testing polarity on transformer bushing CT

[tem123]

Hi,

We had some incident when worker were testing polarity on quite big transformer bushing CT. When they broke the current from the battery into the transformer windind, an arc was initiated, and sometime touched the worker arm even if they had gloves.

We are looking for a safer way to test the polarity. What's your common pratice? We were thinking about a double throw switch, that put the winding to ground immediately after the break (see drawing)

Thanks

 

[RRaghunath]

Being inductive, breaking the current gives rise to huge voltage. It is not a good practice I suppose.
If the bushing is not yet erected, it would be easy to check the CT polarity.
If it is already erected but oil can be lowered to access the other side of the bushing conductor inside the transformer tank (through hand hole), the polarity can be tested without passing current through the windings.
Other option is if the bushing has a test winding. The test winding is meant to test the regular CT secondary windings without accessing the CT primary terminals.

 

[thermionic1]

The best (safest) option is voltage method as outlined in IEEE standards and utilized in many commercial test sets. Apply a voltage on the secondary of the CT, below kneepoint value. Measure induced voltage by utilizing the transformer winding as a connection to the primary. Using an oscilloscope, phase angle meter or similar check the polarity. With the same connection, the ratio can be measured. On a Wye winding this is quite straightforward. With delta windings, one must ground the adjacent (not connected) winding.

There are numerous articles available from the various test equipment manufacturers on connections and theory of test.

 

[ScottyUK]

Short the HV winding via the earth switches. Inject primary current to the LV winding using a variable 3-phase supply, either from a Variac for smaller transformers or from a generator with a lab-type variable DC power supply connected to the field in place of the AVR.

 

[Compositepro]

I suppose you could connect a flyback diode across the transformer primary during the test.