The large-scale application of wind power eases the shortage of conventional energy, but it also brings great hidden danger to the stability and security of the power grid because wind power has no ability for frequency regulation. When doubly-fed induction generator (DFIG) based wind turbines use rotor kinetic energy to participate in frequency regulation, it can effectively respond to frequency fluctuation, but has the problems of secondary frequency drop and output power loss. Furthermore, it cannot provide long-term power support. To solve these problems, a coordinated frequency control strategy based on rotor kinetic energy and supercapacitor was proposed in this paper. In order to ensure the DFIG provides fast and long-term power support, a supercapacitor was used to realize the droop characteristic, and rotor kinetic energy was used to realize the inertia characteristic like synchronous generator (SG). Additionally, the supercapacitor is also controlled to compensate for the power dip of the DFIG when rotor kinetic energy exits inertia support to avoid secondary frequency drop. Additionally, a new tracking curve of DFIG rotor speed and output power was adopted to reduce the power loss during rotor speed recovery.

With the increasing penetration level of wind turbine generators (WTGs) integrated into the power system, the WTGs are enforced to aid network and fulfill the low voltage ride through (LVRT) requirements during faults. To enhance LVRT capability of permanent magnet synchronous generator (PMSG)-based WTG connected to the grid, this paper presents a novel coordinated control scheme named overspeed-while-storing control for PMSG-based WTG. The proposed control scheme purely regulates the rotor speed to reduce the input power of the machine-side converter (MSC) during slight voltage sags. Contrarily, when the severe voltage sag occurs, the coordinated control scheme sets the rotor speed at the upper-limit to decrease the input power of the MSC at the greatest extent, while the surplus power is absorbed by the supercapacitor energy storage (SCES) so as to reduce its maximum capacity. Moreover, the specific capacity configuration scheme of SCES is detailed in this paper. The effectiveness of the overspeed-while-storing control in enhancing the LVRT capability is validated under different levels of voltage sags and different fault types in MATLAB/Simulink.

The lack of inertia and damping mechanism of photovoltaic (PV) grid-connected systems controlled by maximum power point tracking (MPPT) poses a challenge for the safety and stability of the grid. Virtual synchronous generator (VSG) technology has attracted wide attention, since it can make PV grid-connected inverter present the external characteristics of a synchronous generator (SG). Nevertheless, traditional PV-VSG is generally equipped with an energy storage device, which leads to many problems, such as increased costs, space occupation, and post-maintenance. Thus, this paper proposes a two-stage improved PV-VSG control method based on an adaptive-MPPT algorithm. When PV power is adequate, the adaptive-MPPT allows the PV to change the operating point within a stable operation area to actualize system supply-demand, matching in accordance to the load or dispatching power demand; when PV power is insufficient, PV achieves traditional MPPT control to reduce power shortage; simultaneously, improved VSG control prevents the DC bus voltage from falling continuously to ensure its stability. The proposed control approach enables the two-stage PV-VSG to supply power to loads or connect to the grid without adding additional energy storage devices, the effectiveness of which in off-grid and grid-connected modes is demonstrated by typical simulation conditions.

This study introduces a three-phase virtual synchronous motor (VSM) control and its possible application for providing fast-charging service from off-board chargers of electric vehicles (EVs). The main circuit of the off-board charger consists of a three-phase voltage source PWM rectifier (VSR) and a resonant LLC zero-voltage-switching converter. In the proposed control approach, VSM-controlled pre-stage VSR emulates the external characteristics of a synchronous motor (SM), simultaneously, droop control based on charging mode in the VSM can satisfy the demand of the EVs constant-current fast-charging; The post-stage DC–DC converter is responsible for stabilizing the DC bus voltage. The feature of this control strategy is that VSM and fast charging control are implemented by the pre-stage converter, which has better coordination. In the MATLAB, the equivalent synchronous grid of the distribution network supplies to the power battery through the off-board charger, and the effectiveness of the presented control is demonstrated by typical working conditions.

Due to the irreversible energy substitution from fossil fuels to clean energy, the development trend of future power systems is based on renewable energy generation. However, due to the incompatibility of converter-based non-dispatchable renewable energy generation, the stability and reliability of traditional power systems deteriorate as more renewables are introduced. Since conventional power systems are dominated by synchronous machines (SM), it is natural to utilize a virtual synchronous generator (VSG) control strategy that intimates SM characteristics on integrated converters. The VSG algorithm developed in this paper originates from mimicking mathematic models of synchronous machines. Among the different models of implementation, the second-order model is simple, stable, and compatible with the control schemes of current converters in traditional power systems. The VSG control strategy is thoroughly researched and case studied for various converter-interfaced systems that include renewable generation, energy storage, electric vehicles (EV), and other energy demands. VSG-based integration converters can provide grid services such as spinning reserves and inertia emulation to the upper grids of centralized plants, distributed generation networks, and microgrids. Thus, the VSG control strategy has paved a feasible way for an evolutionary transition to a power electronics-based future power grid. By referring to the knowledge of traditional grids, a hierarchical system of operations can be established. Finally, generation and loads can be united in universal compatibility architecture under consolidated synchronous mechanisms.

A large-scale power system breakdown in the United Kingdom caused blackouts in several important cities, losing about 3.2 percent of the load and affecting nearly 1 million power users on 9 August 2019. On the basis of the accident investigation report provided by the UK National Grid, the specific reasons for the sub-synchronous oscillation of Hornsea wind farm were analyzed. The Hornsea wind power system model was established by MATLAB simulation software to reproduce the accident. To solve this problem, based on the positive and negative sequence decomposition, the control strategy of grid-side converter of doubly-fed induction generator is improved to control the positive sequence voltage of the generator terminal, which can quickly recover the voltage by compensating the reactive power at the grid side. Consequently, the influence of the fault is weakened on the Hornsea wind farm system, and the sub-synchronous oscillation of the system is suppressed. The simulation results verify the effectiveness of the proposed control strategy in suppressing the sub-synchronous oscillation of weak AC wind power system after being applied to doubly-fed induction generator, which serves as a reference for studying similar problems of offshore wind power.