Transformer protection

[madomankh]

Hi,
For the MV/LV oil transformer , the transformer mechanical protection like (Buchholz,oil temp,saftey pressure,....)
the trip contact from all those protections are used to trip the upstream MV breaker for the transformer.

Is it possible to use this contacts to trip only the downstream LV breaker of the transformer and keep the upstream energized.

I think it is not possible , but i`ll appriciate so much if any body have the code or standard No. which state and descripe this situatuion.

Thanks

 

[dpc]

Sure it's possible, but if the primary is still energized, tripping the secondary is not going to protect the transformer from an internal transformer fault.

If the transformer is worth all these protective devices, the it needs to be completely de-energized on a fault condition.



David Castor

http://www.cvoes.com

 

[madomankh]

Yes,
that is which i`m belive in that we should trip the upstream but again my question is (is there is any standard we should follow and force us to trip the upstream?)

 

[rbulsara]

Only upstream breaker protects the transformer from its internal faults, not the secondary.

So why do you need a standard to tell you which breakers need be tripped, if there is a fault internal to the transformer?



Rafiq Bulsara

http://www.srengineersct.com

 

[madomankh]

Hi rbulsara,
As i said before it is the logic to trip the upstream to protect the transformer internal fault.

but,imagain you are discussing with your subcontractor and he said it is enought to trip the down stream and no need to trip upstream and you should proof to him the right connections.

Thanks

 

[dpc]

I'm sure your local electrical codes require adequate transformer protection.

David Castor

http://www.cvoes.com

 

[rbulsara]

The sub can say whatever he wants, there is nothing to discuss there.

Rafiq Bulsara

http://www.srengineersct.com

 

[rcw retired EE]

Either the sub thinks the transformer is only fed from the low voltage side (like a generator step-up transformer on an island system) or he believes these protections just protect the transformer from overload. Tripping just the low voltage breaker will remove an overload.

You could explain that the protections respond to many failures not just overload and all power must be removed from the transformer.

There are no codes for good engineering design. It is just understood that the protection will remove energy from the faulty device.

 

[PHovnanian]

Depending on the substation configuration, you may want to trip the low side breakers. Not _just_ the low side breakers, but both low and high side. Its possible that the transformer might be back fed from another low side source, in which case tripping both sides of the suspect unit would be a good idea.

For overload conditions (time overcurrent), there might be an argument for tripping the low side alone (although I'd examine the transformer overload settings coordination with the low side circuit protection). But for things indicative of a fault in or near the transformer (pressure, oil temp or differential zone covering the unit), I'd deenergize the whole thing.

 

[Ohmly]

Have you asked the contractor to explain why he thinks there is no need to trip the breaker.

This could be a simple mis-interpretation.

 

[slavag]

Hi.
From my point of view, you MUST open both CB, from both side of Xfr in case of internal Xfr fault.
Isn't importnat from were you feed this Xfr.
For the oil temp. trip possible check option open only LV side, maybe is only overload situation or problem with cooling system.
But not buchholtz, pressure, etc. protections.
Best Regards.
Slava

 

[madomankh]

Hi guys,
Thanks so much for your valuable replies.
You know the case is a cost issue because the transformer is around 3KM away from the upstream MV circuit breaker and the contractor should lay control cables to send the alarm and trip signals.

Thanks

 

[slavag]

Hi.
It's other issue.
What is a size of Xfr?
I think is more easy for all, add compact GIS or AIS cubicle near to Xfr.

Or, send signals by optical cable.

Good Luck.
Slava

 

[madomankh]

The XFR is 500KVA.
Any way you think that the optical cable will be reliable to cary protection signals or it is recommended to carry the protection signals on hard wires.

 

[slavag]

Hi.
I think for such size of Xfr very easy install additional cubicle near to transformer, some copmact version, it's cheaper.

3km of HW cables...for my pinon is problem, I had lot of problems with long control cables ( about 1.5 and 2 km) EMC!!! and mechanical damages. Such cable isn't controlled.

BTW, 3km of shielded cable is also costly, with installation.
What is a price today, about 10$ per meter, 3000x10=30000$, of course your contractor wouldn't install 3km of cable.

Think about additional cubicle with CT and VT, with metering and protection. Something like to ABB with REJ603 or SE with VIP300, no needed aux power supply.

Best Regards.
Slava

 

[DTR2011]

anyone here think this is a bit much for a 500 kVA xfmr?

 

[madomankh]

Dear smallgreek,
What do you mean by a bit much?
Do you thing that because the xfmr is 500KVA that is mean we don`t need to give it the respect.

For me I think the protection for the lower elecrtrical equipment sizes is very important like the huge one.

Or if any one has different opinion he can provide us with the referance and code which state that we can neglect the protections for low transformer sizes.

That is my main question from the begining of this posts

Thanks

 

[DTR2011]

Hi madomankh,

I mean do disrespect to your unit. I was thinking that $30k would be a bit expensive for a transformer not worth that much in the first place. We often times have units these size which are protected by fuses only, as units this size are generally sealed (Pad mount). Of course, if this was a critical unit, things would be different.

 

[richanton]

I no longer have access to this code, but in USA, I think ANSI C57-12 lists requirements for liquid-filled transformers.

 

[rbulsara]

I agree with smallcreek, this is an overkill for a 500 kVA transformer. Hire a good engineer or talk to some seniors in your organization. It is not a matter of respecting the transformer but justifying cost issues.

There should be local fuses. In the USA, up to 2500 kVA transformers nothing more than fuses or a breaker is justified or used.

Your sub may be thinking or meaning the same, but not conveying his thoughts correctly. Still opening just the secondary breaker is of no use.

Rafiq Bulsara

http://www.srengineersct.com