Selection of passive elements for a three-level inverter based static synchronous compensator

Mathematical models for a three phase three-level static synchronous compensator (STATCOM) working with both fundamental frequency modulation and selective harmonic elimination modulation are described. The mathematical models were obtained by assuming that the power system and the operation of the STATCOM are on a balanced three-phase basis. The mathematical models were simplified to obtain the STATCOM AC side current, the DC capacitor voltage and the ripple for both switching strategies. These simplified design tools were validated using the exact model. The resonance between the DC side capacitance and the coupling reactance was studied using the mathematical model and the resonance frequency for the three-level STATCOM was obtained. The resonance frequency was validated against results of electromagnetic computer simulations. Using the design tools developed and considering the resonance studies, the process for selection of the size of DC side capacitors for a small STATCOM was demonstrated.