Preservation of the environment has become the main motivation to integrate more renewable energy sources (RESs) in electrical networks. However, several technical issues are prevalent at high level RES penetration. The most important technical issue is the difficulty in achieving the frequency stability of these new systems, as they contain less generation units that provide reserve power. Moreover, new power systems have small inertia constant due to the decoupling of the RESs from the AC grid using power converters. Therefore, the RESs in normal operation cannot participate with other conventional generation sources in frequency regulation. This paper reviews several inertia and frequency control techniques proposed for variable speed wind turbines and solar PV generators. Generally, the inertia and frequency regulation techniques were divided into two main groups. The first group includes the deloading technique, which allow the RESs to keep a certain amount of reserve power, while the second group includes inertia emulation, fast power reserve, and droop techniques, which is used to release the RESs reserve power at under frequency events.

Inertia response and frequency control techniques for renewable energy sources: A review

In several countries, the wind power penetration increased tremendously in recent years. To ensure the proper functioning of the power system, some grid operators already require the capability to provide inertial response or primary control with wind turbines. This paper discusses the emulated inertial response with wind turbines by means of the synthetic inertia and the droop control strategy. The behavior of the synthetic inertia strategy is determined by the inertial and the droop constant, whereas droop control only has a droop constant. When these strategies are used, it is important to tune the control parameters depending on the power system to which the wind turbines are connected. Simulations show that it is possible to enhance but also to deteriorate the frequency response of the system, dependent on these parameters. For different power system compositions, the optimal inertial constant is always close to zero. This way, the synthetic inertia strategy reduces to a fast droop control strategy. This is an important outcome, as it means that no differentiation of the frequency is needed to obtain an optimal inertial response from the wind turbines, which is beneficial in terms of robustness. Consequently, droop control is a viable alternative for the synthetic inertia.

Droop Control as an Alternative Inertial Response Strategy for the Synthetic Inertia on Wind Turbines

Flywheel Energy Storage System (FESS) is an electromechanical energy storage system which can exchange electrical power with the electric network. It consists of an electrical machine, back-to-back converter, DC link capacitor and a massive disk. Unlike other storage systems such as the Battery Energy Storage System (BESS), FESS is an environmentally-friendly short- or medium-term energy storage system, which has the capability of numerous charge and discharge cycles. These characteristics make the FESS a suitable choice for different applications in the power system such as power quality improvement, power smoothing, renewable energies integration support, stability improvement, etc. This paper presents an overview on the structures and applications of FESS in power system and Microgrid (MG) and also challenges, problems and future works discussed. It can be a driver for development of FESS applications and also recommends suggestions to use its advantages in other areas. Investigation of different studies shows that FESS, as a developing technology, can play an effective role in the operation of the present and future power system and MG.

Review of Flywheel Energy Storage Systems structures and applications in power systems and microgrids

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.

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Power systems with high renewable energy sources: A review of inertia and frequency control strategies over time

With the increasing penetration of wind power in power systems, it is desirable for wind turbines to have similar characteristics as conventional synchronous generators Conventional generators provide frequency support to the grid through the methods of inertial response and primary and secondary frequency regulation, whereas variable-speed wind turbine generators (WTGs) do not have those desired abilities because they are integrated into the power grid via power electronic converters Although many different control strategies have already been published to enable WTGs to temporarily support the transient frequency, the published strategies may bring various negative effects to the system This paper proposes a new control strategy to compensate for inertia of the wind farm It can improve WTGs' temporary frequency support based on the coordinated control of the WTGs and the energy storage (ES) system The simulation results show that this strategy could provide better performance of temporary frequency support and overcome problems such as system frequency oscillation and a secondary frequency drop The proposed control strategy can be realized in all wind speed conditions with a small-scale ES system

Coordinated Control Strategy of Wind Turbine Generator and Energy Storage Equipment for Frequency Support

This paper introduces a method to improve the primary frequency contribution of grid connected variable speed wind turbine generators (WTGs). Using their energy reserve margins, deloaded WTGs are controlled to provide relief to the grid during depressed frequency conditions. The frequency support from individual WTGs is regulated based on the available reserve, which depends on the prevailing wind velocities. By continuously adjusting the droop of the WTG in response to wind velocities, its primary frequency response is significantly improved in terms of reduced stresses on WTGs during low wind speeds. The impact of variable droop operation on two aspects of WTG operation is investigated-primary frequency contribution and smoothening power fluctuations caused due to changes in wind speed. Also highlighted is the usefulness of this control when adopted by wind farms.

