A Novel Synchronous Reference Frame Transformation Technique for Background Harmonics Elimination and Its Application

In recent years, the background harmonics in the grid are increasing for the substantial increase of non-linear loads. Background harmonics feed into the d-q coordinate system by reference frame transformation, which causes considerable errors in the d-q decoupled control strategy. The traditional phase-locked loop (PLL) solves this problem by using low-pass filters (LPFs). However, with the complication in tuning of LPF, the parameters in time under the condition of changing the operating mode is still difficult to adjust. To deal with such issues, this paper proposes a novel technique to reduce the background harmonic in the d-q coordinate system by means of reference frame transformation. Named nested decoupled synchronous reference frame (NDSRF) transformation, this technique decomposes the fundamental-frequency positive-sequence component and harmonic components by multilayer d-q synchronous transformation. Compared with existing transformation techniques, the proposed NDSRF technique is able to sharply reduce the undesirable transfer of background harmonic without filters. Furthermore, the proposed NDSRF is applicable to the modular multilevel converter (MMC) controller. Finally, the validity of the NDSRF technique and the proposed MMC controller are verified by a three-phase MMC based HVDC system, which is established in PSCAD/EMTDC. The simulation results show that the proposed novel NDSRF technique is superior over existing single synchronous reference frame (SSRF) and decoupled double synchronous reference frame (DDSRF) transformation techniques, in terms of less harmonic distortion for MMC controller under the background harmonics condition.