Research On System Stability In AC/DC Hybrid Transmission With VSC_HVDC

With the gradually increasing of modern power system scale and the network has become increasingly complex, long distance, large capacity between the transmission and the large area of asynchronous interconnected power grid, The development of urban distribution network and environmental protection problem will be increasingly severe. Voltage-source inverter flexible HVDC technology can be passive inverter, does not exist in the ac/dc hybrid transmission system caused by commutation failure conveying power interruption threaten the whole system stability.VSC converter can steady control active and reactive power. In terms of reactive power compensation can have the effect of static synchronous compensator, provide emergency reactive power to support system. Studies of VSC-HVDC will have great significance to improve the ac/dc hybrid power quality, power supply flexibility and reliability, etc.Aiming at the shortcomings of the traditional dc transmission, using the multiple, the three layers of structure design of VSC-HVDC control system. Established the mathematical model of ac/dc system containing VSC-HVDC, inner loop current controller and outer loop power controller were designed by using double loop control strategy of PI regulation, the decoupling of the dq currents can control active and reactive power component of the system, and use the voltage slope control to improve the outer loop controller. Based on MATLAB platform to build the flexible HVDC simulation system, to do the simulation research of both ends of the active network power transmission and supply power to passive network, the result shows that the designed system can be accurate, the active and reactive power independent control and don’t need a communication between the two converter station. When power grid voltage drop is too large, the overcurrent protection of VSC can control the transmission power derating.The slope controller of dc voltage can inhibit the voltage fluctuation. In ac/dc hybrid networks, to do the parameter optimization for power system stabilizer, and use the reactive power compensation capability of VSC enhances the stability of hybrid system. The results show that the parameters of the optimized speed and generator Angle will be stabilized quickly, at the same time the power swing on the line has no effect on dc transmission line basic, VSC can stably control the dc transmission power, using the flexible dc asynchronous interconnection can eradicate the system low frequency oscillation.