Decentralized load frequency control using a new robust optimal MISO PID controller Alireza Yazdizadeh ⇑ , Mohammad Hossein Ramezani, Ehsan Hamedrahmat Power and Water (Abbaspour) University of Technology, P.O. Box 16765-1719, Tehran, Iran article info Article history: Received 20 December 2010 Received in revised form 7 September 2011 Accepted 16 September 2011 Available online 29 November 2011 Keywords: Load frequency control Robust control MISO PID control Optimal control Characteristic matrix eigenvalues abstract Load Frequency Control (LFC) is one of the most important issues in electric power systems. Although Proportional–Integral (PI) controllers are commonly used for LFC systems in industry, they are unable to obtain good dynamic behavior in presence of various load changes and operating conditions because the PI controller parameters are usually tuned based on a classical or trial and error approaches. In this paper, a new decentralized robust optimal MISO PID controller based on Characteristic Matrix Eigen- values and Lyapunov method is proposed as an appropriate technique for Load Frequency Control (LFC) problem. In the proposed method, the tuning of PID controller is stated as an optimization problem in which a combination of quadratic index and maximal complex/real ratio of the closed loop poles is minimized subject to some constraints on Characteristic Matrix Eigenvalues. The proposed method sup- ports a tradeoff between acceptable and desired performance and robustness at the same time. To ensure the good performance of the method, it is applied to a control area which is linked to other control areas of an interconnected power system. This control area includes two large dams, namely, Karoon3 and Dez hydro power plants in KHOZESTAN (a province in southwest of Iran). Simulation results confirm the effi- ciency and consistency of the proposed method. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction In accordance with increasing size, complexity and changing structure of interconnected power systems, Load Frequency Con- trol (LFC) issue is becoming one of the major subjects in these sys- tems. Load demand change in a large area causes a mismatch between electrical energy generation and consumption while it is not fully predictable. The goal of load frequency control is to main- tain frequency and power interchanges variations to its desired (standard) tolerance value for each control area of an intercon- nected power network. Towards this, several LFC algorithms have been proposed to match load demands and the power output of the generators under variable operating conditions. In recent years, application of decentralized control strategies associated with other centralized or multilevel control strategies to the LFC prob- lem has found wide acceptance [1–4] because of considering inter- action between subsystems as a bounded disturbance and design controller separately for each subsystem. Traditionally LFC systems use the well known PI controllers that are tuned based on classical and trial-and-error approaches [5–9]. Nowadays, despite effective progress in control theory, especially in the field of multi-input multi-output control strategy, PID controllers are still the first choice of the designer due to very sim- ple and efficient characteristics of these controllers. On the other hand, optimization and robustness are two important issues which are in center of focus of designers for proposing a controller for an industrial system. In [1–4] different types of decentralized LFC algo- rithms based on sliding mode control, fuzzy approach, frequency response and approximation of the interconnections are presented. A combination of H 1 control and genetic algorithm technique is presented for tuning the PI parameters in [10]. Decentralized LFC method based on structured singular values has been discussed in [11]. The Kharitnov theorem and its results have been used to solve the same problem in [12]. In [13] the decentralized LFC problem is formulated as a H 1 control problem and an Iterative Linear Matrix Inequalities (ILMI) algorithm is used to achieve the PI coefficients. Moreover, other control approaches like adaptive control, variable structure control, fuzzy control and neural network based control schemes are investigated in the various literatures [14–17]. Power exchange and power transmission between several areas in a power network, make the whole interconnected system as a multi-input multi-output coupled large scale system which is affected by disturbances. In such a system, in order to control the frequency in presence of disturbances, an optimization and robust technique for tuning the parameters of a PID controller based on Characteristic Matrix Eigenvalues and Lyapunov method is proposed in this paper. 0142-0615/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijepes.2011.09.007 ⇑ Corresponding author. E-mail address: alireza@pwut.ac.ir (A. Yazdizadeh). Electrical Power and Energy Systems 35 (2012) 57–65 Contents lists available at SciVerse ScienceDirect Electrical Power and Energy Systems journal homepage: www.elsevier.com/locate/ijepes