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This paper proposes a frequency‐control scheme for Doubly Fed Induction Generator (DFIG)‐based (Type‐3) wind turbines to improve the primary‐frequency‐control when the grid power balance is disturbed. The increasing penetration level of renewable energy sources, like wind power plants, reduces the total available inertia of modern grids, which deteriorate the frequency response in case of sudden power‐mismatches. The proposed closed‐loop participation of wind power plant interacts with the thermal units to reduce the frequency nadir and frequency settling‐time, during the inertial and primary stages. The designed disturbance observer decreases the uncertainties in the estimation of grid parameters, which results in robust PI performance in adjusting the ancillary power provided by wind turbines. Certain measures considered within the control loop to limit the rate‐of‐change‐of‐frequency within the permissive range to avoid the protective relays tripping. Comparative simulations studies on modified IEEE 9‐bus and 68‐bus test systems verify the effectiveness and advantages of the proposed method. [ABSTRACT FROM AUTHOR]

This article proposes a flexible control scheme (FCS) of a photovoltaic (PV) power plant to support the voltage stability and provide fast recovery after the grid faults in a power system with high penetration renewable energy sources (RES). To achieve these goals, the proposed FCS includes two control loops: a reactive current (IQ) injection loop and a frequency-damping control (FDC) loop. The IQ injection loop injects IQ in proportion to the terminal voltage of the PV during a fault. After the fault clearance, the FDC loop provides active and/or reactive power modulation to stabilize system frequency; then, the FDC loop puts the system recovery forward by damping out the remaining synchronous machines' frequency oscillations. The FDC loop comprises active power damping function without PV power curtailment and reactive power damping function. We verified the proposed FDC's performance in a two-area system for various penetration levels of inverter-based resources using a PSCAD simulator. We investigated the control schemes' effectiveness by comparing the efficiency and the response in mitigating the post-fault oscillations and restoring the voltage regulation. [ABSTRACT FROM AUTHOR]

Growing trends in the deployment of inverter‐based renewable energy will decrease the inertia and frequency control capability of electric power systems by replacing conventional power plants; thus, the frequency of future power systems might be dynamic. This study proposes a capability‐coordinated frequency control (CCFC) scheme of a virtual power plant (VPP) including adjustable‐speed pumped storage hydropower (AS‐PSH), a wind power plant (WPP), and an energy storage system to support the frequency nadir and reduce the steady‐state error of system frequency. The CCFC scheme is based on a hierarchical‐control structure in which a CCFC organises the output of local frequency control units. To support the frequency nadir, the CCFC dispatches weighted frequency errors that are proportional to the available headroom of the units; thus, the errors are forwarded separately with a system frequency error to the primary control loop of each unit and thereby arrest the frequency nadir at a higher value than a system without the CCFC. To reduce the steady‐state error of the system frequency, the CCFC determines a partial active power command by additionally feeding an integrator of the CCFC with a modified frequency error that depends on the unit with the largest control. [ABSTRACT FROM AUTHOR]

This paper proposes a multi-timescale operational approach for a three-phase unbalanced distribution system containing hourly scheduling at substation level and minutes power flow operation at feeder level. In the substation scheduling model, the objective is to minimize the system cost with variable renewable generations. The given error distribution model of renewable generation is formulated as a chance constraint and derived into a deterministic form by Gaussian mixture model with genetic algorithm-based expectation-maximization. In the feeder scheduling model, the system cost is further reduced with the optimal power flow (OPF) at a timescale of minutes. Considering the nonconvexity of the three-phase unbalanced OPF problem in distribution systems, the semidefinite programming is used to relax the problem into a convex problem and a distributed computation approach is built based on alternating direction method of multiplier. The IEEE 123-bus distribution system, University of Denver campus distribution system, and the IEEE 8500-bus distribution system are used as the test bench for the proposed approach. The numerical results demonstrate the effectiveness and validity of the proposed method. [ABSTRACT FROM AUTHOR]

This paper proposes a torque limit-based inertial control scheme of a doubly-fed induction generator (DFIG) that supports the frequency control of a power system. If a frequency deviation occurs, the proposed scheme aims to release a large amount of kinetic energy (KE) stored in the rotating masses of a DFIG to raise the frequency nadir (FN). Upon detecting the event, the scheme instantly increases its output to the torque limit and then reduces the output with the rotor speed so that it converges to the stable operating range. To restore the rotor speed while causing a small second frequency dip (SFD), after the rotor speed converges the power reference is reduced by a small amount and maintained until it meets the reference for maximum power point tracking control. The test results demonstrate that the scheme can improve the FN and maximum rate of change of frequency while causing a small SFD in any wind conditions and in a power system that has a high penetration of wind power, and thus the scheme helps maintain the required level of system reliability. The scheme releases the KE from 2.9 times to 3.7 times the Hydro-Québec requirement depending on the power reference. [ABSTRACT FROM PUBLISHER]

Over the last decades, renewable energy sources have increased considerably their generation share in power systems. As a consequence, in terms of frequency deviations, both grid reliability and stability have raised interest. By considering the absence of a consensual set of models for frequency control analysis, both for the different generation units (conventional and renewables) and the power system itself, this paper provides extensive and significant information focused on the models and parameters for studies about frequency control and grid stability. An extensive analysis of supply-side and power system modeling for frequency stability studies over the last decade is presented and reviewed. Parameters commonly used and assumed in the specific literature for such simulations are also given and compared. Modeling of generation units are described as well, including both conventional and renewable power plants. • Power system modeling for frequency response analysis is reviewed and discussed. • Conventional supply-side modeling proposals and parameters are revised. • Variable Renewable Energy Source models for frequency analysis are also included. • More than 50 grid and generation unit parameters are described and compared. • Discrepancies of generation units and models are listed from the contributions. [ABSTRACT FROM AUTHOR]

