Frequency Regulation Strategy Research Articles

DOBC-based frequency & voltage regulation strategy for PV-diesel hybrid microgrid during islanding conditions

The integration of renewable energy sources (RES) based distributed energy resources (DER) lead to reduction in overall system inertia which causes large voltage and frequency fluctuations during any intermittency. This necessitates the integration of fast and robust controllers for low-inertia microgrids. Therefore, this paper proposes disturbance observer based control (DOBC) approach for frequency and voltage regulation of microgrid consisting of solar PV and diesel generator (DG) based hybrid microgrid under islanding situations. The DOBC is included as feed-forward control to the synchronous generator-based DG, which handles the real-time power mismatches and regulates frequency and voltage of islanded microgrid. To demonstrate the operational resilience of the designed controller under real-time uncertainties in PV and load, the controller has been evaluated under the most extreme uncertainty scenarios. The suggested controller has been implemented on a MATLAB/Simulink platform, and results have been validated utilising the real-time simulator OPAL-RT. In addition, the efficacy of the suggested control strategy has been evaluated in the presence of communication delays and noisy load circumstances. Results demonstrate the controller's dynamic performance in regulating the frequency and voltage for low-inertia microgrids. Finally, the proposed control technique has been executed on laboratory-scale microgrid to demonstrate the potential of developed controller.

Self-Synchronizing Control and Frequency Response of Offshore Wind Farms Connected to Diode Rectifier Based HVDC System

Light-duty offshore wind power system has been a research hotspot in recent years. The diode rectifier based HVDC scheme is a promising solution and has attracted more and more attention. In this paper, a self-synchronizing control for offshore wind farms connected to diode rectifier based HVDC system is proposed. The active power of each wind turbine is controlled by regulating the amplitude of the AC voltage, while the reactive power is used to adjust the offshore frequency. The proposed control is a decentralized scheme. Thus, no communication network or GPS is required and only the local information is used in the control system. The synchronization of wind turbines is achieved by the proposed self-synchronizing frequency control instead of using phase-lock loop. Besides, in order to make offshore wind farms provide necessary frequency support to the onshore power system, a communication-less frequency regulation scheme with artificial coupling method is proposed. The feasibility and effectiveness of the proposed self-synchronizing control and frequency regulation scheme are verified through simulation studies in PSCAD/EMTDC platform.

Evaluating and Analyzing the Degradation of a Battery Energy Storage System Based on Frequency Regulation Strategies

The capacity aging of lithium-ion energy storage systems is inevitable under long-term use. It has been found in the literature that the aging performance is closely related to battery usage and the current aging state. It follows that different frequency regulation services, C-rates, and maintaining levels of SOC during operation will produce different battery aging rates. In the simulations, the researchers used real frequency data to generate SOC curves based on the Taiwan frequency regulation services under different C-rates and different battery SOC target levels. Then, the aging formula of lithium iron batteries (LiFePO4 battery, LFP battery) and the proposed improved rainflow counting algorithm were used. The capacity aging situation and economy under different usage scenarios were analyzed. The simulation results showed that using a high C-rate and a low SOC level had a higher net profit, and the income of dReg was more than that of sReg. The SOC of BESS has an important impact on the life cycle. Keeping the SOC at a lower level will help prolong the life cycle and increase the net income. In dReg0.5, maintaining the SOC at 30% would yield 8.5% more lifetimes than 50%, 20.6% more lifetimes than 70%.

Frequency Trajectory Planning-based Frequency Regulation Strategy for Wind Turbines Equipped with Energy Storage System

The increase in wind power penetration has weakened the equivalent inertia of the power system, posing a significant challenge to frequency stability. In this paper, a frequency trajectory planning (FTP)-based frequency regulation (FR) strategy is proposed for permanent magnet synchronous generator-based wind turbines equipped with an energy storage system (ESS) on the DC link. The core idea is that the frequency index provided by the grid code is used to plan a safe frequency trajectory when the system frequency fluctuates. The FR power provided by the ESS is controlled by tracking the planned frequency trajectory to compensate for the unbalanced power in the system, provide inertia support, and ensure the frequency stability of the power grid. The proposed strategy can suppress the frequency fluctuation and optimize the FR power of the ESS. It also effectively circumvents the complex parameter design process of virtual inertia control. The simulation model is established in Matlab/Simulink to verify the effectiveness of the control strategy.

