With the increasing integration of renewable energy, the traditional generating units are gradually replaced by voltage source converters (VSCs). VSCs using the traditional current control strategy based on phase-locked loop (PLL) can't provide active power support and frequency regulation under frequency drop. In this paper, power synchronization control (PSC) is applied to substitute the PLL. The PSC output phase and the active power generated by the converter are analyzed when the net frequency decreases. It is proved that, during frequency drop, the PSC output phase leads the phase of the grid voltage at a fixed angle and the converter output power can respond to the change of frequency inherently. In addition, an improved control strategy is proposed to further improve the active support capability of the converter. The theoretical analyses and the effectiveness of the improved control strategy are verified by time simulations.

Multi-terminal DC transmission technology has complex power flow control problems, which require the introduction of a DC power flow controller(DCPFC) to improve the freedom of control. In order to solve the insufficient power flow control ability of existing DCPFC, this article presents a hierarchical control strategy for a system consisting of a distributed control layer and a centralized control layer, to avoid problems caused by overload and uncontrolled power flow. For the distributed control layer, the small-signal model of multi-line capacitance full bridge DCPFC is established, which provides a design basis for the selection of closed-loop proportional and integral (PI) (proportional and integral) control parameters. For the centralized control layer, the deviation of DC voltage in virtual synchronous generator (VSC) terminal and the operating line current ratings are two optimization objectives. The optimal power flow solution obtained from the optimization problem are achieved by DCPFC which enables to regulate VSC terminal voltage. Therefore, line losses in the DC system can be minimized. This article creatively proposes a hierarchical control framework about DCPFC and VSC, further enhances the development potential of DCPFC.

The distributed generations are usually connected to the power grid through converters, but the traditional current control strategy based on synchronous reference frame phase-locked loop (SRF-PLL) is likely to have high current amplitude of negative sequence under deep sag situation. In this paper, active power synchronization control (APSC) is applied to substitute the SRF-PLL. First, the output phase of APSC is analyzed under unbalanced condition. Then, based on the phase estimation and current control strategy, the current amplitude of positive and negative sequence is obtained. The theoretical analysis shows that even under deep sag situation, APSC can lock the grid phase with accuracy and the current amplitude of negative sequence is relatively low. The APSC shows excellent control performance under unbalanced condition. The theorical analysis is verified by time simulations.

With the increasing capacity of renewable energy, the interaction between power electronic equipment such as Voltage Source Inverter (VSI) and weak grid becomes an important issue, which could cause the sub-super synchronous oscillation and threaten the stability of the power system. To enhance the system stability, the influence of SOGI bandwidth in weak grid is analyzed based on the impedance method. First, the small signal model of Second Order Generalized Integrator Phase-Locked Loop (SOGI-PLL) is given. Then, the single phase inverter impedance model with different SOGI bandwidths is proposed. The proposed impedance model shows that though other parameters are the same, the SOGI bandwidth would influence the VSI impedance in the sub-super synchronous frequency band. Further, with the consideration of SOGI bandwidth, the system stability is compared by using the impedance-based Nyquist criterion. The results show that only decreasing the bandwidth of SOGI without changing other parameters could improve the system stability in weak grid. The proposed impedance model and the influence of SOGI bandwidth are validated based on Matlab/Simulink.