| Qns- |
Why is a CT different from a general transformer? |
| Ans- |
A current transfoemer is intended to supply true
samples of power side current to the connected instruments, ( a meter or a
relay). Therefore, accuracy of transformation is the key criteria. In general
transformers, losses and efficiency become key criteria. |
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| Qns- |
Why and where a wound primary (WPL) type CT is
required? |
| Ans- |
Low cost and convenient form of CT is a Bar or Ring
type, popularly called BPL. Low ratio CTs requiring high accuracy or high
burdens cannot be constructed in BPL type. The economic limit is 100/5,
5VA,Cl:1.0 or 200/5, 15VA,Cl:1.0. In case of lower ratio either the burden goes
lower or accuracy goes lower. When high accuracies and/or high burdens are
called for, only solution is a WPL CT. A WPL CT is costlier and combursome in
use too. |
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| Qns- |
What is a BURDEN, how is it selected? |
| Ans- |
BURDEN is the power in VA required to be consumed in
the secondary circuit of the CT containing meters or relays and connecting
leads. VA consumption of meters and relays is specified by their manufacturers.
Sometimes customer specifies OUTPUT VA of the CT. BURDEN and OUTPUT should be
equal in VA. |
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BURDEN GUIDE |
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| INSTRUMENT |
BUDEN in VA |
|
ANALOG | DIGITAL |
| AMMETER |
3.0 0.5 |
| Current coil of Watt,PF,KWH meters |
5.0 0.5 |
| Recording Ammeters |
3.0 0.5 |
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|
| Qns- |
What is the difference in metering and protective
CTs. Can they be interchanged for use. |
| Ans- |
Metering CTs are required to operate accuretely below
120% of rated currents. They should saturate above 5 times of rated current in
order to save the meters from large fault currents occuring on the power
systems.
Protective CTs are required to operate accurately above 100% and upto 10 to 15
times of rated currents, called overloads in order to provide true fault signal
to the relay. Capacity to handle large currents makes the protective CT costler
and bulkier.
Metering and protective CTs cannot be interchanged for use. Use of a metering
CT for protection will defeat the relay as it will saturate earlier than the
operating range. Similarly use of a protective CT in metering will put the
meter at risk of the fault currents as it will not saturate for damaging
currents flowing through the meter. |
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CONTINUE |
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