AUTOMATIC VOLTAGE REGULATOR AND AUTOMATIC LOAD FREQUENCY CONTROL IN TWO-AREA POWER SYSTEM

Transcription

1 AUTOMATIC VOLTAGE REGULATOR AND AUTOMATIC LOAD FREQUENCY CONTROL IN TWO-AREA POWER SYSTEM ABSTRACT [1] Nitesh Thapa, [2] Nilu Murmu, [3] Aditya Narayan, [4] Birju Besra Dept. of Electrical and Electronics Engg., RVS College of Engineering and Technology, Jamshedpur , INDIA In operation of power system, the main purpose is to maintain a good quality and reliable electric power supply. The present a article is aimed to construct the simulink block diagram and obtain frequency response of each area with conclusion of the ACEs. The ALFC system maintain the frequency i.e. regulation of real power and the AVR system maintain the regulation of reactive power of each area and tie line power flow within the specified tolerance. Keywords Load frequency Control, Automatic Voltage Regulator, Interconnected Power System. [1] INTRODUCTION Power system is huge inter connected circuit consisting of generation, transmission and distribution system respectively. The two area system is a form of multi area system of AGC, where a group of generators are coupled internally. The generator turbine tends to have the same response characteristics, such a group of generators are said to be coherent. Thus whole system is referred to as a control area. The two area system deals with the generation and load demands of the two domains. These two or more area systems are interconnected through tie lines.any load change within the area has to be meet by the generators in both areas. Thus we can maintain the constant frequency operation irrespective of load change. The regulation of real power is achieved by load frequency control whereas the regulation of the reactive power is achieved by automatic voltage regulator. Change in real power mainly affects the system frequency and the tie line power. This change is control by the load frequency control. Whereas the change in the reactive power and voltage magnitude is maintain by the AVR loop. LFC has gained in importance with the growth of interconnected systems and has made the operation interconnected system possible. As long as the system frequency is equal to its specified value (assumed value) the difference between an area s actual interchange and its schedule interchange is known as the area control error. The ACE is the most important control operations, which is continuously monitored. At any instant of time, ACE is negative the area is under generating and needs to increase its total generation. Conversely, if the ACE is positive, the area is over generating and needs to decrease its generation. Nitesh Thapa, Nilu Murmu, Aditya Narayan, And Birju Besra 1

2 AUTOMATIC VOLTAGE REGULATOR AND AUTOMATIC LOAD FREQUENCY CONTROL IN TWO-AREA POWER SYSTEM Earlier the control area were manually handled, but now has been switched to automatic. This is known as Automatic generation control. AGC automatically adjust the generation in an area to keep the ACE close to zero(i.e. magnitude close to zero), keeping the net area interchange at the specified value. AGC plays a very important role in power system as it maintain the system frequency and tie line flow at their schedule values during normal period as well as when system is subjected to small perturbations. [2] BASIC GENERATOR CONTROL LOOPS In An interconnected power system each generator is equipped with load frequency control and automatic voltage regulator. The controller is set for a particular operating condition and takes care of perturbation in load demands. Such changes in real power depend on the rotor angle and thus the frequency. Whereas the reactive power depends on the voltage magnitude (on generator excitation). The excitation system time constant is much smaller than the prime mover time constant and its transient decay much faster and does not affect the LFC dynamics. Hence cross coupling between the two loops is negligible, and the load frequency and excitation voltage control are analyzed independently. [3] LOAD FREQUENCY CONTROL The main objective of LFC is to maintain reasonably uniform frequency, such as to divide the load between the generators and to control the tie line interchange schedules. The sensor senses the change in frequency and toe line real power which is measure of the change in the rotor angle that is change in rotor angle to be corrected. The error signals Δf and ΔP tie are amplified, mixed, and transformed into a real power command signal ΔP v which is sent to prime mover to call for an increment in the torque. The prime mover, therefore, brings change in the generator output by an amount ΔP g which will change the values of Δf and ΔP tie within the specified tolerance. The first step in analysis of a control system is mathematical modeling of the system. The two most common functions are the transfer function method and the state variable approach. In order to use the transfer function the system must be linearized. The transfer function models for the following components are obtained [4] GENERATOR MODEL Synchronous generator or alternator is the essential component for power generation. The synchronous generators comprises of two synchronously rotating fields, one is produced by the rotor driven at synchronous speed and excited by dc current and the other is produced in the stator windings by the three phase armature currents. The dc current for the rotor windings is provided by excitation systems. Now days the system uses ac generators with rotating rectifiers known as brushless excitation system. The excitation system maintains generator voltage and controls the reactive power flow. [5] LOAD MODEL The load on a power system consists of a variety of electrical devices. Resistive loads in the electrical power are independent of frequency. Motor load are sensitive to changes in frequency. Sensitivity of frequency depends on the speed-load characteristics of all the driven devices. Nitesh Thapa, Nilu Murmu, Aditya Narayan, And Birju Besra 2

