Primary Frequency Control Research Articles

Application of a novel PIPDF controller in an improved plug-in electric vehicle integrated power system for AGC operation

Previously linearised plug-in electric vehicle (PEV) models are adopted in automatic generation control functioning. However, lesser attention is given towards the exact designing of a PEV unit contemplating the suitable model parameters. Therefore, this paper presents a novel approach for determining the PEV model parameters employing a Bode plot analysis in the primary frequency control operation. Furthermore, a novel proportional–integral internal proportional-derivative feedback controller is proposed for the secondary frequency control application asserting zero frequency and tie-line power deviations under steady-state condition. The proposed controller provides better dynamic stability compared to the existing controllers by adjusting open loop poles of the system. The moth flame optimisation technique is used to tune the gain parameters of the proposed controller. Simulation results, obtained from the case studies under different power scenarios, depict a significant improvement in the dynamic responses of frequency and tie-line power deviations in the presence of the proposed PEV model and optimised controller.

Coordinated Control Strategy and Validation of Vehicle-to-Grid for Frequency Control

The increased penetration of renewable energy sources (RES) and electric vehicles (EVs) is resulting in significant challenges to the stability, reliability, and resiliency of the electrical grid due to the intermittency nature of RES and uncertainty of charging demands of EVs. There is a potential for significant economic returns to use vehicle-to-grid (V2G) technology for peak load reduction and frequency control. To verify the effectiveness of the V2G-based frequency control in a microgrid, modeling and simulations of single- and multi-vehicle-based primary and secondary frequency controls were conducted to utilize the integrated components at the Canadian Centre for Housing Technology (CCHT)-V2G testing facility by using MATLAB/Simulink. A single-vehicle-based model was validated by comparing empirical testing and simulations of primary and secondary frequency controls. The validated conceptual model was then applied for dynamic phasor simulations of multi-vehicle-based frequency control with a proposed coordinated control algorithm for improving frequency stability and facilitating renewables integration with V2G-capable EVs in a microgrid. This proposed model includes a decentralized coordinated control of the state of charge (SOC) and charging schedule for five aggregated EVs with different departure times and SOC management profiles preferred by EV drivers. The simulation results showed that the fleet of 5 EVs in V2B/V2G could effectively reduce frequency deviation in a microgrid.

A Continuous Nonlinear Fractional-Order PI Controller for Primary Frequency Control Application

In this paper, a nonlinear fractional-order PI (NL-FO-PI) controller is proposed for primary frequency control (PFC) of a wind farm based on the squirrel cage induction generator. The new structure composites a fractional-order operator and nonlinear function to achieve better control performance for the PFC system. The benchmarking process is demonstrated by investigating the performance of fractional-order PI (FO-PI) and nonlinear PI (NL-PI) controllers. Initially, the controller is applied to a single-area power system for design and stability study and then extended to the two-area interconnected wind farm to validate the applicability in the more realistic power system. The proposed control method ensures the balance of power and keeps the system frequency within a suitable range. The simulation results demonstrate that the proposed NL-FO-PI controller provides less percentage overshoot, settling time, rise time, and peak time than other controllers.

Grid Support with Wind Turbines: The Case of the 2019 Blackout in Flensburg

The work presented in this paper aims to show how modern wind turbines can help to control the frequency in a small grid which suffers from large power imbalances. It is shown for an exemplary situation, which occurred in Flensburg’s distribution grid in 2019: a major blackout, which occurred after almost two hours in islanding operation, affecting almost the entire distribution grid, which supplies approximately 55,000 households and businesses. For the analysis, a wind turbine model and a grid support controller developed at the Wind Energy Technology Institute are combined with real measurements from the day of the blackout to generate a fictional yet realistic case study for such an islanding situation. For this case study, it is assumed that wind turbines with grid support functionalities are connected to the medium voltage distribution grid of the city. It is shown to what extent wind turbines can help to operate the grid by providing grid frequency support in two ways: By supplying synthetic inertia only, where the wind turbines can help to limit the rate of change of frequency in the islanded grid directly after losing the connection to the central European grid. In combination with the primary frequency control capabilities of the wind turbines (WTs), the disconnection of one gen set in the local power station might have been avoided. Furthermore, wind turbines with primary frequency control capabilities could have restored the grid frequency to 50 Hz shortly after the islanding situation even if the aforementioned gen-set was lost. This would have allowed connecting a backup medium voltage line to the central European grid and thereby avoiding the blackout.

