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.

To reduce the sensitivity of static synchronous compensator (STATCOM) to power or voltage fluctuation induced by renewable energy generation or grid itself, it can add inertia for reactive compensation. Analogizing the virtual synchronous generator (VSG), a virtual synchronous compensator (VSCOM) is proposed in this article, which adds virtual inertia in reactive power regulation. According to the differences and the respective characteristics of VSG and VSCOM, a proportional-differential (PD) controller is proposed to achieve the fast control of dc-side voltage and cooperates with a series integrator to achieve the stable control of dc voltage. Meanwhile, an integral controller is designed for realizing the inertial control of reactive power. The small-signal model of the proposed VSCOM is built, and the design method of the control parameters is analyzed. Meanwhile, the inertial performance of the proposed VSCOM with different control parameters is analyzed from the unit-step response. In weak grid or multiple parallel devices, the proposed VSCOM is more stable than the typical STATCOM in direct reactive power control mode. Finally, experimental results validate the proposed control strategy and parameter design method.

For the wind farm connected to line-commutated converter-based high-voltage direct-current (LCC HVDC) and weak grid at sending end, subsynchronous resonance (SSR) is prone to occur due to the impedance interactions among wind farm, weak grid, and LCC HVDC. First, the mechanism and characteristics of this SSR are analyzed based on the impedance method. With the increase of the output power for wind farm or the decrease of the transmission power for LCC HVDC, the wind farm tends to cause SSR. Moreover, with the decline of the grid stiffness at sending end, the studied system easily causes SSR. Then, a wideband harmonic voltage feedforward control strategy of static synchronous compensator (STATCOM) is proposed to suppress this SSR. For the proposed damping controller of SSR, a constant voltage amplitude-based harmonic extraction method is designed without using filter. The harmonic extraction error of the proposed strategy is a constant |V _n - V _r |, which is induced by the difference between the constant voltage amplitude V _n and actual voltage amplitude V _r . By the theoretical analysis and simulation, this constant error |V _n - V _r | can be automatically counteracted by the PI controller of STATCOM. Finally, experimental results verify the validity of the proposed strategy.

Due to impedance interaction, the sub-synchronous resonance (SSR) issue of the renewable energy generation (REG) system connected to weak grid has recently become prominent. Firstly, the mechanism and characteristics of the SSR occurring in the REG system under weak grid are analyzed based on the impedance method. With the decline of grid stiffness, the REG system can easily cause SSR owing to the impedance interaction between the inductive grid impedance and the capacitive REG system impedance in the low frequency range. To suppress this SSR, an inertia phase locked loop (IPLL) is proposed, which can optimize the impedance of the REG system in the low frequency range. According to the impedance based SSR analysis, the REG system with the proposed IPLL can hardly cause SSR under weak grid. Moreover, the mechanism, characteristics and design method of the proposed IPLL is introduced in detail. By using the proposed IPLL, the SSR issue of the REG system under weak grid can be effectively solved. Finally, the theoretical analysis and the proposed mitigation method of SSR are validated with the simulation and experimental study.

In the current power system, the conversion between dc and ac is widely existing and the dc-side harmonic problem is prominent. To suppress the second-harmonic current at the dc side of a single-stage single-phase inverter, a dc hybrid active power filter (DC-HAPF) structure is presented, which composes of a bidirectional dc-dc circuit-based active power filter (APF) and a CL passive filter. In this article, the CL passive filter is used to mitigate the high-frequency harmonics and the APF is applied to compensate the low-frequency harmonic current. Meanwhile, the influence of the filter parameters on the harmonic suppression is analyzed based on the average switching model. In addition, for the control of the DC-HAPF, a nonlinear unified controller via feedback linearization is proposed, where the voltage and current dual-loop control is converted to a single-loop control of energy. By analyzing the control system stability and DC-HAPF's performance, appropriate control parameters are selected. To verify the feasibility of the proposed topology and control strategy, a 500-W single-stage single-phase inverter with the DC-HAPF is built and a good performance of the dc-side harmonic suppression has been achieved.