Primary Frequency Response Research Articles

The Need for Modeling the Impact of Behind-the-Meter Generation Trip on Primary Frequency Response Through Operational Experiences in Korea Power System

This letter introduces the impact of behind-the-meter generation (BTMG) trip on primary frequency response through recent operational experience in Korea, and examines whether this impact can be appropriately modeled in power system analysis. The most influential parameter for properly modeling BTMG trip is discussed in the case studies, demonstrating that simulation-based studies can reproduce the interesting observation of instantaneous load increase (i.e., the impact of BTMG trip). The outcome of this letter highlights the necessity for modeling BTMG trip in frequency stability analysis, especially for power systems with high penetration of BTMG.

A Study on Frequency Stability and Primary Frequency Response of the Korean Electric Power System Considering the High Penetration of Wind Power

The high penetration of wind power decreases the system inertia and primary frequency reserve while replacing the conventional synchronous generators (SGs). Therefore, if the system operator does not take appropriate action on the remaining generation units (GUs) operation, high penetration of wind power will aggravate the frequency stability. To solve this problem, wind power plants (WPPs) may provide the inertial response and primary frequency response (PFR) to support the frequency stability. However, due to the variability of renewable energy, WPPs may not provide adequate frequency response whenever it is required. This paper proposes an algorithm to determine the operation of GUs to provide appropriate PFR for a power system with high penetration of wind power. Through the proposed algorithm, it calculates the required PFR to restore the decreased frequency stability caused by the high penetration of wind power. Then, while considering the available PFR from WPPs, it redetermines the droop coefficient of SGs governor to provide the sufficient PFR to recover the frequency stability. Finally, the effectiveness of the proposed algorithm is verified on the practical Korean electric power system.

Primary Frequency Stability Support of a DFIG in Association With Pitch Angle Control

For an electric power grid that has large penetration levels of variable renewable energy including wind generation and photovoltaics, the system frequency stability is jeopardized, which is manifest in lowering frequency nadir and settling frequency. This paper suggests an enhanced primary frequency response strategy of a doubly-fed induction generator (DFIG) in association with pitch angle control. The DFIG works in de-loaded operation with a certain reserve power via pitch angle control prior to disturbances for frequency regulation. To address this, a function of the pitch angle is employed that decreases the pitch angle with time to slowly feed the active power to the power gird. The simulation results demonstrate the effectiveness and feasibility of the proposed primary frequency response strategy including the settling frequency and frequency nadir.

Research on the Impacts of the Inertia and Droop Control Gains from a Variable-Speed Wind Turbine Generator on the Frequency Response

System frequency must be kept very close to its nominal range to ensure the stability of an electric power grid. Excessive system frequency variations are able to result in load shedding, frequency instability, and even generator damage. With increasing wind power penetration, there is rising concern about the reduction in inertia response and primary frequency control in the electric power grid. Converter-based wind generation is capable of providing inertia response and primary frequency response; nevertheless, the primary frequency and inertia responses of wind generation are different from those of conventional synchronous fleets; it is not completely understood how the primary frequency and inertia responses affect the given system under various disturbances and available kinetic energy levels. Simulations are used to investigate the influences of inertia and droop control strategies on the dynamic frequency responses, particularly the index of the second frequency drop under various disturbance and wind conditions. A quantitative analysis provides insight into setting of inertia and droop control coefficients for various wind and disturbance conditions to facilitate adequate dynamic frequency responses during frequency events.

Requirements for Interdependent Reserve Types Providing Primary Frequency Control

As renewable energy penetration increases and system inertia levels drop, primary frequency control is becoming a critical concern in relatively small interconnections such as the Electric Reliability Council of Texas (ERCOT). To address this problem ERCOT is implementing a number of market rule changes including the introduction of a new Fast Frequency Response (FFR) reserve type to the electricity market. This FFR reserve type aims to help the traditional Primary Frequency Response (PFR) reserve type in arresting frequency decline in the event of a large generator outage. This paper derives reserve requirements to ensure sufficient reserve to arrest frequency decline before reaching the critical frequency threshold while coupling PFR reserve, FFR reserve, and system inertia. The general reserve requirement places limits on the amount of PFR reserve that can be provided by each unit based on its ramping capabilities. Two such limits are derived from first principles and another is proposed that is capable of accommodating the equivalency ratio introduced in previous work. These PFR reserve limits also provide first principles insight into equivalency ratios, which have only been studied empirically in the past. High-level insights are provided on a large Texas test case.

