Load Frequency Control Model Research Articles

Fractional order PID controller for perturbed load frequency control using Kharitonov’s theorem

In the fast developing world of today, there is an inevident demand of good quality power supply. For this, it is very essential that the present day power systems should possess efficient control algorithms for the load frequency control. Although, in recent years, lot of research has been carried out in this direction however, the parametric uncertainty and rejection of the external disturbances continue to remain important issues for the LFC design engineers. The control algorithms based on fractional order control are known to possess excellent disturbance rejection capability as well as high robustness towards the system parameter variations. Further, a higher level of robustness can be incorporated in the system by using an interval model of the plant while designing the controller. Therefore, in this paper a fractional order PID controller is designed for the interval model of the single area load frequency control using the Kharitonov’s theorem. This technique gives freedom to the design engineer to decide the limits of the parameter variations up to which the controller is required to perform well. This helps in improving the system performance as well as the overall safety of the system by preventing the system breakdown that may occur due to sudden increase in any system parameter. Finally, the performance of the proposed controller is compared to the nominal controllers existing in the literature and evaluated on the basis of the various integral error criterion. It is observed that the proposed controller gives the best performance and is capable of stabilizing the system for a larger range of system parameter variations.

Introducing LQR-Fuzzy Technique with Dynamic Demand Response Control Loop to Load Frequency Control Model

Abstract The present paper indicates a novel approach of LQR-Fuzzy controller to stabilize the frequency in all (normal and emergency) conditions under micro grid environment. Load Frequency Control (LFC) model plays a vital role in electric power system design and operation. From the literature, it is proven that the implementation of DR (Demand Response) is a key to the future micro grid. In practice, LFC-DR model is tuned by conventional controllers like PI, PD, PID controllers, with their fixed gains. However, they are incapable of obtaining good dynamic performance over a wide range of operating conditions and load changes. This paper presents an idea of introducing a DR control loop in the traditional LFC model (called LFC-DR) using Intelligent Controller for a power system. DR communication delay latency in the controller design is considered and is linearized using Pade approximation. The addition of DR control loop guarantees stability of the overall closed-loop system and effectively improves the system dynamic performance. Simulation results show that LQR- Fuzzy Logic Controller based LFC-DR single-area power system have better performance and superiority over a classical controller under any operating scenarios.

Robust load‐frequency control in interconnected power systems

This study describes a robust load-frequency control (LFC) scheme in a two-area interconnected power system with uncertain disturbances. A sector-bounded H∞ control approach is used for compensating of the non-linearities based on a minimum order dynamic LFC model to avoid the complexity of a high-order model. The simulation in time domain from the use of the proposed controller has been compared with the use of a conventional proportional–integral (PI) type controller affected by the same load change. The simulated results have proved the significant improvement over the PI controller and have verified the effectiveness of the proposed robust H∞ controller which can minimise the frequency deviations significantly. This approach treats the tie-line power exchanges in the presences of external disturbance, parameter uncertainties and accidental crashing of generating units as the structural or model uncertainties discussed in this study. Simulation results also show that the proposed H∞ controller guarantees robust performance and fast response under a wide range of operating conditions and system uncertainties.

Stabilising system frequency using HVDC between the Continental European, Nordic, and Great Britain systems

For future efficiency improvement of the frequency containment process (primary control) within European power systems, cooperation between (multiple) synchronous areas using their controllable HVDC interconnections is optioned. However, the differences in system size, HVDC interconnection capacity, and the balancing performance per individual system will have its specific system frequency effect for each different balancing cooperation concept. Consequently, without alignment on cooperation, HVDC balancing might lead to disproportional support between systems, to frequency oscillations, reserve unreliability and non-compliancy, and to network constraints. Therefore, this work assesses frequency quality and associated DC power flows for several balancing arrangements, using a developed load-frequency control model with frequency interdependency for coupled power system. For a trilateral balancing cooperation case study, it is found that certain cooperation concepts result in undesired frequency oscillation and poor frequency quality. However, cooperation where especially fast-response services are shared, such as virtual inertia, show improved system frequency performance. For the case where power imbalances are proportionately distributed among the systems, it is concluded that power transfers over HVDC interconnections are limited and additional control optimisation can be performed. Those concepts with aligned central or coordinated control show best results for a future cooperation for balancing between synchronous areas.

