Eliminating DC Current Injection in Current-Transformer-Sensed STATCOMs

Grid-connected power electronics converters/inverters usually have certain amount of dc component at their ac terminal. They are likely to inject unwanted direct current into the power grid, unless a line-frequency transformer is employed. This study investigates this dc injection problem for static synchronous compensator (STATCOM) and connects the dc injection with current transformer (CT), a current sensing element widely used in high-voltage converters. By introducing CT's model into the STATCOM's model, this study found that using CT for current feedback control can cause large dc injection, if no extra means is taken. Expressions of the dc injection for different controllers were derived. Then, a dc injection elimination method was proposed. This method eliminates the dc component of the output current by building an indirect dc feedback loop; therefore, it can prevent injecting the direct current even when it cannot be sensed. Experimental results taken on a 25-level cascaded multilevel STATCOM prototype verifies the analysis and the proposed method.