Study of the Current Limiting Device Based on Principle of Forced Current Zero

Abstract

With the development trend and rapid increase of the power grid capacity, short-circuit current level is increasing rapidly. How to make sure the steady and reliable operation of the power system is a big challenge. So it is of great importance to limit the short-circuit current effectively. A new kind of current limiting device based on current commutation is proposed in this paper, it consists of a high speed switch[1], counter pulse circuit and a limiting resistor. The function of a high speed switch is to open the fault current fast. The high speed switch can open the current at the forced current zero after a superposition of the counter pulse circuit. Then the current is commuted to a limiting resistor, the current limiting process is completed. According to the short-circuit current phase detection, the sequences coordination of the high speed switch and the counter pulse circuit, and the phase rule of three-phase short-circuit current, the control strategy at any phase of the three-phase short-circuit current is presented here. A circuit simulation is established by the Simulink tool box of Matlab. A lower current limiting experimental prototype is designed. Introduction At present, high-voltage switches of different parameters in different voltage levels are used as the primary actuator in delay protection of the power system in our country. When different types of short-circuit fault in the power system occur, the central control unit sends out the control signal to open the high-voltage switches, separating the fault component in a system from the normal section. However, there are two main problems in today's electric power system: First, due to the mechanical characteristics of circuit breaker itself, the time of breaking short-circuit current is long, from the occur of short circuit faults to the breaker opening to full stroke, it need 2-3 power cycle, meanwhile, fault current is far beyond the normal working current, therefore, the stability of the power system is impacted, electric equipment may be damaged, the system cannot guarantee the quality of electric energy. Second, along with the power dense degree increasing, the fault current level will continue to increase, but the circuit breaker itself is limited. The interrupting capacity of circuit breakers in power system is usually at the level of 63KA, it is with some technical problems to improve the voltage levels of circuit breakers. And replacing a lot of the existing circuit breakers in power system is not realistic. At present, methods for limiting the fault current is partition running. However, this method reduces the connection of the system. It is not conducive to the stability and reliability of the system. The damage of short-circuit faults mainly depends on the volume of short-circuit current, thus, efficiently and quickly limiting the short-circuit current can solve the main problems listed above[2]. The current limiter proposed in this paper based on the principle of current commutation can quickly limit the short-circuit current. International Conference on Intelligent Systems Research and Mechatronics Engineering (ISRME 2015) © 2015. The authors Published by Atlantis Press 1067 Design of fault current limiter based on the current commutation principle For the above context, the fault current limiter we studied based on current commutation can avoid the impact of short-circuit peak current. We also give a discussion of control strategy of the whole cycle. Compared to the fault current limiter based on current commutation at power frequency zero[3,4], its performance improves significantly. This current limiter creates a rapid forced current zero after a superposition of the counter pulse circuit, and then quickly completes the current limiting process. It prevents the power system from larger impacts. The principle of this current limiter: Fig.1 shows a schematic diagram of the current limiter based on principle of current commutation. It consists of a high speed switch, a limiting resistor and a set of counter pulse circuit in the opposite direction (each counter pulse circuit is composed of a capacitor, an inductance, and a thyristor.AC short-circuit fault can occur in the positive half cycle or the negative half cycle, therefore, we build two counter pulse circuit). In addition, the most obvious feature of the current limiter is precise control. The fault current volume and phase detection is conducted through the current detection system. The logical analysis of fault current volume and phase is conducted through the central control unit. After the logic judgment, the central unit outputs the relevant control signals which control the corresponding counter pulse circuit and high speed switch, make the counter pulse circuit and high speed switch complete the current limiting process in cooperation. The high speed switch and counter pulse circuit need to be charged in advance, so the current limiter also includes some charging circuit, etc.