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DEVELOPMENT OF A FUZZY LOGIC BASED VOLTAGE REGULATOR FOR ELECTRIC GENERATING SET

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DEVELOPMENT OF A FUZZY LOGIC BASED VOLTAGE REGULATOR FOR ELECTRIC GENERATING SET

 

 

 

LIST OF TABLES

Table 3.1                                                                34

 

LIST OF FIGURES AND DIAGRAMS

Figure 1.1               Conventional control design                            5

Figure 1.2                Fuzzy control design                                        6

Figure2.1                          Schematic diagram of LFC and AVR of a

synchronous generator                                           9

Figure 2.2                A simple diagram of an AVR                                  10

Figure 2.3                Block diagram of a simple AVR                      12

Figure 2.4              Root locus plot for equation 2.13                    15

Figure 2.5               Terminal voltage response                              17

Figure 2.6               Simulink model for figure 2.5                         18

Figure 2.7                         Block diagram of a simple AVR

compensated with a stabilizer                                 19

Figure 2.8              Terminal voltage step response of CAVR

with stabilizer                                                 20

Figure 2.9               Simulink model for figure 2.7                         21

Figure 2.10            An AVR compensated with PID controller       22

Figure 2.11             Simulink model for figure 2.10                       23

Figure 2.12            Terminal voltage step response with PID                24

Figure 3.1               Fuzzy controller architecture                                  27

Figure 3.2                Fuzzy control system                                      28

Figure 3.3a            Membership functions for the error                        32

Figure 3.3b             Membership functions for ∫Ve                          32

Figure 3.3c           Membership function for the terminal voltage 33

Figure 3.4                  FIS Editor                                                       36

Figure 3.5                 The Rule Editor                                             37

Figure 3.6                 The Rule Viewer                                             38

Figure 3.7               Schematic diagram for fuzzy AVR operation 40

Figure 3.8                 Fuzzy-AVR control loop                                  40

Figure 3.9             Terminal and desired voltage step response

with FPI-AVR                                                 42

Figure 3.10            Fuzzy input voltage to the generator               42

Figure 3.11           A Simple machine infinite bus power system 44

Figure 3.12a         Step response of FPI-AVR to three phase fault 47

Figure 3.12b         Fuzzy input voltage to the generator               47

Figure 4.1               Terminal voltage response                              49

Figure 4.2              Terminal voltage step response of CAVR

with stabilizer                                                 50

Figure 4.3              Terminal voltage step response with PID                51

Figure 4.4a           Terminal and desired voltage step response

with FPI-AVR                                                 52

Figure 4.4b            Fuzzy input voltage to the generator               52

Figure 4.5a           Step response of FPI-AVR to three phase fault 53

Figure 4.5b            Fuzzy input voltage to the generator               53

 

 

 

 

 

 

 

 

 

 

 

CHAPTER ONE

INTRODUCTION

 

1.1 Background of the study

The main objectives of the control strategy in power system is to generate and deliver power in an interconnected system as economically and reliably as possible while maintaining the voltage and frequency in the steady-state within permissible limit [1].

A reliable, continuous supply of electric energy is essential for the functioning of today’s complex societies. Due to a combination of increasing energy consumption and impediments of various kinds concerning the extension of existing electric transmission networks, these power systems are operated closer and closer to their limits.

Deregulatory efforts will tighten the economical constraints under which utilities have to operate their own network or allow or prevent competitors from using it. This in turn will require more precise power flow control which is made possible by phase angle controllers being developed using new power electronic equipment. However, it is to be expected that these highly non-linear components will introduce harmonics and require non-linear control in order to prevent system destabilization.

This situation requires a significantly less conservative power system operation regime which, in turn, is possible only by monitoring and controlling the system state in much more detail than was necessary previously.

In electric power systems, [2], there are three different control levels: Generating Unit Controls which consist of prime mover control and excitation control with automatic voltage Regulator (AVR) and power system stabilization (PSS). The first controls generator speed deviation and energy supply system variable like boiler pressure or water flow. Excitation control aims at maintaining the generator terminal voltage and reactive power output within its machine-dependent limits.