Analysis and Simulations of the Primary Frequency Control during a System Split in Continental Europe Power System

The occurrence of system separations in the power system of Continental Europe has been observed in recent decades as a critical event which might cause power imbalances higher than the reference incident specified per system design, representing an actual challenge for the stability and safe operation of the system. This work presents an analysis and simulations of the primary frequency control in the Continental Europe synchronous area in conditions of system separation. The adopted approach is based on fundamental aspects of the frequency-containment reserve process. The analysis takes an actual event into consideration, which determined the separation of the system in January 2021. The main purpose of the work is the development of specific models and simulations able to reproduce the actual split event. Due to specific arrangements discussed in detail, it is possible to obtain a substantial match between the simulations and the frequencies registered after the system split. The work also provides insight into the importance of the temporal sequence of power imbalances and defensive actions in the primary frequency control process. The models developed in the work are finally used to investigate the separation event under different operating conditions, such as missing defensive actions and low inertia scenarios.

Application of Battery Energy Storage Systems for Primary Frequency Control in Power Systems with High Renewable Energy Penetration

In power systems, high renewable energy penetration generally results in conventional synchronous generators being displaced. Hence, the power system inertia reduces, thus causing a larger frequency deviation when an imbalance between load and generation occurs, and thus potential system instability. The problem associated with this increase in the system’s dynamic response can be addressed by various means, for example, flywheels, supercapacitors, and battery energy storage systems (BESSs). This paper investigates the application of BESSs for primary frequency control in power systems with very high penetration of renewable energy, and consequently, low levels of synchronous generation. By re-creating a major Australian power system separation event and then subsequently simulating the event under low inertia conditions but with BESSs providing frequency support, it has been demonstrated that a droop-controlled BESS can greatly improve frequency response, producing both faster reaction and smaller frequency deviation. Furthermore, it is shown via detailed investigation how factors such as available battery capacity and droop coefficient impact the system frequency response characteristics, providing guidance on how best to mitigate the impact of future synchronous generator retirements. It is intended that this analysis could be beneficial in determining the optimal BESS capacity and droop value to manage the potential frequency stability risks for a future power system with high renewable energy penetrations.

A novel approach for sizing thermal and electrical energy storage systems for energy management of islanded residential microgrid

This paper proposes an energy management scheme (EMS) for islanded operation of a residential Microgrid (MG) using a Lithium-Ion battery (LIB) energy storage system (LIBESS) and distributed generators (DGs). The LIBESS handles primary frequency control and energy management during peak-load period while the dispatchable DGs supply the base load. During peak-load periods, the thermostatically controlled loads (TCLs) like air conditioners (ACs) make up a significant part of MG demand. A Thermal energy storages (TES) is quite similar to a TCL except that it can also store thermal energy for later use. Here, instead of using the ACs, it is proposed to use a TESS to provide the required thermal energy for keeping the inside temperatures of residences within the desired range. Instead of using the ACs, a pre-charged TES can be used to provide the needed cooling thermal energy during islanded operation that power generation capacity is insufficient. By not using the ACs, the total energy consumption of MG and the required LIB capacity for energy management decreases considerably. Various daily scenarios from available recorded data and randomly generated data are used to consider the uncertainties of the variations of the MG load profile, the ambient temperature and the renewable power generations. Also, the outage of the DG with the largest generation capacity is considered as the worst contingency that could ever happen during the islanded operation of MG. The LIBESS sizing results with and without the outage of the DG show that with the ACs being totally replaced with a TES, the overall cost of the TES and the LIBESS is decreased by 33% and 63% respectively than the case that the TES is not utilized.