BESS frequency regulation strategy on the constraints of planned energy arbitrage using chance-constrained programming

Battery energy storage systems (BESS) are regarded as a multi-functional power system participator, participating in the energy arbitrage strategy (EAS), the frequency regulation strategy (FRS) and so on. However, the existing BESS mainly make profit from the EAS instead of the FRS in many countries such as China. Because there is no appropriate control strategy for BESS, when the developing ancillary services market cannot price the contribution of BESS in FRS. Meanwhile, BESS do have redundant power and capacity, if only the EAS is applied. Therefore, it is significant that BESS are involved in the FRS, that the EAS has the priority in BESS utilization. In this paper, a frequency regulation strategy for the user-side BESS is proposed, on the constraints of the planned energy arbitrage. Specifically, the chance-constrained programming is applied to solve the randomness of frequency regulation requirement, ensuring the BESS profit from the EAS. Also, the exiting BESS are fully used with extra profit from FRS. At last, the promising results prove the advantages of the proposed FRS.

Improved System Frequency Regulation Capability of a Battery Energy Storage System

As a large scale of renewable energy generation including wind energy generation is integrated into a power system, the system frequency stability becomes a challenge. The battery energy storage system (BESS) is a better option for enhancing the system frequency stability. This research suggests an improved frequency regulation scheme of the BESS to suppress the maximum frequency deviation and improve the maximum rate of change of the system frequency and the system frequency of the steady state. To this end, the droop control with the input of the system frequency excursions is employed into the controller of the BESS. To improve the frequency-supporting capability and prevent the over-discharging phenomenon, the control coefficient is defined as a proportional function of the instantaneous state of charge of the BESS and excursions of the system frequency. The contributions of the proposed frequency regulation scheme are investigated with various wind power penetration level conditions and disturbances. Results clearly indicate that the proposed frequency regulation scheme of the BESS is able to achieve objectives in terms of enhancing the maximum frequency excursion, the system frequency of the steady state, and the rate of change of the system frequency under various wind power penetration level conditions, initial SOCs, and disturbances.

A novel primary frequency regulation strategy of wind farm based on wind turbine health condition

In order to optimize the primary frequency regulation performance of the wind farm and increase the service life of the wind turbines (WTs), this paper presents a novel primary frequency regulation strategy of the wind farm to maintain the system frequency considering each WT's health condition level. The proposed strategy can make WTs participate in frequency regulation according to their health conditions. Thus, the unhealthy WTs can be prevented from further deteriorating by reducing the load. In this paper, condition monitoring data from WTs is applied to estimate their health condition levels. Then, the power support is estimated according to the supplementary loop which includes inertial control and droop control. An adaptive model predictive control (AMPC) algorithm is employed to improve the control accuracy and mitigate the fluctuation caused by frequency variation. The case study is carried out in a two-area wind-diesel power system to validate the effectiveness of the proposed method. The results show that the proposed strategy optimizes the effect of primary frequency regulation while reducing the load on unhealthy WTs.

Corrigendum: Power Grid Frequency Regulation Strategy for Photovoltaic Plant Based on Multiobjective Harris Hawks Optimization

In the published article, there was an error in affiliation 1. Instead of “School of Mechatronics Engineering, Zhengzhou University of Aeronautics, Zhengzhou 450046, China*”, it should be “**Zhengzhou University of Aeronautics, Zhengzhou 450046, China**”. The authors apologize for this error and state that this does not change the scientific conclusions of the article in any way. The original article has been updated.

A Group-Based Droop Control Strategy Considering Pitch Angle Protection to Deloaded Wind Farms

To promote the frequency stability of a system with high penetration of wind power integrated into it, this paper presents a systematic frequency regulation strategy for wind farms (WFs). As preparation for frequency response, a coordinated deloading control (CDC) scheme combining the over-speed control (OSC) and the pitch angle control (PAC) methods is proposed for wind turbine generators (WTGs) to preserve power reserve. The novelty lies in the consideration of high wind speed situations and pitch angle protection. Then, a group-based droop control (GBDC) scheme is proposed for a WF consisting of WTGs with the CDC. In this scheme, WTGs are divided into two groups for different controls. To improve the frequency response performance and ensure stable operation, the droop coefficients of the WF, groups, and all WTGs are determined according to their frequency regulation capabilities (FRCs). Moreover, pitch angle protection during the frequency response process is considered in this scheme. The effectiveness of the GBDC scheme is verified by comparing it with several existing droop control schemes in various situations.

