This paper proposes a RoCoF droop control, which relieves the transient frequency change process by controlling the energy of the DC side capacitor. Firstly, analyze and classify the control time scale of the wind power system (WPS). Under the electromagnetic time scale, combine the low-pass filter (LPF) and the differential link to obtain the control link of the system. Therefore, the transient process of RoCoF is effectively improved. Secondly, on the basis of the inertia analysis of the original synchronous generator, further derivation and analysis of the RoCoF droop control inertia characteristics. Compared with traditional droop control, RoCoF droop control enables the inverter to have the ability to improve frequency deviation. Then, the mechanism analysis of the bus capacitance voltage to the system inertia support is given, the relationship between capacitor reserve capacity and RoCoF is derived, which verifies the feasibility of capacitor-assisted frequency modulation. Finally, the experimental comparison with the droop control strategy is carried out based on the RT-LAB platform. INDEX TERMS Wind power generation, Droop control, Inertia and damping, Frequency deviation, Rate of change of frequency.

Owing to the increased proportion of new energy power generation, such as wind power and photovoltaics, connected to the island grid, the system powered by the voltage source converter-based HVDC (VSC-HVDC) is prone to oscillate or even lose stability when disrupted. In this study, considering the rapid power compensation (RPC), an active power control strategy has been suggested for the receiving-end converter station of VSC-HVDC that could efficiently suppress the low-frequency oscillation of the island power system. First, the mechanism of VSC-HVDC inhibiting low-frequency oscillation of the island grid is analyzed in this study, and then, it theoretically determines that the damping capacity and inertia level of the rapid power compensation control strategy are stronger than those of conventional droop control and inertia control. Second, the receiving-end converter station switches from the RPC mode to droop control in order to allow the system to have a smooth recovery from the steady-state operation in the later stage of oscillation suppression. Moreover, detailed control logic and state-switching strategies have been designed. Finally, the simulation reflects that the proposed control strategy has a stronger oscillation suppression ability, allowing it to obtain rapid suppression of low-frequency oscillation.

This paper proposes a fast phase capture (FPC) scheme and applies it to the control strategy of grid-tied inverters under imbalanced voltage. Firstly, a method for quickly extracting sequence components of power signals is derived through the symmetrical component method, and a real-time phase calculation method is proposed based on the rotating reference frame, thus avoiding the phase-locked loop (PLL) closed-loop adjustment process. Secondly, the grid-tied inverter suppresses the negative sequence current or power fluctuations through the direct resonance controller, so there is no need to separate the dynamically changing sequence current components. Finally, the experimental comparison with the PLL control strategy is carried out based on the RT-LAB platform. Compared with the previous phase-locking scheme, the FPC scheme has a faster phase-locking speed. Therefore, the fault ride-through capability of the grid-tied inverter is improved. INDEX TERMS Current control, Grid-tied inverter, Imbalanced grid, Phase detection.

No abstract