Primary frequency regulation by deloaded wind turbines using variable droop

This study presents a control strategy for the frequency regulation in a wind–diesel powered microgrid. With wind as a major energy resource, ensuring reliability and quality of power supplied in the system is a great challenge. To reduce the adverse effects caused by wind's variability, intermittency and uncertainty on the system frequency and improve the performance of diesel generator (DG), a solution is explored that involves the use of two different energy storage technologies. A test system is proposed consisting of a wind farm and a DG, supplemented by hydrogen storage with fuel cell (FC) as a long-term and a flywheel (FW) as a short-term energy storage. During low demand or high wind periods, the surplus energy generated is stored as kinetic energy in the FW and as hydrogen gas after water electrolysis. During periods of low wind speed or increased demand, the FW supplies energy by shedding its rotor speed and hydrogen is converted into electricity through the FC. The effectiveness of adding a short-term and a long-term energy storage in enhancing the robustness of wind–diesel system is demonstrated in this study.

Frequency regulation in a wind–diesel powered microgrid using flywheels and fuel cells

In this paper, a detailed ancillary service pricing mechanism must be in place to encourage wind farm owners to opt for deloaded operating strategies. The authors do not agree that “over-speeding” de-loading tends to respond poorly in the high wind-speed regime; on the contrary, the WTG response to frequency regulation will be better during high wind speeds due to higher reserve generation available. As demonstrated in this work, it is possible to reduce the stability related issues significantly by carefully selecting the deloading percent and the droop parameter of the wind turbine generator.

Closure to Discussion on “Primary Frequency Regulation by Deloaded Wind Turbines Using Variable Droop”

This paper discusses a control strategy for the integration of wind turbine generators (WTGs) with fuel cells (FCs), diesel generator (DG) and electrolyzer systems for the regulation of frequency in a microgrid. By incorporating a FC system and electrolyser, the issues related to the fluctuating wind power can be minimized up to certain extend. When the wind power output is high, the surplus energy is stored as hydrogen gas after water electrolysis and during low wind speeds; the stored hydrogen is used for generating electric power using fuel cells. The DG is responsible for the overall frequency balance in the system, which is assisted by WTGs and FCs. The effectiveness of the control strategy was verified through simulations by using detailed models of WTG, FC, electrolyser, hydrogen compressor and storage tank.

Frequency regulation in microgrid using wind — Fuel cell — Diesel generator

This paper presents some of the issues being encountered in the system frequency regulation function when the penetration level of wind energy in the electric grid increases considerably. Enhanced share of wind energy in the system mix will result in a proportional reduction of the conventional generating capacity. The traditional synchronous generators, which form the backbone of large power system, possess certain qualities which are very essential in controlling the grid frequency. Inertia and speed droop are two such important parameters responsible for balancing the system frequency. Since the modern wind turbine generators (WTGs) operate in variable speed mode to extract optimum energy from the wind, the electrical coupling between these generators and the grid will be missing due to the power electronics interfacing. Thus the increasing penetration of wind energy in the existing system will reduce the total inertia and increase the overall system droop, which in turn result in increased frequency deviations after load events in the system.

K. V. Vidyanandan

Papers

Primary frequency regulation by deloaded wind turbines using variable droop

Frequency regulation in a wind–diesel powered microgrid using flywheels and fuel cells

Closure to Discussion on “Primary Frequency Regulation by Deloaded Wind Turbines Using Variable Droop”

Frequency regulation in microgrid using wind — Fuel cell — Diesel generator

Issues in the grid frequency regulation with increased penetration of wind energy systems