This paper proposes a variable proportion coefficient based control scheme for the doubly-fed induction generator based wind turbines (DFIG-WTs) to implement frequency support by regulating rotor speed. To reduce the adverse impact caused by regulating DFIG-WT on the dynamic characteristic of the grid frequency, a two-stage switching control scheme is employed for the proposed method. In the first stage, the variable proportion coefficient is designed for emulating the virtual inertia of the DFIG-WT to provide inertia response. In the second stage, a fuzzy control scheme is employed to design the variable proportion coefficient for both quickly restoring the maximum power point tracking (MPPT) operation of DFIG-WTs and avoiding the secondary frequency dip to system. Case studies are undertaken based on WSCC 9-bus and IEEE 39-bus power systems, respectively. Simulation results show that the proposed control scheme can not only make grid-connected DFIG-WTs provide the friendly frequency support, but also help them to fully use the frequency regulation ability of synchronous generators for quickly restoring the MPPT operation.

The inertia level of power systems decreases with the access of inverter-based generation. Considering electric vehicles (EVs) as a typical controllable load, a consistent collaboration control strategy based on adaptive virtual inertia in multiple charging stations (CSs) is proposed. First, the relationship between system frequency and inertia is analyzed, and virtual inertia control strategies and CS distributed network modeling methods are introduced. Second, considering the frequency recovery process and the collaboration among CSs, an adaptive virtual inertia control strategy is improved to meet the demands of the application scenarios of EV and V2G, as well as a simulation method for CS inertia supporting power. Furthermore, a hierarchical collaborative control strategy for multiple CS frequency regulation is proposed considering the leader-follower consistency algorithm as a secondary control. Simulation and experiment results show that the proposed strategy can provide extra inertia, mitigate frequency deterioration and suppress frequency oscillations, as well as reduce the frequency regulation cost of CSs. This method can provide guidance for the aggregated regulation of demand response at distributed CSs and EVs.

Line-commutated converter based-HVDC (LCC- HVDC) is extensively used in modern power systems. Most of the LCC-HVDCs are implemented with the control strategy termed marginal current control method (MCCM), while their operation is sensitive to disturbance from the AC part of the system. This paper originally reveals the mechanism of two types of oscillations induced by the MCCM after AC disturbances, i.e. the current boundary induced oscillation and the current error induced oscillation. A coordinated marginal current control method (CMCCM) is proposed, aiming at suppressing the aforementioned oscillations and enhancing HVDC recovery performance. Compared to the MCCM that responds only to DC voltage, the CMCCM additionally considers AC voltage, AC current and power factor of the converter. AC grid's parameters are also taken into account. With a more considerate design, the empirical current boundary determined by the MCCM is replaced with an analytical one, to coordinately control DC current. Therefore, better performance of HVDC is achieved. Simulations in RTDS reveal that the proposed CMCCM can suppress the oscillations, speed up HVDC recovery process and moderate transient over voltage of HVDC.

High-voltage direct current (HVDC) transmission is playing an increasingly important role in modern power systems, and the resulted power/voltage stability issue has raised widespread concern. This paper presents an on-line power/voltage stability index (PVSI) for multi-infeed HVDC (MIDC) systems. Different from the existing indices which are developed mainly for off-line and static analysis, the proposed PVSI can be applied in real time. Effects of system changes on stability assessment such as change of system states and control strategies are considered. Thus, helpful guidance can be provided for on-line HVDC stability and controls. The PVSI is originally deduced for single-infeed HVDC systems in an “AC way” by analyzing the power and voltage stability of both pure AC systems and HVDC systems. Moreover, its on-line application in practical MIDC systems is realized by building an equivalent single-infeed model, and utilizing nowadays measurement and communication infrastructures such as wide-area measurement system (WAMS). The effectiveness of the PVSI is verified through simulations in real-time digital simulator (RTDS).

The subsynchronous resonance (SSR) of a double-fed induction generator (DFIG) and its suppression method are studied in this paper. The SSR may be aroused by the interaction between the double-fed induction generator and the series-compensated transmission lines. This paper proposes an expression of the electrical damping for assessing the SSR stability based on the complex torque method. The expression is derived by linearizing the DFIG model at the operating point. When the mechanical damping is neglected, the expression can be used to calculate whether the electrical damping is positive or negative to judge the SSR stability. The expression can quantitatively analyze the impact of the wind speed, the compensation degree, and the parameters of the rotor speed controller and the rotor-side converter controller on the SSR stability. Furthermore, a subsynchronous damping control (SDC) strategy is designed to suppress the SSR. The parameters of the SDC are optimized by particle swarm optimization (PSO) based on the electrical damping. Finally, the above research is verified by the PSCAD/EMTDC time-domain simulations. The results show that the stability of SSR decreases with the decrease of wind speed, the increase of series compensation degree, the increase of proportional coefficient, and the decrease of integral coefficient in rotor speed controller and rotor-side converter. The designed subsynchronous oscillation controller can suppress the SSR of a DFIG.

With the increase in the grid-connected scale of new energy, the ability to flexibility regulate a power system is greatly challenged. Since a variable speed pumped storage (VSPS) unit has a wider power regulation range and higher operation efficiency than conventional pumped storage (CPS), this study focuses on improving system flexibility with the VSPS unit. First, the probabilistic characteristics of forced outages affecting the flexibility of power units are studied. Considering the uncertainty of the net load prediction error, a probabilistic density method is proposed to quantify the flexibility of the power system. Second, in order to improve the flexibility of the system and take into account the comprehensive benefits of the VSPS unit, an optimization model for the VSPS unit to improve the flexibility of the grid was established based on bacterial colony chemotaxis that takes into consideration the operational constraints of the VSPS unit. Finally, to verify the effectiveness of the proposed optimization strategy using simulation, the proposed optimization model is used to compare the advantages of the VSPS unit’s ability to regulate system flexibility with a conventional pumped storage unit.