Guaranteed cost load frequency control for a class of uncertain power systems with large delay periods

This paper investigates the problem of guaranteed cost load frequency control (LFC) for a class of uncertain power systems with large delay periods (LDPs). The model of LFC for power systems with LDPs has been firstly modeled as a switched delay system, which includes a potentially unstable subsystem caused by LDPs and a stable subsystem caused by small delay periods (SDPs). Based on the switching technique and under the constraint conditions of the length ratio and the frequency of LPD, the guaranteed cost load frequency controller can be obtained such that the closed-loop system is exponentially stable and a weighted guaranteed cost upper bound is achieved. Finally, an example is provided to demonstrate the applicability of the proposed method.

Cooperative Control of Power System Load and Frequency by Using Differential Games

Electric power systems are evolving toward smart grids all over the world, which are generally regarded as the next-generation power systems with extensive application of advanced control, communications, and computation techniques. More and more distributed intelligent controllers will be employed in smart grids. Load frequency control (LFC) is a key issue in the traditional interconnected power systems, which constantly requires different control areas (CAs) to share the regulation burden of the CA that lacks regulation capacity by providing the power supports via the tie-line. Such a process imposes extra regulation costs on the helping CAs, for example, the wear and tear of generating units, which may result in the unfairness and the CAs’ deviation from the LFC command. This situation becomes even more serious with the integration of intermittent renewable energy such as wind and solar power. In this paper, cooperative control by using differential games (DGs) is proposed as a possible solution to this problem. With a two-area and a three-area LFC models, a noncooperative equilibrium solution, and two cooperative equilibrium solutions with different time-consistency properties are derived. The case studies compare the proposed three DG-based LFC schemes with the traditional approaches based on proportional-integral control and the optimal control, and the results show that the cooperative control schemes allocate the regulation burden more rationally and ensure the CAs’ sticking to the LFC command, which will be favorable to the stability of power system operation. The capability of conflict solution of DG-based cooperative control makes it a very promising scheme for smart grid control.

Load Frequency Control in Isolated Micro-Grids with Electrical Vehicles Based on Multivariable Generalized Predictive Theory

In power systems, although the inertia energy in power sources can partly cover power unbalances caused by load disturbance or renewable energy fluctuation, it is still hard to maintain the frequency deviation within acceptable ranges. However, with the vehicle-to-grid (V2G) technique, electric vehicles (EVs) can act as mobile energy storage units, which could be a solution for load frequency control (LFC) in an isolated grid. In this paper, a LFC model of an isolated micro-grid with EVs, distributed generations and their constraints is developed. In addition, a controller based on multivariable generalized predictive control (MGPC) theory is proposed for LFC in the isolated micro-grid, where EVs and diesel generator (DG) are coordinated to achieve a satisfied performance on load frequency. A benchmark isolated micro-grid with EVs, DG, and wind farm is modeled in the Matlab/Simulink environment to demonstrate the effectiveness of the proposed method. Simulation results demonstrate that with MGPC, the energy stored in EVs can be managed intelligently according to LFC requirement. This improves the system frequency stability with complex operation situations including the random renewable energy resource and the continuous load disturbances.

Frequency Stabilization for Multi-area Thermal–Hydro Power System Using Genetic Algorithm-optimized Fuzzy Logic Controller in Deregulated Environment

—This article develops a model of load frequency control for an interconnected two-area thermal–hydro power system under a deregulated environment. In this article, a fuzzy logic controller is optimized by a genetic algorithm in two steps. The first step of fuzzy logic controller optimization is for variable range optimization, and the second step is for the optimization of scaling and gain parameters. Further, the genetic algorithm-optimized fuzzy logic controller is compared against a conventional proportional-integral-derivative controller and a simple fuzzy logic controller. The proposed genetic algorithm-optimized fuzzy logic controller shows better dynamic response following a step-load change with combination of poolco and bilateral contracts in a deregulated environment. In this article, the effect of the governor dead band is also considered. In addition, performance of genetic algorithm-optimized fuzzy logic controller also has been examined for various step-load changes in different distribution unit demands and compared with the proportional-integral-derivative controller and simple fuzzy logic controller.

