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Check the cross cables in the transformer

Check the cross cables in the transformer

Check the cross cables in the transformer

if i have a transformer 22/0.4 kv and at the 0.4 side i have 4 cable per phase
the cable from the transformer 0.4 kv side go to the main cabinet and connect there to the bus bar r,s,t,n
there is a test that i can check If there is cross-linking on cables without disconnect the cable and check continuity?


RE: Check the cross cables in the transformer

If you can safely isolate the HV side and LV side, and have a controllable voltage source like a 110 VAC adjustable test set used for protection testing, you may be able to inject low voltage across one phase at a time.
Connect the source to r & s phases and have meters monitoring test set output current and voltage. Slowly raise the voltage and monitor current. If any cables are cross connected, there will be current flow but no voltage. If everything is clear, the voltage will increase as the test set supplies the transformer excitation current. Lower the test output to zero and try on other phases.

No need to go to a very high voltage, 5-10 Volts might be enough. (There is a possibility of false results if the cables are long enough that the V drop through the cables is 5 volts).

This test requires safety awareness by the testing personnel. High voltages could be present on the other terminals.

We have found phase to phase bus duct faults on the secondary of transformers this way. A straight ohmmeter or other insulation resistance test can't identify the short because the transformer winding is essentially shorting the phases together and you read zero ohms on a good system.

We did this after a bus to transformer connection blew up when energized. Transformer and bus duct test results were good prior to energization, but the tests were done prior to connecting the flexible braids between the bus and the transformer bushings. Improper installation of copper shielding tape on the bolted connections shorted the phases. A good NETA technician showed us how to check for phase-phase faults using this voltage injection method.

RE: Check the cross cables in the transformer

Similar to rcwilson's experience:
Called in to verify the connections of multiple cables from a transformer before the final connection to bus.
The transformer terminal box was cramped and many of the faint phase markings on the cables had been wiped off.
We used an extension cord, a clamp-on ammeter and an electric kettle.
We shorted the high voltage bushings, just in case.
We connected one of the lv terminals to ground at the transformer end. We connected 120 Volts in series with the electric kettle (About 1200 watts) and connected the 120 Volts to each cable in turn. In the transformer terminal box we were able to identified the current carrying cable with a clamp-on ammeter and verify that it was connected to the proper phase.
The test was quick and easy and easy to understand by the customers people.
Our firm got some nice follow-up work as a direct result of the good impression that the test made on the owners electrical people.

A previous contractor doing a similar transformer install crossed the cables. When the plant electrical super closed the old oil circuit breaker, it blew all of the oil out of the breaker all over the switch room. They shared this story with me after I had verified the connections and put their minds at ease about the connections.
"Why not the best?"
Jimmy Carter

RE: Check the cross cables in the transformer

Meggering phase to ground should work. Just open the main breaker.
Megger A phase while grounding B and C.
Megger B phase while grounding C and A.
Megger C phase while grounding A and B.
A zero reading will indicate a cable is not on the phase it should be.

RE: Check the cross cables in the transformer

Sorry Uncle Bob.
The winding resistance on the low voltage side of a large transformer is so low that all cables are effectively shorted together by the transformer windings.
As rcwilson points out, the voltage drop in the cables may be more than the voltage drop across a winding. Resistance tests are so low as to meaningless.
The best way to test is to inject a current and monitor the current.
In some installations such as bus ducts where it is not possible to clamp an Amprobe around a conductor, rcwilson's test may be the best way to go.
With cables that are easily clamped with an Amprobe, my suggestion may be easier.

"Why not the best?"
Jimmy Carter

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