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Transfromer Insulation Resistance Test
3

Transfromer Insulation Resistance Test

Transfromer Insulation Resistance Test

(OP)
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?  
                            

RE: Transfromer Insulation Resistance Test

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.  

RE: Transfromer Insulation Resistance Test

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.

RE: Transfromer Insulation Resistance Test

(OP)
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?

  

RE: Transfromer Insulation Resistance Test

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.

RE: Transfromer Insulation Resistance Test

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.  

RE: Transfromer Insulation Resistance Test

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.

RE: Transfromer Insulation Resistance Test

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.  

RE: Transfromer Insulation Resistance Test

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.

RE: Transfromer Insulation Resistance Test

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.  

RE: Transfromer Insulation Resistance Test

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".  

RE: Transfromer Insulation Resistance Test

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

RE: Transfromer Insulation Resistance Test

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

RE: Transfromer Insulation Resistance Test

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

RE: Transfromer Insulation Resistance Test

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.


 

RE: Transfromer Insulation Resistance Test

(OP)
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...  

RE: Transfromer Insulation Resistance Test

(OP)
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?   

RE: Transfromer Insulation Resistance Test

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

 

RE: Transfromer Insulation Resistance Test

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)
 

RE: Transfromer Insulation Resistance Test

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).  

RE: Transfromer Insulation Resistance Test

Nightfox1925, The formula you are referring was first suggested in a small booklet "A stitch in time" by AVO (now Biddle).It was then seen in Transformer Maintenance Guide by TMI of SD Meyers.In TMG first edition(1981)it was properly quoted, but in third edition(2004)it is seen wrongly mentioned.It was also quoted in IEEE/ANSI C57.125-1991 Failure investigation(appendix A)

In the formula E shall be volts and not kV.It shall be the phase voltage of the winding and not phase to phase.
If it is a three phase unit KVA shall be three phase rating and not per winding kVA.

R will be IR at 20C with guard.If measurement is without guard,C shall be 16.With oil filling, C will be 1.5 &0.8 respectively.For oil filled trfs, IR will double with every 10C fall in temp and halve with every 10C rise in temp.For dry type, I am not sure,but variation must be of the same order.

With the above background, if you calculate, assuming it is a 3pahse unit, minimum  IR value at 20C with guard will be 464 megohm,very near to NETA value.

There are many similar formulae,graphs  for minimum IR followed at various parts of the world.My view is the  dont give much importance on these but go by earlier benchmarked values.These values are changing based on materials used and technology.10-20years back oil filled transformers were showing 100-600 meghom as normal IR.But today it is 10,000-30,000meghom due to the change in the  quality of oil(due to change in manufacturing method of oil).Today's oils are with high resistivity that give very high values for trf IR,but will not be any representaion of moisture inside or condition of insulation.But of course this is  the easiest way to check the insulation condition of a transformer and will be followed in future too.

Once I was perplexed by a running HV transformer with zero IR for HV winding.It was found out that solidly grounded neutral bushing(a SRBP condenser bushing) was carrying rain water inside the expansion chamber of bushing!

The IEEE formula says that in case of 3 phase unit IR will come down by 1.7 factor.But my experience is it will come down by 3.Logically also it must be correct as three resistances in parallel will bring down R by 3.Some 20 years back one utility ordered 2 nos 315 MVA 400 kV auto trfs.Later utility found out  that one  of the unit cannot be transported to  proposed site by road.So they asked  us to make it in to single phase units to form a three phase unit.So I had the opportunity to measure IR of a P MVA 3 phase trf and P/3 MVA single phase unit with identical windings,under same condtions.Single phase units showed 3 times more IR than three phase unit.
Let us discuss PI & DAR on another occasion.

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