Model-free control for frequency response support in microgrids utilizing wind turbines

Islanded microgrids integrated with renewable energy resources bring additional challenges to the primary frequency control due to their power-electronic interfaced generations. Developing an efficient control strategy for inertia emulation and frequency response support of microgrids is becoming an important issue. Wind turbine dynamics, however, will degrade the emulated inertial and primary responses with most traditional control strategies. On the other hand, compensation methods often require the knowledge of system models at the design stage and state estimation for feedback once deployed online. To overcome these challenges, this paper explores a model-free control (MFC) strategy to emulate the desired inertia and support the frequency response of a diesel-wind microgrid system. The proposed MFC strategy utilizes a model reference concept to obtain guaranteed inertial response and improve the frequency response of microgrids. This work considers different control strategies with different operating points. The proposed controller is implemented and verified using the modified 33-bus full nonlinear model of the three-phase diesel-wind system in Simulink. Simulation results demonstrate the improved frequency response by precisely emulating the desired inertia using the MFC strategy.

Frequency-Based Time-of-Use Pricing with Multiple Battery-buffered Smart Loads

This work discusses a new time-of-use energy pricing structure based on grid frequency. Under the proposed pricing structure, energy cost depends on the time of frequency deviation and the period during which energy is used. This pricing mechanism is intended for use with multiple behind-the-meter home batteries as part of a frequency-controlled smart load strategy known as a battery buffered smart load. The load energy consumption from the grid and energy storage rates thus depend on the frequency deviation and hours of use, which means that the proposed pricing encourages battery-using customers to not only utilise energy arbitrage, shifting peak demand but also to contribute to demand-side primary frequency control. The second contribution of this paper is the presentation of an operation strategy for multiple buffered battery smart load units in which charge and discharge decisions are made according to off- and on-peak period information to ensure continuous provision of primary frequency control under the proposed time of frequency division pricing. A MATLAB simulation of the proposed pricing and operation modes of the BBSL was conducted with multiple battery-buffered smart load to assess feasibility.

Analysis of Control Strategies Based on Virtual Inertia for the Improvement of Frequency Stability in an Islanded Grid with Wind Generators and Battery Energy Storage Systems

Rising levels of non-synchronous generation in power systems are leading to increasing difficulties in primary frequency control. In response, there has been much research effort aimed at providing individual electronic interfaced generators with different frequency response capabilities. There is now a growing research interest in analyzing the interactions among different power system elements that include these features. This paper explores how the implementation of control strategies based on the concept of virtual inertia can help to improve frequency stability. More specifically, the work is focused on islanded systems with high share of wind generation interacting with battery energy storage systems. The paper presents a methodology for modeling a power system with virtual primary frequency control, as an aid to power system planning and operation. The methodology and its implementation are illustrated with a real case study.

Highly Robust Observer Sliding Mode Based Frequency Control for Multi Area Power Systems with Renewable Power Plants

This paper centers on the design of highly robust observer sliding mode (HROSM)-based load frequency and tie-power control to compensate for primary frequency control of multi-area interconnected power systems integrated with renewable power generation. At first, the power system with external disturbance is model in the state space form. Then the state observer is used to estimate the system states which are difficult or expensive to measure. Secondly, the sliding mode control (SMC) is designed with a new single phase sliding surface (SPSS). In addition, the whole system asymptotic stability is proven with Lyapunov stability theory based on the linear matrix inequality (LMI) technique. The new SPSS without reaching time guarantees rapid convergence of high transient frequency, tie-power change as well as reduces chattering without loss of accuracies. Therefore, the superiority of modern state-of-the-art SMC-based frequency controllers relies on good practical application. The experimental simulation results on large interconnected power systems show good performance and high robustness against external disturbances when compared with some modern state of art controllers in terms of overshoots and settling time.