Frequency Stability Constrained Microgrid Scheduling Considering Seamless Islanding

The frequency stability of microgrids is of paramount importance due to their low inertia and high share of renewable energy sources. In this paper, a frequency stability-constrained microgrid scheduling (FSC-MS) model is developed based on a time-dependent discretized frequency response model considering seamless unintentional islanding events. The optimal dispatch of controllable units, as well as optimal upward and downward primary reserves, are determined with a minimum operating cost while ensuring frequency stability criteria are maintained in their safe ranges following an unintentional islanding event, such as rate of change of frequency (RoCoF) and frequency nadir and overshoot. The limitation of generators with respect to primary frequency response is formulated in the form of primary reserve capacity and primary ramp rate limits. A cost-based model is presented to describe the available primary reserve of synchronous generators. The proposed FSC-MS model is formulated as a mixed-integer linear programming (MILP) problem using Benders decomposition. Simulation results on a medium voltage microgrid test system demonstrate the effectiveness of the proposed model to meet the frequency stability constraints while minimizing the total operating cost.

Adaptive Neuro-Fuzzy Based Load Frequency Control in Presence of Energy Storage Devices

Energy storage technologies are utilized for improving the primary frequency control in complex electrical systems. In this paper, the modeling and simulation of a two-area power system is done to evaluate and compare the impact of three different energy storage applications on load frequency control performance. Capacitive energy storage (CES), battery energy storage (BES), and superconducting magnetic energy storage (SMES) are considered for the study. On the basis of peak overshoot and settling time, the performance of these energy storage devices is compared. The power system consists of thermal, wind, and solar resources. All nonlinearities are incorporated in the system model. Both conventional and artificial intelligence (AI) based controllers are tested for the case study. The simulations are carried out under multiple operating conditions and penetration levels of renewable sources. It is found that the primary frequency response of the test system is improved in the presence of the storage facilities and the neuro-fuzzy controller. The simulation results exhibit that the SMES displays the best performance indices. The prime contribution of the paper is to investigate and compare the response of the storage devices in a realistically modeled power system with renewable resources using both conventional and AI controllers.

Frequency Event Detection and Mitigation in Power Systems: A Systematic Literature Review

Modern power systems characterized by complex topologies require accurate situational awareness to maintain an adequate level of reliability. Since they are large and spread over wide geographical areas, it is inevitable that failures will occur. Various generation and transmission disturbances, such as generator and transmission line tripping and load disconnection, give rise to a mismatch between generation and demand, which manifest as frequency events. These events can take the form of negligible frequency deviations or more severe emergencies that can precipitate cascading outages, depending on the severity of the disturbance and efficacy of remedial action schema. The impacts of such events have become more critical recently due to increased levels of renewable penetration and distributed energy resources, which have caused a decline in system synchronous inertia. Due to the repercussions, it is indispensable to arrest such disturbances on time by activating primary frequency control measures. In this paper, a comprehensive systematic literature review is presented on the techniques used for event detection in power systems and the methods of primary frequency response in modern power systems. The paper also highlights the impacts of severe frequency events within power systems.

High-Fidelity Model of Stand-Alone Diesel Electric Generator With Hybrid Turbine-Governor Configuration for Microgrid Studies

Diesel electric generators are an inherent part of remote hybrid microgrids found in remote regions of the world that provide primary frequency response (PFR) to restore system frequency during load or generation changes. However, with inverter-based resources (IBR) integration into microgrids, the IBR control provides a fast frequency response (FFR) to restore the system frequency. Hence, supplementing PFR with FFR requires a sophisticated control system and a high fidelity diesel electric generator model to design these control systems. In this work, a high-fidelity model of a diesel electric generator is developed. Its parameters are tuned using a surrogate optimization algorithm by emulating its response during a load change to a 400 kVA Caterpillar C-15 diesel generator, similar to those found in remote microgrids. The diesel electric generator model consists of a synchronous machine, DC4B excitation with V/Hz limiter, and a proposed modified IEEE GGOV1 engine-governor model (GGOV1D). The performance of the GGOV1D is compared with simple, Woodward DEGOV, and a standard IEEE GGOV1 engine-governor model. Results show that error in the diesel electric generator's response to load changes using the GGOV1D model is lower with an improved frequency response during the arresting and rebound period than the other engine-governor models.