The Strategy of Considering the Participation of Doubly-Fed Pumped-Storage Units in Power Grid Frequency Regulation

Facing the challenges of grid stability brought by the large-scale access of new energy to the grid, as one of the important means of frequency regulation of the grid, the doubly-fed pumped-storage units (DFPSU) have great practical significance in the study of the joint frequency regulation strategy with the thermal power unit (TPU). In order to reduce the frequency regulation pressure on the grid caused by the large-scale entry of new energy sources, this paper proposes a joint frequency regulation strategy for the TPU and DFPSU. Based on establishing the optimal scheduling model of the system, taking into account the minimum total operating cost and the optimal time as goals, the chaotic particle swarm optimization (CPSO) is proposed to solve the model. Finally, through MATLAB simulation, the relevant examples are studied and analyzed, and the simulation results verify that the joint frequency regulation strategy in this paper can effectively improve the frequency regulation performance of the unit and the operation economy is good.

Adaptive Power Compensation-Based Frequency Regulation Strategy of Wind Turbine System

Given the increasing penetration of wind power generation, the power system faces great challenges in frequency regulation due to reduction in system inertia. This paper proposes an adaptive power compensation based frequency regulation (FR) strategy of the wind turbine system (WTS). In a frequency transient event, the proposed strategy exploits power excitation method to estimate the disturbance level, which enables the WTS to timely and accurately participate in FR. After the system frequency and the rate of change of frequency meet their requirements, the droop control is adopted to smooth the WTS out of the FR mode and further improve the frequency quality. By analyzing the operation mode and determining the switching logic, detailed implementation of the proposed strategy is developed. Its excellent frequency response performance is verified via results of hardware-in-the-loop experiment and MATLAB/Simulink-based simulation.

Significance of SMES Devices for Power System Frequency Regulation Scheme considering Distributed Energy Resources in a Deregulated Environment

Nowadays, the restructuring of power systems is extremely urgent due to the depletion of fossil fuels on the one hand and the environmental impact on the other. In the restructured environment, the incorporation of renewable energy sources and storage devices is key as they have helped achieve a milestone in the form of microgrid technology. As the restructuring of the power system increases, there are several types of generation sources, and distribution companies express their interest in trading in a deregulated environment to operate economically. When considering the power system deregulation, the contract value deviates in some situations, resulting in an imbalance between the generation and the energy consumption, which can bring the system into a power outage condition. In particular, load frequency control has been a great challenge over the past few decades to ensure the stable operation of power systems. This study considers two generation sources: mini-hydro in GENCO-1 and 3 and microgrid (combination of wind, fuel cell, battery storage, and diesel engine) in GENCO-2 and 4. It is two equal-area networks; in area-1, GENCO-1 and 2, and in area-2, GENCO-3 and 4 are considered, respectively. In addition, a FOPID controller and two ancillary devices, such as a unified power flow controller and a superconducting magnetic energy storage system, have been incorporated. Three different test networks have been formed according to the contract value, such as unilateral, bilateral, and agreement violations. The simulation results show that ancillary devices and controller participation significantly enhance the system response by reducing the frequency and tie-line power fluctuation. To validate the efficacy of the proposed method, respective performance indices and percentages of improvement have been obtained. Finally, this study demonstrated the effectiveness of the proposed restructured power system in a deregulated environment.

Primary frequency regulation scheme applicable to LCC – VSC series hybrid HVDC considering AC voltage stability at receiving end

