Frequency Regulation Function Research Articles

Sequential frequency regulation strategy for DFIG and battery energy storage system considering artificial deadbands

To address the issues of the mechanical stress of doubly-fed induction generator (DFIG) and the service life of energy storage systems (ESSs) resulting from excessively and frequently participate in grid frequency regulation (GFR), this paper suggests a sequential GFR of the DFIG and ESS considering the artificial dead-bands. Firstly, the system frequency deviation expression is derived considering the dead-band to analyze the influences of the dead-bands on the grid frequency dynamics. Secondly, based on the artificial dead-bands, the GFR process is divided into three zones: the GFR zone of DFIG, the transaction zone of DFIG and ESS, and the GFR zone of ESS. To avoid the large mechanical stress of the wind turbines and shallow charging and discharging phenomenon of the ESS, the DFIG provides GFR function to mitigate the system frequency fluctuations, while ESS provides frequency regulation function to arrest the grid frequency variation taking into account the condition of state-of-charge constraints for severe frequency disturbances. Finally, simulations are conducted to verify the effectiveness of the proposed strategy under varying wind speed conditions combined with large disturbance. The results show that the proposed sequential GFR can maintain the frequency support while reducing the negative effects of mechanical fatigue of the DFIG and service life of ESS.

Optimized Non-Integer Load Frequency Control Scheme for Interconnected Microgrids in Remote Areas with High Renewable Energy and Electric Vehicle Penetrations

Renewable energy systems (RESs) have introduced themselves as vital solutions for energy supply in remote regions, wherein main utility supply systems are not available. The construction of microgrid (MG) systems is useful candidate for proper control and management with hybrid RESs. However, RESs-based MGs face reduced power system inertia due to the dependency of RESs on power electronic converter systems. Accordingly, preserving nominal operating frequency and reduced deviations in tie-line power are crucial elements for proper operation of interconnected MGs in remote areas. To overcome this problem, load frequency control (LFC) systems have proven featured solutions. Therefore, this paper proposes a new non-integer LFC method based on the fractional order (FO) control theory for LFC in interconnected MGs in remote areas. The proposed control is based on the three degree of freedom (3DoF) cascaded 1+proportional-integral-derivative-accelerated (PIDA) controller with FOPI controller, namely 3DoF 1+PIDA/FOPI LFC scheme. The proposed 3DoF 1+PIDA/FOPItakes the advantages of the accelerated term of PIDA control to improve power system transients, regarding maximum overshoot/undershoot and settling times. Additionally, it employs outer loop to reduce errors and faster inner loop to mitigate disturbances effects. The contribution of plug-in controlled electric vehicles (EVs) are considered to enhance the frequency regulation functions. An optimized design of the proposed 3DoF 1+PIDA/FOPI LFC scheme is proposed using the newly developed hybrid equilibrium optimizer (EO)-slime mould optimization (SMA) algorithm (namely EOSMA optimizer). The EOSMA combines the features of the EO and SMA powerful optimization algorithms. A two interconnected MGs in remote areas with RESs and EVs inclusions with high penetration levels is selected to verify the proposed 3DoF 1+PIDA/FOPI LFC scheme and the EOSMA optimizer. The results show high ability of the proposed controller and design scheme to minimize MGs’ frequency and tie-line power fluctuations and to preserve frequency stability and security.

Delay Compensation Control Strategy for Electric Vehicle Participating in Frequency Regulation Based on MPC Algorithm

With the continuous increase of renewable energy, the power system will face the problem of insufficient frequency response capability. Electric vehicles (EVs) combine the characteristics of mobile energy storage and controllable loads and can provide capacity support in frequency control. However, as a distributed frequency regulation resource, there is inevitably a communication delay in the process of electric vehicles participating in frequency regulation, which will affect the stability of the system frequency. Therefore, this paper proposes a frequency regulation delay compensation control strategy based on model predictive control (MPC). Firstly, the model predictive control is utilized to design an electric vehicle frequency response model controller and establish a frequency regulation model of the power system containing electric vehicles; then, by establishing the selection rules of the control signal, the multi-step control signal is dynamically adjusted during the rolling optimization of the predictive model to form a delay compensation mechanism considering delay and packet loss; finally, multi-scenario simulation is conducted, and the results show that the proposed strategy can roll-optimize the auxiliary frequency regulation function of electric vehicles, significantly reduce the influence of communication time delay, and improve the system frequency stability.

