meggering through a transformer
meggering through a transformer
(OP)
I need to test some insulation integrity.
Its a 3 phase system, with three single phase tx.s. I can disconnect on the high side of the tx only.
Its a typical pole tx. from the utility co.s overhead line to a commercial bldg.
Do you think I can still get a reliable meggar reading? Will my 5000v fluke megger even work?
Its a 3 phase system, with three single phase tx.s. I can disconnect on the high side of the tx only.
Its a typical pole tx. from the utility co.s overhead line to a commercial bldg.
Do you think I can still get a reliable meggar reading? Will my 5000v fluke megger even work?





RE: meggering through a transformer
To check the LV-insulation, you'll have to test from the LV-side.
Regards
Ralph
Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon
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RE: meggering through a transformer
RE: meggering through a transformer
You can megger the LV-side with the transformer in the circuit. No problem. Just disconnect any grounding points (I am not sure how the transformer is grounded) and megger between phases and ground. If the reading is not good, you might have to disconnect the transformer to determine if the fault is on the cables or in the transformer.
Just be sure to use the correct voltage to megger - 5kV seems a bit high for a commercial building.
Regards
Ralph
Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon
Make the best use of Eng-Tips.com
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RE: meggering through a transformer
Rugged
RE: meggering through a transformer
And I agree 5kv is too high for this app.
Why would I need to disconnect ground?
RE: meggering through a transformer
You have insulation between windings and insulation from the coils to the nearest ground! if you want to test the low voltage insulation you need to apply to the X1-X2 and put your gaurd lead on the H1-H2 terminals. What the gaurd does it gaurds the reading from the hv terminals and reads the rest of the output giving you the low voltage insulation.
Now if you want to test the whole insulation system between the HV and LV connect LV TO HV doesnt matter which is applied and gaurd the ground and you will read CH+CL!
you also have to disconnect everything from the transformer or you readings are not accurate!
RE: meggering through a transformer
RE: meggering through a transformer
RE: meggering through a transformer
Due to the fact that your network is running on a certain voltage-level, the whole network (subjected to this voltage) should be insulated to handle it. Because there should be no connection to ground under normal operating conditions, you can, in theory, megger your whole network towards ground to detect isolation failures. Practical however, in big networks, you might run into problems doing it. (The network becomes just to big, takes longer to charge-up during megger-tests, etc.)
Thus, in my opinion:
Open the Incoming Breaker at the LV-side and disconnect the incoming neutral (if any) at the same location.
Disconnect the grounding-point of the transformer. (Not sure how your system is grounded, star point to ground, corner delta grounded, etc. You'll rarely find such a system to be ungrounded) You'll have to disconnect the grounding-point to ensure there are no path to ground during the test.
Megger the cables with the transformer included, towards ground. If the reading is fine, it means that the transformer and cable insulation are in a good condition. If not, you'll have to disconnect the transformer from the cables, and megger each on its own to determine which (cables or transformer) is defective.
Regards
Ralph
Just one last note:
Be on the check-out for VTs connected directly on the cables, although I do not think it will be present on your network. If they are wye-wye-connected, you'll have to disconnect it totally or disconnect the grounding point on the primary side.
Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon
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RE: meggering through a transformer
Only insulation you will be able to measure is line to ground, and that too with all tx ground connections disconnected. With tx windings connected, the phase to phase insualtion will obviously read zero and will render your test results meaningless.
Also in the past I have noticed with generators that If the LV side of is wye connected, it will give you a skewed reading of meggaring, specially if the tranformer is old. For more accurate results each phase winding needs to be isolated from the others.
RE: meggering through a transformer
Thanks for the replies,
Dave
RE: meggering through a transformer
The only reason I proposed it was because the transformer can't be disconnected.
Regards
Ralph
Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon
Make the best use of Eng-Tips.com
Read the Site Policies at FAQ731-376
RE: meggering through a transformer
With all due respect, you need professional help. This forum is no substitute for that.
Hire a professional testing company. Your understanding of the system and electrical basics seem so limited that if you don't involve more experienced or professional people you may end up damaging the equipment or even worse hurting someone including yourself.