A Study on Evaluation and Improvement for Interruption Performance of Molded Case Circuit Breaker

Abstract

The circuit breaker is basically a device that separates and protects the load from the fault-current. Therefore, it is always installed at the point where the load protection is required, and various circuit breakers are installed from the transmission system to the power distribution system. In a low-voltage distribution system, the Molded Case Circuit Breaker(MCCB) is a widely used device and this device is the closest device to the consumer. Fundamentally, when the fault current inflow to the MCCB, the trip-unit of MCCB detects it and the operation-unit separates the moving electrode and the fixed electrodes. At that time, the arc discharge occurs between electrodes and the hot-gas with high temperature and high pressure is generated also. Then, generated arc between electrodes is stretched and bent toward the splitter plate through influenced by the lorentz force and arc runner, gas pressure. In this process, the arc voltage between electrodes increases with its stretching due to the resistance of the arc proportional to the arc voltage. When the arc reaches to the splitter plate, electrically, the arc voltage rapidly increases by the voltage drop of the splitter plate and physically, the arc is stretched, cooled, divided by the splitter plate. After then, due to stretching and cooling, division in the splitter plate, the arc is extinguished at the novel current-zero point (This point is faster than conventional current-zero due to energy consumed by the splitter plate voltage drop and it make the fault current down. It is called the current-limitation effect). This is the basic interruption process of the conventional MCCB. However, considering the residual hot-gas, the interruption process is not over yet. The hot-gas generated by arc discharge when separation of contacts reduces the dielectric recovery strength between electrodes and if the dielectric recovery strength is lower than the recovery voltage applied from the power source when the fault current is interrupted, the re-ignition occurs. When the re-ignition is occurred, arc discharge is regenerated between electrodes and the fault current flows again. Therefore, even if the MCCB was tripped, it can not protect the load and leads to an electric leakage accident. In order to prevent such accidents in advance, it is necessary to grasp the dielectric recovery characteristic by the hot-gas after the current-zero of the MCCB and appropriately install. Also, for improve the interruption performance, consideration of effect by the hot-gas after current-zero is helpful when design the MCCB. This is the main purpose of this paper. In this paper, development of the evaluation technique for the interruption performance of the MCCB by measuring the dielectric recovery voltage is done and the influencing factors to the interruption performance by evaluating domestic and oversea products are analyzed. Then, by synthesizing the influence factors, novel splitter plate structures were proposed for improving the interruption performance of the MCCB by experiment and simulation.