Study On Dynamic Factors Forced Zero-crossing Technique Based On DC Interruption

In recent years, with the development of HVDC system, the application of HVDC is not limited to ships, subway and light rail. It can also be applied to the construction of large power grid. As an important device for interrupting DC current, the demand of the DC circuit breakers become larger and larger, high reliability is required, too. Lower voltage and higher current is applied to achieve the high current-capacity operation in vehicle DC system. When a failure occurs, the fault current rises to a very high level, fault current should be limited to a low level in a short time in order to avoid the impact on the circuit breakers and other electrical equipment. In the HVDC system, DC converter station is adopted for DC interruption, which has lower economy and flexibility when comparing to the DC circuit breaker. Therefore, the study of DC circuit breaker has practical significance.In this paper, artificial zero-crossing interruption technique is implemented when AC is superimposed on DC. Artificial current zero-crossing is the most essential feature of AC interruption, which provides best opportunity to extinguish AC arcs. In this process, dielectric recovery after artificial current zero is influenced by opening speed, the moment of applying transfer current, the change rate of current when zero-crossing and magnetic field between contact gap, so are extinguishing properties. First, vacuum arc model used for current transfer process simulation is built using ATP. Variation of current and voltage under different factors are analyzed. Simulation of the impact of magnetic field inside gap on arc is undertaken using ANSOFT. In the control system, which controls the circuit effectively, the main control unit is ATmegal6L, combined with LCD monitor12864, keyboards and other devices. High-speed repulsive mechanism is applied in the experiment. There are significant changes in speed by changing the pre-charging voltage of separating brake capacitors.Arc images are collected through experiments under different factors, their features are extracted. The change of frequency has significant impact on magnetic field, which is found through simulation. Distribution of the magnetic field has an important relationship with structure of the contact surface. By means of arc images gray-processing, combustion process can be clearly seen mainly on the cathode contacts. With the zero-crossing slope gradually increases, the erosion of the anode enhances. When the contacts separation is between0.5-3mm, at the same contact separation, the smaller of speed, the larger of arc burning area, and the erosion of contacts is more severe; When the contacts separation is between3-8mm, arcing on the surface of the cathode is aggravated with increasing velocity, which lead to severe erosion of the cathode surface. In summary, there is a certain open speed so that the arcing between the cathode and the anode arc is more uniform and erosion of contacts is relatively slight.