Comparison of PLL-Based and PLL-Less Vector Current Controllers

This article presents a detailed and rigorous comparison between two vector current control (VCC) approaches for voltage source converters (VSC). A PLL-less VCC (also called the direct power control) has recently been proposed in several major publications with the assumption that it will improve the performance of conventional PLL-based VCC during weak grid conditions. In this article, first, we introduce an enhancement to both the PLL-based and PLL-less VCCs by using an optimal multivariable control design for both of them. Then, we show that both controllers perform equally during normal and weak grid conditions. However, the PLL-less VCC produces errors when the grid frequency variations occur and eventually becomes unstable when the grid is also weakened. Furthermore, the PLL-less VCC cannot produce a clean output current when unbalanced grid disturbances occur. This article proposes a method to address this latter issue. It also proves that the filters used in PLL-less VCC introduce similar dynamics to those of PLL. This article concludes that the PLL-based VCC is still the preferred choice for the grid-interfaced VSCs. Laboratory-scale experimental results are also included to validate the simulation results.