Automatic Voltage Regulator Generator

7/28/2019 Automatic Voltage Regulator Generator

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AUTOMATIC VOLTAGE REGULATOR (AVR)

-I will talk today with you about an important equipment usedin power system utilities, it is the Automatic VoltageRegulator (AVR). From its name it is a regulator which

regulates the output voltage at a nominal constant voltagelevel.Role of AVRAVR (Automatic voltage regulator) has following roles. 1- To regu late generator term inal vo ltage.Mainly generator under no-load condition, AVR regulates thegenerator voltage to voltage setter (90R).

*AVR detects terminal voltage and compare with voltagesetter (90R).*AVR regulates field current via the Exciter.*Generator terminal voltage is regulated by field current.

Vt < 90R _ Field current will be increaseVt > 90R _ Field current will be decrease

2-To adju st MVars (Reactive power).When the generator connected to power grid, AVR adjustreactive power by regulate generator voltage.
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MVar (Reactive power: Q) is regulated by generator terminalvoltage. Therefore AVR can regulate MVars.

Vt is increased _ MVars will be increase

Vt is decreased _ MVars will be decreaseHence;

To increase MVars _ 90R raiseTo decrease MVars _ 90R lower

3-To imp rove the power system stabi l i ty .There are two stability-Transient stability Improved by AVR-Dynamic stability . Improved by PSS (power systemstabilizer)
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* Impro ve the Transient s tabil i ty

Transient stability is improved by high initial responsecharacteristic. In the fault condition, Field voltage isincreased to keep the generator voltage constantly. If theexcitation response is slow, it will not able to keep voltageand the generator cannot keep synchronizing.

* Impro ve the Dynamic state stabil i ty
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Dynamic stability is improved by Power System Stabilizer(PSS). PSS is provided in order to improve the power systemdynamic stability. PSS will control the excitation to reducethe power swing rapidly.

4-To supp ress the ov er-vol tage on load reject ion .When the load rejection, field current and field voltageshould be reduced rapidly to keep terminal voltageconstantly and prevent overvoltage.
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Voltage Control of A-C GeneratorsAuthor:E.E. Kimberly

The voltage regulation of alternating-current generators is greater than that of direct-

current shunt generators of comparable size. Correction of terminal voltage is usuallyaccomplished by automatic voltage control. A device known as a voltage regulator iscommonly used for this purpose.

The action of the Tirrill regulator is as follows: In Fig. 20-6 a transformer, with its primary

connected across the terminals of the alternator whose voltage is to be regulated, has its

secondary connected to a solenoid a. Assume that the alternator voltage is at the desiredvalue and that the regulator parts are therefore at rest. Assume next that additional load is

connected to the alternator so that its terminal voltage falls. The decreased pull of solenoid

a will permit the weight w to open the contacts b. This action de-energizes coil c andreleases the armature dt which closes the contacts at e. These contacts short-circuit the

portion of the exciter field rheostat which is in use; and the exciting generator voltage risesand boosts the field of the alternator, so that there is a rise in its terminal voltage. The

increased alternator voltage causes solenoid a to be strengthened, and so the contacts at bare closed again. The reclosing of contacts b restores the circuit through coil c, causing the

contacts e to open and the exciter field rheostat to be inserted again in the exciter field; andthe excitation of the alternator is again decreased. This cycle of events takes place so

rapidly that the rise and fall of the alternator voltage back and forth across the normal or

average is not perceptible. Coil / is almost, but not quite, strong enough to hold thecontacts e open, so that only a small change in current through coil c is required to move

armature d. This arrangement increases sensitivity and speed of response. Coil g tends toprevent overshooting of the voltage and to eliminate the action known as "hunting."
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Fig. 20-6. Circuit Diagram of A-C Generator With Exciter and Tirrill Voltage

Regulator

Such a regulator will control the voltage of the a-c generator within close limits, even with

large sudden changes in load. It is also possible to cause the terminal voltage to rise with

increase in load, so as to compensate for loss of voltage at the load because of impedancevoltage drop in the intervening feeders.

Fig. 20-7 shows a direct-acting voltage regulator of a simpler type, as built by the GeneralElectric Company. In series with the shunt field of the exciter is a resistance unit consistingof resistance plates in series. The ends of the plates, with contact tips forming a resistance

stack, are held together by a spring; and, when not in use; the plates are short circuitedand offer no resistance.


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Fig. 20-7. Resistance-Stack Type of Voltage Regulator

When the alternator is started and its voltage approaches the rated voltage, it energizes a

torque motor through a potential transformer and rectifier; and the torque motor, actingagainst the spring of the resistance stack, causes the stack contacts to open one by one and

thus inserts added resistance into the exciter field circuit. This action lowers the excitervoltage, and so lowers the generator voltage and thus causes some of the contacts to

reclose as the torque motor relaxes. These operations would be repetitive, and thegenerator voltage would tend to drift or "hunt" above and below the desired normal voltage.To prevent this hunting, a stabilizing transformer is connected across the exciter-armature

terminals. As the exciter voltage rises, a voltage is generated in the secondary of thestabilizing transformer. This generated voltage, acting through the rectifier, tends to nullifythe rise in voltage on the torque motor and so to stabilize the action and prevent"overshooting"


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Automatic Voltage Regulator Generator