System Generation Control which determines active power output such that the overall system generation meets the system load. It further controls the frequency and the tie line flows between different power system areas. Transmission Control monitors power and voltage control devices like tapchanging transformers, synchronous condensers and static VAR compensators.    From the view point of system automation, [3], Generating Unit Control is a complete closed-loop system and in the past a lot of effort has been dedicated to improve the performance of the controllers. The main problem for example with excitation control is that the control law is based on a linearized machine model and the control parameters are tuned to some nominal operating conditions. In case of a large disturbance, the system conditions will change in a highly non-linear manner and the controller parameters are no longer valid. In this case the controller may even add a destabilizing effect to the disturbance by for example adding negative damping.

These problems provide an important motivation to explore other modern control techniques like fuzzy logic control.

The system frequency is affected by changes in real power while changes in reactive power affect the voltage magnitude. While the Load Frequency Control (LFC) controls the real power and frequency, the Automatic Voltage Regulator (AVR) controls the reactive power and terminal voltage magnitude. Because the excitation time constant is much smaller than the prime mover time constant, its transient decays much faster. For this reason, the cross-coupling between the LFC loop and AVR loop is negligible and hence the load frequency and excitation voltage control can be analyzed independently.

In modern large power system interconnected networks, manual regulation is not feasible and therefore automatic generation and voltage regulation equipment are installed on each generator.

The increasing technological demands and performance requirements call for complex production and manufacturing systems that in turn requires sophisticated control systems. To satisfy the product quality requirement in a flexible production environment, an advanced control techniques that can deal with uncertainty and non-linear ties need to be introduced. Hence the need for fuzzy logic control which can handle both linear and non-linear system. Fuzzy logic is a paradigm for an alternative design methodology which can be applied in developing both linear and non-linear systems [4].

Ndubisi and Agu [5] puts it thus, Fuzzy logic concept incorporates an alternative way which allows one to design a controller using a higher level of abstraction without knowing the plant model.

Conventional modeling and control approaches based on differential equations are often insufficient, mainly due to the lack of precise formal knowledge about the process to be controlled. Unlike the conventional control, if the mathematical model of the process is unknown we can design fuzzy controllers in a manner that guarantees certain key performance criteria.

Lee et al [6] states that while the conventional control starts with a mathematical model of the process and controllers are designed based on the model, fuzzy logic control on the other hand starts with heuristics and human expertise knowledge (in terms of fuzzy IF-THEN rules) and controllers are designed by synthesizing these rules. An important source of information to the fuzzy design is the knowledge of the plant operators, control engineers and process designers. Fuzzy logic control (FLC) reduces the time and complexity in analyzing the differential equations involved in the conventional control and hence in the overall design development cycle as depicted in Figures 1.1 and 1.2 respectively.

 

 

 

 

Conventional Design Methodology

 

Fig 1.1: Conventional control design

 

 

 

Fig.1.2: Fuzzy control design

 

Using the conventional approach, our first step is to understand the physical system and its control requirements. Based on this understanding, our second step is to develop a model which includes the plant, sensors and actuators. The third step is to use linear control theory in order to determine a simplified version of the controller, such as the parameters of a PID, PI controllers. The fourth step is to develop an algorithm for the simplified controller. The last step is to simulate the design including the effects of non-linearity, noise, and parameter variations. If the performance is not satisfactory we need to modify our system modeling, re-design the controller, re-write the algorithm and re-try.  With fuzzy logic the first step is to understand and characterize the system behavior by using our knowledge and experience. The second step is to directly design the control algorithm using fuzzy rules, which describe the principles of the controller’s regulation in terms of the relationship between its inputs and outputs. The last step is to simulate and debug the design. If the performance is not satisfactory we only need to modify some fuzzy rules and re-try.   Although the two design methodologies are similar, the fuzzy based methodology substantially simplifies the design loop. This results in some significant benefits, such as reduced development time and simpler design. As reported in [7], seven fuzzy subsets were employed to develop software written in C++ in the design of a fuzzy AVR of a controller for a synchronous generator.

Fuzzy experts like Lofti Zadeh proved that the greater the number of fuzzy subsets, the better the performance of the controller. The cumbersome nature of the C++ language was reduced by the use of the MATLAB software.       In this work, a rule based fuzzy logic controller is developed for controlling the terminal voltage and reactive power of a synchronous generator. Eleven fuzzy subsets were utilized to enhance the performance of the fuzzy controller. The work is arranged in this order; first the conventional control approach is discussed, followed by the fuzzy logic control approach. A comparison is made between the results of the two control approaches. All the simulations are carried out using MATLAB software package. The results show reduction in percent overshoot, rise time, peak time, settling time and overall responses when fuzzy logic approach is applied.