Cooperation of large-scale wind farm and battery storage in frequency control: An optimal Fuzzy-logic based controller

Grid integration of renewables and battery energy storage systems and its consequent synchronous machines retirement may drive power systems into low-inertia conditions with high risk of frequency instability which is associated with lack of sufficient inertia and primary frequency response previously supplied by synchronous machines. Therefore, there is a need to figure out alternative solutions to compensate the lack of system frequency dynamic supports in future renewable-dominated power systems. In this context, this paper aims to highlight wind farm capabilities, particularly doubly-fed induction generator (DFIG) and permanent magnet synchronous generator (PMSG), in system primary frequency control, similar to conventional synchronous units, while also investigates the synergy between the wind unit cooperation with large-scale battery storages. First, a novel dynamic model is presented which takes into account large-scale wind farms, as well as utility-scale batteries, equipped with frequency-sensitive active power reference scheme so as to control the system frequency violations following contingencies. The proposed frequency control model also includes power-electronics interface model, its associated control loops, and a novel Fuzzy-logic based controller. The proposed Fuzzy-logic based controller along with a wash-out filter allows combined wind-battery system to estimate the system active power mismatch, emerged from a contingency/trip, to determine the consequent frequency variations, and therefore to deliver fast frequency response in a robust and reliable way to arrest the mentioned frequency distortions. The proposed Fuzzy-based controller is then designed via optimizing its model parameters, including the membership function parameters, thereby improving its efficiency in frequency control provision as well as active power control. The optimization process is performed via a widely used artificial bee colony (ABC) algorithm while a multi-objective function is considered. The proposed frequency control model is then evaluated in the 14-generation low-inertia Australian test system with 30% wind penetration. This work tries to provide a deep insight on how to utilize wind farms for frequency support and how to wind-battery frequency response may positively interact with nearby converter-based resources, i.e., photovoltaic units. The proposed Fuzzy-based coordination approach for large-scale wind-battery units can be a potential solution to deal with frequency stability problems in future power systems with low-inertia conditions including multiple nearby converter-based resources.

PV-Supercapacitor Cascaded Topology for Primary Frequency Responses and Dynamic Inertia Emulation

Owing to rapid increase in PV penetration without inherent inertia, there has been an unremitting deterioration of the effective inertia of the existing power systems. This may pose a serious threat to the stability of power systems during disturbances if not taken care of. Hence, the problem of how to emulate Synthetic Inertia (SI) in PV Systems (PVS) to retain their frequency stability demands attention. Super Capacitor (SC)-based storage become an attractive option over the other energy storage types because of its high-power density, burst power handling capability, faster response and longer life cycle. Considering this, the authors here propose a novel PV-SC Cascaded Topology (PSCT) as a cost-effective approach to emulate SI by integrating a low voltage SC to a high voltage grid-connected PVS. The proposed PSCT helps in operating the SC as a voltage source rather than a current source. Thus, it eliminates the high gain requirements of the SC interfacing converters. The aim is to target two main frequency response services, i.e., Primary Frequency Response (PFR) and Synthetic Inertial Response (SIR), using a novel common control scheme, but without affecting any other energy intensive services. The authors introduced a Droop-Inspired (DI) method with an adjustable inertia constant to emulate dynamic inertia so that a wider range of Rate of Change of Frequency (RoCoF) values can be serviced with a limited storage. A very streamlined analysis was also carried out for sizing of the SC stage based on a simple Three-Point Linearization (TPL) technique and DI technique with a limited knowledge of the disturbance parameters. The whole system was initially validated in a MATLAB Simulink environment and later confirmed with the OPAL-RT Real-Time Simulator. The investigated response was subject to variation in terms of control parameters, changes in solar irradiance, grid frequency variation, etc.

Enhancement of frequency stability of power systems integrated with wind energy using marine predator algorithm based PIDA controlled STATCOM

This paper proposes a novel optimized Proportional-Integral-Derivative-Acceleration (PIDA) controlled static synchronous compensator (STATCOM) to improve the frequency response of power systems through reactive power modulation. The proposed controller can maintain the system frequency within permissible limits upon load change or generation loss. The gains of the PIDA controller are fine-tuned using the Marine Predator Algorithm (MPA), which is a newly metaheuristic optimization technique. Moreover, the performance of the MPA-PIDA controlled STATCOM is tested in the two-area four-machine system (Kundur system) and the New England IEEE 39-bus system. The results show that the MPA-PIDA controller can provide better frequency response compared with the multi-band controller presented in the previous literature. In addition, the robustness of the proposed controller is verified when case studies are integrated with wind generation. The results show that the primary frequency response is greatly improved by using the MPA-PIDA controlled STATCOM in all studied cases. Moreover, the results are given in the form of time domain simulations using MATLAB/SIMULINK.

