1 CHAPTER 1 INTRODUCTION 1.1 PREAMBLE Load Frequency Control (LFC) or Automatic Generation Control (AGC) is a paramount feature in power system operation and control. The continuous monitoring is needed for the sufficient supply and uninterrupted power. In order to ensure good quality and reliability to the consumers and to obtain the above said criteria, it is mandatory to interconnect all the power system which includes thermal and hydro-thermal system. 1.2 THE GENESIS OF THE THESIS LFC deals with the problem of delivering the demanded power to the load with minimum transient oscillation. Modeling and analyzing of multi area LFC with Governor Dead Band (GDB) and Generation Rate Constraint (GRC) non-linearities is a complex problem. The current scenario of the power system is the interconnection of different power plants. Stand alone power system is basically a dynamic device, and has the tendency to become unstable after a severe disturbance. Therefore, there is a need for an effective design and control to maintain its stability. Further, an effective control system is required for interconnected operations. But the problem in interconnected power system is damping out the frequency and tie line power oscillations. If no adequate damping is provided, the oscillations may persist for a long time causing disintegration of system.
2 Consequently, extensive research work is being carried out in order to provide a reliable and good quality of power to the end users. 1.3 LITERATURE SURVEY Interconnected power system is necessary to meet the power balance, however, with the increase in numbers it gets complicated. The problem is classified into two different heads and it is analyzed in the following sections. 1.3.1 Load Frequency Control of Interconnected Thermal System Concordia and Kirchmayer (1953) analyzed a tie-line power and frequency deviation of an interconnected thermal system. Nathan Cohn (1957) proposed a new method for the interconnection strategy called tie-line bias control. This gives a proper dimension to bias settings. Klopfenstein (1959) established a practical procedure to find dead band non-linearity for thermal plants. Milan (1972) developed a proportional-integral optimal linear regulator LFC for two area and three area interconnected system without considering non-linearities. IEEE committee report (1973) presented the mathematical modeling of speed governing for thermal system steam turbine, speed governing for hydro system and hydro turbine. demello et al (1973) presented the model of the prime mover system including representation of boiler pressure effects. This effect includes steam flow on boiler drum pressure. Taylor et al (1979) analyzed stochastic performance of automatic generation control including speed governor dead band effects. These test results are compared with real time data.
3 Kothari et al (1981) analyzed AGC studies for two area interconnected thermal system considering generation rate constraint nonlinearity. The results were presented for optimum PI gains. Stability margin was also discussed with various sampling periods. Further, Tripathy et al (1984) developed a mathematical model for governor dead band non-linearity using describing function approach. Automatic generation control of two area reheat thermal system with new Area Control Error (ACE) was addressed in depth by Kothari et al (1989). Chun-Fung Lu et al (1995) introduced Battery Energy Storage (BES) for interconnected LFC system considering Governor Dead Band (GDB) and Generation Rate Constraint (GRC), which is very effective in damping the oscillation caused by load disturbance. The BES model is suitable for charging and discharging mode operation. Chown and Hartman (1998) described the design, implementation and operational performance of a Fuzzy Logic Controller (FLC) to AGC. Al-Hamouz et al (2000) developed a variable structure controller using genetic algorithm for single area thermal LFC system. Nanda et al (2003) implemented FLC to AGC considering GRC non-linearity for two area interconnected thermal system. This FLC results are compared with conventional integral controller. Ghoshal (2003) presented fuzzy gain scheduling for conventional integral gain AGC for radial and ring connected three equal power system areas without considering non-linearities. In LFC system, application of fuzzy logic is further enhanced by Ilhan Kocaarslan et al (2005). In this study, Fuzzy Gain scheduled Proportional and Integral (FGPI) controller is implemented.
4 Hassan et al (2008) proposed Fuzzy Gain scheduled Integral and Derivative (FGID) controller for AGC of two area non-reheat power system. Shayanfar et al (2009) proposed multi variable characteristic loci method for two area power system without considering non-linearities. Ndubisi Samuel (2010) developed an intelligent fuzzy logic controller applied to multi area load frequency control system without nonlinearities, Boiler Dynamics (BD) and reheat turbines. Gayadhar Panda et al (2010) proposed modified Genetic Algorithm (GA) based optimal selection of parameters for automatic generation control of multi area electric power system considering dead band non-linearity. Based on the conceptual frame of reference, the proposed load frequency control of interconnected thermal system considers non- linearities and boiler dynamics, using Fuzzy Logic (FL) controller that minimizes the steady state error, settling time and frequency peak that enhance smooth supply of power. 1.3.2 Load Frequency Control of Interconnected Hydro-Thermal System Concordia and Kirchmayer (1954) analyzed and observed that the performance of interconnected hydroelectric power generation areas is affected by frequency and tie-line power controllers. Klopfenstein (1959) presented the practical method to find governor dead band non-linearity for hydro system. The IEEE Committee (1972) reported the dynamic models for hydro turbine in power system studies. In the report, modeling of speed governing system for hydro turbines is presented.
