A transient damping method (TDM), introducing the frequency of a virtual synchronous generator through a high-pass filter (HPF) to the power reference, is usually adopted to stabilize the system against disturbances. However, different from a small-signal stability perspective, this letter reveals that the gain of HPF should not be too large due to the transient stability issues. Thus, an optimal parameter design method of the TDM is proposed for the improvement of transient stability. The experimental results validate the theoretical findings.

The power-synchronization control (PSC) method of grid-connected converters, which is based on the active power orientation, is a promising solution for high-penetration renewable power generation systems. Unfortunately, synchronous frequency resonance (SFR) is inevitably found in PSC-controlled converters, especially when connected to inductive networks, which seriously damages the stability of the system. The relatively small equivalent resistance of the inductive network is regarded as the main cause of SFR, and a virtual resistor (VR) is usually employed to dampen this kind of oscillation. However, another important reason for SFR is the nonminimum phase effect of the loop gains, which is discussed in this article. Considering the dynamics of the grid equivalent inductance, the small-signal model of PSC-controlled converter is established. By this, the mechanism of SFR and the nonminimum phase effect are revealed. Furthermore, a conjugate poles elimination control method based on auxiliary branches introduced into the power control loops is proposed to suppress SFR, which can also broaden the control bandwidth compared with the VR method. Finally, the theoretical analysis is verified by simulations and hardware-in-loop experimental results.

This paper develops a flexible power regulation and limitation strategy of voltage source inverters (VSIs) under unbalanced grid faults. When the classical power theory is used under unbalanced grid faults, the power oscillations and current distortions are inevitable. In the proposed strategy, the extended power theory is introduced to compute the power feedbacks together with the classical power theory. Based on the combination of the classical and extended power theory, the proposed strategy can achieve the sinusoidal current provision and the flexible regulation between three common targets, i.e., constant active power, balanced current, and constant reactive power. Meanwhile, the proposed strategy is associated with a power limiter, which is capable to keep the currents under the pre-defined threshold and to compute the maximum apparent power for better utilization of the inverter capacity. With this power limiter, the rated inverter capacity is fully used for both the active and reactive power provisions under unbalanced grid faults. Using the proposed power regulation and limitation, the VSI can avoid overcurrent tripping and flexibly regulate its power under unbalanced grid faults. All the conclusions are verified by the real-time hardware-in-loop tests.

Concerning the difficulty in unified modeling of direct-drive wind farm with virtual inertia and the unknown cause of low frequency oscillation, a detailed model of integrated dissipation energy of direct-drive wind farm is built. First, the dissipation energy model of wind generator and the integrated dissipation energy model of direct-drive wind farm containing virtual inertia and PLL is derived and built. On this basis, according to different sources of integrated dissipation energy, it is decomposed into free dissipation energy, resonant dissipation energy and forced dissipation energy. These three components respectively characterize the contribution of inherent damping, generator-grid coupled resonance and generator-grid interaction to the stability of system. And then, how the variation of wind speed and generator location affects the stability of system is analyzed quantitatively using the integrated dissipation energy. Finally, the model of IEEE 10-machine 39-bus system connected with direct-drive wind farm is built in RTDS for hardware-in-the-loop tests. Simulation results verify that, the detailed model exhibits three different oscillation states, and each generator in the wind farm contributes differently to system oscillation. The traditional equivalent model of wind farm only exhibits one oscillation state and cannot reflect the difference in contributions of generators, thus it may wrongly identify the stability of system, even aggravate oscillation.

This letter proposes a unified power control method for DFIG-DC system, which can seamlessly transfer from grid connected mode to standalone mode without changing control strategy and vice versa. There is no need to detect the interconnection switch to identify whether it is grid connected or standalone. The dc voltage and stator active power are both positive correlations with the magnitude of rotor current vector, which indicates that these two objectives can be combined as the unified power to generate the rotor current reference. The stator frequency can be controlled by the rotating speed of rotor current vector without calculating stator frequency which can reduce the parameter dependency. Finally, the experiments based on a 1 kW DFIG-DC setup is carried out to verify the proposed unified power control method.