Provision of Primary Frequency Response as Ancillary Service From Active Distribution Networks to the Transmission System

This paper deals with the provision of primary frequency response (PFR) as ancillary service (AS) from active distribution networks (ADNs) to the transmission system (TS). In particular, two methodologies are developed. The first one aims to quantify the PFR capability range of the ADN. This range is defined by determining the range of the aggregated, i.e., equivalent, active power - frequency <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$P(f)$ </tex-math></inline-formula> droop curves that can be provided at the point of interconnection (POI) with the TS. The second one targets to optimally control <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$P(f)$ </tex-math></inline-formula> droop curves of individual distributed energy resources (DERs), installed in the premises of the ADN, to guarantee specific frequency regulation characteristic at the POI. This frequency regulation characteristic is expressed by means of a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$P(f)$ </tex-math></inline-formula> droop curve. Both methods are tested on two discrete distribution systems. Several test cases are examined to demonstrate their implementation. Additionally, comparisons against conventional approaches and time series simulations are conducted to evaluate the performance of the proposed methods.

A Market Clearing Mechanism Considering Primary Frequency Response Rate

Prior-art market mechanism designs in the literature do not well distinguish the primary frequency reserve resources with different response rates. This letter proposes a performance ratio that quantifies the relative effectiveness of generators and flexible demand resources in frequency regulation considering their response rates. The performance ratio is incorporated into a real-time market clearing model to optimize both upward and downward primary frequency reserve resources. The proposed method is validated based on the IEEE 118 bus system.

A decentralized non-linear dynamic droop control of a hybrid energy storage system bluefor primary frequency control in integrated AC-MTDC systems

Power-sharing between energy storage systems (ESSs) is one of the significant challenges in a hybrid energy storage system (HESS). For primary frequency control through multi-terminal DC (MTDC) systems interfacing renewable resources, a decentralized control method based on non-linear dynamic droop control (NLDDC) is proposed in this study for a hybrid energy storage system. Battery energy storage systems (BESS) and super capacitor (SC) make up the hybrid energy storage device in this article. The proposed strategy, which employs dynamic droop gain to distribute power between the SC and the battery, outperforms the previous droop regulation. The SC will be in charge of compensating the high-frequency demand, whereas the BESS will be in charge of compensating the low-frequency power demand, thanks to a decentralized hierarchical energy management process. As a result, the proposed approach improves system transient stability following grid faults, while enhancing primary frequency response under net load variability and generation outage. Finally, simulation results utilizing the Cigre MTDC benchmark and the IEEE 39-bus test system are provided to validate the proposed structure. The NLDDC parameters are fine-tuned using krill herd (KH), an intelligence algorithm with biological inspiration.

Potential of Slip Synchronous Wind Turbine Systems: Grid Support and Mechanical Load Mitigation

Wind power penetration into existing electrical power systems continues to experience year-on-year growth. Consequently, modern wind turbine systems (WTS) are required to comply with relevant grid codes and provide ancillary grid services to assist with overall grid stability. Adhering to these grid codes and services can cause additional mechanical loading on WTS, which can result in a reduction in service life of some of the drivetrain components, and instability if a sufficient means of damping is not present in the drivetrain. In this paper, a dynamic simulation model of a Type 1, direct grid-connected, fixed-speed (FS) slip-synchronous wind turbine system (SS-WTS) is developed to investigate its dynamic stability in response to the additional mechanical loads imparted onto it during transient events on the grid. The SS-WTS is not equipped with a power converter and, consequently, an understanding of its dynamic stability is critical to evaluate its ability to assist with grid services and maintain stability during transient grid conditions such as low-voltage ride-through (LVRT) events. An analytical transfer function model of a 1.5 MW geared direct grid-connected SS-WTS was derived and implemented in MATLAB/Simulink. It was found that the SS technology provides significant damping to the WTS drivetrain while maintaining dynamic stability during a severe LVRT event. Moreover, it was found that the degree of damping is directly proportional to the value of rated slip, and that high-speed drivetrains provide a greater degree of damping for a given value of rated slip. Furthermore, it is shown that the SS-WTS has the ability to assist with grid services such as primary frequency response, short-circuit strength, and reactive power compensation.