5 Kothari et al (1988) developed an optimal control strategy to automatic generator control of a hydrothermal system. Kusic et al (1988) presented the major concepts which are used to design an AGC system for a hydro type. Also experimental results are presented to verify theoretical procedures. Multi area interconnected power system with and without nonlinearities are analyzed by Malik et al (1988). A self tuning controller for a two area hydro-thermal load frequency control problem, considering generation rate constraints is presented by Swain and Mohanty (1995). Chown et al (1998) enumerates design procedures, implementation and performance of a fuzzy logic controller to automatic generation control. Nanda and Mangla (2004) investigated automatic generation control of an interconnected hydro thermal system using conventional integral and fuzzy logic controller. It has been concluded that, presence of FLC in both areas and small step perturbation in both areas simultaneously guarantees zero steady state error. Ibraheem et al (2004) proposed a new method of interconnection between two areas and presented a modeling of hydro-hydro power system for AGC through asynchronous tie-line. Swain (2006) adopted generation rate constraint non-linearity to single area hydro power system with fuzzy logic controller. In this study the significant improvement is that step load disturbance is varying from 1% to 5% to the system. In particular, dead band effects of speed governor is not considered in this work. Nanda et al (2006) analyzed the feasibility of electric power governor instead of mechanical governor in hydro system. Also in thermal
6 area, two stage reheat turbine is incorporated for an interconnected hydro thermal system. Prabhat Kumar et al (2008) developed an optimal control of three area power system with two areas thermal and one area hydro with EHVAC/HVDC links. Srinivasa Rao et al (2009) presented an analysis on dynamic performance of two area hydro-thermal system interconnected through A.C-D.C transmission links. The effect of thyristor controlled phase shifter series with the A.C tie line in the presence of integral controller is analyzed. Surya Prakash et al (2009) proposed the impact of addition of slider gain with fuzzy logic controller for LFC of interconnected hydro-thermal power system. The proposed load frequency control of interconnected hydrothermal system considering non-linearities with fuzzy logic controller reduces the frequency deviation, megawatt power mismatch in highly loaded condition, thereby, delivers a reliable power to the consumer. 1.4 OBJECTIVES The quantum of literature published in the area revealed that a real power system component has more non-linear characteristics such as GDB and GRC. For the practical purpose of simulation, all non-linear characteristics should be incorporated to achieve the exact results. Thereby, the proposed design suitable for the secondary load frequency controller. The main objectives of this thesis are as follows: To propose an exact model of electric power system with the incorporation of governor dead band and generation rate
7 constraint non-linearities and boiler dynamic in both the areas for interconnected thermal LFC system. To propose a proper hydro-thermal system model guiding non-linearities even in hydro area. To propose a fuzzy logic controller to LFC problem, it eliminates the conventional PI controller in the case of both interconnected thermal and hydro-thermal system. To compare the performance of the proposed controller with the conventional controller for thermal and hydro-thermal power system. 1.5 OVERVIEW OF THE THESIS The existing load frequency control mechanism has twin design goals, change in frequency and tie-line power deviation in case of interconnected system. The former is important because it is the prime parameter that involves the basic frequency consisting criteria. It also requires that when the number of interconnection increases, change in frequency parameter also increases which will make the system more complex. The tieline power deviation is another critical factor since the efficient utilization of net interchange power is also of major consideration. Infect, there are two kinds of tie-line, power absorption and power delivery. Tie-line power absorption is felt during the sudden load change in generator of the same area. Tie-line power delivery is the result of delivering MW power to the neighbouring system which is already interconnected through tie-line, when sudden load disturbances occur in neighbouring system. Furthermore, more interconnection is required for irrespective of the type of system to deliver uninterrupted power supply.
