Frequency regulation scheme of doubly-fed induction generators (DFIGs) are implemented to improve the frequency stability achieving by rotor speed control. However, the coupling dynamics between the maximum power point tracking operation control loop and frequency regulation control loop would impair the frequency regulation capability. To eliminate the adverse influences, firstly, analytical derivation for coupling output power dynamics is represented when adopting frequency regulation scheme of the DFIG. Then, an adaptive frequency droop feedback control based on power tracking operation strategy, which is a decoupling control scheme for frequency regulation, is proposed to respond the dynamic grid frequency without reserving power. The frequency deviation-based droop control with dynamics gains is used to calculate the coefficient of power tracking operation of the DFIG. The system stability with various power tracking operation of the DFIG is analyzed, in addition, the stable boundary is used to limit the setting of coefficient of power tracking operation. Simulations are carried based on an EMTP-RV simulator. It is found that the suggested adaptive frequency droop feedback control based on power tracking strategy can provide dual frequency regulation capability and reduce the maximum frequency excursion under various scenarios.

The cost of Energy Storage System (ESS) for frequency regulation is difficult to calculate due to battery’s degradation when an ESS is in grid-connected operation. To solve this problem, the influence mechanism of actual operating conditions on the life degradation of Li-ion battery energy storage is analyzed. A control strategy of Li-ion ESS participating in grid frequency regulation is constructed and a cost accounting model for frequency regulation considering the effect of battery life degradation is established. The estimated operating life and annual average cost of the Li-ion ESS under different dead bands and SOC set-points are calculated. The case studies show that the estimated operating life of the Li-ion ESS under the actual operating condition differs significantly from the nominal life provided by the manufacturer under the standard condition and the full discharge mode. This paper provides an accurate costing method for the ESS participating in grid frequency regulation to help the promotion of the ESS to participate in the ancillary service market.

Power electronic devices interfaced doubly fed induction generators (DFIGs) cause a reduction in the power system inertia and further the power system becomes vulnerable to severe disturbances. DFIGs provide fast frequency response (FFR) strategies to the power system by increasing the output power. Nevertheless, these strategies produce a large frequency second drop or a slow rotor speed restoration. This article suggests a FFR strategy of a DFIG based on variable power point tracking control to boost the frequency support capability with grid-friendly rotor speed recovery. Furthermore, the benefit of the proposed strategy when participating in FFR is calculated. The test results illustrate that the proposed FFR strategy enhances the performance of the frequency nadir, size of SFD, and benefits of auxiliary FFR service. Hence, the proposed strategy provides a promising solution of ancillary service for FFR from a wind turbine generator to power systems.

The challenges of modern power systems with a high level of renewable generation penetration will impose increased ancillary service on Photovoltaic (PV) systems, including frequency regulation. In this paper, a frequency regulation strategy for PV systems without energy storage is proposed. The proposed strategy is divided into two layers: the frequency regulation layer and the power reserve control layer. In the power reserve control layer, a novel power reserve control is proposed as the core element and novel feature. The existing power reserve control for PV systems must estimate the maximum available power, which is difficult because of the variation in environmental conditions and complex modeling of PV systems. Instead, a power tracking control method based on a curtailment power-current curve is proposed. It can automatically track a given reserve ratio under any environmental condition without an irradiance sensor. In the frequency regulation layer, the combination of droop control and virtual inertia control is used to adjust the given power reserve ratio. Simulation results show that the proposed strategy offers good accuracy and tracking speed and can significantly improve the frequency response of the power system.

Large scale wind power integration has a negative influence on the frequency response. Assistant measurement improves the frequency stability of power systems under high wind penetration. The Proportional Curtailment Strategy (PCS) for wind turbines provides a primary frequency reserve for power systems. To solve the worthless curtailed wind power, the PCS is used to improve the utilization of wind power curtailment. Then the wind turbine and the lithium battery Energy Storage System (ESS) provide primary frequency reserves together. Different control strategies of ESS have been proposed based on the different methods for selecting valid reserves. The economic benefits of different control strategies have been compared based on the same frequency regulation reserve. The optimal control strategy is the maximum method. The economic benefit of the maximum value method is ¥4,445,300.

Aiming at the problem that the mechanism of sub-synchronous oscillation in photovoltaic power generation connected to weak AC system is unknown in low frequency band, a small signal model of photovoltaic power generation connected to weak AC system is established, its oscillation modes are analyzed, the participation of each state variable in each oscillation mode is calculated, and the promotion of damping torque method is put forward. Damping torque modeling is used to reveal the oscillation mechanism, identify the leading link, analyze the influence of working conditions, controller bandwidth and grid stiffness on system stability, and finally design additional ones according to the conclusion of mechanism analysis. The results show that the increase of input power, the decrease of DC voltage loop bandwidth and PLL loop bandwidth, and the decrease of grid stiffness will lead to the decrease of system damping and stability. Finally, the electromagnetic simulation model of photovoltaic power generation connected to weak AC system is built in PSCAD/EMTDC platform to verify the effectiveness of the damping torque modeling analysis results and the effectiveness of SSDC.