ANPC-Waveshaper-Based Medium-Voltage STATCOM With Natural Capacitor Voltage Balancing During Negative-Sequence Current Compensation

This article introduces a medium voltage (MV) multilevel STATCOM which is configured around an active neutral point clamped converter (ANPC) and waveshapers (WS) (named as ANPC-WS). The use of ANPC allows equal voltage sharing among the semiconductor switches and also makes it suitable for MV applications. The WS consists of cascaded half-bridge (CHB) submodules (SMs). Increased voltage levels can be achieved by increasing the number of CHB SMs across the WS. The ANPC part of the converter is operated at the fundamental frequency of the grid, which reduces the switching losses of the converter. In this work, the STATCOM operation of the converter has been studied for both positive as well as negative sequence current compensation. In traditional three-phase multilevel converters, e.g., CHB and modular multilevel converter (MMC), operation during unbalanced results in capacitor voltage deviation, which requires zero sequence voltage (ZSV) or zero sequence current (ZSC) injection or circulating current injection. The ZSV or ZSC or circulating current injection not only increases voltage/current rating, but also makes the control complex. The ANPC-WS converter has inherent symmetry that allows capacitor voltage balancing without additional ZSV or ZSC injection. This has been verified mathematically. Fault-tolerant operation of the converter after an SM is bypassed due to a fault has also been presented. After an SM fault, the converter can be successfully operated with minimum changes to the control. A three-stage capacitor voltage balancing control has been developed to take care of both normal and post-SM fault operation. A prototype of the proposed converter has been developed in the laboratory to evaluate the performance and to validate the claims.