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Transformer protection

Transformer protection

Transformer protection

(OP)
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   

RE: Transformer protection

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
www.cvoes.com

RE: Transformer protection

(OP)
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?)

RE: Transformer protection

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

RE: Transformer protection

(OP)
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

RE: Transformer protection

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

David Castor
www.cvoes.com

RE: Transformer protection

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.

RE: Transformer protection

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.

RE: Transformer protection

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.

RE: Transformer protection

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

RE: Transformer protection

(OP)
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  

RE: Transformer protection

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

 

RE: Transformer protection

(OP)
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.

RE: Transformer protection

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

RE: Transformer protection

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

RE: Transformer protection

(OP)
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

RE: Transformer protection

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.

RE: Transformer protection

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

RE: Transformer protection

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

RE: Transformer protection

You mention mechanical protection only.  Do you have overcurrent or differential protection?  If so, what do these trip?  If the overcurrent protection is on the source end of the 3 km feeder, and is set low enough to see all transformer faults, and there is no problem with tripping other feeder loads for a transformer fault, then this is probably enough for a 500 kVA transformer.
 

RE: Transformer protection

(OP)
rbulsara
I agree with you that fuses is enough for small rating transformer but this may be only for dry transformers.

But in the oil transformer can you explain if it is enough such fuses why the transformer manufacturer equiped the transformer with all the mechanical protection like (Buchholz,oil temp,saftey pressure,....).

You think only to trip the downstrem to shed the load and to keep transformer energized !!!!!!!!!!!!

jghrist
We have also
51, overload current
50 instantaneous current
67N ground directional

But also,
If the electrical protection is enough why we use mechanical protection??????

I think transformer internal faults is also important and need to be protected.

Thanks for your feed back

really i wanna to understand



   

RE: Transformer protection

Hi.
Electrical is electrical protection, mechanical is mechanical.
Isn't covered one by second, maybe can say, differntial protection covered buhholtz, but also isnt correct. But differential protection not used for 500kVA transformer.

Dry transfrmet is also not include only electrical protection, for example it include protection based on Pt100.

today, with low prices of RMU, I don't see any benefit of fuses used.

Check price of RMU with CB.

Best Regards.
Slava

RE: Transformer protection

Never even have considered putting that much protection on a 500 kVA transformer.  We usually use that kind of protection to limit the damage to an expensive power transformer, and in relative terms this one is not.  If you have an internal problem in the unit, chances are it may well cost more to repair than it is worth.  Not necessarily the case with a large power transformer if damage can be minimized and is worth the effort.  And not to mention the down time you will have while a 500 kVA unit is being repaired, so you will probably have a spare transformer anyway.

You probably have 20+ amps at full load on the primary, so unless single phasing is a big concern, I would not consider a breaker.  You could also use current limiting fuses.

I would fuse it, load it up, and have a spare available.

Alan

RE: Transformer protection

Are the 50/51/67N on the primary or secondary?  What do they trip?

Are there any other loads on the 3 km feeder other than the transformer?

RE: Transformer protection

jghrist ask a very right Q's!
What about protection sensetivity and selctivity?
Are covered SC on the secondary?

jghrist, maybe we can recommned a very old type of protection, grounding switch on the primary, closed by any protection devices and 67N will opened main CB.

Best Regards.
Slava

RE: Transformer protection

(OP)
The 50/51/67N on the primary and trip the transformer upstream breaker.
and there is no-load except the transformer in the 3km side.  

RE: Transformer protection

There are probably over 1,000,000 single phase, oil filled, distribution transformers in North America 1n the 7.5 KVA to 100 KVA range that are protected by fuses. Some have some type of overload breaker or indication. Many are protected by fuses alone.
There has to be a lot of 500 KVA that have only fuse protection or fuse plus overload protection. The extra protection that you have will not prevent transformer faults, it will just detect them sooner. Then what do you do?
1> Just buy and install another transformer for less than the protection package cost.
2> Send the faulty transformer to a shop to find out that it does have a fault but it will be cheaper to replace the transformer than to fix it. More money wasted and more time wasted.
3> What often happens with a transformer that size is that it will be re-energized several times to see if it really should have tripped offline. If it could have been repaired after the first trip, regardless of cost, it probably can't be repaired after several re-energizations.
What is the cost of 3 kilometers of control line relative to the transformer cost?

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Transformer protection

Madomankh,

Are you saying that you have a 500kVA transformer and the point of isolation is 3km away ?. Doesn't this raise some health and safety issues in terms of isolation for maintenance ? As a minimum I would have expected a remote transformer like this to be fitted with a throat mounted or free standing/pole mounted HV switch/RMU in which case this could be shunt tripped by your Buchholtz, REF or overtemperature protection. Alternatively theres always the good old fashioned fault thrower !!  http://www.hss-ltd.com/assets/files/Fault%20Thrower.pdf

FPP

RE: Transformer protection

Quote:

The 50/51/67N on the primary and trip the transformer upstream breaker.and there is no-load except the transformer in the 3km side.
Are the overcurrent relays at the transformer location or at the source end of the 3 km feeder?  If the relays are at the breaker, why can't the mechanical protection trip the breaker by the same method as the overcurrent relays?  

Set the 51 relays to see secondary fault currents and coordinate with the low side overcurrent protection.  This will provide protection for internal faults.  Set the 50 relay above maximum secondary fault level and below primary fault level.  Set the 67N sensitively, assuming a delta-wye transformer, because it will not see any secondary load or fault current.  IMHO, it is not worth the expense to install a circuit to trip the upstream breaker with the mechanical protection.  The cost of the transformer and the probability of a fault that will be tripped by the mechanical protection but not by the overcurrent relays are not high enough to justify the expense.

  

RE: Transformer protection

There is a lot of technical discussion of value in this post.

My first question gets back to contract mananagement, and why you are letting the tail wag the dog.  You should be specifying to the "sub" what you want, and if he has better suggestions, then he should have to prove to you that it is valid, not the other way around.

Many good points have been made as to more standard ways of protecting transformers, especially of this size.  To ask the question of "why the transformer manufacturer equipped the transformer with all the mechanical protection like (Buchholz,oil temp,saftey pressure,....)." when asking if fuses are enough, is a little naive.  The transformer manufacturer will sell you what you specify.  If you want all those bells and whistles on a transformer, they will gladly supply it, because they will make a percentage profit on whatever you ask for.

We have 10s of thousands of oil filled transformers being protected by fuses, successfully for many decades now.  A 500kVA transformer is a critical part of the network from a reliability point of view, but it can be very easily protected, and if it happens to fail, it can be very easily replaced at very little cost - in fact, the cost of the transformer will probably be outweighed by the cost of isolation, installation, testing and restoration.

As smallgreek said, no disrespect, but you've got to decide your protection regime, your risk level and how you can meet this.  Don't forget the extra maintenance costs that you will require for any additional protection systems, and you also need to count the cost of a maloperation of your bucholtz system as part of the overall running cost of the system.

ausphil

RE: Transformer protection

To show a proof please refer code ref. NESC 2007 Clause 153

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