A Novel Controller for Model with Combined LFC and AVR Loops of Single Area Power System

In this study, a novel controller is designed to study low frequency oscillations for load frequency control (LFC) and voltage control of a single area power system. For more accuracy in dynamic and steady state responses, mutual effects between LFC and automatic voltage regulation (AVR) loops are investigated in a combined simulink model of LFC and AVR loops. The effectiveness of the proposed controller is first simulated on model with LFC loop alone. The proposed controller is a hybrid of neural network and fast traversal filters. The proposed hybrid controller requires less number of samples for training of weights, thus making the system fast. To study the coupling effects of AVR and LFC loops, dynamic performance of a complete system model for low frequency oscillation studies comprising of mechanical and electrical loops is done with the proposed controller.

Introducing Dynamic Demand Response in the LFC Model

Demand response (DR) has proved to be an inevitable part of the future grid. Much research works have been reported in the literature on the benefits and implementation of DR. However, little works have been reported on the impacts of DR on dynamic performance of power systems, specifically on the load frequency control (LFC) problem. This paper makes an attempt to fill this gap by introducing a DR control loop in the traditional LFC model (called LFC-DR) for a single-area power system. The model has the feature of optimal operation through optimal power sharing between DR and supplementary control. The effect of DR communication delay in the controller design is also considered. It is shown that the addition of the DR control loop increases the stability margin of the system and DR effectively improves the system dynamic performance. Simulation studies are carried out for single-area power systems to verify the effectiveness of the proposed method.

A Framework for Economic Load Frequency Control Design Using Modified Multi-objective Genetic Algorithm

This article deals with the coordination of security-constrained economic dispatch and load frequency control in an interconnected power system. The realistic and performance optimization inherent of the load frequency control (LFC) and security-constrained economic dispatch are fully considered without simplifying assumptions. For this purpose, modeling security-constrained economic dispatch as a discontinuous control action in the continuous frequency response model of a power system is well addressed. Considering conflict behavior of LFC and security-constrained economic dispatch beside the powerfulness of the multi-objective genetic algorithm (GA) to solve high-dimensional problems with conflicted objective functions makes it attractive for the automatic generation control coordination problem. The employed security-constrained economic dispatch utilizes the advantages of dynamic economic dispatch to achieve more realistic results. The GA is used to compute the decentralized control parameters and centralized generation levels of the on-line units to achieve an acceptable operating point. A significant modification in convergence speed has been performed by using LFC model properties in corporation with the genetic algorithm, so the proposed method gives considerable promise for implementation in multi-area power systems. The efficiency of the proposed algorithm and modification is demonstrated on a three control area power system.

New approach using structure-based modeling for the simulation of real power/frequency dynamics in deregulated power systems

Load frequency control (LFC) is one of the signicant ancillary services in the electricity markets. The major concerns with LFC models have long been their dynamic response and complexity, as well as market price sensitivity. To cater to the complexity of the dynamic model that is introduced by the LFC and algebraic constraints by the transmission network, the dierential-algeb raic equation is transformed into a set of ordinary dierential equations. This is done by dierentiating the network constraints under a certain assumption that can be held throughout the system. To make the model suitable in a market environment, the real power (MW) variable is substituted by the energy ($) variable to emphasize the price signal in the market. The obtained results show that the model is much easier to understand and simpler to implement. The computational complexity of the model is further reduced. Based on the performed simulations, the proposed LFC model functions better in a deregulated environment. A test case comprising 5 buses and 3 machines, with a 2-region system, is used to address the advantages of the proposed LFC model.

An Observer-Based Robust Load Frequency Control

A full-order observer-based state feedback linear controller design approach for a single area load frequency problem is presented. The load frequency control model incorporates integral control for improved command tracking response and disturbance rejection. A full state feedback is implemented through full-order state observer design. State feedback with integral controllers design is carried out through a pole-placement technique using symmetrical root locus method. Parametric model uncertainty has been considered and stability robustness study has been carried out assigning a bound on the deviations in parameters of single area load frequency model. The proposed design methodology achieves performance satisfying the specified stability margins. The methodology provides a control over peak values of the frequency and control signal deviations which may be utilized to meet hardware constraints. A numerical example illustrates the effectiveness of the developed methodology.

Robust Load Frequency Controller Design Based on a New Strict Model

This article presents an approach to design robust load frequency controllers. In view of the drawbacks of the conventional load frequency control model, a new strict model is derived from the detailed area model. Based on the derived model, the robust load frequency controllers are designed to restrain the disturbances. Since the robust controllers with high order are adverse to practical application, the balanced truncation method is adopted to reduce the order of the controllers. Finally, the designed controllers are tested on a typical two-area four-machine system, and the results verify the validity of the presented approach.