Analysis of Voltage and Frequency Influenced by Control Function Selection of Smart Inverter based on Voltage Class of Grid Connection Point

In recent, smart inverter is focused on as a solution for the problems on voltage and frequency stability caused by mass introduction of PV. Smart inverter has many functions to keep voltage and frequency within the proper range. Its effectiveness significantly depends on which function is activated but it has not been clarified sufficiently. This paper investigates influence on voltage and frequency of combined use of voltage regulation function and frequency regulation function of smart inverter. In the analysis, smart inverters connected to transmission network and ones connected to distribution network are modeled separately, and appropriate function activation in each network is investigated. Results of simulation study shows that precedence setting in the combined use of multiple function sometimes makes interference among them and deteriorates performance of smart inverter. The dependencies on the performance of introduction in transmission and distribution network by the analysis, and importance of selection of activating function considering introduction amount in transmission and distribution network is recognized.

Prediction technology and application of primary frequency regulation capability of thermal power unit

As one of the important functions of grid connected thermal power generation units, the primary frequency regulation function aims to increase and decrease load rapidly when the grid frequency changes, so as to achieve the stability of grid frequency. The primary frequency regulation capability of the unit is easy to be affected by the current main parameters of the unit operation and the operation mode of the equipment, which will lead to the phenomenon that the primary frequency regulation capability of the unit is insufficient. This paper introduces the basic principle of primary frequency regulation of thermal power units, and puts forward the main technical methods for prediction of primary frequency regulation capacity of units. Through the real-time prediction of the primary frequency regulation capability of the thermal power generation unit, the operation parameters and equipment operation mode of the thermal power generation unit can be changed in time, so as to avoid the lack of primary frequency regulation capability when the grid frequency changes. Finally, a typical industrial case is used to verify the effectiveness of this technology.

Characteristic Analysis and Optimal Regulation of Primary Frequency Regulation Condition in Low Water Head Area Based on Hydraulic-Mechanical-Electrical Coupling Model of Pumped Storage Unit

Pumped storage power station is an important regulating tool for peak load regulation and frequency regulation of the power grid, especially its primary frequency regulation function, which is of profound significance for the safety and stability of the power grid. As the core equipment of the pumped storage power station, the reversible design of the pump turbine makes it easy to have hydraulic fluctuation and mechanical instability when the unit runs to the “S” characteristic area, which will cause the frequency oscillation of the generator under the condition of primary frequency regulation. Therefore, some innovative work is studied in this paper: (1) an accurate hydraulic-mechanical-electrical coupling system mathematical model of pumped storage unit regulation system (PSURS) is established based on full characteristic curve of the pump turbine and seventh-order synchronous generator and excitation system; (2) the dynamic response characteristics of primary frequency regulation of pumped storage unit (PSU) under different water heads and different frequency disturbances are analyzed by numerical simulation; (3) in view of the unstable condition of the unit under the large frequency disturbance when it operates in the low head and small load area, the objective optimization function considering the ITAE index of hydraulic, mechanical, and electrical factors is proposed; and (4) fractional-order PID controller and the bacterial-foraging chemotaxis gravitational search algorithm (BCGSA) combined optimization strategy is used for PSURS optimization regulation and parameter optimization. The results show that the joint optimization strategy proposed in this paper has smaller objective function value, and makes the PSURS pass through the unbalanced area quickly, with better primary frequency regulation speed and smaller regulation depth.