Marine hydrokinetic power generation is a relatively new type of renewable generation. Its predecessors, such as wind power generation, hydropower plant generation, geothermal generation, photovoltaic generation, and solar thermal generation, have gained a lot of attention because of their successful implementation. The successful integration of renewable generation into the electric power grid has energized the power system global communities to take the lessons learned, innovations, and market structure to focus on the large potential of marine hydrokinetic to also contribute to the pool of renewable energy generation. This article covers the broad spectrum of marine hydrokinetic generation. The state of the art of power take-off will be discussed. Types of electrical generators will be presented, and the options for implementation will also be presented. [ABSTRACT FROM PUBLISHER]

This paper introduces the concept of constant volt/hertz operation of offshore wind power plants (WPPs). The deployment of offshore WPPs requires power transmission from the plant to the load center inland. Because this power transmission requires submarine cables, there is a need to use high-voltage direct current (HVDC) transmission, which is economical for distances greater than 50 km. In the concept presented here, the onshore substation was operated at 60 Hz synced with the grid, and the offshore substation was operated at variable frequency and voltage, allowing the WPP to be operated at constant volt/hertz. In this paper, a variable frequency at rated volt/hertz operation was applied to a Type 3 doubly fed induction generator (DFIG) wind turbine generator. The size of the power converter at the turbine can be significantly reduced from 30\% of the rated power output in a conventional Type 3 turbine to 5\% of the rated power. The DFIG allows each turbine to vary its operating speed with respect to the other turbines. Thus, small wind diversity within the WPP can be accommodated by the DFIG, and the collector system frequency can be controlled by HVDC to follow large variations in average wind speed. [ABSTRACT FROM AUTHOR]

With the advance of power electronics, direct-driven permanent magnet synchronous generators have drawn increased interest to wind turbine manufacturers due to its advantages over other variable-speed wind turbines. This article studies permanent magnet synchronous generator characteristics under the general d-q control strategy in the rotor-flux-oriented frame so as to benefit the development of advanced permanent magnet synchronous generator control technology. Compared to traditional approaches, the specific features of the article are (1) a steady-state permanent magnet synchronous generator model in a d-q reference frame, (2) a simulation mechanism that reflects the general permanent magnet synchronous generator d-q control strategy, (3) an integrative study that combines generator-converted power with extracted wind power characteristics, and (4) a joint investigation incorporating both steady-state and transient evaluations. Extensive simulation-based analysis is conducted to study how permanent magnet synchronous generator characteristics are affected by different d-q control conditions and are interacted with wind power drive characteristics in power generation and speed regulation of a permanent magnet synchronous generator wind turbine. [ABSTRACT FROM AUTHOR]

The main objective of this special issue is to collect and disseminate publications that highlight recent advances and breakthroughs in the area of renewable energy resources. The use of these resources for production of electricity is increasing rapidly worldwide. As of 2015, a majority of countries have set renewable electricity targets in the 10%?40% range to be achieved by 2020?2030, with a few notable exceptions aiming for 100% generation by renewables. We are experiencing a truly unprecedented transition away from fossil fuels, driven by environmental, energy security, and socio-economic factors.Electric machines can be found in a wide range of renewable energy applications, such as wind turbines, hydropower and hydrokinetic systems, flywheel energy storage devices, and low-power energy harvesting systems. Hence, the design of reliable, efficient, cost-effective, and controllable electric machines is crucial in enabling even higher penetrations of renewable energy systems in the smart grid of the future. In addition, power electronic converter design and control is critical, as they provide essential controllability, flexibility, grid interface, and integration functions. [ABSTRACT FROM AUTHOR]

Traditionally, inertia in power systems has been determined by considering all the rotating masses directly connected to the grid. During the last decade, the integration of renewable energy sources, mainly photovoltaic installations and wind power plants, has led to a significant dynamic characteristic change in power systems. This change is mainly due to the fact that most renewables have power electronics at the grid interface. The overall impact on stability and reliability analysis of power systems is very significant. The power systems become more dynamic and require a new set of strategies modifying traditional generation control algorithms. Indeed, renewable generation units are decoupled from the grid by electronic converters, decreasing the overall inertia of the grid. 'Hidden inertia', 'synthetic inertia' or 'virtual inertia' are terms currently used to represent artificial inertia created by converter control of the renewable sources. Alternative spinning reserves are then needed in the new power system with high penetration renewables, where the lack of rotating masses directly connected to the grid must be emulated to maintain an acceptable power system reliability. This paper reviews the inertia concept in terms of values and their evolution in the last decades, as well as the damping factor values. A comparison of the rotational grid inertia for traditional and current averaged generation mix scenarios is also carried out. In addition, an extensive discussion on wind and photovoltaic power plants and their contributions to inertia in terms of frequency control strategies is included in the paper. • Extensive review about inertia values for power systems and wind power plants. • Wind turbine and photovoltaic installation frequency control: inertial response. • New proposal to estimate the averaged inertia constant of power systems. • Estimation and comparison of averaged inertia constant in regions and countries. • Relevant information for grid frequency studies with high renewable integration. [ABSTRACT FROM AUTHOR]