Decentralized load frequency based on H∞ control

This paper presents a decentralized load frequency control (LFC) based on H_∞ optimal control theory with an observer. A few LFC schemes have been proposed based on the optimal control theory, but they have not considered the change of system parameters in operation and the characteristics of load disturbances in a target system. In this paper, H_∞ robust control is introduced to address such problems. Owing to its practical merit, the proposed control scheme is a decentralized LFC. Employing observer theory, the proposed method requires only frequency and tie‐line power deviation in each area. Numerical simulations are shown to demonstrate the effectiveness of the proposed method. H_∞ control was proven to show greater effectiveness of damping disturbance over the conventional optimal control by the design of control systems aimed at restricting the H_∞ norm of its transfer function. In particular, when a decentralized LFC is applied, by reducing the system size, H_∞ norm is easier to dampen; thus H_∞ control is more effective in the decentralized control. Future research topics include the design of H_∞ control system with a weight on frequency response. © 2001 Scripta Technica, Electr Eng Jpn, 136(3): 28–38, 2001     

Robust Load Frequency Control with Dynamic Demand Response for Deregulated Power Systems Considering Communication Delays

Communication networks are used in load frequency control (LFC) for transmitting remote measurements and control commands, and in demand side response (DSR) for aggregating small-scale controllable loads. This paper investigates modeling and controller design for LFC together DSR in a deregulated environment, considering multiple time delays introduced by the usage of communication channels. Time delay model of the deregulated multi-area LFC with dynamic demand control (DDC) is obtained at first, in which a typical thermostatically controlled appliance, air conditioner, is used for DDC. A robust proportional integral derivative (PID) load frequency controller is designed, through the H_∞ performance analysis and the particle swarm optimization (PSO) searching algorithm, to deal with the load disturbances and multiple delays in the LFC loop and the DDC loop. Case studies based on a three-area deregulated LFC system demonstrate the effectiveness of the proposed load frequency controller and the performance improvement from the DDC. Simulation results show that the DDC can increase the delay margin of the LFC scheme. Moreover, several delay stable regions are revealed via simulation method.     

H∞ load frequency control of interconnected power systems with communication delays

This paper considers the problem of power system load frequency control design incorporating the effect of using open communication network instead of a dedicated one for the area control error signals. To have this, we appropriately consider time-delays in the ACE signals. A delay-dependent two-term H_∞ controller design has then been proposed using linear matrix inequalities. Comparison of effectiveness of the proposed two-term controller with that of existing one-term and two-term controller designs establishes the superiority as well as applicability of the present design for the LFC problem.     

Frequency Regulation in Hybrid Power System Using Iterative Proportional-Integral-Derivative H ∞ Controller

This study considers frequency regulation in a hybrid power system consisting of conventional and distributed generation resources. The performance of two controllers—an H _∞ design via linear matrix inequalities and an iterative proportional-integral-derivative H _∞ via linear matrix inequalities—is assessed to maintain frequency deviation profile in acceptable limits. In the latter control design, the iterative linear matrix inequality approach is used to tune proportional-integral-derivative controller parameters subjected to H_∞ constraints in terms of the iterative linear matrix inequality. The efficacy of the control law and disturbance accommodation properties is shown. The robustness of these controllers is demonstrated in the hybrid power system with different load disturbance conditions, wind power, and parameter variations. Controller performance is compared with a suboptimal controller to demonstrate its superiority. It is found that the second controller design has satisfactory disturbance rejection properties and robustness against parameter variations over a wide range of conditions.     

Power system transient stability enhancement by STATCOM with nonlinear H∞ stabilizer

A robust control design is presented for a single machine infinite bus system (SMIB) with a STATCOM. The controller for the nonlinear system is designed using the recently developed nonlinear H_∞ theory. The approach combines state feedback exact linearization with linear H_∞ principle, which avoids the difficulty of solving Hamilton–Jacoby–Issacs inequality. Simulation results with a number of disturbances like torque pulses and three-phase faults on the generator show that the proposed robust controller can ensure transient stability of the power system over a wide range of operating points     

Stabilization control for multimachine system using decentralized H∞ excitation controller realizing terminal voltage control and damping control