 

 1.2 Statement of the problem

Fuzzy logic control is a non-mathematical decision algorithm that is based on an operator’s experience.   This type of control strategy is suited well for nonlinear systems such as the synchronous generator, which exhibits non-linearity between the field current in and the armature voltage out.

 

1.3 Aims and Objectives

  1. To understand the methods of developing a fuzzy logic based voltage regulator for electric generating set
  2. The understand the relationship between fuzzy logic based voltage regulator and functionality of electric generating set.

 

1.4 Motivation

To solve a this nagging problem with limited functionality of generating sets in Nigeria owing to poor supply of electricity in Nigeria. This research is aimed at boosting power supply.

 

1.5 Scope of the study

This work and research is acarried on in Enugu south lga of Enugu state.

 

1.6 Limitation of the study

Time constraint for this research, limited access to certain useful data for this research are factors

1.7 Definition of terms

 

DEFINITION OF TERMS

   
Rise time: The time for a system to respond to a step input and attains a response equal to the magnitude of the input.
Peak time: The time for a system to respond to a step input and rise to a peak response.
Overshoot: The amount the system output response proceeds beyond the desired response.
Settling time: The time required for the system output to settle within a certain percentage of the input amplitude.
PID controller: A controller with three terms in which the output of the

controller is the sum of a proportional term, an integrating term, and a differentiating term, with an adjustable gain for each term.

Steady-state error: The error when the time period is large and when the transient response has decayed, leaving the continuous response.

 

 

 

1.8 List of symbols and abbreviations

 

 

KR            Sensor gain

KA                  Amplifier gain KE                  Exciter gain KG                  Generator gain τR                  Sensor time constant τA                  Amplifier time constant τE                  Exciter time constant τG                  Generator time constant

Vtss                  Steady-state response of the AVR Vs(s)               Output voltage of the sensor

Vref                 Reference voltage

Vt                   Generator terminal voltage

VR                  Input voltage to the exciter

VF                  Field voltage

PID                Proportional Integral Derivative

PI                   Proportional Integral

Vref                         Reference/specified voltage to generator

Vt                            Terminal voltage of the generator

V1                         Voltage error (Vref – Vt ) ∫Ve                           Integral of the error, ∫ Ve  e(t)               Error

Int. e       Integral of the error e

U                    Output of the fuzzy rules (input to the plant) g1                    Scaling gain for tuning of the membership functions for e (t) g2                   Scaling gain for tuning of the membership functions for “int. e” g 0                    Scaling gain for tuning of the membership functions for u

d(t)                 Power angle of the generator in radian                  w(t)                Rotor speed of the generator in rad/sec. D                    Damping coefficient of the generator H                     Inertia coefficient of the generator

w0                   Speed of the generator at the operating point

Pm                   Motor mechanical power in p.u.                  

Pe (t)               Active electrical power delivered by the generator in p.u.

E f (t)              Equivalent EMF in the excitation coil in p.u.

E, f (t)           Transient EMF in the generator coil in p.u.

E,d (t)            Transient direct axis EMF of the generator

Eq ,(t)          Transient EMF of the generator in the quadrature axis  Xd                 Direct axis reactance of the generator

X ,d                Transient direct axis reactance of the generator

X q, (t)             Transient reactance in the quadrature axis in the generator in p.u.

Id                Direct axis current of the generator

Iq                 Quadrature axis current of the generator I f                Excitation field current

Vs                            Infinite bus voltage in p.u.

U f                          Output of the PI fuzzy controller

X ds                 Stator reactance in the direct axis

SMIB             Single Machine Infinite Bus

LFC                Load frequency control

AVR              Automatic Voltage Regulator

MATLAB      Matrix Laboratory

PSS                Power system stabilization

VAR              Volt Ampere Reactive FLC               Fuzzy Logic Control

PT Potential Transformer
NV Negative Very
NL Negative Large
NB Negative Big
NM Negative Medium
NS Negative Small
Z Zero
PS Positive Small
PM Positive Medium
PB Positive Big
PL Positive Large
PV Positive Very
FIS Fuzzy inference System
D/A Digital/Analogue
FLPSS Fuzzy Logic Power System Stabilizer
FPI-AVR Fuzzy Proportional Integral-Automatic Voltage Regulator
CAVR Conventional Automatic Voltage Regulator
G Generator
CB Circuit Breaker

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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