Optimization and service lifetime prediction of hydro-wind power complementary system

Wind power is the most potential clean renewable energy. However, due to the uncertainty of wind energy, its grid connection is limited, resulting in wind curtailment. To address these challenges, a novel model of combining wind power and hydropower is proposed by using reservoir energy storage in this paper, which uses dual cycle control, i.e. the primary frequency control and power complementary control to ensure the power quality and realize the requirement of high proportion wind power grid connection, and two indexes: the capacity ratio of wind power to hydropower (α) and the capacity ratio of the complementary system to the power grid (β), four objective functions as well as a service lifetime model are proposed to optimize the complementary system. The research shows that there are three typical variations, i.e., the variation caused by abrupt wind speed, the positive and negative co-variations between wind power and objective power. The positive co-variation is beneficial to the stability of hydro-wind power complementary system, while the other variations bring certain risks. Furthermore, under the premise that hydropower undertakes dual tasks: regulating power quality and compensating wind power, the stability and power quality of the complementary system are guaranteed when β is within 25%. If hydropower units are not responsible for regulating peak demand,β will increase from 25% to 40%. In addition, according to the proposed lifetime model, if the frequency deviation meets the allowable value, the maximum service lifetime of hydropower units reduces by 15%. Otherwise, the service lifetime will decrease rapidly. The results provide reference for developing variable renewable energy in the future.

A Control Strategy for Battery Energy Storage Systems Participating in Primary Frequency Control Considering the Disturbance Type

In power systems, various types of disturbances can randomly affect the active power balance, which can result in unexpected frequency changes. A battery energy storage system (BESS) is an effective technique to assist power system primary frequency control. In this work, a comprehensive self-adaptive strategy considering load disturbance types is proposed that enables BESS participation in the primary frequency control of power grids. First, the different types of load disturbances in the power system are divided into two categories, namely, step disturbance and continuous small disturbance. Then, the advantages of virtual inertia control and virtual droop control are taken into full consideration. Model A and Model B are proposed to match the two load disturbance types. Model A is used to rapidly prevent sudden changes in frequency under step disturbance conditions without restricting the state of charge (SOC) of the BESS. Under the conditions of continuous small disturbances, we further propose Model B in which primary frequency control is realized while considering SOC recovery. Compared with the previous primary frequency control methods, the presented approach significantly improves the stability of complicated and changeable power systems. Meanwhile, it fulfills the frequency control requirements with superior frequency control performance.

A Novel Frequency Control Scheme for Autonomous Microgrid Using Cooperative Application of Supercapacitor-Battery HESS, Photovoltaics, LED Lighting Loads and TCLs

The supercapacitor-battery hybrid energy storage system (SBHESS) has been commonly used to reduce the stress of fast power variations on the battery, and hence, prolong its lifetime. However, the utilization of the supercapacitor (SC) significantly increases the cost of energy storage system. Considering the potentials of the photovoltaic systems, the smart LED lighting loads and the thermostatically controlled loads in primary frequency control (PFC), three novel PFC schemes are proposed for each of them. They are enabled to cooperatively participate in the PFC of islanded MG alongside the SBHESS. Their cooperative application alongside the SBHESS decreases the participation share of the SC in the PFC and hence, reduces the size and the cost of the SC and its converter which consequently decreases the SBHESS cost. In this regard, the genetic algorithm is employed to tune the controller coefficients of the PFC schemes and the distributed generators to limit and minimize frequency deviations within an acceptable range, and also minimize the size of SC and its converter. The results show that the proposed cooperative application considerably decreases the size and the cost of the SC and its converter. This is while the SBHESS battery life is still extended the same as the case in which the SBHESS is used alone. As the frequency control results are similar with and without the proposed cooperative application and the imposed cost is much higher in the latter case, the proposed cooperative application increases the cost-effectiveness of the SBHESS.

Model for Frequency Dynamics in an Islanded Microgrid and Primary Frequency Control Based on Disturbance Compensation

Islanded microgrids have low inertia due to a large penetration of non-inertial inverter based power sources. In such systems, the primary frequency controller (PFC) faces the issue of a higher rate of change of frequency (RoCoF) and large peak frequency deviation in case of a sudden change in load or generation loss. The frequency control becomes more challenging as the variation in the frequency of different sources is not synchronized. This paper proposes a model for frequency dynamics in an islanded microgrid comprised of both inverter based distributed generators (DGs) and synchronous generators (SGs). The model is developed considering the asynchronous variation of frequency among the SGs. Based on the developed model, a novel disturbance compensation-based PFC is proposed. The PFC controls the reference power of a battery energy storage system (BESS) which is operated in grid-following mode and compensates for the power imbalance in the microgrid. The performance of the proposed model and the PFC is verified using Typhoon real-time hardware-in-the-loop simulation.

