Yushi Koyama

System fault test of SiC device applied 6.6kV transformerless D-STATCOM

This paper reports the investigation results for transformerless Distribution-Static Synchronous Compensator (D-STATCOM) with Modular Multilevel Converter (MMC) topology. The D-STATCOM has cascaded Silicon-Insulated Gate Bipolar Transistor (Si-IGBT) inverter cells and Silicon Carbide-Junction Field Effect Transistor (SiC-JFET) inverter cells. SiC-JFET inverter cells are operated with Pulse Width Modulation (PWM) since the low switching losses. The Si-IGBT inverter cells have higher DC voltage than the SiC-JFET inverter cells, and they are operated with one-pulse operation that output one positive and one negative pulses during a cycle. MMC topology and high voltage Si-IGBT inverter cells realized connecting the D-STATCOM to 6.6 kV distribution system without transformer. A prototype model of the D-STATCOM rated at 6.6 kV, 100 kVA was built and its field test was executed. The experimental results prove the stable rated operation and Fault-Ride-Through performance.

One-pulse control for modular multilevel converter based STATCOM

This paper reports an investigation results for one-pulse control methods for modular multilevel converter (MMC) based static synchronous compensator (STATCOM). MMC-STATCOM has cascaded full-bridge inverter cells in each phase arm, and the phase arms are configured with delta connection. In proposed MMC-STATCOM, an one-pulse control is applied. With one-pulse control, every cell outputs one positive and one negative pulse during a system voltage cycle. Since switching frequency is very low, extremely low switching losses can be provided. Cells have a capacitor, and the capacitor voltages should be balanced. Average voltages of the capacitors in each phase arm can be balanced by injecting zero sequence current which flows through delta connection loop. Individual capacitor voltages among arm cells are balanced by sorting algorithm. Cells are sorted by the capacitor voltage and pulse-order (PO) is determined. According to PO, the output of the cells turns ON and OFF, and one-pulse voltage is output. Lower voltage cell is charged more and higher voltage cell is discharged more, therefore the capacitor voltages are balanced. However, the capacitor voltage cannot be controlled in the cells which are not involved with the output. The uncontrolled cells have the last order or previous ones in PO. The capacitor voltage of the uncontrolled cell decreases because of self-discharge. Conventional sorting algorithm is insufficient. In proposed control method, to solve this problem, put the last order cell in a controllable order. This control allows all the capacitor voltages to be balanced including the uncontrolled cells. The proposed control methods were confirmed by experimental model MMC-STATCOM. The STATCOM with one-pulse control operated under steady state and the unbalanced system voltages. The STATCOM kept stable operation under the both conditions. And all the capacitor voltages were balanced even it has the uncontrolled cells.

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.