Frequency Regulation Support Research Articles

An optimal hybrid control strategy for supporting frequency regulation considering fatigue load mitigation of wind turbines

An optimal hybrid control strategy for supporting frequency regulation considering fatigue load mitigation of wind turbines

Enhancing Frequency Regulation Support through Several Synthetic Inertial Approaches for WDPS

The aim of this paper is to propose an enhancement to the primary frequency control (PFC) of the San Cristobal Island hybrid wind–diesel power system (WDPS). Naturally, variable speed wind turbines (VSWT) provide negligible inertia. Therefore, various control strategies, i.e., modified synthetic inertial control, droop control and traditional inertial control, if introduced into VSWT, enable them to release hidden inertia. Based on these strategies, a WDPS has been simulated under seven different control strategies, to evaluate the power system performance for frequency regulation (FR). Furthermore, the student psychology-based algorithm (SBPA) methodology is used to optimize the WDPS control. The results show that modified synthetic inertial control is the most suitable approach to provide FR. However, further exhaustive research validates that droop control is a better alternative than modified synthetic inertial control due to the negligible system performance differences. In addition, droop control does not require a frequency derivative function in the control system. Therefore, the hybrid system is more robust. Moreover, it reduces the steady state error, which makes the power system more stable. In addition, a pitch compensation control is introduced in blade pitch angle control (BPAC) to enhance the pitch angle smoothness and to help the power system to return to normal after perturbations. Moreover, to justify the performance of hybrid WDPS, it is tested under certain real-world contingency events, i.e., loss of a wind generator, increased wind speed, fluctuating wind speed, and simultaneously fluctuating load demand and wind speed. The simulation results validate the proposed WDPS control strategy performance.

Cyber-physical system planning for VPPs supporting frequency regulation considering hierarchical control and multidimensional uncertainties

Renewable energy resources with uncertain output are being connected to the power grid, which puts great pressure on the grid frequency regulation (FR). Virtual power plants (VPPs), which aggregate numerous flexible resources, have significant potential for participation in FR services. However, the FR capability of VPPs is often unreliable due to uncertain resource potential and unreliable communications, which limits their FR applications. Therefore, a cyber-physical system (CPS) planning method considering multidimensional uncertainties for hierarchical VPPs to provide reliable FR capacity is proposed in this paper. Firstly, a fast and reliable hierarchical architecture for VPPs is presented. Then, the reliable capacity model of VPPs for service planning is established within this architecture. The minimum probability scenario method based on the Pauta criterion and the failure mode and effects analysis (FMEA) method are used to quantify the impact of information-physical uncertainty in the model. Based on this, a CPS planning model for VPPs with the objective of minimizing investment and the constraints of normal capacity, reliable capacity and delay under various scenarios is constructed. By linearizing the model, the optimal solution for coordinating flexible resource pool composition, edge terminal allocation and communication can be solved. Numerical simulations on the improved IEEE 33-node and 123-node systems demonstrate the effectiveness of the proposed method.

Reliable Frequency Control Support Scheme Based on Wind Power Generator Combined with Rechargeable Energy Storage System Applying Adaptive Power Reference

To reduce carbon emissions in the atmosphere, the utilization of renewable energy sources has been on the rise. However, as their integration level increases, grid system operators require higher performance of the frequency response service for renewable energy sources, especially wind power generators (WPGs). Conventional frequency control schemes release kinetic energy depending on the fixed and adjustable gains in the system difference loop between the standard and current system frequency; however, these conventional schemes cannot provide frequency support outside of the rotor’s speed operating region. In this work, a frequency regulation support strategy employing a WPG and lithium-ion battery based on an adaptable power reference is implemented. This is accomplished by assigning different roles to the WPG and battery. As the primary frequency control support, the WPG uses a frequency deviation loop with adaptable gain which depends on the speed of the rotor and the difference in frequency. Additionally, to assist with the frequency control support, the battery operates based on its state-of-charge (SOC) and rotor speed of the WPG. For investigating the capability of the suggested technique, an IEEE 14-bus system is employed. Qualitative wake effect analysis is further presented in the study to determine the feasibility of the proposed approach which consists of the hybrid WPP–battery system for frequency regulation. The main limitations of this study and further research studies that can be performed in the future to improve the performance of the proposed technique are presented. The scenario study results show that the minimum frequency point during a synchronous generator trip obtains a higher value than conventional ones in the suggested strategy by releasing more stored energy from the WPG and the battery.

