Transfromer Insulation Resistance Test 3

[nightfox1925]

I'm a little confused on some basics. Minimum allowable insulation resistance can be calculated using the formula

IR = CE / Sqrt(kVA) in Megohms

wherein C = 30 (for dry type)
E = phase to phase voltage

Question: is the E in volts or kilovolts?

1)As per NETA, this IR value allowed is based from measurements: winding-to-winding and EACH winding to ground.

Will this calculated allowable min IR be applicable if I were to measure HV winding to LV winding and ground, LV winding to HV winding and ground, HV winding and LV winding to ground?

In performing the measurement arrangement in 1) above, will the megger lead connected to the phase being measured and the ground lead will be connected to tank ground with the remaining HV phases and all LV phases short circuited to ground?

 

[edison123]

E is in KV. Otherwise, you would need millions of megohms to qualify the trafo for service !

Yes, your test method of keeping one winding hot and other windings and tank grounded and connected to the megger ground is correct.

 

[edison123]

BTW, it would be HV winding to LV winding & Ground and LV winding to HV winding & Ground.

You would not be able to measure the IR individual phase of the windings.

 

[nightfox1925]

Thank you very much edison123...am a little surprised why one of our client's test records shows phase A to ground, phase B to ground, phase C to ground (for each primary and secondary).

BTW, the 1999 edition of NETA, section 7.2.1.1.2.2 (Electrical tests for dry type transformers)as quoted: "Perform insulation resistance tests winding-to-winding and EACH WINDING-TO-GROUND" with test voltage in accordance with table 10.5."

The term "each winding-to-ground" sounds tricky to me and somehow ambiguous. Any comments?

 

[edison123]

Here the winding means HV winding, LV winding and tertiary winding, if any. It is definitely not each phase of the winding.

I have never seen a trafo (dry or liquid filled) where you can dismantle the winding connections unless you open it up. Even then, some connections are permanently brazed together.

 

[Zogzog]

Hey, could be worse, I have seen "Test reports" that have phase to phase IR measurements (Megger test) on them.

Edison, dont forget HV to LV windings test.

Nightfox, dont forget DAR/PI tests, just as important. And the readings you get are meaningless unless you temp correct them.

And 1999 NETA spec is a little dated, you should get the 2007 MTS.

 

[edison123]

Zog

HV to LV+G and LV to HV+G tests will cover the HV to LV intrinsically. You need to do HV to LV only if one or both these tests show a problem.

Also, DAR/PI tests for liquid filled trafos have no meaning. I am not so sure about dry types.

 

[Zogzog]

NETA specs require a PI for large dry type ADN liquid filled, why do you say they have no value?

HV-LV+G and LV-HV+G readings that are borderline could mask a bad HV-LV for PF testing, but for IR I see your point. However, again it is a required test per NETA MTS.

 

[edison123]

PI and DAR are all about insulation properties like absorption current, capacitive current & conduction current, which are applicable to solid insulation like MV motor and generator windings, which are basically capacitances.

In oil filled trafos, the oil is the main insulant and the paper/board insulation is there to create a physical device for channeling the oil. I have tried to measure PI in large and medium trafos and never gotten more than 1. If you had different experience, I would love to hear about it.

As I said, I am not so sure about dry type trafos.

Again, HV to LV IR is required, only if HV to Ground IR and LV to Ground IR were done.

 

[Zogzog]

I have seen a few Pi's >1.0 on oil filled but I agree it is rare. Dry type is more common, it is a solid insulations and for testing purposes not much different than a motor. Almost always >1.0 PI on dry types (Large) and DAR is measured on small dry types.

You have a very good point about the IR HV-LV, I am in tight with the NETA SRC and will make a recommendation that the spec reflects something like "IF HV-LV/G or LV-HV/G" is below spec test HV-LV instead of HV-LV as a required test. Thanks for the insight on that, makes good sense, maybe the SRC has some different idea about that.

 

[edison123]

Zog

Thanks for that nugget about PI being more than 1. I will keep my eyes peeled next time around because I had pretty much given up on measuring PI based on my "experience".

 

[nightfox1925]

Thanks for all the insights gentlemen....Now I am on the right track on this.

 

[oldfieldguy]

One good argument for making DC transformer insulation resistance tests is that most facilities will have the equipment to make the test themselves if there is a suspected problem with the unit.

We typically include the DC tests as part of the standard test package for transformers since it is a NETA specification.

However, if a transformer trips off line during the night, the owner's own staff will not likely have access to other transformer test equipment in most facilities, but they will have a megohmmeter and can perform that test and compare it with results form previous tests.

old field guy

 

[NCTHAI]

For oil filled transformer, measurement of PI does not make sense. By principle, in the IR tests you measure leackage current. When oil (Insulation layer) is so much (any where oil layer thickness is 10 mm), measureing device can not drive enough current to make steady flow of leackage current and hence PI > 1 is rare.

In any case measurement of PI is for Dry insulation not for wet insulation like oil filled transformers.

NC

 

[edison123]

NCTHAI

"measureing device can not drive enough current to make steady flow of leackage current".

Actually, the current is determined by the resistance in the oil, not by the quantity. That's why you get low IR values (high leakage current) when the oil contains water. Going by your logic, large trafos with huge amounts of oil and high oil thickness between the coils and the ground should always show high IR's.

PI is all about various stages of current during the sudden application of DC to a capacitor and only MV motor/generator winding insulation comes close to that model.

IEEE std 43 deals well with various aspects of IR measurements of rotating machines.

 

[nightfox1925]

Hi again edison123, I got a copy of the 2007 NETA and it has Table 100.5 at page 203 which tabulates Transformer Insulation Resistance Testing. In this table, it indicates that for transfomer coil voltage rating of 0-600V, the recommended minimum insulation resistance in Megohms for "Dry" is "500 Megohms"

If I am to apply the formula above, then for a 500kVA dry type transformer at 600VAC would give me

IR = 30(0.60)/(sqrt(500)) = 0.80 Megohm

This is fine with the 1Megohm/1kV rule but it is very less for that 500 Megohm requirement as per NETA ATS Table.

I think I am confused or getting mixed up here. help...

 

[nightfox1925]

can anyone comment on what is the minimum IR value for dry type transformer, 600V and below, 500kVA...

Is 500 Megohm minimum IR correct per NETA ATS?

 

[Zogzog]

Yep, 500M (ATS and MTS) for 0-600V, temperature corrected of course, cant stress that enough.

 

[edison123]

I don't have NETA copy with me, so I don't know the whole context of its requirements.

However, I can say a 500-meg-min requirement for a 600 V, 500 KVA trafo is an overkill.

I would be happy with 100 meg at an ambient of 30 deg C (As Zog says, can't stress the temp. effect on IR enough)

 

[Zogzog]

NETA sets its standards pretty high, a reading of < those on table 100.5 is just cause for further investigation and should be brought to the attention of the owner (Remmember, NETA is all 3rd party testing).