Traditional grid-connected inverters (TGCI) could suffer from small-signal instability owing to the dynamic interactions among inverters and a weak grid. In this letter, the small-signal sequence impedance model of the virtual synchronous generator (VSG) is built, and the sequence impedance characteristics of the VSG and the TGCI are compared and analyzed. The sequence impedance of the TGCI is mainly capacitive in the middle-frequency area, and the impedance amplitude is quite high. By contrast, the sequence impedance of the VSG, being consistent with the grid impedance characteristics, is generally inductive, and the impedance amplitude is quite low. Based on the sequence impedance model and the Nyquist stability criterion, the influence of the grid stiffness, the number of paralleled inverters, and the phase-locked loop (PLL) bandwidth on the stability of the VSG and the TGCI systems is analyzed. The stability analysis results show that the TGCI loses stability easily, whereas the VSG still works well without PLL restrictions under an ultraweak grid or with a large number of inverters connected to the grid. Therefore, the VSG is more suitable than the TGCI for achieving high penetration of renewable energy generation in an ultraweak grid from a system stability viewpoint. Finally, the experimental results validate the sequence impedance model and the stability analysis.

The premise that the virtual synchronous generator (VSG) can actively support the weak grid is that the VSG can operate stably. In this paper, the small-signal sequence impedance models of the voltage-controlled VSG and the current-controlled VSG are built and the sequence impedance characteristics of these two types of VSGs are compared and analyzed. The sequence impedance of the voltage-controlled VSG, being consistent with the grid impedance characteristics, is generally inductive. By contrast, the sequence impedance of the current-controlled VSG is mainly negative impedance in the middle- and high-frequency areas and the impedance amplitude is quite low. Based on the built sequence impedance models and the impedance stability criterion, the influence of the weakness of the grid on the stability of these two types of VSG grid-connected system is analyzed. The results of stability analysis show that the current-controlled VSG is prone to instability while the voltage-controlled VSG remains stable. Therefore, the voltage-controlled VSG is more suitable than the current-controlled VSG for grid-connected renewable energy generation in an ultra-weak grid from the point of view of system stability. Finally, experiment results validate the stability analysis.

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.

With the wide application of non-linear loads and the large-scale access of distributed energy generations based on power electronics equipments, power quality problems in the distribution network are increasingly serious with new characteristics. Further in-depth research is of great significance in theory and practice. This paper provides an overview of power quality analysis, compensators, and control technologies under the new situation of smart grid. It focuses on the topologies and control methods for power quality conditioners, especially new characteristics of power quality and applicable control technologies in microgrids and distributed power plants. Finally, trends and prospects of power quality control technology are introduced, which is important to achieve security and efficient operation of the smart grid.

In recent years, the virtual synchronous generator (VSG) concept has been widely studied to integrate renewable energy sources. However, instability occurs due to the implementation of the dc-link voltage controllers under the weak grid, and its mechanism remains unclear, which is investigated in this article. At first, the wideband dq -frame impedance models of the VSGs with the dc-link voltage controllers for two cases are established. Then, the stability analyses of the VSGs are compared based on these impedance models. It is revealed that the interaction dynamics between the dc-link voltage loop and the active loop lead to the negative resistor behavior of the q–q channel impedances for both VSGs, which induces the emerging oscillations of the system in a weak grid. Besides, as a useful design guideline, the parameter design of the VSGs is proposed to enhance the system stability. Finally, the experimental results obtained from a 100-kW prototype system show good agreement with simulated results, validating the impedance models and theoretical analysis.