As the requirements of economical operation and reliability on power grid are enhanced gradually nowadays, the existing under frequency load shedding (UFLS) scheme is not quite fit for the modern power system that integrates high wind power. In this paper, the impacts of high wind power penetration on the UFLS are discussed thoroughly. A novel adaptive load shedding (LS) scheme is presented taking the high wind power penetration into account. In the proposed scheme, the equivalent inertia constant (EIC) is calculated accurately to improve the power deficit accuracy so as to reduce the error of LS. The dynamic correction of power deficit is able to solve the negative effects of the wind power output random reduction/the wind generator tripping. Besides, the locking criterion is capable of avoiding the influences of the wind power output random increase on the LS, thus cutting down the LS costs and even preventing the frequency overshoot. Moreover, in terms of the LS parameters setting, the coordination of the low frequency protection of the wind generator and the frequency threshold is addressed. The location and capacity model of LS, which is based on the load characteristics, can ameliorate the frequency recovery process. Finally, the validity and robustness of the proposed scheme are verified in the simulations on the IEEE-39 bus system with high wind power penetration.

With an increase in the High-Voltage Direct Current (HVDC) infeed, the strength of the receiving-end AC grid decreases, leading to transient voltage instability. Voltage source converter (VSC)-HVDC stations have a large unit capacity and high controllability, offering great potential for voltage support of the receiving-end grid. A grid-following/grid-forming (GFL–GFM) hybrid control can improve the oscillation stability of VSC stations under both strong and weak grid conditions; however, most relevant studies have focused on oscillation stability, while little attention has been paid to transient voltage regulation performance. Moreover, a quantitative analysis method for the transient active- and reactive-power characteristics of the hybrid control is lacking. This paper proposes a series-connected GFL/GFM hybrid control strategy along with a quantitative dynamic power analysis method. By establishing the closed-loop transfer function model, the steady-state power control performance and transient reactive-power response of the proposed control are quantitatively analyzed. Electro-Magnetic Transient (EMT) simulation results verify that, compared with the existing hybrid synchronization-type control, the proposed series-connected scheme exhibits superior performance in mitigating transient low-voltage and overvoltage issues, with the minimum voltage dip improved from 0.3 p. u. to 0.8 p. u. and the maximum overvoltage after fault clearance decreasing from 1.38 p. u. to 1 p. u.

Grid-following (GFL) renewable energy stations may experience phase-locked loop (PLL)-dominated small-disturbance synchronization instability in weak grids, while grid-forming (GFM) devices can improve the damping of this mode. However, most current studies directly approximate GFM as an ideal voltage source without providing rigorous proof. There is also a lack of in-depth analysis on how the parameters of GFM affect its enhancement effect on the small disturbance synchronization stability of GFL. To address this gap, this paper establishes an ordinary differential equation model of the GFL/GFM hybrid system and proposes a fourth-order polynomial approximate root method suitable for polynomials with two pairs of weakly damped conjugate complex roots. Based on this method, the approximated form of the PLL damping is given, and the effect of the inertia time constant, damping coefficient, and capacity of a virtual synchronous generator (VSG) on the PLL mode damping was analyzed analytically. Moreover, it was proven that VSG with a certain level of inertia can play a role close to the ideal voltage source in improving the small disturbance synchronization stability of GFL. Finally, an electromagnetic transient simulation model was established on MATLAB/Simulink, and these conclusions were verified.