EXPERIMENT 13 AUTOMATIC VOLTAGE REGULATOR

AIM: To construct a model for Automatic Voltage Regulator and to obtain the step response, using

MATLAB/SIMULINK. SOFTWARE REQUIRED The software required is MATLAB 7.5 Theory: The Generator excitation system maintains generator voltage and controls the reactive power flow. The sources of reactive power are Generators, Capacitors, and reactors. The generator reactive powers are controlled by field excitation and other supplementary methods of improving the voltage profile on electric transmission systems are transformer load-tap changers, switched capacitors, step-voltage regulators and static-var control equipment. The primary means of generator reactive power control is the excitation control using Automatic Voltage Regulator(AVR). The role of an AVR is to hold the terminal voltage magnitude of a synchronous generator at a specified level. Mathematical Model of AVR: The mathematical modeling of AVR is derived as follows: a) Amplifier Model The excitation system amplifier may be magnetic amplifier, rotating amplifier or modern electronic amplifier. The amplifier is represented by a gain KA and time constant T and the transfer function is Typical values of KA are in the range of 10 to 400, time constant T is very small in the range of 0.02 to 0.1 sec and often neglected. b) Exciter Model Assume that for some reason the terminal voltage │V│ would decrease. This immediately results in an increased “error voltage” e which, in turn, causes increased values of vR, ie, vf, and if. The d axis generator flux increases as result of the boost in if, thus raising the magnitude of the internal generator emf E and terminal voltage V.

The time constants τE values in the range of 0.5 – 1.0 sec. c) Generator Model

VR(s)/ Ve(s) = KA /(1+ sτA)

VF(s)/ VR(s) = KE /(1+ sτE)

The synchronous machine generator emf is a function of the machine magnetization curve, and its terminal voltage is dependent on the generator load. In the linearized model, the transfer function relating the generator terminal voltage to its field voltage can be represented by a gain KG and a time constant TG and the transfer function is These time constant are load dependent. KG varies from 0.7 to 1 and τG between 1.0 to 2.0sec from full-load to no-load d) Sensor Model The voltage sensed through a potential transformer and, in one form, it is rectified through a bridge rectifier. The sensor modeled be a simple first order transfer function, given by τR is small and range lies between 0.01 to 0.06sec The overall block diagram for Automatic Voltage Regulator is given below:

Result:

The Simulink block diagram for Automatic Voltage Regulator is constructed and the step response is also obtained for a given specified gain and time constants.

Comparator Amplifier Exciter field Generator field

kA / (1 + sTA) Ke/ (1+sTe) KF/ (1+sT’do)

Δ vR

Δ vf Δ│V│

Δ│V│ref Δe

Δ│V│

+

KR/ (1+sTR)

Sensor

Vt(s)/ VF(s) = KG /(1+ sτG)

VS(s)/ Vt(s) = KR /(1+ sτR)

Block Diagram of MATLAB