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

This paper is to investigate the inertial dynamics of virtual-synchronous-controlled (VSynC) doubly fed induction generator (DFIG)-based wind turbines (WTs). VSynC, different from the conventional synchronization method based on phase-locked-loop (PLL) synchronizing technique, makes DFIG-based WT synchronize with power grid via the active power control (APC), and thus provide the desired inertial support to power grid. Further, an effective approach for describing the inertial dynamics of DFIG-based WT with VSynC is proposed by establishing the WT's electromechanical motion equation. The approach synthetically considers the impacts of the WT's different controller parameters, operating points, and, in particular, the variations of mechanical power caused by the rotational speed or pitch angle changes during the inertial response period. It also makes the essential of DFIG-based WT's inertia clearer, which, as a matter of fact, is controllable and manifests frequency-dependent characteristics, and noticeably differs from the fixed inertia time constant featured in synchronous generator (SG). The impacts of different controller parameters and operating points on single WT's frequency response characteristics are studied. Simulated results also validate the superiority of VSynC on inertial support capability and operation stability to the typical PLL-based vector control (VC), especially for weak grid conditions. Finally, the frequency response on wind power plant (WPP) level is initially explored and further research to improve VSynC is discussed.

On Inertial Dynamics of Virtual-Synchronous-Controlled DFIG-Based Wind Turbines

This study discusses synthetic inertia from the perspective of a transmission system operator and compares it to fast frequency response based on frequency deviation. A clear distinction of the meanings between these concepts is discussed, the basis of which is a description of their characteristics. A contribution and the purpose is the clarification of these concepts in addition to share the perspectives of a transmission system operator. The frequency response of a power system based on the Nordic system is examined for future scenarios with large amounts of wind power. Conclusions are drawn regarding the benefit of synthetic inertia compared with fast frequency response based on frequency deviation.

https://ietresearch.onlinelibrary.wiley.com/doi/pdf/10.1049/iet-rpg.2017.0370

Synthetic inertia versus fast frequency response: a definition

As renewable power generation becomes more prevalent, the problem of frequency stability has become a particular concern of transmission system operators (TSOs), especially those of small power transmission systems. Traditional wind generation systems do not provide frequency regulation because they are decoupled from the power grid. Therefore, as conventional thermal generators are replaced by wind generators, the issue of frequency regulation for wind generation systems has become increasingly important. To release the kinetic energy stored in the rotating mass, inertia and droop control loops can be added into the controller of a wind turbine (WT). This work proposes an advanced control strategy with the time-varying gains of two control loops. In the proposed strategy, the gains are determined based on the desired frequency-response time. Moreover, the initial gain of the control loop is determined based on the wind speed, considering the operating condition of each WT in a wind farm (WF). The effectiveness of the proposed method is verified using an actual power system, revealing that it can be used to improve frequency regulation in a power grid.

Frequency regulation at a wind farm using time-varying inertia and droop controls

To address the system operability challenges due to the continuous reduction of system inertia by increasing penetration of renewable power generation, this paper proposes a new synthetic inertia control (SIC) strategy for doubly fed induction generator (DFIG)-based wind turbine generators. The proposed SIC scheme enables individual DFIG generators to contribute an effective inertial response which helps to stabilise the rate of change of frequency and minimize the maximum frequency deviations in particular under severe disturbance conditions. To achieve the proposed SIC, lithium-ion supercapacitors are required in the dc link of the DFIG power electronic conversion system to increase extra energy storage capability. The DFIG's dc voltage operates at various levels following the derived SIC algorithms for energy exchange with the connected ac grid, and the conventional control system for DFIG is modified to implement the algorithms. The functions and system benefits of SIC scheme are verified using a DFIG model connected to a power system and compared with the simulations of conventional DFIG control scheme. Practical issues such as size, cost, weight, connection configuration, and charging/discharging rate of lithium-ion capacitors for the SIC are discussed and relevant recommendations are presented.

Synthetic Inertia Control Strategy for Doubly Fed Induction Generator Wind Turbine Generators Using Lithium-Ion Supercapacitors

The need for inertial response provided by wind turbines (WTs) has been discussed in the industry for more than five years now. Yet, as of today only very few grid codes include specific requirements. Hence, the number of wind farms in commercial operation using such frequency control features is low, and knowledge of the real capabilities and limitations of inertial response from WTs is limited. The objective of this article was to summarise and to assess two years of operational experience with inertial response provided by type 4 WTs installed in a 138 MW wind farm in the Canadian province of Quebec. In order to put the topic into context, existing and anticipated future grid code requirements in respect to inertial response as well as related performance criteria were summarised. Data from high-frequency measurement devices recorded at various operating conditions and grid situations was downloaded and processed. The actual performance was compared with the expected characteristics for each of the measurements, which allowed identifying areas for further development. Results of dynamic simulations demonstrated that the methodology and the models used for frequency control studies need to be improved.

Operational experiences with inertial response provided by type 4 wind turbines

Die Erfindung betrifft ein Verfahren zum Betreiben einer einen Generator aufweisenden Windenergieanlage (WEA) zum Einspeisen elektrischer Leistung in ein elektrisches Versorgungsnetz, wobei nach einem ersten Betriebszustand mit einer ersten Leistungsabgabe und einer ersten Drehzahl in einen zweiten Betriebszustand mit einer zweiten Leistungsabgabe und einer zweiten Drehzahl gesteuert wird, wobei zum Steuern in den zweiten Betriebszustand eine zum Einspeisen verfugbare aerodynamische Leistung bestimmt wird, aus dieser verfugbaren Leistung eine Solldrehzahl bestimmt wird und aus der Solldrehzahl eine an dem Generator einzustellende Sollleistung vorgegeben wird. The invention relates to a method for operating a a generator having wind turbine (WT) for feeding electrical power into an electrical supply network, controlled by a first operational state with a first power output and a first speed in a second operating state having a second output and a second speed is said to control in the second mode of operation an available for feeding aerodynamic performance is determined from this power available, a target speed is determined and from the target speed, a target power to be set at the generator is given.

Verfahren zum Betreiben einer Windenergieanlage A method for operating a wind power plant

The invention relates to a method for operating a a generator having wind turbine (WT) for feeding electrical power into an electrical supply network, controlled by a first operational state with a first power output and a first speed in a second operating state having a second output power and a second rotational speed is, wherein an available for feeding aerodynamic performance is determined for controlling in the second operating state, a target speed is determined from the available power and a target power to be set at the generator is determined from the target speed.

Nikolay Mihov

Papers

Operational experiences with inertial response provided by type 4 wind turbines

Verfahren zum Betreiben einer Windenergieanlage A method for operating a wind power plant

A method of operating a wind power plant