3 [6] PRIME MOVER MODEL The prime mover is the main source of mechanical power, such as hydraulic turbines at waterfalls, steam turbines whose energy comes from the burning of coal, gas, nuclear fuel, and gas turbines. The model of the turbines relates change in mechanical power output ΔP m to changes in steam valve position ΔP v. Different types of turbines vary widely in characteristics. The simplest prime mover model for non reheat steam turbine can be approximated with a single time constant T T. The time constant T 1 is in the range of 0.2 to 2.0 seconds. [7] GOVERNER MODEL When the generator electrical load is suddenly increased, the electrical power exceeds the mechanical power input. The power deficiency is supplied by the kinetic energy stored in the rotating system. The reduction in kinetic energy causes the turbine speed and the generator frequency to fall. This change in speed is sensed by the turbine governor which senses the speed to a new steady state. The earliest watt governors sense the speed by means of rotating fly balls and provide mechanical motion in response to speed. However modern governors use electronic means to sense speed changes. 1. Speed governor: The essential parts are the centrifugal fly balls driven directly or through gearing by the turbine shaft. This mechanical mechanism provides upwards and downwards vertical movement with respect to the decrease and increase in speed respectively. 2. Linkage Mechanism: This mechanism system transfers the fly ball movement to the turbine valve through a hydraulic amplifier proportional to change in speed. It also provides feedback from the turbine steam valve movement. 3. Hydraulic Amplifier: It consists of a pilot valve and main valve and a piston arrangement which provides the necessary mechanical forces needed to operate the steam valve. Therefore low level pilot valve movement is converted into high power level piston valve movement. 4. Speed Changer: The speed changer consist of servomotor which can be operated manually or automatically to provide steady state output setting for turbine. It s downwards movement opens the upper pilot valve so that more steam is flush to turbine under steady condition, reverse happens when speed changer upward operated. [8] AUTOMATIC GENERATION CONTROL When the load on the system is increased, the turbine speed drops before the governor can adjust the input of the steam to the new load. As the change in the value of speed diminishes, the error signal becomes smaller and the position of the governor falls and gets closer to the point required to maintain steady speed. However the constant speed will not be the set point, and there will be offset. The one technique to Nitesh Thapa, Nilu Murmu, Aditya Narayan, And Birju Besra 3

4 AUTOMATIC VOLTAGE REGULATOR AND AUTOMATIC LOAD FREQUENCY CONTROL IN TWO-AREA POWER SYSTEM restore the speed or frequency to its nominal value is to add an integrator. The integral unit monitors the average error over a period of time and will overcome the offset. The integral action is known as the reset action because it has the ability to return a system to its set point. Thus, as the system load change continuously, the generation is adjusted automatically to restore the frequency to the nominal value. This technique is called as the automatic generation control (AGC) In an interconnected system consisting of several pools, the role of the automatic generation control system is to divide the loads among system, station generators so as to achieve the maximum economy and control the scheduled interchanges of tie line power while maintaining a reasonably uniform frequency. During large transient disturbances and emergencies, AGC is bypassed and other emergency controls are applied. Modern power system network consists of a number of utilities interconnected together and power is exchanged between utilities over tie lines by which they are connected. Automatic generation control plays a very important in power system as its main role is to maintain the system frequency and tie line flow at their scheduled values during normal period and also when the system is subjected to small step load perturbations. Many investigations in the field of automatic generation control of interconnected power system have been reported over the past few decades. [9] AGC IN SINGLE AREA SYSTEM With the primary LFC loop, a change in the system load will result in a steady state frequency deviation, depending on the governor speed regulation. In order to reduce the frequency deviation to zero, we must provide a rest action. This rest action can be achieved by introducing an integral controller to act on the load reference setting to change the speed set point. The integral controller increases the system type by one which forces the final frequency deviation to zero. The integral controller gain K must be adjusted for a satisfactory transient response. [10] AGC IN MULTIAREA SYSTEM In power system a group of generators are closely couple internally and swing unison. Further the generator turbines must have same response characteristics. The AGC of a multi area system can be realized by studying first the AGC for a two area system. During normal operation, the real power transferred over the tie line is given by P 12= E 1 E 2 X 12 sin δ 12 Where X12= X1+Xtie+X12 and δ 12 =δ 1 δ 2 Then the tie line power deviation taken on the form P 12 = P s( δ1 δ 2 ) The tie line power flow appears as a load increases in one area and the load decrease in the other area, depending on the direction of the flow. The direction of flow is dictated by the phase angle difference. If δ 1 > δ 2,power flows from area 1 to area 2 and vice versa. Conventional LFC is based on the line bias control, where each area tends to reduce the control error (ACE) to zero. The control error for each area tends to consists of linear combination of frequency and tie line error. The general equation for ACE is given by ACE = n j=1 P ij + K i ω Nitesh Thapa, Nilu Murmu, Aditya Narayan, And Birju Besra 4