In this paper, to achieve both damping of power system oscillation and terminal voltage control simultaneously on a multimachine power system, we propose a decentralized H_∞ excitation controller. In the proposed method, H_∞ control via the Normalized Coprime Factorization approach is used to achieve the proposed design idea. By the Normalized Coprime Factorization approach, the weighting function in H_∞ control design is simplified, and output feedback controllers that take into account the realities and constraints of the power systems are designed. The proposed controller is subjected to model reduction of H_∞ controllers, and is transformed to a discrete system to perform digital control by computer systems in consideration of application to a real system. We verify that the proposed excitation controller can achieve both damping of power system oscillation and terminal voltage control by computer simulations of a multimachine power system. © 2004 Wiley Periodicals, Inc. Electr Eng Jpn, 147(1): 33–41, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.10254     

Robust decentralized load frequency control of interconnected power system with Generation Rate Constraint using Type-2 fuzzy approach

The Load Frequency Control (LFC) problem has been a major subject in electrical power system design/operation. LFC is becoming more significant recently with increasing size, changing structure and complexity in interconnected power systems. In practice LFC systems use simple Proportional Integral (PI) controllers. As the PI control parameters are usually tuned, based on classical approaches. Moreover, they have fixed gains; hence are incapable of obtaining good dynamic performance for a wide range of operating conditions and various load changes, in multi-area power system. Literature shows that fuzzy logic controller, one of the most useful approaches, for utilizing expert knowledge, is adaptive in nature and is applied successfully for power system stabilization control. This paper proposes a Type-2 (T2) fuzzy approach for load frequency control of two-area interconnected reheat thermal power system with the consideration of Generation Rate Constraint (GRC). The performance of the Type-2 (T2) controller is compared with conventional controller and Type-1 (T1) fuzzy controller with regard to Generation Rate Constraint (GRC). The system parametric uncertainties are verified by changing parameters by 40% simultaneously from their typical values.     

Robust controller design of microturbine and electrolyzer for frequency stabilization in a microgrid system with plug-in hybrid electric vehicles

This paper proposes a new robust controller design of microturbine (MT) and electrolyzer (ES) in a control and monitoring system (CMS) for frequency stabilization in a microgrid system with plug-in hybrid electric vehicles (PHEVs). In the studied microgrid, the MT is normally used to provide the main power to the loads while the ES absorbs the power from the system to produce the hydrogen as the fuel input for the power generation of the fuel cell. On the other hand, the large numbers of PHEVs are utilized in the consumer side. The concurrent charging powers of PHEVs cause a problem of severe frequency fluctuation in the microgrid. To solve this problem, the frequency stabilization of CMS is performed by controlling the power output of MT and ES. The controller structure of MT and ES is a proportional integral with a single input. To enhance the tracking performance and the robustness against system uncertainties of the designed MT and ES controllers, the control parameters are optimized by shuffled frog leaping algorithm based on specified-structure mixed H₂/H_∞ control technique. Simulation results not only show the frequency stabilization effect against the random charging power of PHEVs but also the high robustness of the proposed robust MT and ES controllers against the system parameters variation.     

Decentralized coordinated control for large power system based on transient stability assessment

For the purpose of enhancing transient stability of large power systems, this paper focuses on an issue of decentralized coordinated control. A modified equal area criterion (MEAC) is firstly proposed as the transient stability judgment criterion of multi-machine power systems. Then, a hierarchical decentralized coordinated excitation control is designed, which consists of both upper level coordinated control and lower level decentralized control. Based on the transient stability assessment, the coordinated controller determines whether to send coordinated control signal to lower level decentralized controllers. Moreover, the decentralized coordinated controller is designed by using H_∞ robust control method so as to deal with the uncertainties of system. Finally, simulation studies test effectiveness of the proposed control.     

Active vibration control of a motor system by using H∞ control theory

This paper considers the vibration control of a motor system which has a motor and a load connected with a flexible shaft. However, this system often generates a shaft torsional vibration. Traditional methods of treating this problem to adjust the PID controller so that the closed-loop frequency response is slower than that of the vibration mode. On the other hand, one method has already been proposed in which the vibration is suppressed by a disturbance observer. This paper proposes a new approach based on H_∞ control theory. For comparison, a PI control system based on classical control theory also is constructed. The results of several experiments show that compared with the PI control system, the H_∞ control system is effective in suppressing the vibration. Further, the H_∞ controller obtained in the study consists of a PI controller and a series compensator that functions as an active vibration controller.     

