Study On Back-to-back VSC-HVDC Control Strategy Used For AC Looped Electromagnetically Coupled Power Network

With the development of higher voltage power grid, the electromagnetic loop has become hidden danger of power system because of the increasing transmission load. Open-loop, a conventional approach for elimination of loop operation, may cause the reduction of reliability and flexibility. VSC-HVDC is considered for loop-opening. In this paper, a new control strategy is proposed, which can separate the power grid from AC system fault and open the electromagnetic loop rapidly. The main works in this paper as follow:(1)Mathematical models of VSC positive sequence components and negative sequence components are established respectively within rotating reference frame. A control system model is established for VSC-HVDC using direct current control. Parameter setting models of outer loops are established. Objective function for optimizing parameters is structured under ITAE criterion, solving by genetic algorithm.(2)A control strategy to suppress positive sequence q-axis component in short-circuit current with consideration of AC voltage stability is proposed. Negative sequence component in fault current is suppressed though feed-forward decoupling control, after extracting positive and negative sequence component from AC voltage when the AC system is unbalanced.(3)A control strategy to suppress positive sequence d-axis component in short-circuit current in order to reduce the fluctuation of DC voltage is proposed. This strategy consists of modifying reference of active power and modifying limit amplitude of positive sequence d-axis current, which coordinated with the strategy of modifying positive sequence q-axis current reference. When the DC voltage drops or rises to a certain value in the case of three-phase short-circuit, the reference of active power from AC system injected to DC system becomes zero, which isolate the fault from the line and open the electromagnetic loop.