5 The area bias k i determines the amount of interaction during a disturbance in the neighboring areas. An overall satisfactory performance is achieved when K is selected equal to the frequency bias factor of that area. Thus the ACEs for a two area systems are ACE1= P 12 + B 1 ω 1 ACE2= P 21 + B 2 ω 2 Where P 12 and P 21 are departures from scheduled interchanges. ACEs are used as actuating signals to activate changes in the reference power set points, and when steady state is reached, P 12 and ω will be zero. The integrator gain constant must be chosen small enough so as not cause the area to go into a chase mode. [11] SIMULATION OF TWO AREA CONTROL SYSTEM Computer has provided engineers with immense mathematical powers, which can be used to simulate dynamic system without the actual physical setup. Simulation of dynamic systems has proved to be immensely useful when it comes to control design, saving time and money that would otherwise be spent in prototyping a physical system. Simulink is software add on to the MATLAB software. We can build idealized linear models to explore more realistic nonlinear models. Power system parameters taken for the design of the governor system controller is enlisted below : Parameters Two area system Area 1 Area 2 Turbine time constant 0.5 sec 0.6sec Governor time constant 0.2sec 0.3sec Frequent-sense load coeff Governor speed regulation Inertia constant Base power 1000MVA Nitesh Thapa, Nilu Murmu, Aditya Narayan, And Birju Besra 5

6 AUTOMATIC VOLTAGE REGULATOR AND AUTOMATIC LOAD FREQUENCY CONTROL IN TWO-AREA POWER SYSTEM [12] SIMULATION RESULT Simulation Result for single area: Automatic Voltage Regulator:- Automatic Load Frequency Control:- Simulation Result for two area: Automatic Load Frequency Control:- Nitesh Thapa, Nilu Murmu, Aditya Narayan, And Birju Besra 6

7 [13] CONCLUSION This shows that the static change in the tie power following a step load in any area should be zero, provided each area can accommodate its own load change. In any area if need of power during emergency should be assisted from other areas. From simulation result we have seen that the integrator gain constant are adjusted for a satisfactory response and frequency deviation returns to zero with settling time of approximately 20 sec. REFRENCES [1]. Elgerd O I and Charles Fosha E, Optimum Megawatt- Frequency Control of Multi area Electric Energy Systems. [2]. Elgerd OI, Electric Energy Systems Theory: An Introduction, New York: McGrawHill, [3]. Wood A J and Woolen Berg BF, Power Generation Operation and Control, John Wiley and Sons, [4]. Shayeghi H, Jalili A and Shayanfar H A, Load Frequency Control Strategies: A State ofthe-art survey for the researcher. [5]. Bassi S J, Mishra M K and Omizegba E E, Automatic Tuning of Proportional Integral Derivative (PID) Controller using Particle Swarm Optimization (PSO) Algorithm. [6]. Ibraheem and Singh O, Design of particle swarm optimization (PSO) based automatic generation control (AGC) regulator with different cost functions, Journal of Electrical and Electronics Engineering Research. [7]. Jain S K, Chakrabarti S and Singh S N, Review of Load Frequency Control Methods, Part-I Introduction and Pre-Deregulation Scenario, International conference on Control, Automation, Robotics and Embedded Systems (CARE). [8]. Jain S K, Chakrabarti S and Singh S.N., Review of Load Frequency Control Methods, Part-II Post- Deregulation Scenario and Case Studies, International conference on Control,Automation, Robotics and Embedded Systems (CARE). Nitesh Thapa, Nilu Murmu, Aditya Narayan, And Birju Besra 7