Design And Construction Of An Automatic Voltage Regulator Generator Power Plant

ABSTRACT

A voltage regulator provides a stable voltage independent of the load current, temperature and AC line voltage variations. A voltage regulator may use a simple feed-forward design or may include negative feedback. It may use an electromechanical mechanism, or electronic components. Depending on the design, it may be used to regulate one or more AC voltages.

Electronic voltage regulators are found in many devices. In power station generator plants, voltage regulators control the output of the plant. In an electric power distribution system, voltage regulators may be installed at a substation or along distribution lines so that all customers receive steady voltage independent of how much power is drawn from the line.

 CHAPTER ONE

1.0                                                        INTRODUCTION

The automatic voltage regulator is used to regulate the voltage. It takes the fluctuate voltage and changes them into a constant voltage. The fluctuation in the voltage mainly occurs due to the variation in load on the supply system. The variation in voltage damages the equipment of the power system. The variation in the voltage can be controlled by installing the voltage control equipment at several places likes near the transformers, generator, feeders, etc., The voltage regulator is provided in more than one point in the power system for controlling the voltage variations.

In AC system the voltage can be controlled by using the various methods likes booster transformers, induction regulators, shunt condensers, etc.

1.1                                         BACKGROUND OF THE PROJECT

While some small generators have an inherent ability to produce a reasonably constant voltage as the load varies, it is clear from the previous explanations that some form of automatic voltage control is required in the usual form of generator. The Automatic Voltage Regulator (AVR) already referred to in the preceding sections is based on a closed-loop control principle.

The output voltage is converted, usually through a transformer or resistor network, to a low voltage dc signal, and this feedback signal is subtracted from a fixed reference voltage to produce an error signal.

The error signal is processed by a compensator before being amplified to drive the rotor excitation current. The change in rotor excitation current produces a variation in output voltage, closing the control loop. If the gain of the control loop is large enough then only a small error is required to produce the necessary change in excitation current, but a high gain can lead to instability in the circuit, with oscillations in the output voltage. The purpose of the compensating circuit is to enable small errors to be  handled in a stable way. The most common form of compensator is a PID circuit in which the error is amplified proportionately (P), integrated (I) and differentiated (D) in three parallel circuits before being added together. Many AVRs have adjustment potentiometers which allow the gains of each channel to be varied in order to achieve the best performance. The integral term enables compensator output to be achieved at zero error, and this produces the minimum error in output voltage.

The continuous improvement in power electronic controls and processor power is bringing further advances in voltage and speed control, with more flexible protection of the generator and its connected circuits.

This consists of a permanent-magnet generator driven by a variable-speed engine and feeding a power electronic frequency-changer circuit, which delivers output at constant frequency. A microprocessor is used to control the switching of the output devices and to regulate the engine speed depending upon the load applied to the generator. At low power demand the engine speed is reduced to minimize noise, increase efficiency and extend life. The result is a saving in the volume and weight of the generator.

1.2                                             OBJECTIVE OF THE PROJECT

The main objective of this work is design an electronics device used in power station generator plants, to control the output voltage of the plant.

Other objectives are:

1. To improve the power system stability.
2. Improve the Transient stability: Transient stability is improved by high initial response characteristic. In the fault condition, Field voltage is increased to keep the generator voltage constantly. If the excitation response is slow, it will not able to keep voltage and the generator cannot keep synchronizing.

• Improve the Dynamic state stability: Dynamic stability is improved by Power System Stabilizer (PSS). PSS is provided in order to improve the power system dynamic stability. PSS will control the excitation to reduce the power swing rapidly.

1. To suppress the over-voltage on load rejection: When the load rejection, field current and field voltage should be reduced rapidly to keep terminal voltage constantly and prevent overvoltage.

1.3                                         SIGNIFICANCE OF THE PROJECT

1. To regulate generator terminal voltage: Mainly generator under no-load condition, AVR regulates the generator voltage to voltage setter (90R).
2. AVR detects terminal voltage and compare with voltage setter (90R).

• AVR regulates field current via the Exciter.

1. Generator terminal voltage is regulated by field current.
2. To adjust MVars (Reactive power).
3. When the generator connected to power grid, AVR adjust reactive power by regulate generator voltage.

• MVar (Reactive power: Q) is regulated by generator terminal voltage. Therefore AVR can regulate MVars.

1.4                                              PURPOSE OF THE PROJECT

The main purpose for using voltage regulation is financial – to avoid the costs associated with equipment damage and downtime caused by poor voltage levels. That is to design a device that will protect device from damage due to poor voltage level.

1.5             FACTORS CONTRIBUTING TO VOLTAGE LEVEL FLUCTUATION

1. Location on the local distribution line;
2. proximity to large electricity consumers;
3. proximity to utility voltage regulating equipment;
4. seasonal variations in overall system voltage levels;
5. load factor on local transmission and distribution system

 1.6                                                 SCOPE OF THE PROJECT

Automatic Voltage Regulator is a device which maintains the Generator output terminal voltage. To be more accurate, AVR is a controller which always compares the Generator output terminal voltage V_t with the set reference voltage V_ref and as per the error signal i.e. (V_ref – V_t) it changes the filed excitation of Generator to maintain constant terminal voltage V_t.

 1.7                APPLICATION OF THE AUTOMATIC VOLTAGE REGULATOR

The main functions of an AVR are as follows.

1. It controls the voltage of the system and has the operation of the machine nearer to the steady state stability.
2. It divides the reactive load between the alternators operating in parallel.
3. The automatic voltage regulators reduce the overvoltages which occur because of the sudden loss of load on the system.
4. It increases the excitation of the system under fault conditions so that the maximum synchronising power exists at the time of clearance of the fault.

When there is a sudden change in load in the alternator, there should be a change in the excitation system to provide the same voltage under the new load condition. This can be done by the help of the automatic voltage regulator. The automatic voltage regulator equipment operates in the exciter field and changes the exciter output voltage, and the field current. During the violent fluctuation, the ARV does not give a quick response.