A sliding mode frequency control scheme for wind turbines participating in frequency regulation support in power systems

With high penetration of wind power, the apparent inertia of power systems is decreased, which seriously affects the safety and stability of the system frequency. Therefore, a novel sliding mode frequency control (SMFC) scheme is proposed for the wind turbines participating in frequency regulation support (FRS) in power systems, which can significantly improve the dynamic characteristics of the system frequency. At first, a power system frequency response model considering wind turbines participation in FRS is derived. In this model, the load disturbance, wind power disturbance and synchronous generator frequency regulation dynamics are combined as a lumped disturbance. Then, the design of the proposed SMFC scheme, which combines the super-twisting sliding-mode disturbance observer (STSMDO) with the super-twisting sliding-mode frequency controller (STSMFC), is introduced. The proposed STSMFC has ability to tune the system dynamics easily by choosing suitable sliding surface matrix via eigenvalue assignment method or optimal sliding manifold design. In such a frequency control scheme, the stability of the system frequency is guaranteed by the STSMFC, whereas the STSMDO is employed to estimate the lumped disturbance. Finally, numerical simulations verify the effectiveness and superiority of the proposed SMFC scheme.

Reinforcement Learning-Based Optimal Battery Control Under Cycle-Based Degradation Cost

Battery energy storage systems are providing increasing level of benefits to power grid operations by decreasing the resource uncertainty and supporting frequency regulation. Thus, it is crucial to obtain the optimal policy for utility-level battery to efficiently provide these grid-services while accounting for its degradation cost. To solve the optimal battery control (OBC) problem using the powerful reinforcement learning (RL) algorithms, this paper aims to develop a new representation of the cycle-based battery degradation model according to the rainflow algorithm. As the latter depends on the full trajectory, existing work has to rely on linearized approximation for converting it into instantaneous terms for the Markov Decision Process (MDP) based formulation. We propose a new MDP form by introducing additional state variables to keep track of past switching points for determining the cycle depth. The proposed degradation model allows to adopt the powerful deep Q-Network (DQN) based RL algorithm to efficiently search for the OBC policy. Numerical tests using real market data have demonstrated the performance improvements of the proposed cycle-based degradation model in enhancing the battery operations while mitigating its degradation, as compared to earlier work using the linearized approximation.

Frequency Regulation From Electrified Railway

With the improvement of traffic electrification, the coupling between power and transportation systems becomes more and more intensified. In this paper, electrified railway is firstly introduced into power system to support frequency regulation under high penetration of renewable energy sources. A novel frequency regulation framework consisting of day-ahead capacity estimation, frequency control parameters dispatch, and real-time response is constructed for electrified railway to provide primary frequency control support. Motion equations of electric trains under regulation are developed in detail. To fulfill the frequency regulation requirement and ensure punctual arrival, a multi-train joint response strategy is proposed to coordinate different electric trains for primary frequency control. The frequency regulation capacity estimation problem and frequency control power dispatch problem are established based on the multi-train joint response strategy. A novel sequential cutting plane algorithm is developed to linearly relax the nonlinear capacity estimation problem and power dispatch problem with ordinary differential equation constraints and iteratively tighten the relaxation area to improve solution optimality with desirable efficiency. Comprehensive numerical studies verify the effectiveness of the proposed primary frequency control framework, indicating the high potential of electrified railway to support frequency regulation of modern power systems.

An Interoperable Communication Framework for Grid Frequency Regulation Support from Microgrids.

