the post arc conductivity is used to represent the dielectric strength recovery of VCB, and the results show that the rate of rise of transient recovery voltage (TRV) mainly influences the post arc dielectric recovery rate after 1.5 μs, while the rate of decrease of short circuit current prior to current zero only influences the initial post arc dielectric recovery rate. This model is expected to provide an effective way to predict the breaking performance of VCB.

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.