Test winding in Bushing current transformer

[prc]

In power transformers, usually bushing current transformers ( both for protection and metering)are provided as back up. For convenience of testing it is usual to provide a one turn test winding in addition to the secondary winding. During service, test winding will be kept open with one end earthed.Is this a normal practice everywhere? In case more than one turn is provided for test winding is there any issue as the secondary will be always shorted through load.How much current rating is required for test winding?

 

[PHovnanian]

How much current rating is required for test winding?

It depends on what sort of test you need to run after commissioning the installation. Rated current isn't required to verify turns ratio or polarity. The primary current needed would depend on the requirements of the test equipment being used. I've heard figures like 10 Amps.

If you need to test the CT's accuracy over its operating range (for metering calibration purposes), then you may have to approach rated current.

 

[inpran]

prc,
Yes, that is the usual practise followed in reactors/transformers provided with a test point. But I'm yet to come across a case where there is more than one turn test winding.

The rating of these points, as pointed by Phovnanian, depends on the manufacturer and application I suppose. The maximum rating I have come across is 20A.

 

[prc]

inpran, the vendor claims that for correct and easy measurement more turns in test winding is advantageous. Since there is a shorted secondary to cancel the primary AT, trust a multiturn test winding ,kept open will not create over voltages.

 

[DTR2011]

Testing CT with voltage method eliminates the need for this. The transformer conductors are used as the primary for test purposes. See IEEE C57.13