Characterization and optimization of soft switched multi-level converters for STATCOMs

In this work, three types of multi-level converters are compared for use in STATCOMs, and the flying capacitor multi-level (FCML) converter is regarded as the most promising topology. The problem in using the FCML converter for STATCOMs is that the flying capacitor becomes extremely large and the capacitors increase the manufacturing costs and volume, because it is difficult to increase the switching frequency of high-voltage devices. Therefore, the Auxiliary Resonant Commutated Pole (ARCP) technique, which is a softswitching technique for PWM converters, is applied to the FCML converter. The design policy of this work is to reduce manufacturing costs. Thus, minimizing the flying capacitor is regarded as being most important. To design this converter, it is necessary to calculate the total losses. The switching losses of the devices used in FCML converters depend on the DC-link voltage, flying capacitor voltages, and ARCP parameters (resonant capacitors and resonant inductors). Therefore, an automatic device characterization system has been developed and several high-voltage devices have been characterized. Furthermore, a loss-implemented simulation to calculate the operational losses of ARCP converters is also developed in this work. Then, ARCP parameters, main switches, auxiliary switches, and flying capacitors are optimized by loss-implemented simulation to design a five-level FCML converter (5 MVA, 6.6 kV). Finally, the total losses and the manufacturing costs of the hard- and soft-switched FCML converters are compared and discussed.