With the increasing penetration of wind power, the effective inertia of the power system reduces. Besides, a series of interactive instability issues including sub-synchronous oscillations and harmonic oscillations were reported due to the weak grid effects. Broadly speaking, those issues are closely related to the PLL and its tuning. Therefore, recent literature make efforts to avoid using the PLL for converter control, e.g., the virtual synchronous generator (VSG) control. However, the VSG control usually employs multiple loops, where a fast switching frequency of the converter is required to decouple each loop's dynamic. This prerequisite is usually not met for wind power converters with high capacity and low switching frequency. To address these issues but also inherit the merits of a VSG on the grid-integration, this paper employs a new concept of PLL-less control and applies it to the Type-IV wind turbine, in which the grid-synchronization is realized by the dynamics of dc capacitor voltage. The virtual capacitor control is designed and added to the machine-side converter to eventually deliver adequate inertia to the grid. To justify the effectiveness of the proposed control, both the soft start-up validation and the thorough analysis of the overall smallsignal stability are presented. Several concerns of vital importance regarding the virtual capacitor design and stabilization control are discussed, where the mechanism of stability is revealed through the complex-power coefficient-based analysis. On this basis, a stabilization control method is proposed, which can enlarge the stable range of virtual capacitor coefficient and enhance the inertial response effect. Finally, the performance of the proposed method on the inertial response and the weak grid operation is evaluated by time domain simulations in PSCAD/EMTDC, which is proven effective overall.INDEX TERMS Wind power generation, weak grid, inertial response, stability analysis, state space model.

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The terminal-connected series dynamic braking resistor (SDBR) is applied to assist the lowvoltage ride-through (LVRT) of the doubly-fed induction generator (DFIG). With the fault current and switch-in of the SDBR, the stator voltage oscillates, thus the constant stator voltage drop assumption is invalid and the effect of the converter current control is weakened with the changing voltage-oriented reference frame. In this paper, the xy frame of the point of common coupling is applied to the converter control to avoid oscillation of the reference frame. The analytical expression of fault current with the SDBR and constant converter current control is derived. To evaluate the LVRT effect, the analytical analysis of the LVRT transient is carried out. The resistance of the SDBR is optimized based on an index combining the capabilities of the DFIG to provide the active power support and damp the electromagnetic torque oscillation. The uncertainties of the fault scenario are considered in the optimization algorithm by applying the probabilistic method. Simulation results show that the improved LVRT effect of the DFIG is realized with optimization to the SDBR resistance and its switch-in criterion.INDEX TERMS Doubly-fed induction generator, low-voltage ride-through, series dynamic braking resistor, constant current control, analytical fault current expression, fault uncertainties, probabilistic evaluation.

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The control of the power conversion system (PCS) in a battery energy storage system has a challenge due to the existence of grid impedance. This paper studies an impedance model of an LCL-based PCS in the d-q domain. The feature of a PCS connected to a weak grid is unveiled by use of an impedance model and a generalized Nyquist criterion. It is shown that the interaction between grid impedance and the PCS destabilizes the cascaded system in certain cases. Therefore, this paper proposes a novel control method that adopts virtual resistors to overcome this issue. The improvement in the control loop leads the PCS to a more stable condition than the conventional method. Impedance measurement is implemented to verify the correctness of the theoretical analysis. Experimental results obtained from a down-scaled prototype indicate that the proposed control method can improve the performance of the PCS under a weak grid.