Transformer odd behavior 1

[7anoter4]

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?

 

[slavag]

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

 

[7anoter4]

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

 

[slavag]

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

 

[7anoter4]

Hi slavag
It is very clear now!
Many thanks!

 

[slavag]

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

 

[7anoter4]

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