lighting transformer protection 1

[SELEC]

Hi, Guys

I am doing the coordination study for 600V system. What is bothering me is, it is very hard to coordinate between Transformer damage curve with MCB & inrush current, which is very high. According to codes, primary CB shouldn't set higher than 125% FLA, because we don't want dry type Transformer to be over loaded, in any case. I am wondering if there is a good solution for this.

Thank you in advance.

 

[alehman]

At 125%, coordination with the inrush will be difficult for a typical MCCB. Per the NEC, if you have secondary protection set <125%, you can set the primary as high as 250% which gives you more flexibility.

What are you using for inrush time/current?

When coordinating with the damage curve, sometimes you must sacrifice some low-current (overload) protection on the primary and try to cover it with the secondary protection.

 

[davidbeach]

Good luck. The smaller the transformer the harder it is to do much that is very useful in terms of protecting the transformer to the damage curves.

While not doing anything to violate code, the first given is that you have to not trip on inrush, but getting a good number for inrush is not easy. I always used 10x @ 0.1s and never heard of a breaker that wouldn't hold for inrush. Next thing to keep in mind is that nothing is gained in most situations by having coordination between the primary and secondary breakers; it doesn't matter which one trips, the outage is the same in either case, and if they both trips there isn't any more outage. Pushing the primary breaker to the maximum allowed by code increase coordination with the secondary and provides more clearance at the inrush point, but worsens the protection of the damage curve, but the damage curve is for through faults (internal faults do lots of damage really fast) so the secondary main can protect the transformer against all through faults except those in the 25 feet (or less) of conductor between the transformer and the breaker.

The NEC does not address protection of the transformer from the standpoint of the damage curve, so not protecting it does not create code issues if the NEC is your governing code, other codes may vary.

If you are stuck with thermal-magnetic trip units, you might as well give up all hope any elegant solution at this point. If you can use solid state trip breakers on both sides of the transformer, you can do far better than you could with thermal-magnetics. If you want to do a real knock-out job, use protective relays on shunt-trip breaker and ease the trip curve in just where it needs to go. May cost more than the transformer to implement a relay scheme, but you will get far better protection and coordination; I never found it worthwhile to even think about it on any project though.

Unless there is some super critical process being handled by the transformer, just do the best you can with the available choices and let it go. Lighting wouldn't qualify in my book as super critical since there will always be a separate source of egress lighting even if the transformer is toasted.

As alehman says, at least if you are bound by the NEC, you can push the primary overcurrent up well beyond 125% is you have secondary overcurrent, and you couldn't feed a lighting panel, per the NEC, without a secondary main.

 

[SELEC]

Thank you for the valuable inputs. Actually I did set up primary CB at 1.25 FLA according to CEC and NEC. we won't allow dry type transformers to be over loaded by arranging the load on them. I also sized CB to avoid tripping on the inrush current. It is not worthwhile using a shunt relay to protect the tranformer. I think that's the best I can do.

 

[RalphChristie]

Maybe a little off-track:


You can not protect a transformer against overloading with a overcurrent relay. Overcurrent protection is used entirely for the clearance of faults, although with the settings usually adopted some kind of overload protection may be obtained.

Overload protection should be done by oil and winding temperature devices, or relays that have similar tripping characteristics to the termal constant of the transformer.




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[waross]

Hello SELEC
I realize that this is of the topic of coordination, but if you want to protect the transformer based on the damage curve it may be of interest.
What size of transformer are you planning? Is this the main service transformer or lighting transformers throughout the plant.
Overloading and Overcurrent:
Overloading is a load current in excess of the transformer rating.
Overcurrent is used to describe short circuit conditions.
Dry type transformers are rugged and dependable. The smaller ones used for in plant use typically have no additional protection other than the code requirments.
An overload on a transformer results in an exponential loss of expected service life. Overloads on a transformer are often a creeping phenomena. Over time, additional loads are added and each load pushes you farther up the "Loss of service life" curve.
On a large transformer consider a temperature alarm, or overcurrent alarm. This can give you warning of overload conditions when you still have time to take preventitive action and avoid unexpected outages. A small cronic overload may shorten the life expectancy of the transformer from 20 to 15 years. An alarm at this point may be a better choice than waiting for an overload to reach the shutdown level.
I concur with davidbeach
yours

 

[davidbeach]

Per the NEC, a lighting transformer will have secondary protection, so your primary protection, by code, can be 250% of FLA.

 

[SELEC]

Our lighting tranformer( 45kvA) dosen't have secondary protection, because it secondary feeder is oversized, which is allowed by code. As davidbeach said, it is not worthwhile to use relay to protect such a small transformer. We decided to reduce the load on the transformer to avoid the overloading of the transformer.

 

[davidbeach]

Please explain what you mean by

Our lighting tranformer( 45kvA) dosen't have secondary protection, because it secondary feeder is oversized, which is allowed by code.

particularly if you are using the NEC, please cite the specific section. Having spent about 19 years doing that type of work, that is a concept I never heard of or say in the code.

 

[waross]

Hi david beach
SELEC may be in Canada. With appologies, my old (1998) Canadian code allows this practice. CEC 1998, Rule 26-256
I understand that NEC mandates 125% overcurrent protection on the secondary. CEC allowed 125% protection on either the primary or secondary. If the protection is on the primary, the secondary conductors must be rated 125% or more.
If the secondary is protected at 125% the primary breaker may be omitted if the feeder is protected within similar limits to NEC. eg; 300% vs 250%
Allowing for my very old library.
respectfully

 

[SELEC]

Yes.I am in Canada.CEC 26-256 allows 125% protection on the primary side.The secondary conductor is 50% oversized and it is less than 5 meters run.