Power XFMR Testing 5

[jacphi]

Hi all,
i was involved with testing of a 7.5 MVA, 13.8kV-4.16kV transformer. the following points were noted.

1,The Liquid-Temp Indicator (thermometer)installed was with capillary and not direct stem type. Does ANSI C57.12.10 states that the Liquid-Temp Indicator (thermometer) shall be direct stem mounted in a closed well.Is it a violation of ANSI Standards?

2,The Load-Loss for the 7.5 MVA transformer was calculated by a computer program.this was done by injecting Approx 70% of the rated high voltage current. i.e. 219.6Amps instead of 313.77 Amps.Does ANSI C57.12.90 permit injecting only 70% instead of 100%, of the HV rated Current?

3,The oil analysis performed on sample taken from the 7.5MVA Transformer, did not give a satisfactory result. Four samples were taken from the same transformer and only one sample passed the Dielectric Voltage Breakdown test. The test was performed using ASTM-D877 test method. the sample which passed the test was kept undisturbed for an hour after the sample was taken and for the others the test was done immediately, after the sample was taken from the transformer. why is the oil which was kept for a while passing the test?

i need immediate help and request all the experts out there to help me out.
regards,

 

[RalphChristie]

jacphi:

First, I am talking out of an IEC-perspective, not an ANSI perspective. I have no idea of how the ANSI-standards look like.

1. Not sure, altough I can't really see any problems if it is remote. I have seen many transformers with the temperature relays inside a seperated box next to the transformer. (For top oil temp and winding temp)

2.The temperature of the windings at which the test is carried out must be measured accurately and also the test must be completed as quickly as possible to ensure that winding temp doesn't change during the test. Should several copper-loss and impedance tests be required on a transformer (i.e. on various tappings) then it is advisable to carry out these tests at reduced currents, in no case at less than half the rated current, and correct the results to rated values of current. (IEC 76) I think ANSI would have a similar standard.

3.If the sample gives any cause for suspicion that the water content is high, i.e. if there is free water present or if the oil should fail the electrical strengh test, then it is desirable to determine the actual water content by carrying out a Karl Fischer test (IEC 814)

Sorry that I was not helpful from an ANSI-viewpoint.

Regards
Ralph

 

[jnims]

1. This is a common practice to get the dial closer to eye level. The actual temperature measurement is at the well located near the top of the tank.

2. C57.12.90-1999, Clause 11.5.2.1 which describes the short-circuit method for simulating load to measure the winding temperature rise tests states:

"When test equipment limitations dictate, operating at a value lower than rated current, but not less than 85% of rated current, is permissible".

3. I can't address this issue.

 

[stevenal]

I cannot find any such wording in 5.1.3. for temperature indicators. The winding temperature simulator does require direct mounting per 5.9.1.2.
ANSI standards are usually referenced in a purchase specification. The purchase specification may deviate from and overrule ANSI if written properly. We prefer low mounted gages that can easily be seen and reset and let ANSI handle the other details not directly addressed in our spec.

 

[jacphi]

hi jnims,
as mentioned in C57.12.90-1999, Clause 11.5.2.1

When test equipment limitations dictate, operating at a value lower than rated current, but not less than 85% of rated current, is permissible.

does this applies for load loss measurement also or only for winding temperature rise tests?
regards,

 

[jnims]

I don't find any statement that allows lower than rated current for the load loss test. The rated current referred to is the transformer's base rating (usually, but not always,the ONAN rating for power transformers). In Clause 9.3.3 of C57.12.90-1999 where is discusses the load loss test procedure for two-winding and autotransformers:

"With one winding shorted-circuited, a voltage of sufficient magnitude at rated frequency is applied to the other winding and adjusted to circulate rated current in the excited winding."

There is slightly different wording for autotransformer. I am assuming you have a two-winding transformer.

 

[jacphi]

thanks jnims,
does that mean that for measuring load loss, rated current is to be applied and for winding temp rise test not below 85% is to be applied?
is my intrepretation of the ansi std correct???
regards,

 

[jnims]

That is the way it appears. 11.5.2.1 includes equations to correct for currents other than rated.

 

[jacphi]

thanks jnims,
now i understand why the inspector did not accept the test.by the way our test booth does not have the facility to apply 100% rated current. is there any alternate method you can sugest??? or else we are doooomed......

 

[buzzp]

One comment on your oil samples, were the failed samples taken first? We generally bleed off a little oil before we pull a sample to allow any moisture, debris in the valve to be flushed out. Other points about air escaping over time on the sample that was allowed to sit seems possible.

 

[jacphi]

hi buzzp,
yes the failed samples were taken first. but no oil was bleeded out.
yes even we feel that it might have failed due to air.
regards,

 

[buzzp]

Perhaps, a quick flush of the piping after opening the valve may help get a better, more consistent sample. This is something I have our field guys do when they take a sample from a transformer.

 

[HVTester]

Addressing problem 3
We generally check for free water and drain off until free of water then take the sample. When back at the lab we always leave the sample at least an hour until it reaches ambient temp of the lab. This eliminates any suspicious results you seem to be getting.
Phil