Research On The Control Method And PWM Strategy Of STATCOM

STATCOM based on cascade multilevel inverters is a new type of reactive power compensator, which can generate or draw reactive power from the power network. The main object of this paper is to deal with the control strategy of STATCOM used in distribution system under balanced loads and unbalanced loads, and new PWM strategy for Cascade STATCOM.First, the principle of STATCOM is analyzed in detail. By theoretical analysis, the mathematic model based on the i_p-i_q (real and reactive current) is modeled, which reflectsthe characteristics of STATCOM. Then, on the basis of this model, the multivariable nonlinear control method based on inverse-system is introduced. Consequently a pseudo linear i_p - i_q decoupled system is obtained and the fast and decoupled closed-loop control for reactivecurrent is achieved, and the control system based on DSP is designed. Triangular carrier phase shifting-switch frequency optional PWM (PS-SFO-PWM) for Cascade STATCOM is presented. The new PWM strategy brings about the effect of high frequency carrier without the increase of switch frequency, and at the same time improves the use of DC side voltage. Last, the control strategy of STATCOM under the unbalance loads is investigated. The compensation way with the instantaneous value of negative-sequence voltage to adjust the pulse width of balance compensation gains good characteristics, which can compensate negative-sequence current caused by loads effectively and quickly.