Renewable energy sources, which are controllable under the management of the microgrids with the contribution of energy storage systems and smart inverters, can support power system frequency regulation along with traditionally frequency control providers. This issue will not be viable without a robust communication architecture that meets all communication specification requirements of frequency regulation, including latency, reliability, and security. Therefore, this paper focuses on providing a communication framework of interacting between the power grid management system and microgrid central controller. In this scenario, the microgrid control center is integrated into the utility grid as a frequency regulation supporter for the main grid. This communication structure emulates the information model of the IEC 61850 protocol to meet interoperability. By employing IoT’s transmission protocol data distribution services, the structure satisfies the communication requirements for interacting in the wide-area network. This paper represents an interoperable information model for the microgrid central controller and power system management sectors’ interactions based on the IEC 61850–8–2 standard. Furthermore, we evaluate our scenario by measuring the latency, reliability, and security performance of data distribution services on a real communication testbed.

Cooperative control framework of the wind turbine generators and the compressed air energy storage system for efficient frequency regulation support

This paper presents a cooperative control framework of the wind energy conversion system (WECS) and the compressed air energy storage (CAES). The proposed framework is mainly based on the coordination between the two units to improve the overall frequency response and mitigate the impacts of wind power uncertainty. Besides, the presented coordination aims to resolve the contradiction issue between the speed recovery system and the frequency regulation signal in the WECS. Accordingly, the CAES is involved in the frequency regulation under simultaneous charging and discharging mode to act as a complement unit of the WECS which assist to take the benefits of WECS fast inertia support and recompense the WECS limitations from obeying the load demand. The effectiveness of the proposed coordination has been examined under different scenarios and load profiles by using MATLAB/SIMULINK simulation. The results have confirmed the ability of the proposed coordination to improve the system frequency response compared to the uncoordinated control operations. Furthermore, the proposed control is able to limit effectively the impacts of the load and wind power fluctuations in the system frequency.

Primary frequency regulation support through deloaded offshore renewable power generators with HVDC‐link

Primary frequency regulation support through deloaded offshore renewable power generators with HVDC‐link

Overview of Frequency-Control Technologies for a VSC-HVDC-Integrated Wind Farm

In modern power systems, the increasing penetration of renewable energy resources has reduced the overall system inertia. However, the intermittent nature of wind power generation reduces frequency stability, which is a crucial issue. Consequently, modern power systems require that renewable energy sources, such as offshore wind farms, support the frequency regulation. High voltage direct current (HVDC) systems based on voltage source converters (VSCs) are currently being used to connect wind farms with the main grid. By adjusting the DC-link voltage of the VSC-HVDC, a capacitor can absorb or release energy to provide frequency support. In addition, the VSC-HVDC system can coordinate wind farms to support frequency regulation and thereby achieve better performance. Thus, this paper reviews and compares various frequency-control strategies for a VSC-HVDC-connected wind farm. This work implements in PSCAD/EMTDC a simulation of typical frequency-control techniques to verify their effectiveness and robustness. Furthermore, this paper shows the advantages and drawbacks of each frequency regulation method. Other auxiliary services for supporting frequency regulation, such as by energy storage systems, are also discussed. Finally, this paper provides complete recommendations for frequency regulation techniques for VSC-HVDC-integrated wind farms.

Distributed Control of Battery Energy Storage Systems for Improved Frequency Regulation

In this paper a distributed control strategy for coordinating multiple battery energy storage systems to support frequency regulation in power systems with high penetration of renewable generation is proposed. The approach is based on an online convex optimisation framework that considers both the operating costs of storage systems and the frequency regulation requirements. The proposed framework uses a dynamic regret based on the last available information up to the current point in time of renewable generation and demand, which are uncertainty sources. The approach provides more accurate control than the currently employed prediction-based algorithms and it can be implemented in a distributed manner using a multi-agent system framework that facilitates plug-and-play functionality. The effectiveness and scalability of the proposed strategy is assessed through several case studies.