A Robust Cascade Controller for Load Frequency Control of a Standalone Microgrid Incorporating Electric Vehicles

Abstract

Intermittency in the output power of renewable and green energy sources (RGES) and low inertia of a standalone microgrid (SMG) result in large frequency deviations. Use of energy storage systems (ESSs) alleviate the SMG frequency deviations in an adorable manner but their high cost and low power density calls for alternative sources to balance the mismatch between power supply and demand. In recent years, utilization of the battery of an electric vehicle (EV) to minimize the frequency deviations has gained a lot of attention. Consequently, this paper proposes a robust and newly developed bio-inspired Salp Swarm Optimization (SSO) algorithm based PI-PD cascade controller for load frequency control (LFC) of the SMG integrated with the EVs. To demonstrate the efficacy of the proposed controller, its performance has been compared with other well-known controllers and algorithms considering diverse SMG operating scenarios. Simulation results distinctly prove the superiority of the proposed controller over the other controllers. Also, robustness of the proposed controller has been tested subject to ±50% variation in certain SMG parameters. Results clearly justify the robustness of the proposed controller. Additionally, operational stability of the SMG has been appraised through Eigenvalue and Bode diagram analysis for all the scenarios.     

Fixed-order controller for reduced-order model for damping of power oscillation in wide area network

The interconnected power systems are complex and stabilizing control design still remains challenging task. The use of wide area monitoring system (WAMS) offers an integrated measurement-based and model-based control, which suits to the operation of large electric power system (EPS), along with online analysis. This paper presents a study on fixed-order controller design for equivalent network of coherent generator in order to stabilize inter-area electromechanical oscillations in the system. Firstly, the coherent generators in each area of large EPS are determined by mutual information theory, which represents the dynamic equivalence. Then network of each area with input–output variables of the selected generator that participates dominantly is reduced to lower size by square-root variant of balanced truncation algorithm. The dynamics and important oscillation modes are verified in equivalent representation of each area. Finally a local controller (decentralized) in each coherent area and a centralized controller between two coherent areas for selected generator are designed by reducing the H^∞ norm of its closed loop transfer function as much as possible. These controllers feed supplementary control signal in addition to one fed by local conventionally tuned PSS. The decentralized controller for selected generator is fed by local bus power or generator’s speed signal. On other hand, the centralized controller uses difference of power flow/speed of generators as input signal to dampen the oscillations between equivalent networks of two areas. The simulation results reveal effective damping of power/speed oscillations achieved by designed controller with respect to conventional PSS implemented. The robustness of controller is verified for heavy and light load operating conditions.     

Load frequency control in deregulated environments via active disturbance rejection

Linear active disturbance rejection control (LADRC) method is investigated for the load frequency control (LFC) of power systems in deregulated environments. The connections between one area and the rest of the system and the effects of possible contracts are treated as a set of new disturbances besides the system load. LADRC uses an extended state observer (ESO) to estimate the disturbances and compensates them quickly. Thus it can achieve good disturbance rejection performance and is a good candidate for LFC design. The proposed method is tested on two power systems. Simulation results show that the LADRC is simple to tune for load frequency control systems, and good performance can be achieved.     

Performance of a grid-connected wind generation system with a robust susceptance controller

Wind turbine driven induction generators are vulnerable to transient disturbances like wind gusts and low voltages on the system. The fixed capacitor at the generator terminal or the limited support from the grid may not be able to provide the requisite reactive power under these abnormal conditions. This paper presents a susceptance control strategy for a variable speed wound-rotor induction generator which can cater for the reactive power requirement. The susceptance is adjusted through a robust feedback controller included in the terminal voltage driven automatic excitation control circuit. The fixed parameter robust controller design is carried out in frequency domain using multiplicative uncertainty modeling and H_∞ norms. The robustness of the controller has been evaluated through optimally tuned PID controllers. Simulation results show that the robust controller can effectively restore normal operation following emergencies like sudden load changes, wind gusts and low voltage conditions. The proposed robust controller has been shown to have adequate fault ride through capabilities in order to be able to meet connection requirements defined by transmission system operators.     

