Study On High Power AC Electronic Load

AC electronic load is a power electronic equipment which can take place of conventional real impedance load. It can simulate a fixed or variable load, and power energy can even feedback to AC grid. The original intention of electronic load is to use in AC power source burn-in test. Presently, foreign and civil researchers'research on AC electronic load mainly aim at controlling the average active power and reactive power supplied by AC source. But in these cases, power source output current is often distorted and therefore different from that in real load circumstance. There have been some AC electronic load products in market, which can drive power source output current same as that in real load circumstance, but their power is not so high and power energy is dissipated during testing which cause much waste. These two types of electronic load all have disadvantages, therefore they can only apply in power source test field and the effect of test is not so good. In this paper a new type of AC electronic load is investigated which converges the advantages of the two types of electronic load above. Its input side is completely equivalent to the real impeadance load and can be adjusted flexibly. It is presented in this paper that this AC electronic load can apply in dynamic physical simulation of power system. It can simulate the reactance of transmission line and the resistance of load and their changes, such as short circuit, break of transmission line, changes of load capacity and so on. It can greatly improve the flexibility, reliability and automation degree of dynamic physical simulation, decrease testing time and testing cost.Firstly, DC link AC-DC-AC converter is adopted as main topology structure and the input rectifier and output inverter consist of PWM rectifier. Secondly, Hysteresis PWM current control based on bipolar modulation is adopted as control mode. Then the realization method of fixed impedance and fixed current mode of electronic load is discussed. Control strategy and calculation arithmetic of reference current in these two modes is presented. In fixed impedance mode, reference current is obtained by solving circuit equation in numerical method. This method is of precision, real time and good dynamic characteristics. In fixed impedance mode, electronic load can simulate resistant-reactant impedance, resistant impedance and resistant-capacitive impedance. In fixed current mode, four-quadrant control is realized. The PSCAD/EMTDC simulation results show that electronic load can reliably simulate steady impedance and current, and impedance transition or current transition as well. When the output voltage of power supply is fluctuating, it can still work at given impedance and current. The two modes are realized therefore. With good dynamic characteristics, it is presented that this AC electronic load can apply in dynamic physical simulation of power system, instead of conventional load such as reactance and resistance which represent the transmission line and load seperately. PSCAD/EMTDC simulation results show that it can exactly simulate power system short circuit fault. The low order component of DC voltage and switch frequency of electronic load is then investigated, which can be used in prototype design. Based on theory study and simulation study, a prototype is then developed. The intelligent power module is used to construct the switch circuit. DSP is used as kernel controller and control software is developped. The experimental results show that the proposed control method is effective and the design function of AC electronic load is accomplished.