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CAPACITOR VOLTAGETRANSFORMER

Capacitor Voltage Transformers (CVT) is used forvoltage metering and protection in high voltagenetwork systems. Accordingly they are either

measuring type or protection type voltagetransform.

They transform the high voltage into low voltageto be processed in measuring and protection

instruments such as relays and recorders.

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Diagram!

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Coupling capacitors for power line carrier

applications.Capacitor elements consist of Aluminum foil

insulated with paper and polypropylene "lmimpregnated with mineral insulating oil.

#$% unit is "lled with free mineral insulatingoil.

&n CVT the primary voltage is applied to aseries capacitor group. The voltage across one

of the capacitor is taken to au'iliary voltagetransformer. The secondary of au'iliaryvoltage transformer is taken formeasurement.

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Construction and

Working:-

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A capacitor voltage transformer (CVT) or capacitance coupledvoltage transformer (CCVT) is a transformerused inpower systemsto step down e'tra high voltagesignals andprovide a low voltagesignal for measurement or to operate aprotective relay.

&n its most basic form the device consists of three parts twocapacitorsacross which the transmission line signal is split aninductive elementto tune the device to the line fre*uency and atransformerto step down the voltage for the instrumentation orprotective relay.

CVTs are typically single!phase devices used for measuringvoltages in e'cess of one hundred kilovolts where the use ofwound primary voltage transformers would be uneconomical.

&n practice capacitor C+is often constructed as a stack of smaller

capacitors connected in series. This provides a large voltage dropacross C+and a relatively small voltage drop across C,.

The CVT is also useful in communication systems. CVTs incombination with wave traps are used for "ltering high fre*uencycommunication signals from power fre*uency. This forms acarrier communication networkthroughout the transmissionnetwork.

http://en.wikipedia.org/wiki/Transformerhttp://en.wikipedia.org/wiki/Power_systemshttp://en.wikipedia.org/wiki/Extra_high_voltagehttp://en.wikipedia.org/wiki/Low_voltagehttp://en.wikipedia.org/wiki/Protective_relayhttp://en.wikipedia.org/wiki/Capacitorhttp://en.wikipedia.org/wiki/Inductorhttp://en.wikipedia.org/wiki/Transformerhttp://en.wikipedia.org/wiki/Power_line_carrier_communicationhttp://en.wikipedia.org/wiki/Power_line_carrier_communicationhttp://en.wikipedia.org/wiki/Transformerhttp://en.wikipedia.org/wiki/Inductorhttp://en.wikipedia.org/wiki/Capacitorhttp://en.wikipedia.org/wiki/Protective_relayhttp://en.wikipedia.org/wiki/Low_voltagehttp://en.wikipedia.org/wiki/Extra_high_voltagehttp://en.wikipedia.org/wiki/Power_systemshttp://en.wikipedia.org/wiki/Transformer

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Main A!ications o" CVT in #V N$t%orks:

Voltage $easuring They accurately transformtransmission voltages down to useable levelsfor revenue metering protection and controlpurposes

&nsulation They guarantee the insulation

between -V network and V circuits ensuringsafety condition to control room operators

They can be used for /ower ine Carrier (/C)coupling

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R$ason "or Fai!ur$ o" CVT:

0rinkles on aluminum foil

/oor soldering *uality

/oor *uality of paper1horting of transformer cores

2ailures of circuit components

3usting of coupling bolts (between 4ange and

emu tank)ooseness of core bolts

#ntry of moisture in capacitor stacks

$oisture entry due to poor gasket *uality

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T$st "or CVT:

M$asur$&$nt o" Caacitanc$' Tan d$!ta

$easurement the Capacitance and Tan delta of the capacitor units. Themeasurement should be made with an applied voltage of +5kV

L$akag$ T$st

Clean the entire surface of the capacitor unit with a clean cloth. Check visuallyfor anyleakages. Also check the oil level in the electromagnetic unit.

Ratio Error T$st on t($ CVT

#rect the complete CVT. #arth the -2 bushing and the tank. Apply a voltage of,,5 V or 6+7 Vbetween the primary terminal and earth.$easurement thevoltage on the secondary side.

M$asur$&$nt o" Insu!ation R$sistanc$ )On!* "or PT o" EM+,

$easure the insulation resistance between the secondary windings.

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