The penetration of high proportion renewable energy generation in the load center reduces the inertia of the power system and causes frequency stability problems. Series hybrid HVDC (e.g., BaiHeTan-JiangSu) with landing point at load center should play an important role in primary frequency regulation. Thus, this paper proposes a control scheme that coordinates line-commutated converter (LCC) and voltage-source converter (VSC) operation of the series hybrid HVDC (SHHVDC) and couples inertia emulation control with droop control (CIEDC). The CIEDC absorbs or releases the energy stored in the augmented capacitor of VSC for inertia emulation through regulating DC voltage. Then the DC voltage of LCC is coupled with the DC voltage of VSC through the voltage distribution coefficient. Meanwhile, the DC voltage of the inverter side is used to transmit the frequency information to the rectifier side to change the DC current order value without communication-based. Next, The equivalent DC input resistance based on communication-free is introduced to obtain the reactive power variation caused by LCC, which is transmitted to the reactive power control link in VSC to suppress the AC side converter bus voltage fluctuation. Finally, The CIEDC control scheme is verified by case studies in the event of slight AC fault and step load variation compared with the Fixed-Inertia Emulation Control Scheme (FIEC) and conventional droop control (CDC) in AC bus voltage stability, frequency deviation, rate of change of frequency (RoCoF).

A novel Green Leaf‐hopper Flame optimization algorithm for competent frequency regulation in hybrid microgrids

AbstractA proficient frequency regulation scheme has been explored in this work for two interconnected hybrid microgrids () comprised of multiple renewable energy sources, like solar‐thermal units, wind turbine generator units, solar photovoltaic, biogas turbine, biodiesel generator, and storage units. A new optimization algorithm has been formulated to replicate the photo‐tactic swarming behavior of green leafhoppers named Green Leaf‐hopper Flame optimization algorithm (GLFOA) to tune the system controllers (proportional integral (PI)/proportional derivative (PD)/proportional integral derivative (PID) controllers). The superiority of GLFOA is confirmed by comparing the performance with several popular optimization algorithms. To assess the stability of the several rigorous tests have been conducted considering availability/nonavailability of renewable resources, gusty wind, real‐time variation of solar radiation and wind velocities along with the random load disturbances. GLFOA tuned PID controllers were found to be the best to contain the frequency deviations. The maximum peak overshoot, undershoot, and settling time obtained by GLFOA tuned PID controllers are 0.01051 Hz, −0.00764 Hz, and 14.15 s, respectively, which are within an acceptable limit. Further, there is an improvement of 0.15% in the performance index obtained by GLFOA over its counterpart obtained by grey wolf optimization, confirming the superiority of the GLFOA tuned controller.

Hierarchical Distributed Frequency Regulation Strategy of Electric Vehicle Cluster Considering Demand Charging Load Optimization

In view of the problems that the disorderly demand charging load increases the power supply pressure and the control of large-scale cluster is difficult, which will appear during the process of large-scale electric vehicle cluster participating in frequency regulation, this article proposes a hierarchical distributed frequency regulation strategy of electric vehicle cluster considering the optimization of demand charging load. First, the demand charging plans of electric vehicles are optimized to suppress the fluctuation of wind power generation and system load, and the reserve capacity of electric vehicle cluster is predicted in real time. Then, in order to improve the control performance of large-scale cluster, a hierarchically distributed control framework is established, which enables the traditional unit to assist the electric vehicle through the real-time assignment of system frequency regulation task, and realizes the individual-level orderly scheduling of electric vehicles based on the distributed sorting. The simulation results of two-area interconnected system show that the proposed frequency regulation strategy can not only adjust automatically for ensuring expected frequency regulation effect but also reduce the power supply pressure on the grid side, as well as achieve efficient and reliable scheduling of electric vehicle cluster.

Autonomous Dual Active Power-frequency Control in Power System with Small-scale Photovoltaic Power Generation

Active power control of the photovoltaic (PV) power generation system is a promising solution to regulate frequency fluctuation in a power system with high penetration of renewable energy. This paper proposes an autonomous active power control of a small-scale PV system for supporting the inertial response of synchronous generators and power-frequency control. In the proposed control approach, an effective grid frequency regulation scheme is realized using slow- and fast-frequency responses. A low-pass filter based frequency measurement is used for slow-frequency response, while direct frequency measurement is used for fast-frequency response. The designed dual droop characteristic-based control is shaped to achieve a smooth transition between slow- and fast-frequency responses. The performance of the proposed control approach is demonstrated for serious disturbance scenarios, i. e., considerable power-load imbalance and generation trip. In the power-load imbalance test scenario, the proposed control approach works properly within the normal frequency deviation region even when the frequency deviation exceeds that region occasionally. In the generation trip test, the frequency deviation is mitigated quickly, and the employed droop control is smoothly transferred from the slow- to fast-frequency responses.

