Under weak grid, wind farm can easily cause subsynchronous resonance (SSR) owing to the impedance interaction between wind farm and grid. In this paper, a synchronous compensator with battery energy storage system (STATCOM/BESS) is connected in parallel with grid to suppress this SSR. Meanwhile, analogizing virtual synchronous generator (VSG), an enhancing grid stiffness control strategy is proposed for STATCOM/BESS with low rated active power and high rated reactive power. According to the designed control strategy, the demand quantity of battery for STATCOM/BESS can be reduced by more than 50%, which can decrease the costs and simultaneously improve the security. Since the impedances of grid and STATCOM/BESS controlled by the designed control strategy both are basically inductive, the parallel impedance of grid and STATCOM/BESS is smaller than the single grid impedance, which equivalently enhances grid stiffness from the point of view of impedance. Moreover, as the proposed suppression method of SSR does not need to modify other controllers of power station nor detect harmonics, it is a general suppression method of the SSR induced by impedance interaction between power station and weak grid. Finally, experimental results verify the validity of the proposed suppression strategy for SSR.

Analogizing the sub-synchronous resonance (SSR) phenomenon of the series-compensated grid-connected system based on synchronous generator, the virtual synchronous generator (VSG) also has the low-frequency oscillation problem owing to the impedance interaction between VSG and series-compensated network. Stability of the series-compensated grid-connected system controlled by VSG is analyzed by the impedance-based Nyquist stability criterion. Due to the inductive impedance of VSG, VSG is easy to couple with the series-compensated network in the low-frequency areas, which can lead to the SSR. With the increase of the series compensation level, the stability of VSG becomes worse and the induced oscillation frequency becomes higher. Therefore, a mod-ulation signal feedforward active damping control strategy with time delay compensation is proposed to suppress this SSR, which directly feeds the active-damping control signal to the PWM modulator for reshaping the output impedance of VSG from the inductance to the resistance-inductance. Moreover, a value range of virtual resistance is given with considering the influence on the regular operation of VSG. Finally, simulation and experimental results verify the correctness of the SSR analysis and the feasibility of the proposed active-damping control strategy.

In islanded microgrids with complex feeder networks, distributed generation units (DGs) and loads can be freely connected to any node, leading to changes in the feeder impedance between nodes and the microgrid structure. However, traditional harmonic sharing control methods encounter difficulties with microgrids with complex topologies. Additionally, existing techniques essentially require constructing additional harmonic virtual impedance, possibly resulting in conflict between voltage quality and harmonic sharing effectiveness. To address this issue, this paper proposes an adaptive harmonic impedance reshaping method based on a consensus algorithm. The proposed method adaptively reshapes the inverter harmonic impedance by introducing harmonic current feedforward compensation control and impedance reshaping factor adaptive control, which can achieve accurate harmonic sharing while greatly improving the voltage quality of the microgrid. The proposed method avoids constructing additional harmonic virtual impedance and can be applied to microgrids with complex feeder networks and variable structures without acquiring prior microgrid configuration knowledge. Moreover, the proposed method is a fully distributed control, where the microgrid central controller (MGCC) is not required. Furthermore, the control parameter design method based on modeling and stability analysis is discussed in detail. Finally, simulations and experiments verify the feasibility and effectiveness of the proposed method.

For the distributed generation units (DGs) using virtual synchronous generator (VSG) control algorithm connected to the islanded microgrid with high penetration of nonlinear load, there is always a trade-off between harmonics sharing accuracy and voltage quality when using conventional harmonics sharing methods. To address this issue, an impedance-based adaptively reshaping method is proposed in this paper, which is based on harmonic current feedforward compensation control and impedance reshaping factor adaptive control, assigning a variable harmonic impedance reshaping factor for each VSG to improve the controllability of VSG harmonic impedance, thus simultaneously enhancing nonlinear load sharing and voltage quality. Moreover, a droop-based methodology for impedance reshaping factor tuning is presented to adaptively reshape harmonic impedance for each VSG without acquiring the line parameters in advance. The proposed method also enjoys the advantage of avoiding introducing additional virtual harmonic impedance, and thus it is immune from the inverter closed-loop gain throughout the dominant harmonic frequencies. Furthermore, the stability analysis and the key control parameter design are discussed in detail. Finally, the effectiveness of the proposed method is verified by simulation and experimental results.

In the series-compensated grid-connected system (SCGCS), there is an impedance interaction between the inverter impedance and the grid impedance that is prone to cause resonance in the SCGCS. In this paper, firstly, considering the effects of the phase-locked loop (PLL), current-loop, and frequency coupling, the broadband impedance model of the SCGCS is established. The stability of the SCGCS is analyzed by the impedance-based Nyquist stability criterion. It is found from the stability analysis that the impedance interaction between the inverter impedance and the grid impedance is the leading cause of the resonance. An impedance reshaping based resonance suppression method is proposed to suppress the resonance. The phase characteristics of the inverter equivalent output impedance are reshaped from the perspective of impedance. The phase margin at the intersection frequency of the inverter impedance and the grid impedance is improved. The proposed resonance suppression approach mainly consists of reshaping the current loop impedance and the novel phase-locked loop impedance. Finally, simulations and experiments are used to verify the feasibility of the resonance analysis and the effectiveness of the proposed control strategy.