8 The possibilities have paved the way for an extensive research in the load frequency control domain to reduce change in frequency error and tie-line power deviation error. The research is carried out by using conventional Proportional-Integral (PI) controller which is optimized by Integral Square Error (ISE) technique. This technique contains a unique parameter such as frequency and tie-line power deviation. To resolve the major problem of frequency and tie-line power deviation during external disturbance, PI controller is preferred. This controller provides an adequate control performance in order to reduce the error(s) to provide guarantee power delivery. Load frequency control of an interconnected system poses a challenging task, while frequent changes in frequency and tie-line power imply an imbalance between generation and delivery. On one hand, frequency consistency is required for a stable operation of area itself. On the other hand, a balanced tie-line power flow of operation requires for overall safe operation of interconnection. Therefore, the properties cited for stand alone power system are not adoptable for interconnected power system. To address the challenges of more interconnection networks, most of the researchers have studied how to reduce control mechanism to maintain the power balance equation. Three ways to achieve this goal are: Incorporate all non-linearities in the system. Interconnect all areas with same size. Proposed controller should have adequate control performance even with non-linearities.
9 Based on these ideas, several control strategies are proposed recently. Even now, interconnected system with non-linearities and boiler dynamics is far from well established model. The principal idea of the three ways mentioned above is to reduce the error. This leads to the reduction of the power imbalance. However, too much of gain cannot be allowed to reduce error. Therefore, a certain trade-off should be found between controller gain and error in order to reduce total frequency and tie line error. Another important issue that is usually ignored during modeling is the dead band effect in hydro area. For example, in the interconnected hydrothermal system, there will be significant backlash effect in hydro governor. It should be incorporated otherwise situation can further worsen, in other words optimal controller gain cannot be obtained. With relevance to these observations, it is aimed that it would be ideal, if a system model could be developed to overcome the drawbacks of the existing system model. The outcomes are the new simulation model developed and it is presented in the thesis. In the avenue of LFC of interconnected thermal system, this new proposal strives to deliver the optimum control action and as close as possible to the consumers requirements. Hence, this dissertation strives to bring out the best effort of AGC interconnected thermal and hydro-thermal system with fuzzy logic controller will strongly succeed in providing an optimal trade-off between total generation and efficient delivery. Simulation is performed in order to incorporate the newly developed efforts for some system model that requires a guaranteed power balance and reliability is corroborated through simulation.
10 It is seen from the system topological requirements that generation and utilization are the important factors to be considered in power system structures that contribute to best effort service. At present there is no established model and controller that is available to realize a number of interconnected power systems in LFC environment. It can be made possible with this efficient proposed power system model. 1.6 ORGANIZATION OF THE THESIS This dissertation focuses on providing fuzzy logic based load frequency controller for interconnected thermal and hydro-thermal system in an efficient way. This dissertation is divided into five chapters including the introduction. Chapter two delineates an overview to provide modeling of interconnected thermal and hydro-thermal system. It includes the nonlinearities such as GRC, GDB and BD on both the system model. The simulation results confirm that the proposed both models articulate the original system. In chapter three, load frequency control of interconnected thermal system with fuzzy logic controller is proposed for reliable operation of power system. An ISE technique is applied to obtain optimum control gain of conventional PI controller. Simulation results of open loop responses with PI controller are presented. In order to increase the system performance fuzzy logic controller is proposed. In the proposed FL controller, the various control rules may change during simulation to accommodate error. In other words, FLC does not take a single control action with respect to changing error, and hence the proposed solution methodology is working well.
11 Load Frequency Control of Interconnected Thermal and Hydro-Thermal System with Fuzzy Logic Controller Development of Interconnected Thermal System Model Development of Interconnected Hydro-Thermal System Model Open Loop Responses Open Loop Responses Design of PI Controller using Integral Square Error Method Design of PI Controller using Integral Square Error Method Responses with PI Controller Responses with PI Controller Development of Fuzzy Logic Controller: 1. Fuzzification 2. Rule Base 3. Defuzzification Implementation of Fuzzy Logic Controller Comparing the Performance of Developed Controller for Interconnected Thermal and Hydro-Thermal System Figure 1.1 Approach Flow Diagram
12 The simulation results are presented to show that the proposed FL controller mechanism can achieve better performance. This solution chooses three parameters to judge the proposed controller s performance. These are the frequency peak, settling time and steady state error. The flow diagram of the proposed approach is shown in Figure 1.1. In chapter four, load frequency control of interconnected hydrothermal system with FL is proposed. ISE method is used to find conventional PI controller gains. FL controller takes the input as area control error and change in area control error. Simulation results show that the proposed reliable fuzzy logic load frequency controller maintains low frequency peak, low settling time and zero steady state error. This is justified through area control error. A comprehensive summary of conclusion obtained from this research work is also presented in the fifth chapter.