Grid Frequency Regulation Support From Back-to-Back Motor Drive System With Virtual-Synchronous-Generator-Based Coordinated Control

The virtual synchronous generator (VSG) control, which enables inverter-interfaced distributed generators to possess inertia like synchronous generators, provides a promising solution to the lack of inertia of a future power grid. The energy buffer emulating the kinetic energy variation is essential to this technique. In this article, we propose a new control scheme for a smart motor, i.e., a back-to-back motor drive system which temporarily utilizes the kinetic energy stored in rotating loads. With this system, significant frequency regulation support can be achieved through merely a control mechanism update, and without any dedicated energy storage system. To achieve it, the VSG control is applied to the grid-side converter of the back-to-back motor drive system, and a coordinated control between the VSG control and the motor speed control is proposed. The proposed coordinated control has a faster frequency support response and a lower sensitivity to grid voltage unbalance and distortion than previous control schemes based on frequency measurement. The parameter tuning of the proposed control is discussed based on stability analyses and verified by simulation studies. The effectiveness of suppression of grid frequency fluctuation by the proposed system is verified through experimental results obtained with a commercial blower.

Frequency regulation support from aggregation of air conditioners based on the trigger value local update strategy

As the penetration of renewable energy sources increases, demand side resources like air conditioners (ACs), whose electricity consumptions account for a large share, present remarkable potentials as frequency reserves to maintain the electricity supply-demand balance. This study proposes a hybrid control framework based on a central controller and local controllers, under which air conditioning loads managed by the load aggregator are constructed as a virtual generator for frequency regulation. A well-founded setting and updating method of frequency deviation trigger values in consideration of lock on and off constraints is designed. In the level of the central controller, on the basis of priority lists, the actual droop curve composed of ACs is constructed and the initial frequency deviation trigger values are determined. In the level of the local controllers, on/off states of ACs are switched autonomously according to their trigger values. In order to prevent response power deviations caused by lock on and off constraints, a trigger value local update strategy is developed to adjust the frequency deviation trigger values in the local controllers, which would not increase communication burden between the central controller and local controllers. Simulation studies suggest the effectiveness of the proposed method.

Design of Resonance Damper for Wind Energy Conversion System Providing Frequency Support Service to Low Inertia Power Systems

Due to the higher proportion of non-synchronous power resources in microgrids, adequate frequency regulation supports are required to maintain acceptable system frequency stability. As one of the sources for frequency regulation support in microgrid operation, via proper droop controller operations, wind energy conversion systems (WECS) could deliver kinetic energy stored in their turbines to the grid when needed. However, as the system inertia becomes smaller, the droop controller's response to system disturbances could cause the WECS to resonate with the grid and lead to torsional stress and fatigue damage in the WECS drivetrain. To protect the WECS against such damage, a resonance damper based on pole-zero cancellation technique is proposed in this paper. Simulation results depict the possible resonance phenomenon, and the reduction of undesired fatigue damage by the proposed damper retrofitted to the droop controller while improving the overall frequency regulation. Sensitivity analysis results are also provided to verify its robustness for practical applications.

A Dual VSG-Based M3C Control Scheme for Frequency Regulation Support of a Remote AC Grid Via Low-Frequency AC Transmission System

This paper proposes a low-frequency ac (LFAC) transmission system capable of providing frequency regulation support for a remote ac grid aiming at enhancing its frequency stability. The LFAC transmission system using modular multilevel matrix converters (M3Cs) for a direct ac-ac conversion is receiving noticeable attention as an alternative solution for a long-distance transmission system. One of its potential applications is the system connection between a large ac grid and a remote ac grid, whereas the latter usually suffers from a fluctuant frequency due to lack of inertia. The core of this work is addressing a novel control scheme of the remote ac grid-side M3C, which makes both interactions with the LFAC system and the remote ac grid behave like a synchronous generator using a dual virtual synchronous generator (VSG) control scheme. This control scheme can enhance the frequency regulation in the remote ac grid, which the existing control schemes cannot provide. Average dc capacitor voltage versus active power (V ave -P) droop control is proposed to coordinate the power flow between the VSG controls applied in both sides of the M3C. To demonstrate the problem of the existing control scheme, as well as the benefits offered by the proposed control scheme, transient performance including load variation and fault events in the remote ac grid is studied and examined in the EMTDC/PSCAD software environment.

