Control scheme studies of voltage source type superconducting magnetic energy storage (SMES) under asymmetrical voltage

Symmetry of system voltage is very important to the performance of superconducting magnetic energy storage (SMES). Both theoretical analysis and simulation results show that, negative sequence current and 2nd harmonic power will be injected into the system, when conventional symmetrical fire is used in power conditioning system (PCS) of SMES. Two indices are used to evaluate the performance of PCS of SMES: the relative error of average value and 2nd harmonic component of active power supplied by SMES. To improve the performance of SMES, symmetrical vector control is put forward in this paper. According to the objective, there are three kinds of symmetrical vector control: positive sequence voltage control, zero negative sequence current control and constant active power control. Their principles are presented and improvements on the performance of SMES are verified by electromagnetic transient simulation. It shows that all those three methods decrease the relative error of average value of active power supplied by SMES. Zero negative sequence current control and constant active power control can also limit the relative error of 2nd harmonic component of active power supplied by SMES effectively.