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Farhad Yazdi
10-09-2013 01:31 AM

Replace transformer's circuit breaker with fused disconnector switch

Can we replace transformer's circuit breaker with fused (fusible) disconnector switch and use the thermometer of transformer to protect transformer against overload?
What is the shortcomings?
Suppose the fused (fusible) disconnector switch can be operated by thermometer's trip command.
How is the situation in your country?
10-09-2013 10:52 PM
Top #2
Alan Maltz
10-09-2013 10:52 PM
What is the rating range of the transformer that you are trying to protect? I can only assume that the circuit that you are protecting is non-essential or inside a facility where there are maintenance personnel 7x24, otherwise if the fuse blows you will be out of service until it can be replaced. A breaker can be reset much quicker and easier.

I assume you are referring to the Hot-Spot thermometer. I have seen it as a two stage process, first Alarm below your desired trip point, then Trip via protective relaying. The Alarm provides a warning that the transformer loading should be looked at first, most operators don't want to simply trip without warning, unless of course the rate of rise is too fast. I have also seen the trip manually blocked in some emergency situations where it is critical that the transformer stay online as long as possible despite the temperature rise.
10-10-2013 12:53 AM
Top #3
Aijaz Hussain
10-10-2013 12:53 AM
What is capacity of transformer in KVA or MVA as few years back Transformers having capacity upto around 10 MVA were protected through Liquid Fuse but were not much reliable & also achieving coordinated protection was an issu, but now a days CB has taken its place together with relay actuation. Protecting a transformer through thermometer for overloading is not recommended..for how much time you want to allow a transformer in overloaded state? any mismatch will either cause early tripping or late tripping and both are unwanted.
10-10-2013 02:57 AM
Top #4
Robert Beltz
10-10-2013 02:57 AM
Well, it all depends on the transformer. If this is a 225kVA with a 480v primary, yes, the fuse is fine. In fact fused primary is typical up to 2500kVA with up to 13.8kV primary. However, larger transformer cost justifies primary protective relaying and circuit breakers.

The primary voltage and transformer size dictate protection. As far as thermal, this is also reasonable. However I have used a water spray with cooling fans on the radiators to cool on more than one occasion. Remember, secondary protection will do more for thermal than primary.

I hope this is of some value.
10-10-2013 05:43 AM
Top #5
Gary Fox
10-10-2013 05:43 AM
Winding temperature indicators on liquid type transformers respond to a combination of both the liquid temperature and the heating produced by a resistor in the indicator well that is connected to a CT montioring one phase current. So this indication is an approximation and not necessarily indicative of the true hot spot temperature in a transformer.
10-10-2013 08:02 AM
Top #6
Jozef Szmigielski
10-10-2013 08:02 AM
I can say we can. Without explanation. For detail explanation the price is $15000.00
10-10-2013 10:12 AM
Top #7
Arthur Simplina
10-10-2013 10:12 AM
Why would you do that? Is this due to financial reasons? Remember that transformers are very expensive equipment (not considering the loads connected to this).
10-10-2013 12:51 PM
Top #8
Randy Beltz
10-10-2013 12:51 PM
I suggest that you give great consideration to the advice that Mr. Maltz and Mr. Robert Beltz provided. A fused disconnect is the most basic form of protection and although it will work to protect your transformer it does not generally have the ability to work in conjunction with temperature indicators/monitors. Also, as Mr. Fox from GE indicated and Mr. Hussain further explained, the winding temp indicator, as a result of the way it derives its information, only provides an "average" of internal temperatures and is not recommended as a way to determine whether a transformer should, or should not be taken offline. As Mr. Simplina indicated, transformers (assuming we are talking about transmission or distribution class transformers) are very expensive and not quickly replaced. A well designed and implemented protection schema should include various forms of monitoring and related responsive devices and tactics (either vacuum circuit breakers or less preferably, fuses) on both the primary and secondary of your transformer. Anything less and you jeopardize both continuity of service and the life of the equipment. Good luck.
10-10-2013 03:25 PM
Top #9
John Heidenreich
10-10-2013 03:25 PM
I agree with all of the gentlemen who recommend CBs instead of the fused disconnector system. If you do use disconnector, how are you going to turn the transformer off for maintenance purposes? If you want to use the disconnector to turn off the transformer, you must have the disconnector installed with flicker blades to help extinguish the arcs. So you will need to add the cost of flicker blades to your disconnector solution when you do the sums.
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