A 35 kV/8 Mvar cascaded transformerless static synchronous compensator (STATCOM) has been developed as a star configuration with self-excitation. With multiple power supplies, each unit can be automatically bypassed when a fault occurs to implement (N-1) uninterrupted operation. The compensation uses the optimized carrier phase-shift PWM method, instantaneous current control and a capacitor balancing strategy based on the voltage difference to give fast dynamic reactive power compensation. The STATCOM also coordinates with the voltage and reactive power control (VQC) system. Tests show that the STATCOM efficiency reaches 99%, the degree of unbalance among units is not greater than 3.3%, and the response time is below 10 ms when a voltage sag is detected which maintains the power system. The power factor is also related to the coordination between the STATCOM and VQC systems. Experimental data and field operation results both show that the excellent performance of the transformerless 35 kV/8 Mvar STATCOM as a new solution for voltage and reactive power control in substations.
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