A New Design of Wide-Area MIMO Coupling Robust HVDC Supplementary Controller

A new multi-input multi-output (MIMO) robust coordination approach for the controller design of multi-infeed high voltage direct current (HVDC) system is proposed. This design will enhance the damping of certain low frequency oscillation mode. The dominant mode ratio (DMR) is applied to choose the appropriate feedback signals, while time delay of wide-area signals is considered. The Total Least Squares-Estimation on Signal Parameters via Rotational Technique (TLS-ESPRIT) algorithm serves in identifying the MIMO system model. To reach the best control effect and disturbance suppression level, the mixed H₂/H_∞ robust theory with regional poles placement is introduced to design the controller. The new MIMO design approach avoids system decoupling in complex situation, and it uses the interactions of different control loops to enhance the system's stability when most MIMO HVDC controllers are designed by decoupling strategy. Simulation results of a multi-infeed HVDC system validate the control effect and robustness of the proposed method.     

负荷频率系统的LFC模糊监控器设计

应用在LFC控制中常规的模糊调整增益PI控制器和I控制器,只针对单一区域的负荷频率进行控制,并不考虑实际模型中的互相扰动.其中最突出的问题是由于不限制联络线上的功率流动,电能总是从频率高区域流向频率低区域,加剧了频率高区域的控制负担,再加上各区域控制动作的不协调,使得当扰动在不同的时间和幅值时引起系统调节过程的急剧恶化,导致整个系统不稳定.提出一种基于模糊MAMDANI推理算法的模糊监督控制器,采用各个区域的频率偏差作为控制器的输入,监督控制器作为一种前馈补偿,设计输出为一个ACE的倍数,加快调节过程,到达稳态.经过实验仿真数据对比,说明提出的监督控制器能有效提高系统的稳定性.     

Robust adaptive tracking control of uncertain discrete time systems

In this paper, the problem of robust adaptive tracking for uncertain discrete‐time systems is considered from the slowly varying systems point of view. The class of uncertain discrete‐time systems considered is subjected to both 𝓁_∞ to 𝓁_∞ bounded unstructured uncertainty and external additive bounded disturbances. A priori knowledge of the dynamic model of the reference signal to be tracked is not completely known. For such problem, an indirect adaptive tracking controller is obtained by frozen‐time controllers that at each time optimally robustly stabilize the estimated models of the plant and minimize the worst‐case steady‐state absolute value of the tracking error of the estimated model over the model uncertainty. Based on 𝓁_∞ to 𝓁_∞ stability and performance of slowly varying system found in the literature, the proposed adaptive tracking scheme is shown to have good robust stability. Moreover, a computable upper bound on the size of the unstructured uncertainty permitted by the adaptive system and a computable tight upper bound on asymptotic robust steady‐state tracking performance are provided. Copyright © 2005 John Wiley & Sons, Ltd.     

Load frequency control and automatic generation control using fractional-order controllers

Recently, fractional calculus has received extensive attention and research. Accordingly, there is an increasing interest in fractional-order (FO) dynamic systems and controllers. The widely used classical integer-order proportional-integral controller and proportional-integral-derivative controller are usually adopted in the load frequency control (LFC) and automatic generation control (AGC) to improve the dynamic response and to eliminate or reduce steady-state errors. This paper utilizes the FO controllers to improve stability and response of LFC and AGC system. The paper uses the integral of the time-weighted absolute error performance index for optimal controller design. The paper investigates LFC and AGC for both isolated and interconnected power systems and shows that FO controllers perform better than classical integer-order controllers in theses systems.     

Hybrid Neuro Fuzzy approach for automatic generation control in restructured power system

In this paper, a hybrid combination of Neuro and Fuzzy is proposed as a controller to solve the Automatic Generation Control (AGC) problem in a restructured power system that operates under deregulation pedestal on the bilateral policy. In each control area, the effects of the possible contracts are treated as a set of new input signal in a modified traditional dynamical model. The prominent advantage of this strategy is its high insensitivity to large load changes and disturbances in the presence of plant parameter discrepancy and system nonlinearities. This newly developed strategy leads to a flexible controller with a simple structure that is easy to implement and consequently it can be constructive for the real world power system. The proposed method is tested on a three-area hydro-thermal power system in consideration with Generation Rate Constraint (GRC) for different contracted scenarios under diverse operating conditions. The results of the proposed controller are evaluated with the Hybrid Particle Swarm Optimisation (HCPSO), Real Coded Genetic Algorithm (RCGA) and Artificial Neural Network (ANN) controllers to illustrate its robust performance.