Control strategy for unsymmetrical operation of HVDC-VSC based on the improved instantaneous reactive power theory

During an unsymmetrical fault period, the AC voltage will become unsymmetrical, for a temporary fault this period may last more then thirty cycles, which is much larger than the response time of the HVDC-VSC. In order to improve the stability and reliability of the power system, it is desired that the HVDC-VSC system remain operation in the fault period. Besides, even in normal operation conditions, the voltage of a distribution system may be unsymmetrical. It is well known that the VSC (voltage source converter) is quite sensitive to the negative sequence component of the AC voltage. When there is a negative sequence component in the AC voltage, a second harmonic DC voltage will be produced, and the reactive power output of the VSC will ripple at the frequency of 100 Hz. For HVDC-VSC system, a ripple of do voltage will lead to a ripple of the transmitted power, and thus cause other VSCs in the system operate unsymmetrically, even those VSCs are connected to a symmetrical AC system. So unsymmetrical operation can spread through the do line. What most serious is that if the amplitude of the negative sequence component is high enough, safety of the valves and stability of the HVDC-VSC controller will be endangered. So it is necessary to study control strategy for unsymmetrical operation of HVDC-VSC system. In this paper, a feedback linearization control strategy for symmetrical system is developed first, and then the unsymmetrical condition is considered, and a feedback linearization control strategy for unsymmetrical system is proposed. The validity of the proposed control strategy has been verified by digital simulation using NETOMAC.