VLF testing through transformer disconnects.

[young_dad]

Wondering anyone has a reference or specification showing it is acceptable to test through a transformers load break disconnects with the transformer disconnected and both sources ON. Specifically looking for ABB oil filled. I know it is done all the time but I'm curious if anyone has a spec saying that a 44kV RMS test for one hour is ok.

[nwilliams]

Wondering anyone has a reference or specification showing it is acceptable to test through a transformers load break disconnects with the transformer disconnected and both sources ON. Specifically looking for ABB oil filled. I know it is done all the time but I'm curious if anyone has a spec saying that a 44kV RMS test for one hour is ok.

IEEE 400.2 states "[w]hen a switch or disconnect type device is used to isolate the cable circuit from the rest of the system, the ability of the device to sustain the VLF ac test voltage and maintain isolation while the
other end is under normal operating voltage shall be checked with the manufacturer."

IEEE 400.2 also states, albeit in reference to Tan Delta testing, "[i]n addition, some utilities may have components connected to the cable circuit being measured, e.g., oil-filled switches, that cannot be removed but can influence the test results. Most users of dielectric response techniques choose to measure the entire cable
system response that would include the responses from all terminations, cable, and joints within the circuit."

So this is the closest I could find to what you're looking for. I hope it answers your question.

[Kalbi_Rob]

Wondering anyone has a reference or specification showing it is acceptable to test through a transformers load break disconnects with the transformer disconnected and both sources ON. Specifically looking for ABB oil filled. I know it is done all the time but I'm curious if anyone has a spec saying that a 44kV RMS test for one hour is ok.

Table 2 of IEEE C57.12.90-1999 shows the ability to perform dielectric field testing of the HV bushing as a percentage of maximum system operating voltages vs allowable duration. That is with the transformer switch closed, with the transformer switch open you technically fall under IEEE Std C57.19.00-1991 for bushing test which allows for much higher voltages than C57.12.90, so you are well within the acceptable dielectric test values for testing 35kV cables using VLF for acceptance testing.
It would be highly discouraged to test with the transformer un-isolated as lower frequency will cause the transformer to saturate in accordance with Std C57.19.00 section 10.7.2 creating a condition for potential catastrophic damage to the transformer upon energization.

[young_dad]

IEEE 400.2 states "[w]hen a switch or disconnect type device is used to isolate the cable circuit from the rest of the system, the ability of the device to sustain the VLF ac test voltage and maintain isolation while the
other end is under normal operating voltage shall be checked with the manufacturer."

IEEE 400.2 also states, albeit in reference to Tan Delta testing, "[i]n addition, some utilities may have components connected to the cable circuit being measured, e.g., oil-filled switches, that cannot be removed but can influence the test results. Most users of dielectric response techniques choose to measure the entire cable
system response that would include the responses from all terminations, cable, and joints within the circuit."

So this is the closest I could find to what you're looking for. I hope it answers your question.

Thank you for your reply. I'm comfortable with the fact that the test is going through other apparatus. The issue is whether or not the equipment manufacturer allows for the sustained test at a higher than rated voltage.

[nwilliams]

Thank you for your reply. I'm comfortable with the fact that the test is going through other apparatus. The issue is whether or not the equipment manufacturer allows for the sustained test at a higher than rated voltage.

I see. It looks like the standard that Kalbi_Rob referenced has you covered. Another thing you could do is see any tests that the manufacturer did on the transformer/disconnects. Make sure that the Overpotential used by manufacturer testing meets or exceeds your 44 kV RMS. If the manufacturer used DC, make sure the peak voltage of your VLF test doesn't exceed the DC overpotential. However, if the manufacturer didn't test for a duration of an hour, it'll probably have to be a judgement call.

[Kalbi_Rob]

Thank you for your reply. I'm comfortable with the fact that the test is going through other apparatus. The issue is whether or not the equipment manufacturer allows for the sustained test at a higher than rated voltage.

That is where Table 2 comes into play. Manufacturer's are still required to follow IEEE in performing the test. Their withstand test in Table 2 places the voltage approximately same as VLF, but that would be a test performed through the transformer, so the switch connections can be held at higher voltages for dielectric withstand installation tests. Your only performing an Acceptance test.

[young_dad]

Wondering anyone has a reference or specification showing it is acceptable to test through a transformers load break disconnects with the transformer disconnected and both sources ON. Specifically looking for ABB oil filled. I know it is done all the time but I'm curious if anyone has a spec saying that a 44kV RMS test for one hour is ok.

I got information straight from the manufacturer. Testing through the transformer OK for this model. This was only for the specific case of going through the loop feed with xfmr off. I didn't ask about testing into an 'off' switch.