Real-time Monitoring System and Method of PFC Capability of Power Grid Based on Data Analysis

The development of wind power and other new energy sources has a great impact on the stability of power system frequency. By analyzing the characteristics of the primary frequency control(PFC) assessment standard of the power grid, one real-time monitoring method of the unit’s PFC capability of the power grid based on data analysis is proposed. The Power network dispatching department can fully grasps the overall frequency regulation capability of the operating units. It can improve the grid’s response methods to deal with high power gaps and ensure the safe and stable operation of the grid.

The Impact of Primary Frequency Control of WTs on Ultra-Low Frequency Oscillation

Since entering the 21st century, as wind power develops rapidly, wind power plays an increasingly important role in the safe and stable operation of power systems. Since the wind turbines under traditional control don’t provide frequency support, the frequency dynamics of the power grids after load disturbance will be deteriorated. Thus, wind farms are required to equip primary frequency control in many countries. Most existing researches on primary frequency control of wind turbines focus on control strategies and the impact on system frequency, but few studies pay attention to the impact of primary frequency control of wind power on ultra-low frequency oscillation. This paper first proposes an analysis framework for ultra-low frequency oscillations in power systems with wind power. On this basis, a motion equation model of wind turbines is established. Finally, an extended three-machine nine-node system is used as an example to analyze the impact mechanism of primary frequency control on ultra-low frequency oscillation. The impact mechanism provides reference for the design of primary frequency of wind turbines in some regional power grids with ultra-low frequency oscillation.

Analysis on parameter switching methods for hydropower dominant system

With the rapid development of DC asynchronous interconnection, in order to ensure the frequency quality, higher requirements are put forward for the primary frequency control of hydropower units. In the high proportion hydropower system, it is difficult for the governor parameters to meet the requirements of primary frequency control and dynamic stability simultaneously. In order to solve the problem, this paper proposes a recommended parameter switching method by referring to the Nordic system. The parameter optimization methods before and after switching are proposed, and the effectiveness of the method is verified by time domain simulation.

Primary Frequency Control Using Motor Drives for Short Term Grid Disturbances

Industrial motor systems make up a quarter of all electric sales in the United States. Variable speed drives (VSDs) can provide energy efficiency savings to the customer by regulating motor speed based on specific and varying needs. In addition to the benefits provided to the customer, VSDs can provide support to the grid through ancillary services. The Center for Ultra-Wide-Area Resilient Electric Energy Transmission Networks (CURENT) developed a power electronics converter-based grid emulator to allow testing of various power system architectures and demonstration of key technologies in monitoring, control, actuation, and visualization. This paper proposes using an active front-end VSD's connected motor load to provide frequency regulation to a large scale power grid. Each part of the emulator is described including motor and power electronics model and control. The proposed frequency regulation is implemented in VSDs and modeled in both a transmission system in EMTDC/PSCAD and verified on CURENT's hardware testbed.

Adaptive Frequency Responsive Control for Wind Farm Considering Wake Interaction

With the increasing share of wind power, it is expected that wind turbines would provide frequency regulation ancillary service. However, the complex wake effect intensifies the difficulty in controlling wind turbines and evaluating the frequency regulation potential from the wind farm. We propose a novel frequency control scheme for doubly-fed induction generator (DFIG)-based wind turbines, in which the wake effect is considered. The proposed control scheme is developed by incorporating the virtual inertia control and primary frequency control in a holistic way. To facilitate frequency regulation in time-varying operation status, the control gains are adaptively adjusted according to wind turbine operation status in the proposed controller. Besides, different kinds of power reserve control approaches are explicitly investigated. Finally, extensive case studies are conducted and simulation results verify that the frequency behavior is significantly improved via the proposed control scheme.