Functional orientation analysis of primary frequency regulation and inertia control of DFIG

With the penetration of wind power increasing continuously, the inertia and the capability of primary frequency regulation of grid are declining. To deal with this problem effectively, additional primary frequency regulation control and inertia control adopted in wind turbines are necessary. However, at present, the functional orientation of primary frequency regulation control and inertia control is still unclear. It is urgent to clarify the difference between the two kinds of control methods and to make clear the demand of power grid for them. In this paper, based on analysis of main methods of primary frequency regulation and inertia control of doubly-fed induction generators (DFIG), the functional orientation distinction between inertia support function and primary frequency regulation function is studied from perspective of control principles and control results. The different effectiveness of different control functions on the frequency regulation process in different scenarios are analyzed by simulations.

Public transit service frequency and fares with heterogeneous users under monopoly and alternative regulatory policies

We present a model of public transit service under monopoly when potential users differ in their willingness to pay and value of time. The transit operator chooses service frequency and the fare to maximize a weighted sum of profit and consumers’ surplus. Profit-maximizing and social-surplus-maximizing frequency decisions are compared using a unified framework that includes results of previous studies as special cases. The prevalence of the Mohring Effect and the need for subsidization are investigated. Four types of regulatory policies are then considered: fare regulation, frequency regulation, goal or objective function regulation, and fiscal regulation whereby the operator receives a subsidy based on consumers’ surplus or demand. A numerical example is used to assess the relative efficiency of the regulatory regimes, and illustrate how the solutions depend on the joint distribution of willingness to pay and value of time.

Smart grid energy storage controller for frequency regulation and peak shaving, using a vanadium redox flow battery

Grid connected energy storage systems are regarded as promising solutions for providing ancillary services to electricity networks and to play an important role in the development of smart grids. Thus far, the more mature battery technologies have been installed in pilot projects and studies have indicated their main advantages and shortcomings. The main concerns for wide adoption are the overall cost, the limited number of charging cycles (or lifetime), the depth of discharge, the low energy density and the sustainability of materials used. Vanadium Redox Flow Batteries (VRFB) are a promising option to mitigate many of these shortcomings, and demonstration projects using this technology are being implemented both in Europe and in the USA. This study presents a model using MATLAB/Simulink, to demonstrate how a VRFB based storage device can provide multi-ancillary services, focusing on frequency regulation and peak-shaving functions. The study presents a storage system at a medium voltage substation and considers a small grid load profile, originating from a residential neighbourhood and fast charging stations demand. The model also includes an inverter controller that provides a net power output from the battery system, in order to offer both services simultaneously. Simulation results show that the VRFB storage device can regulate frequency effectively due to its fast response time, while still performing peak-shaving services. VRFB potential in grid connected systems is discussed to increase awareness of decision makers, while identifying the main challenges for wider implementation of storage systems, particularly related to market structure and standardisation requirements.

Decentralized Vehicle-to-Grid Design for Frequency Regulation within Price-based Operation

The utilization of electric vehicles has been suggested to support the frequency regulation of power system. Assuming that an intermediate aggregator exists, this study suggests a decentralized vehicle-to-grid operation scheme in which each vehicle-to-grid aggregator can behave independently of the power system operator. To implement this type of decentralized operation, this study adopts a price-based operation that has been proposed by many researches as an alternative operation scheme for the power system. In this environment, each vehicle-to-grid aggregator can determine its participation in vehicle-to-grid service in consideration of its residual energy of aggregated system and real-time market price. Consequently, the main purpose of this study is to verify whether or not the vehicle-to-grid power can effectively support the current frequency regulation function within the price-based operation scheme. Specifically, a frequency regulation method is proposed based on the real-time price signal, and a feedback controller for battery management is designed for decentralized vehicle-to-grid operation.

Dynamic Power Conditioning Method of Microgrid Via Adaptive Inverse Control

Different microsources have different frequency regulation functions and capabilities. The droop control can allocate power among the microsources according to the operation demand during system dynamics; however, the steady-state frequency often deviates from the rated value because of the droop characteristics. To ensure the precise condition of power and the stability of frequency even in a low-voltage network, this paper puts forward an improved droop control algorithm based on coordinate rotational transformation. With the ability to accurately regulate the unbalance power, this method realizes self-discipline parallel operation of microsources. Furthermore, an adaptive inverse control strategy applied to modified power conditioning is developed. With an online adjustment of modified $P-f$ droop coefficient for the frequency of microgrid to track the rated frequency, the strategy guarantees maintaining the frequency of microgrid at the rated value and meeting the important customers' frequency requirements. The simulation results from a multibus microgrid show the validity and feasibility of the proposed control scheme.

