Research On Fault Control Strategy Of VSC-HVDC System

VSC-HVDC is the high voltage direct current transmission system, which is based on turn-off thyristor IGBT and PWM technology. It can instantaneously realize the fast decoupling control of active power and reactive power, transmit power to passive network, restrain synchronous oscillation, and constitute a multi-terminal DC system.Under the strong disturbance of high voltage, large current, many components of VSC-HVDC are subjected to overvoltage or overcurrent impulse. The double frequency active power oscillations and large overcurrent are occurred, when the converter of VSC-HVDC system is exposed to unbalanced faults in the AC grid. The performance of the system is affected. The safety of converter station equipment is threatened.The mathematical model of VSC-HVDC is established in this thesis. The inner loop current decoupling controller and outer loop controller are studied, when the system is under balanced voltages. The simulation model of VSC-HVDC is established using PSCAD simulation software. The simulation results show that the system has good steady and dynamic performance. When the AC side of converter is under unbalanced voltages, the sequence component real-time detection system is proposed using the dual second order generalized integrator. It can quickly detect the positive and negative sequence component of fault current and fault voltage. It has certain robustness for grid frequency disturbance. The proportional resonant controller in the inner current decoupling loop is proposed. According to the instantaneous power theory, the setting values of inner current controller are calculated using the virtual flux components. Verified by PSCAD digital simulation, the designed controller under the condition of asymmetric fault on the AC side of converter can effectively eliminate overcurrent and double frequency active power oscillation.In this thesis, when the VSC-HVDC system operating under balanced voltage conditions, the proposed control strategy has fast response and dynamic performance. When the asymmetric fault occurs, it can effectively restrain the double frequency active power oscillation and DC voltage oscillation. The operation ability of the system is improved.