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Transformer odd behavior

Transformer odd behavior

Transformer odd behavior

(OP)
In a Connection Substation, 3 generators of 100 MW each and 26 step-down transformers of 20 to 40 MVA are connected .
An usual test of measuring instrument over all the Station was performed.
One of the transformer connection [40 MVA Yo/D ] shows an odd result. One of the phases was less than 10% loaded.
The transformer its self does not show any damage or strange behavior. It seems that this transformer was over sized,
[future development was expected], so for years nobody paid attention.
Within few months the tap changer was checked so all the tank was open . On the medium voltage side[D] a phase
winding was founded  interrupted[disconnected], may be from the beginning, as installed.  
How could the transformer  work properly for many years in this conditions?

RE: Transformer odd behavior

Hi 7anoter4 .
This xfr. operated in parallel with other xfr's.
What is avg. load on this xfr.?
This 10% I assume you see on the secondary.
Regards.
Slava

RE: Transformer odd behavior

(OP)
Hi slavag,
The phase diagram test was done on the high voltage[110 kV] side only.
The transformer did not work parallel with other xfr.
The average load was between. 45%  to  50% [seen from 110 Kv.]
One phase was loaded less than 10%[may be 5% or less].
Thanks

RE: Transformer odd behavior

It's O.K.
You don't have any problem with xfr.
Your transformer work as two phase xfr to 3pf.
Don't forget, it's Delta side.
Current in connected phases was less from I nominal, your load was 40-50%.
Any protection don't see this problem , only NPS on MV side
of xfr., but you haven't it.
 O/C,  Overload, diff protection are not sensetive to this case.
Good Luck.
Slava

RE: Transformer odd behavior

(OP)
Hi slavag
It is very clear now!
Many thanks!

RE: Transformer odd behavior

Hi 7another4.
Could you please send me what is a setting of NPS function in generator protection. ( for alarm and for trip).

RE: Transformer odd behavior

(OP)
Hi slavag
The main supply of this Connection Substation are 4 incoming overhead lines from the national network.
The generators are mainly for backup. All generators are connected by means of high voltage cables and
including step-up transformers are located far from substation and belong to the national network authority.
I do not know exactly the protection setting but I think all generator is provided with a ABB [may be still BBC] NPS
That means longitudinal an transversal differential , directional[definite or inverse or both] overcurrent, stator and rotor earth fault protection and other as overvoltage and undervoltage and so on.
Our step-down transformers were provided with an usually protection as differential, definite time overcurrent, earth fault,overheating and gas[Buchholtz]. A busbar differential protection and an anti-oscillation trip device was provided also.
If is a system tendency to oscillate than this device trip the incoming lines and all the Connection Substation works
in insland, supplied only by the generators connected with.
Best regards

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