Chance-Constrained Frequency Regulation with Energy Storage Systems in Distribution Networks

One of the applications of energy storage systems (ESSs) is to support frequency regulation in power systems. In this paper, we consider such an application and address the challenges of uncertain frequency changes, limited energy storage, as well as distribution network constraints. We formulate a bi-level optimization problem that includes the operation objectives of the system operator and the ESSs, using chance constraints to account for uncertain frequency changes. The frequency regulation decision of the system operator depends on the ESSs decision to participate in the regulation service as well as the distribution network constraints. Due to the interdependencies between the ESSs demand fluctuations and the distribution network power flow changes, the system operator requires the ESSs’ operation information for frequency regulation decisions, which may not be available from the ESSs. Therefore, we propose a decentralized algorithm such that the system operator and the ESSs can pursue their own operation objectives, while ensuring the distribution network constraints are satisfied. We evaluate the performance of our method on IEEE 37-bus and 123-bus test feeders by considering combinations of ESSs with different sizes. Simulation results demonstrate that our approach can successfully coordinate the ESSs to regulate the frequency deviations.

Hybrid control of DFIGs for short‐term and long‐term frequency regulation support in power systems

This paper proposes a new hybrid frequency control strategy for doubly fed induction generator (DFIG)-based wind turbines (WTs) to simultaneously provide short-term and long-term frequency regulation support to the connected power system. The kinetic energy stored in the rotating mass and the mechanical power reserve of the WT are coordinately configured to participate in frequency recovery at both primary and secondary stages. A frequency response model (FRM) is also derived to analyse the mutual interactions between WTs and the power system. Based on the FRM, the impacts of relevant parameter variations on the overall system stability can be investigated through Nyquist criterion and modal analyses and the stable operation range of proposed strategy could be easily identified. Theoretical and numerical results have both verified the effectiveness of the proposed control strategy and its advantages over existing methods.

A BESS Sizing Strategy for Primary Frequency Regulation Support of Solar Photovoltaic Plants

This paper proposes a strategy for sizing a battery energy storage system (BESS) that supports primary frequency regulation (PFR) service of solar photo-voltaic plants. The strategy is composed of an optimization model and a performance assessment algorithm. The optimization model includes not only investment costs, but also a novel penalty function depending on the state of charge (SoC). This function avoids the existence of a potential inappropriate SoC trajectory during BESS operation that could impede the supply of PFR service. The performance assessment algorithm, fed by the optimization model sizing results, allows the emulation of BESS operation and determines either the success or failure of a particular BESS design. The quality of a BESS design is measured through number of days in which BESS failed to satisfactorily provide PFR and its associated penalization cost. Battery lifetime, battery replacements, and SoC are also key performance indexes that finally permit making better decisions in the election of the best BESS size. The inclusion of multiple BESS operational restrictions under PFR is another important advantage of this strategy since it adds a realistic characterization of BESS to the analysis. The optimization model was coded using GAMS/CPLEX, and the performance assessment algorithm was implemented in MATLAB. Results were obtained using actual frequency data obtained from the Colombian power system; and the resulting BESS sizes show that the number of BESS penalties, caused by failure to provide PFR service, can be reduced to zero at minimum investment cost.

Electric Vehicles Battery Wear Cost Optimization for Frequency Regulation Support

Due to their capacities and quick response, Electric Vehicle (EV) batteries can be used to support a number of power grid services and form a Vehicle-to-Grid (V2G) system. When aggregated and properly managed EV batteries can provide important ancillary services such as peak load levelling and frequency regulation. EVs can also provide various demand response services and help in renewable energy integration. The major challenge for having a wide-scale V2G system to effectively provide the above services is the availability of power which is limited by the battery degradation and the battery cycle life. The battery cycle life is inversely proportional to the charge/discharge cycles the battery goes through during its operation. Therefore, the charge/discharge operation should be optimized to maximize the benefit for both the EV owners and the grid operator. In this paper, we develop an EV charge/discharge optimization model that incorporates frequency regulation and electricity prices from both real and forecasting models into the objective function of the model. We develop a prediction and optimization model to reflect the effects of dynamic and static electricity and regulation prices on the battery cycle life. We present a case study for the charge/discharge scheduling problem utilizing real, predicted regulation and electricity hourly pricing.