Transformer Problem

[byjuvc]

Recently we faced the following problem:
Breaker of 1000 KVA TR. tripped and after checking LT ACB Breaker tried at no load but did not hold then TR. Meggered and results found satisfactory then again tried but did not hold .The Mobile TR. was put in service and supply of the area restored . TR .Particulars are as under.
TR. Make: -------CGL
Year of mfg:-------2002
Capacity:-----------1000 KVA
DYN11

In the morning TR. gain checked and IR values found as under.

HT TO EARTH ----10000 MEGA OHM

LT TO EARTH ----10000 MEGA OHM

HT TO LT------------10000 MEGA OHM

Keeping all LT leads and earth disconnected Breaker put on and TR. hold but when tried after connecting earth on Neutral Bushing Breaker did not hold.

Supply at LT Bushing was also checked and no supply found when earth of Neutral was disconnected.

Load on TR. taken on 26.12.06 was as under.
R PH.----936 AMP
Y PH.---857 AMP
B PH 969 AMP

NEUTRAL---200 AMP

The Tr has been sent to workshop and we are awaiting their verdict.
Did anyone face a similar situation ?

Thx

Byju

[alexlights]

when you say it 'does not hold', i'm guessing you are getting relay targets. what relays are taking you out?

Are you using the same breaker for the mobile transformer?

[byjuvc]

when you say it 'does not hold', i'm guessing you are getting relay targets. what relays are taking you out?

Are you using the same breaker for the mobile transformer?

Ya the relay targets are reached and the breaker trips. The relay is a WIP1 fitted in an ABB RMU. It is set for 56 A. Yes the same breaker is used for the mobile Transformer but with a different set point that is 36 A since it is a 630 kVA . part of the load has been shifted to other LT feeders ( back feeded).

[nolacbd]

Sounds like you have a winding failure inside of the transformer. Could be a turn to ground failure. Some of the cellulose on the winding may be bad or there is water in the oil. Do you guys have anybody that can hipot the xfmr? In the shop we used to put a hipot on the xfmr and put primary voltage at 400 cycles. You should not have any leakage current besides maybe a few microamps. There is also a test that can be performed called a Dissolved Gas Analysis. You take an oil sample and the lab analyzes the quantity of hydrocarbons in the oil. From that they can tell if something has burned up. We also have a shop that can test oil for water with a hipot. A Megger may not reveal problems because of the low voltage that it applies. Just a few thoughts, Good Luck.