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

 

[jghrist]

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.

 

[madomankh]

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

 

[slavag]

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

 

[racobb]

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

 

[jghrist]

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?

 

[slavag]

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

 

[madomankh]

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.

 

[waross]

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

 

[Freeportpower]

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 Thrower.pdf

FPP

 

[jghrist]

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.

 

[ausphil]

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

 

[IndianTechEngg1001]

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