Power Factor Limit on Old Transformers

At its age, it really doesn't owe you anything. Doble's standards are based on many years and units of experience - you should trust them. Even filtering the oil won't improve the quality of the rest of the winding insulation, which is probably severely deteriorated due to oxidation and heating over the years. Just becuse you improved the PF value doesn't mean you eliminated the aging of the transformer - you just improved the oil quality some, but the rest of the unit is still OLD. My suspicion if PF was at 30 and the oil quality was bad is that this transformer was run hard during those years, so an expected 40-year lifetime could be reduced.

However, just because the PF is over Doble limits doesn't mean the transformer will fail any minute. It COULD, but there's no way to predict. It will probably last a shorter time than a transformer with better insulation quality. My recommendation is to get money in the budget to buy a replacement, even if you keep running this one. Otherwise your boss will be on your back asking why the lights are out when it actually does fail.

The insulation resistance results were:

HV-earth: 2.4 GΩ

HV-LV: 3.5 GΩ

LV-earth: 4.2 GΩ

The ratio results were within standard of +/- 0.5%, oil PF is 0.1 %, oil DVB is 38 kV (ASTM D877) and water content is 10ppm.

Power factor for CH + CL = 1.2% ; CL= 1.1%

It is only the CH value that is above the Doble standard of 2% max for power transformers. The test engineer refuses to take into consideration the above test results and sticks to the above normal CH value as his reason to not allow the energization of the unit.

I do not know if there is a separate standard for distribution transformers inasmuch as the impressed voltage is low compared to power transformers.

Due to budgetary constraints we are forced to recondition this unit until such time that it pass the 2% PF barrier, if it ever could. I was just hoping to drive some sense into this guy by referring to other existing standards if there are any.

THE BDV INDICATE THE OIL IS NOT FULLY DRY AND THE PPM IS ON THE HIGHER SIDE.

CONDUCT FILTRATION OF THE COMPLETE TRANSFORMER.

THE SUCTION SHOULD BE AT THE BOTTOM AND DISCHARGE AT THE TOP.

FILTRATION SHOULD BE DONE WITH A VACUUM OF -760MMOF HG AND TEMPERATURE 70DEG c.

WHAT IS THE WEATHER CONDITION?

AS ALL THIS TESTS ARE SUPPOSED TO BE CONDUCTED IN A RELATIVELY DRY AND NON HUMID ATMOSPHERE.

WHAT IS THE CONDITION OF THE BUSHINGS?

AS YOU ARE DOING TAN DELTA ALONG WITH THE BUSHINGS ,CLEAN ALL THE BUSHINGS THOROUGHLY WITH CRC-1:1:1- AND LINT FREE CLOTH.

DISCONNECT ALL CONNECTIONS BOTH PRIMARY AND SECONDARY.

AND AGAIN CONDUCT PF TEST YOU MIGHT GET A BETTER RESULT.

FIRST LET ME CORRECT YOU.

THIS IS NOT POWER FACTOR BUT IS CALLED DIELECTRIC DISSIPATION FACTOR(DDF).

FOR AN OLD EQUIPMENT LIKE THIS REFERR THE OLD RECORDS.

SEND THE OIL SAMLE FOR A DGA ANALYSIS ,ANALYSE THE VALUES IF THE VALUES ARE OK .

THEN CHECK THE POLARISATION INDEX(BETWEEN HV-EARTH-R10/R1 THIS VALUE SHOULD BE GREATER THAN 1.3 AND THE OIL BDV WITH 2.5MMGAP SHOULD BE GREATER THAN 50KV FOR 5 AVERAGE TESTS).

IF YOU GET THE ABOVE MIN VALUES ,IN THAT CASE EVEN AFTER YOU GET LOW DDF HIGHER THAN 2PERCENT YOU CAN TEST CHARGE THE TRANSFORMER WITH LOWER SETTINGS FOR 24HOURS .

IF IT HOLDS THEN YOU CAN GO AHEAD AND LOAD THIS TRAFO FURTHER.

PF and DF at lower values are almost the same. European countries commonly stick to DF while Americans to PF.

Unfortunately there are no records of PF results for this equipment as it was not tested for PF back then.

DGA will be of no value as the oil is new. PI is more than 2. The company standard for DBV at 2.5mm gap (ASTM D877) is 30 KV which was passed.

I've already energized the equipment from the secondary side with just 220 V, 3 phase (HV terminals open) and there were no abnormalities noted for three days. However, the test engineer stuck to his 2% PF standard and would not put the equipment online.

I think stealth moderation has gone wrong on this one, I recall the original post being titled PF Limit on old transformers, no mention of Power Factor.

Chrono, you should have given the complete Doble PF results right away. While CH of 2.3% is over the recommended limit, it is not severely so. The CH+CL and CL are acceptable. While I would still suggest that you consider budgeting for a replacement unit, I do not agree with the Doble consultant who says that you could not energize the transformer. However, its longer-term life and reliability are in question.

As for Mr. bsekhar's comment (Please turn off your Caps-Lock!) on your oil test results, 38kV dielectric is quite acceptable (ASTM D-877 calls for 27kV minimum breakdown) and 10ppm water is very good for aged, in-service oil (ASTM D-1533 spec is 34ppm max). Also, the Doble Power Factor test IS essentially the same as the Dissipation Factor or Tan-Delta test, except it has the Doble trademarked brand name on it. All of these tests do the same thing: measure the capacitive leakage current across a dielectric, which is what electrical apparatus insulation is. (Aside: TonyS, I think this is what threw you off. If you don't know about insulation power factor testing, it just sounds wrong.)

Anyway, based on all the testing you have done, I have to agree that this transformer could be put back in service, with the understanding with your management that it is an older transformer near the end of its operating life, and that at some point within the next few years it could possibly fail (of course any transformer can!) and should be replaced.

Thanks PeterT for your comments and explanation. I know that this transformer could be energized based from the other test results. When energized and loaded the winding will get a little more hotter than the oil and some water from the cellulose insulation will migrate to the oil. I told the test engineer that if he Doble test it after a couple of weeks it will pass his criteria. But nope, he ignores all relevant test results and insists that if the equipment fails the Doble test all other tests after it would be irrelevant.