CURRENT TRANSFORMER

[ssardana]

What parmaters must be checked during the process and Final Testing QA of Current Transformer upto 132 KV ?
What is the basic difference between the Measuring and
Protective CT & PT ?

 

[GRPatel]

Please refer a thread under subject Switchgear/boards/relays/motors engineering

 

[busbar]

ANSI-rated instrument transformers have two general classes—metering- and relay-class.

Relay class devices can generally be successfully used for metering, but the reverse can be a serious mistake.

Reference IEEE C57.13 and C12.11

 

[scottf]

For VTs, metering (measureing) and protection (relay) designs are essentially the same. In the US, most wound-type VTs come with a metering accuracy rating, which can be used for relaying purposes (overkill).

CTs are a different story. Metering accuracy CTs are designed to be accurate at currents present during steady-state conditions (i.e. nominal current +/-?%). Relay rated CTs are designed to maintain some accuracy during system faults. In the ANSI market, they are designed to be +/-10% accurate up to 20 times rated current and are rated in terms of the voltage that can be developed across the secondary winding.

In the IEC world, there are many ways to rate relay class CTs. 10P20 at some VA is the close equivalent to the ANSI C class. C800 would be close to the IEC 10P20 - 200 VA, assuming a 5A rated secondary. Most IEC installs use 1A secondary and the math gets a little more confusing.

It is true that most relay rated cores have a metering accuracy at nominal current, however, the factory test procedure for testing relay and metering cores is quite different and normally the routine test reports will not give enough information about the relay rated cores to assume a suitable metering accuracy.

Hope this helps.

 

[scottf]

Sorry...forgot to address your QA question. Since you mention 132 kV, I assume you're in the IEC world. From an insulation standpoint, IEC requires/recommends a hi-pot test on primary and each secondary to ground, partial discharge test (done with hi-pot test), insulation capacitance measurment (before and after hi-pot) and dissipation factor (insualtion power factor). Of course an accuracy test is also required and this varies with how the unit is rated. The applicable standard is IEC 44-1 (was IEC 286).

The partial-discharge criteria varies with insulation type, i.e. oil-filled vs. dry-type.

 

[saif1112001]

what is the basic defination of CT and PT in power stations.

 

[scottf]

CT = current transformer
VT = voltage transformer

They step either current or voltage from the line level to some level that meters and relays are able to handle. In the US, normally the secondary voltage of VTs is 69/115V and the secondary current rating of CTs is 5A.

Their main performance parameter is how accurately they make the transformation.