Power reserve control strategy in the left‐side of maximum power point for primary frequency regulation of grid‐connected photovoltaic systems

The traditional method of operating photovoltaic (PV) systems on the right side of the maximum power point (MPP) may cause the voltage to be close to the open-circuit voltage of the PV panel, resulting in a large oscillation of the PV power. In contrast, the change in dP/dU value of the curve on the left of the MPP is small, which is more conducive to adjusting the output power of the PV. A frequency regulation control strategy is proposed for a single-stage PV power plant (PVPP), maintaining the PV string operating on the left side of the MPP. This paper proposes an optimal step-size calculation method based on the flexible power tracking evaluation function. This calculation method is suitable for PV operating on the left side of the MPP and can improve the dynamic and steady-state performance of the PV power tracking process. In addition, the system response is analyzed by establishing a small-signal model with PV operating on the left side of the MPP. Finally, simulations and experiments verify the correctness and accuracy of the proposed primary frequency regulation strategy and step-size calculation method.

Power Grid Frequency Regulation Strategy for Photovoltaic Plant Based on Multi-Objective Harris Hawks Optimization

To settle the issue of balance between two objectives, i.e., photovoltaic (PV) power station output power maximization and frequency regulation (FR) signals response, a novel PV reconfiguration strategy is proposed in this work, which maximizes the output power through PV reconfiguration, and meanwhile utilizes the energy storage system (ESS) to decrease the PV plant generated power’ deviation from FR signals. Above all, a model of PV-storage power station reconfiguration is designed to minimize the power bias of both rated power and FR signals. Then, the multi-objective Harris hawks optimization (MHHO) is used to obtain the Pareto front which can optimize the above two objectives due to its high optimization efficiency and speed. Subsequently, the optimal compromise solution is selected by the decision-making method of VIseKriterijumska Optimizacija I Kompromisno Resenje (VIKOR). Aiming to substantiate the efficacy of the proposed technique, the case studies are carried out under partial shading condition (PSC) with constant and time-varying FR signals. The simulation results show that, compared with the situation without optimization, the power deviations of the two objectives are reduced by 25.11 and 75.76% under constant FR signals and 23.27 and 55.81% under time-varying FR signals by proposed method, respectively.

A cooperative management strategy for battery energy storage system providing Enhanced Frequency Response

The increased integration of renewable energy sources in the power grid reduces system stability, which translates to more frequency fluctuation. The UK National Grid Electricity Transmission (NGET) has introduced different frequency regulation strategies—Enhanced​ Frequency Response (EFR), and more recently Dynamic Containment (DC) to mitigate this issue. Compared to DC, the EFR includes an adjustable envelope for state of charge (SOC) management which results in better flexibility of the battery energy storage system (BESS). This paper conducts technical analysis and comparison on the two EFR services designed for BESS, and further proposes a cooperative control strategy combining the advantages of the two services based on EFR in order to achieve a better solution for both power grid and BESS. Case studies were carried out using real NGET frequency data to illustrate the schemes and verify the proposed strategy. EFR service 2 shows better frequency performance for frequency regulation while EFR service 1 behaves better in SOC management. The proposed strategy manages to incorporate the strength of both EFR services, which can be beneficial to the grid and BESS.

Hybridization of battery with pico hydel for frequency regulation of microgrids using synchronverter control

Frequency regulation of autonomous microgrids with intermittent renewables is a tedious task, which requires additional support from energy storage systems (ESS). Ideally, the system requires a faster-acting ESS to arrest the initial frequency deviation and high energy capacity ESS to restore the frequency to its nominal value. Pico hydel energy storage (PHES) is one such high energy capacity ESS developed from the agricultural pumping systems. However, PHES has a slower response and cannot switch rapidly between pumping and generation modes. Hence, this paper incorporates a battery with PHES to achieve a faster dynamic response and absorb high-frequency power dynamics. Additionally, a frequency regulation strategy with synchronverter control is proposed to mimic synchronous machines, especially to reduce the rate of change of frequency (ROCOF) and frequency nadir. Firstly, a small-signal modelling approach is proposed, and it was utilized to assess the system stability through eigenvalue analysis. Subsequently, transient simulations were carried out to gauge the performance of the synchronverter strategy with vector control for frequency regulation. As a result, the frequency nadir and ROCOF were substantially reduced with the synchronverter strategy compared to the vector control based on the simulation results.