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?

 

[prc]

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.