Overload Protection for Transformers

[HamidEle]

I might be asking a silly question,

We have a transformer, which is fed from 4160V systrem. On the primary, we have fuse and relay to provide Short circuit and overload protection respectively. On the secondary, we have a circuit breaker(600V) to provide short circuit and overload protection. However, the primary relay is not large enough and overload trip will occur. I am thinking of disabling it since downstream Circuit breaker can provide Overload protection as well, also this is only short period of service, which is about 15mininutes. Any comments would be appreciated.

 

[Marmite]

In principle it sounds OK to me. It would be unusual in any case to trip the primary circuit breaker for an overload on the LV side that can be cleared by tripping the LV circuit breaker. Are you sure that the primary side relay is providing overload protection though? What are the curve and settings?
Regards
Marmite

 

[HamidEle]

Yes, Marmite,
Both primary and secondary sides are providing O/L protection. We set primary settings higher than secondary settings.

 

[alehman]

If this installation is governed by the National Electrical Code you should take a look at Article 450. Primary fuses must not exceed 300% of full load current.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[waross]

You must have short circuit protection on the primary of a transformer. Overload protection may be applied on the secondary side only.

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

 

[slavag]

Hi.
Not needed.
Maybe.. possible only connect PT100 on the transformer ( if is dry) to some monitoring, like to DGPT.

Best Regards.
Slava

 

[oldfieldguy]

It would work. It could be justified as a temporary fix to get around an equipment failure while you wait for replacement parts, etc. That's a compromise to be made "in the heat of battle", so to speak.

I would not choose it for a permanent installation.

In the terms of cost, if you're already forking out money for a 4160 volt installation, the added cost a of a dedicated control power transformer (CPT) is relatively small.

If you're modifying aging equipment to upgrade protection and control, it makes no sense to add several bad failure modes by using a PT to do the CPT's job.

System protection is not well served by compromises of this depth. Most of us in system protection tend to be conservative because most of us are still remembering horror stories that start with "that should be good enough" and "I never expected THAT to happen."

old field guy

 

[jghrist]

The fuse and relay on the secondary do not protect for a transformer internal fault.

 

[waross]

Hamid:
By CEC rule #26-252(4) and Table 50 you can use fuses up to 200% of the transformers FLC if the transformer impedance is between 7.5% and 10%.
If the transformer is less than 7.5% impedance you may fuse the primary at 300%.
This is contingent on proper overload protection on the secondary side.
Again from Table 50, For a breaker that is not more than 250% of the rated current of the transformer.

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

 

[HamidEle]

Thank you guys for all the inputs. It is justified to diable this primary overload protection as a temperary means. But we will have to have the primary O/L protection since the primary fuse rating is higher than 200% FLA.

 

[waross]

Hamid:
If you want a definite answer, you must tell us:
A> KVA?
B> Wet or dry?
C> Setting of secondary breaker?
D> % Impedance?

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

 

[alehman]

... and what codes apply here.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.