Fast and accurate acquisition of current components is a key factor for an active power filter to realize transient control under unbalanced conditions. In this letter, a robust real-time algorithm, which rapidly separates the positivesequence component (PSC) from multiple decaying dc (DDC) components, dc bias component, negative-sequence component, and harmonics, is proposed. To this end, first, the multiple DDC components are detected in the multiple disturbance and unbalanced grid context, by making use of the periodicity of remaining components. The dc bias can be obtained accordingly, followed by the detection of a compound signal encompassing the positive-sequence component, negative-sequence component, and harmonics. Specifically, the compound signal can be extracted precisely, based on the detected DDC and dc bias components, with one grid cycle response time, or approximately, within half grid cycle. A switching logic of the two approaches is designed to shorten the overall convergence time and improve steady-state accuracy. The PSC is then effectively extracted by constructing the virtual orthogonal signal of the compound signal and using dq-frame filtering. Compared with the existing transient control techniques, the proposed scheme guarantees one grid cycle response time and simultaneously suppresses multiple disturbances. Finally, experimental results verify the effectiveness of the proposed method.
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.
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