STATCOM control for operation with unbalanced voltages

In view of STATCOM operation analysis, special attention must be paid to the operation with unbalanced network voltages. In this paper a control algorithm that enables STATCOM operation with unbalanced voltages is presented. The algorithm is based on the compensator mathematical model in the d-q coordinate system. In case of an unbalanced voltage sag the compensator unbalances the generated voltages to limit the negative sequence currents. The problem of low-order harmonics generation is solved with the use of appropriate switching function modulation that enables the use of a relatively small capacitor on the dc side. The proposed algorithm was tested by means of simulation of STATCOM operation in the PSCAD program. The algorithm shows a good transient response also in the case of relatively deep voltage sags.