A Transformerless 6.6-kV STATCOM Based on a Hybrid Cascade Multilevel Converter Using SiC Devices

We have developed a full-scale prototype of transformerless static synchronous compensator (STATCOM), rated at 6.6 kV and 100 kVA, based on a hybrid cascade multilevel converter using SiC devices. The topology employs multivoltage converter cells, Si and SiC semiconductor devices, and hybrid modulations in the converters. One phase of the STATCOM has two Si insulated-gate bipolar transistor converter cells and two SiC-JFET converter cells connected in series. The SiC cells are operated with high-frequency pulse width modulation. The Si cells carry higher dc voltage, resulting in smaller converter volume by reducing the number of cascaded cells. To minimize the loss, the Si cells are operated with one-pulse. The balance control of dc voltage of the hybrid cells is also discussed. We tested the developed STATCOM in a full-scale 6.6-kV distribution grid. The experimental results show a stable rated operation and fault-ride-through performances. Moreover, we analyzed its loss and volume and discussed them based on the developed STATCOM. The estimated loss was approximately 0.3% at rated power, which is comparable to an all-SiC configuration. It also has an advantage in its small volume compared with other configurations. These analyses revealed that the hybrid configuration is a suitable topology for low-loss and small-size distribution STATCOM.