Decentralized load frequency control in deregulated environments

Decentralized load frequency control (LFC) problem for multi-area power systems in deregulated environments is studied in this paper. A decentralized PID tuning method is proposed by assuming that the tie-line power flows are disconnected. The general transfer function model of LFC for decentralized control design of deregulated power systems is derived first, and then an IMC-PID method is used to design the local load frequency controllers. A method to analyze the stability of the decentralized LFC for conventional environments is also extended to the deregulated case. The proposed method is tested on a four-area power system under various operating conditions. Simulation results show that the proposed design method is simple to follow, can achieve good performance, and results in implementable PID controllers.

On feasibility of regional frequency-based emergency control plans

Decentralized and regional load-frequency control of power systems operating in normal and near-normal conditions has been well studied; and several analysis/synthesis approaches have been developed during the last few decades. However in contingency and off-normal conditions, the existing emergency control plans, such as under-frequency load shedding, are usually applied in a centralized structure using a different analysis model. This paper discusses the feasibility of using frequency-based emergency control schemes based on tie-line measurements and local information available within a control area. The conventional load-frequency control model is generalized by considering the dynamics of emergency control/protection schemes and an analytic approach to analyze the regional frequency response under normal and emergency conditions is presented.

On the Design of Variable Structure Load Frequency Controllers by Tabu Search Algorithm: Application to Nonlinear Interconnected Models

A variable structure controller (VSC) with chattering reduction feature applied to interconnected load frequency control (LFC) problem is presented. Formulating the design of VSC as an optimization problem and utilizing tabu search (TS) algorithm provides a simple and systematic way of arriving at the optimal feedback gains and switching vector values of the controller. In addition, this will cut down the need for nonlinear or coordinate transformation as reported before. The tested interconnected LFC model incorporates nonlinearities in terms of generation rate constraint (GRC) and a limiter on the integral control value. In order to guarantee the enhancement of the system, dynamical performance and chattering reduction of the VSC, different objective functions were investigated in the optimization process. In addition, the complexity of the controller is reduced by using only the accessible states in designing the VSC. Comparison with previous LFC methods reported in literature validates the significance of the proposed VSC design.

Adaptive approach to load shedding including pumped-storage units during underfrequency conditions

An adaptive approach is proposed to load shedding including the consideration of pumped-storage units during underfrequency conditions. Based on the idea of average or uniform frequency behaviour, a load-frequency control model is utilised in the scheme design. At this stage, Taiwan Power Company (Taipower) owns ten pumped-storage units that amount to about 13% of the total rating capacity. These units can be operated in generating mode during peak period, to reduce generation cost, or operated in pumping mode during off-peak period, to consume the surplus power. As the difference between the peak load and off-peak load of Taipower is considerable, pumped-storage units are playing a critical role in Taipower operations. Once disturbances take place in off-peak periods, pumped-storage units can be thus shed with a high priority. As they are useful in both restoring system frequency and decreasing the degree of service interruptions, these pumped-storage units have been included in the proposed load-shedding scheme design and tested through the utility data. From test results and comparisons with the existent utility shedding scheme, they have demonstrated the feasibility and practicality of the method for such applications.

An automatic load shedding scheme including pumped-storage units

An automatic load shedding scheme considering the pumped-storage units at Taiwan Power Company (Taipower) is proposed in this paper. Taipower owns ten pumped-storage units which can be shed with higher priority when operated in pumping mode in order to restore the system frequency. In this way, the amount of customer load required to be shed can be significantly decreased. The system reliability and stability can also be enhanced. In the paper, a load frequency control model is developed. An automatic load shedding scheme that is adaptable to Taipower operation is thus developed. The proposed method has been validated through the utility data and compared with the utility shedding scheme. Test results solidify the feasibility and effectiveness of the proposed method for the applications.

Stability of Load Frequency Control System for Interconnected Power Systems with Governor Dead-Band Nonlinearity

The domain separation method is used for an appropriate choice of parameters on the basis of a linear model of load frequency control. A Popov plot is then used to check if the system is stable with governor dead-band nonlinearity.