Finding Interturn faults in PT 1

[charz]

How to find Inter turn faults in a potential transformer?
The ratio tests on the PT indicates a no fault and the Insulation resistance too were normal.
How to know if the fault is in the primary winding (or) secondary winding?

 

[scottf]

Excitation test would be my first try. You'll need original excitation curve to compare to, or a like unit to compare to, or get with the factory.

How do you know there is a turn-to-turn fault to begin with? What is the voltage class of the PT?

 

[amps21]

Agree with Scottf. could attempt FRA and compare standard tests, if available.

 

[prc]

Measure single phase excitation current at 230V and compare with earlier figures or on similar units. In case of an inter turn fault it will increase more than 2-3 times.

 

[charz]

Hi Scottf,

Due to surge voltage, surge arresters got damaged in a Incoming feeder and so checked the PT's before re-energizing. while checking we were discussing about this.

Hi PRC,
Is the excitation test for PT's same as excitation test for CT (saturation check). But how does this tell, whether the inter turn fault is at Primary winding or secondary winding?

 

[scottf]

charz-

To do the excitation test, apply voltage to the secondary with a variac and record the current drawn. ** Caution that performing this test on a VT will result in high voltages on the primary winding **

If the high voltage surge did damage the transformer, the damage almost certainly will be in the high voltage winding. Of course, if you have a shorted turn on the primary or a shorted turn on the secondary, it doesn't really matter which it is, the VT is lost either way.

 

[prc]

In power transformers I used to identify the faulty winding like this- conduct the excitation test on both windings and express the current as times of the previous reading under identical conditions. The faulty winding will show more times than the other.

 

[charz]

Scottf and PRC,

Thank you for your replies. I have one more question, Why doesn't the potential transformer gets saturated during the peak voltage in every AC cycle. Say, If the PT is rated 10kV/0.11kV, during every ac cycle, the peak voltage reaches 10 x 1.414 which is 14.14kV. Normally the PT's have a continuous voltage rating of 1.2 times the nominal voltage, so here the continuous rating would be 12kV.

 

[oldfieldguy]

We called it the 'reverse excitation' test, but using a Variac, apply voltage starting at zero to the secondary winding.

As previously noted, take precautions because you will possibly induce high voltage on the HV terminals.

It has been my experience that a PT that has failed due to shorted turns will draw very high current on the secondary at very low voltages.

This is the method we commonly use to verify bad PT's. They often pass ratio tests and insulation integrity tests, but reverse excitation finds them every time.

old field guy