Frequency response support assessment from uncertain wind generation

Large share of uncertain generation from Wind Power Plants (WPPs) in the power systems has a detrimental impact on system frequency stability, as WPPs inherently do not provide inertial response support. Currently, extensive research is being carried out to envisage Synthetic Inertia (SI) & Primary Frequency Response (PFR) support from WPPs. Modelling of inertial response support from WPPs and its consideration in stochastic scheduling requires investigation. System inertia condition varies over a day and hence, accurate estimation of PFR requirement is a challenging task. This paper develops a novel approach to model SI & PFR support from WPPs and simultaneously estimate the system inertia condition & PFR requirement in a day-ahead stochastic scheduling framework. Wind generation uncertainty is characterized by probabilistic scenarios. IEEE Reliability Test System (RTS) is used to carry out the case studies at different wind penetration levels. Numerical results demonstrate that the SI and PFR support from WPPs reduce the overall cost by 15% and wind curtailment reduction up to 35%. SI support from WPP adds around 12% to overall system inertia. The proposed model could be enhanced for the optimal tuning of inertial, droop constants, and modelling of multiple uncertainties.

A Fast Frequency Control Based on Model Predictive Control Taking Into Account of Optimal Allocation of Power From the Energy Storage System

The displacement of synchronous generators by converter interfaced energy renewable resource essentially reduces the system inertia, which can trigger under-frequency load shedding relay. To support the frequency containment, virtual inertia control (VIC) and fast frequency control (FFC) based on energy storage system (ESS) have been developed. Most of the existing VIC did not take into account the power constraints of the ESS. Although some of the FFC methods account for the ESS constraints, they ignore the intermediate steady-state (ISS) operating point of the primary frequency response (PFR). To counteract these problems, a model predictive control (MPC) based FFC, which considers the power limitations of the ESS, the frequency deviation constraint, and the ISS frequency tracking capability, is proposed in this paper. The proposed MPC only requires few local measurements, and the system parameters including inertia constants, onset time of inertia response and the ISS time of the PFR can be accurately estimated. To achieve the optimal control action and reduce computational burden, quadratic programming (QP) is incorporated in the proposed MPC. The proposed MPC is tested on a real Taiwan power system. The fast response and robustness of the proposed MPC has been justified by comparing with the state-of-the-art method.

Operation and Control of a Hybrid Power Plant with the Capability of Grid Services Provision

The integration of distributed power plants that rely on renewable energy sources (RESs) is a major challenge for system operators (SOs) due to the variable nature of the input energy (e.g., wind and solar irradiation) to these power sources. A key solution to such a challenge is to coordinate and combine the power generation of these sources such that their behavior is closer to a conventional and dispatchable power station, taking into account the limitations imposed by the battery storage system (BESS), so it is seen as a hybrid power plant (HPP) from the SOs’ viewpoint. This paper develops a model of HPP that encompasses two generation technologies, wind and photovoltaic farms, which are assisted by a BESS. The paper proposes a comprehensive control method that can smooth the HPP output with minimized energy rejection whilst enabling the HPP to provide synthetic inertia and primary frequency response, which are grid-code compliant. The proposed control method is validated through various scenarios, which are implemented on a detailed electromechanical test system modeled in MATLAB/Simulink. The results show and quantify the achieved improvement on stabilizing the HPP capacity factor under variable wind speed. The HPP also enhances the system response to frequency events.

Chance-constrained energy and multi-type reserves scheduling exploiting flexibility from combined power and heat units and heat pumps

District heating systems can provide considerable flexibility for electric power systems through combined heat and power units and heat pumps. This paper proposes a chance-constrained two-stage energy and multi-type reserves scheduling scheme for integrated electricity and heating systems to handle both wind power forecast errors and the outage of the largest generator. The combined heat and power units and heat pumps not only provide following reserves for offsetting wind power forecast errors under normal conditions, but also provide primary frequency response reserves for arresting system frequency decline after the outage of the largest generator. The primary frequency response reserves from combined heat and power units and heat pumps are optimized satisfying system steady-state frequency requirement while considering the reserve costs. To manage the risk level of load shedding caused by wind power forecast errors, chance constraints are adopted to achieve the trade-off between the sufficiency of following reserves and system economic efficiency. The nonlinear and nonconvex scheduling model is reformulated as a mixed-integer linear program via linearization and convex approximation based on conditional value-at-risk. The effectiveness of the proposed scheduling scheme in improving the system frequency regulation, system economic efficiency, and wind power integration is verified through the case studies on a 6-bus and 6-node integrated electricity and heating system and a regional large-scale test system.