Back-to-back voltage-source converters (VSCs) enable Type 3 wind turbines (WTs) to operate with zero-power transfer, and this allows them to restore the system both in black-starting and stand-alone loaded stages. Therefore, this paper first identifies the prerequisites for Type 3 WTs to act as voltage-source interfaces in the case of bulk system outage and reveals their inherent deficiency for system restoration from the voltage/frequency and output power (P-f and Q-V) relationships. These analyses provide the basic guidance for subsequent control updating. Afterwards, to allow Type 3 WTs to provide black-starting capacity, a concomitant control strategy and a restoration sequence are proposed. The uncovered P-f relationship is reinforced by self-maintained frequency control for the subsequent stand-alone loaded stage. The mechanism of frequency matching among multiple WTs is also discussed. Furthermore, a mathematical analysis for the impacts of controller parameters on the frequency characteristics of a Type 3 WTs-based stand-alone system is developed, thereby providing the guidance for control design. Eventually, a specific islanded system, merely consisting of Type 3 WTs and some passive local loads, is constructed to verify the feasibility of Type 3 WTs for system restoration and to validate the analytical results.

As wind turbines (WTs) become more prevalent, there is an increasing interest in actively controlling their power output to participate in the frequency regulation for the power grid. Conventional frequency regulation controllers use fixed gains, making it difficult for the WT to adjust its kinetic energy uptake to its operating conditions and to collaborate effectively with other WTs in the wind farm. In addition, the design of conventional frequency controllers does not consider their impacts on the mechanical structure. To address these issues, in this article, we model the cooperative frequency control problem for all the WTs in a wind farm as a decentralized partially observable Markov decision process and use a multiagent deep reinforcement learning algorithm to solve it. We also develop a grid-connected wind farm simulation model based on MATLAB/Simulink and OpenFAST, which can reflect the detailed interactions between the electrical and mechanical components of WTs. Simulation results show that the proposed strategy is effective in reducing frequency drops and has less impact on mechanical structure deflections compared with traditional methods.

Phase-locked loop (PLL) control provides a typical synchronization mechanism for voltage-source converters (VSCs) integrated into strong grids. However, such a synchronization mechanism is hardly adaptive to the high-impedance weak grid neither to the islanded grid. In the scope of low-inertia weak grid issues, additional frequency control is conceived as an effective way for VSC-based devices to improve the frequency dynamics. The involved two controls are studied in two separate areas. In fact, additional frequency control combined with PLL is an adaptive synchronization mechanism with the ability to adapt to impedance changes in the power grid, which allows VSCs to remain synchronization stability under either strong grid, weak grid or islanded grid, as demonstrated in this letter. Recognition of this synchronization mechanism provides a helpful insight into the stable operation of PLL-synchronized VSCs in different grid conditions.

This paper is committed to developing a thorough analysis on the electromechanical oscillations (EOs) of DFIG wind turbines (WTs), taking into consideration the implemented frequency control and drivetrain dynamics of the mechanical system. Some basic recommendations for mitigating EOs would be hence attained. To this end, a linearized model of DFIG WT is proposed to present the interplay between frequency control and the dynamics of WT's drivetrain and electrical system. Then the conventional torque analysis method is used to establish the basic effects of the frequency control on the damping of the drivetrain and electrical system frequency dynamics. Therein, the adverse effects could incur detrimental EOs, viz., drivetrain torsional oscillations (TOs) and electrical system low-frequency oscillations (LFOs). To mitigate such TOs and LFOs, this paper proceeds with an investigation of different alternatives to stabilize the torsional dynamcis of drivetrain and the frequency dynamics of electrical system, meanwhile, to minimize undesired impacts caused by their interactions. Simulation studies on two test systems validate (i) the basic concepts obtained from theoretical analysis and (ii) that the findings can be generalized to multi-machine systems.

This paper presents a dynamic analysis for ultra-low frequency oscillations (ultra-LFOs) observed in the wind-hydropower hybrid systems. In such systems, DFIG wind turbines (WTs) are required to be frequency-responsive with providing the inertia and frequency support. First, this paper establishes an analytical model to capture the dynamics of frequency-responsive DFIG WTs at the electromechanical timescale. A systematic analysis is then conducted on a 2-machine system (include a hydraulic generator (HG) and an aggregated DFIG WT) to reveal WTs’ dynamic behaviors and their interference mechanism with the HG, accounting for the system ultra-LFO mode. The result shows that WTs’ frequency control, speed control, MPPT control and pitch control would be involved in the system ultra-LFOs, but they have different effects under different operating modes. Cases studies are carried out on the 2-machine system and a modified 10 machine 39-bus New-England power system. Based on the modelling effort and simulation studies, some recommendations are made for using DFIG WTs to help damp the ultra-LFO in wind-hydropower hybrid systems.

The renewable energy resources integrating HVDC transmission systems, consisting of grid-following and grid-forming converters, have complex physical and control dynamic interactions. Although the impedance analysis method is advisable to study these interactions, impedance modeling is still challenging when taking various converter control schemes into account. This article proposes a modular three-port admittance modeling framework for the hybrid AC/DC power systems. This approach separates the modeling process into three levels: controller, converter, and system levels. The current and voltage feedback loops are subdivided into multiple submodules at the controller level, and the submodule connection method is proposed to obtain the small-signal model of the unified main controller, which thus can eliminate the repetitive work caused by the diversity of control schemes. Due to the modular modeling advantage, the method is also versatile and scalable, as well as using unlimited and expandable interconnecting. In addition, the modeling separates the entire system into three components, and the stability criterion of the hybrid AC/DC power systems is derived by regrouping their transfer function matrix models. Then, the proposed modeling method is applied to assess the stability of hybrid power plants integrating HVDC systems with grid-following and grid-forming sources. The conclusions can be used to plan GFM converter capacity in power System plants, the installation location of HVDC transmission systems, and other scenarios. Simulations are carried out to validate the correctness of the impedance modeling method and stability criterion. Finally, the effect of the different capacity ratios of grid-forming sources to renewable energy power plants on system stability is discussed.