Analysis on Primary Frequency Regulation Function Tests for Thermal Power Units

With the increase of the load capacity and the development of the automation degree, the power dispatching center has a higher requirement for the primary frequency regulation function of the unit aircrew. After a certain period of time when the unit has been put into operation, the primary frequency regulation function must have the conditions to be put into operation, so that the unit can be involved in grid frequency at any time when the conditions are satisfied and the power supply quality can be fully guaranteed [1]. The regulation performance of primary frequency regulation plays a vital role in maintaining the stability of the unit aircrew and the power grid, so it is very necessary to conduct the frequency tests for the thermal power units [2]. In this paper, we describe the disturbance tests and recovery tests under different load points and different network frequencies. Through the analysis, we can draw the conclusion that the primary frequency regulation function of unit 1 can fully meet the requirement of the frequency regulation of the power grid.

Active Power Control Strategy with Auxiliary Frequency Regulation Function of Wind Farms

The fluctuations and uncertainties of wind power increase the frequency regulation pressure of conventional power plants. This paper introduces an active power control strategy with auxiliary frequency regulation function of wind farms. With this active power control strategy, wind farms can work in normal operation mode or in auxiliary regulation mode. Wind farms in normal operation mode can trace the power command to the utmost extent. When the system frequency drops large, to respond this change, the wind farms can switch to auxiliary regulation mode automatically. Simulation results show that the wind farms can assist the conventional power plants to regulate the system frequency effectively.

Designing Rule for Price-Based Operation With Reliability Enhancement by Reducing the Frequency Deviation

In a deregulated power system, the independent system operator (ISO) becomes the central entity which is responsible for the reliable system operation and the real-time balancing. ISO can provide necessary functions through the market mechanism. However, the possibility of a price-based operation is suggested as well, in which time-varying price coordinates non-cooperative players' actions. In the price-based operation, the main objectives of operation are also to keep the system frequency to the nominal value and to manage the line congestion. In terms of the frequency, the price-based operation should at least assist the frequency regulation functions by reducing the frequency deviation. For this purpose, this paper tries to present a method to reduce the frequency deviation and proposes the application of PID tuning rule to the design of price-based operation. In addition, a modified asymmetric control structure is proposed to obtain a further reduction of the frequency deviation. The performance and suitability of the designing rule are verified through simulations using the IEEE 39-bus network.

Phase compensated gas turbine governor for damping oscillatory modes

With market deregulation, there is constant pressure to utilise existing assets in more effective ways in order to achieve high levels of performance and as governor technologies mature the ability of governors to achieve much more than the standard power–frequency regulation function increases. Thus, this paper has focused on a more active use of governor control for a gas turbine to provide improved system stabilisation and performance via the inclusion of phase compensation in the governor control loop. Due to the decoupled nature of the mechanical power and excitation control loops, performance improvement via governor control does not interfere with generator voltage regulation, which is a drawback of conventional generator damping provision via a power system stabiliser (PSS). In addition, the mechanical power control loop is also less affected by the operating condition of the power system and is hence more robust. It is shown that inclusion of appropriate phase compensation in the governor control loop can improve dynamic and transient stability, either alone or in conjunction with a PSS in the exciter control loop, without adversely interfering with voltage control or changing steady state power–frequency regulation.

Study on a system frequency response model for a large industrial area load shedding

After reviewing the classical, simplified system frequency response (SFR) models for load shedding, the paper presents an up-to-date high order system frequency response model for load shedding of a large industrial area. The model possesses the dynamic performance of integrated turbine speed governors (reheat, non-reheat and gas turbine, etc.) and includes equivalent inertia constant of the motive loads during power system islanding. It also considers primary frequency regulation functions and the capability limits of turbines. For an industrial power system connected with the grid, an applicable under-frequency load shedding scheme considering minimum permissible non-shedding frequency is given in detail.