meggering through a transformer

[bogie418]

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?

 

[RalphChristie]

Not sure if I interpret your question correctly, but no, you can not test insulation integrity of the LV-side from the HV-side. If you'll megger from the HV-side, you will only test the insulation on the HV-side.

To check the LV-insulation, you'll have to test from the LV-side.

Regards
Ralph

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[bogie418]

Sorry if I wasnt clear. I dont care about the tx. but I cant remove it from my cct. Its the secondary conductors Im worried about. I think they were exposed to heat so I want to test the insulation. Im worried the tx might screw up my megger readings. ie maybe put a load on my tester or some how alter the readings??

 

[RalphChristie]

Ok, it seems to be more clear now.

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

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[ruggedscot]

Oh and remember have the power isolated - you cant megger with voltage present ! And you may just scare yourself if you try it live, worse you could end up well you know fried.

Rugged

 

[bogie418]

I realize you cant megger hot and I also that the tx isnt conductively connected from LV side to HV side. My question is more about the load that would present by the tx (even with the power off) and if a megger would be affected by this.
And I agree 5kv is too high for this app.
Why would I need to disconnect ground?

 

[able357]

Okay so first off you should test the transformer at 1kv! To test the LV side of a single phase transformer you need to short the h1 to h2 with some shooting wire or bare copper wire! you also need to short the LV side (X1-X2).

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!

 

[bogie418]

ahhhh....I already said I cant remove the tx, and I also said I dont want to test the tx.

 

[able357]

what point to what point are you testing! I never tested the conductors that go to the tx so im curious!

 

[RalphChristie]

Bogie:

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.

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[rbulsara]

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.

 

[bogie418]

I agree there could be grounding on the LV side But being three individual tx's, each phase is isolated with the HV side open, has me wondering how and where its grounded. I think Ill give it a go, and if the readings are crap Ill look at isolating the tx and/or the ground. IF I cant do that Ill have to cut the PVC and inspect the suspected area. Ill keep you guys posted.
Thanks for the replies,
Dave

 

[RalphChristie]

The test is just to ensure the cable insulation integrity.

Its the secondary conductors Im
worried about.

The only reason I proposed it was because the transformer can't be disconnected.

I dont care about the tx. but I cant remove it from my cct

Regards
Ralph

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[rbulsara]

bogie418:

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.