Control Technology Of STATCOM Under Load Unbalance Condition

As a freqently used power quality controller, static var compensator(STATCOM) is widely used in both transmmission system and distribution system,and it can solve the power quality problems elegantly like reactive compensationand voltage compensation at point of common coupling (PCC). Due to thecomplication of distribution system, the load unbalance is always existing, whichleads to the unbalance of current and voltage in power grid. As a result, the powerquality for customers is reduced to some extent, and the electric equipments in thepowe grid are badly influenced. Therefore, the compensation of voltage at point ofcommon coupling under load unbalance condition is a problem urgently to besolved.In this paper, the operating principles under different control objects areintroduced firstly. And the mathmetical model of STATCOM under load balancecondition is built using Kirchhoff’s law. On this basis, the mathmetical model underload unbalance condition is derived combined with symmetrical componnentmethod. The precondition of positive and negative sequence current separate controlis also analysed.Secondly, the paper proposes a novel equivalent circuit model of STATCOMfor voltage compensating at point of common coupling. The small signal method isused to linearize the nonlinear model, and the transfer function of input and ouput isobtained. It is rigorously shown that the STATCOM system of voltage compensatingat point of common coupling is non-minimum phase, through the analysis of transferfunction and equivalent circuit.Then, the adverse effects of STATCOM under load unbalance condition areanalysed theoretically, the simulation experiment is also used to prove the result. Toeleminate the effect, a voltage compensation control algorithm of STATCOM isproposed based on positive and nagetive sequence double current loop. The controlalgorithm can maintain the stability of the voltage at point of common coupling, andeliminate the voltage unbalance factor. The MATLAB simulation experiments verifyits effectness.Finally, the STATCOM hardware-in-the-loop simulation experiments based ondSPACE are completed. The results show that the proposed control algorithm cansolve the unbalance of voltage at point of common coupling effectively, and thevoltage magnitude is also regulated quickly. The control system has a good responsespeed, control accuracy and robustness.