Breaker sometimes trips on transformer inrush current 7

[Andy32821]

I have a technical question. I'm hoping you can help me out.

At the local convention center they have seventy-five 300 kva GE K Factor transformers for temporary show power.

Each transformer is protected by a GE 500 amp SGLA36AT0600 current limiting circuit breaker.

These transformers have no loads for days at a time and the convention center would like to turn the transformers off during downtime.

(They are unloaded during switching and installed in an air conditioned tunnel).

This could save about $500 a day on the power bill.

I have reports of the breakers sometimes tripping when the transformers are re-energized. (I assume this is inrush current at a bad time in the cycle.)
I can find no problems with the transformers.

My Question:

Will re-energizing the tripped breaker two or three times in a row damage the breaker or damage the transformer?

Is it ok to use this breaker to turn this unloaded transformer on/off a few times a week?

Thanks,

Andy

 

[EEJaime]

Andi32821,
You did not specifically state wether or not the transformers are loaded when they are re-energized. This has been assumed by many of the posts above, but it is my experience that at conventions, lighting shows, etc..., these units are randomly loaded, and they are assigned to any number of exhibitors whom will set up their shows at various times in multiple stages. It is definitely possible that some of these have pre-wired displays or facilities which will be energized in one step, but I doubt that a large percentage would be. Also, when taking down the shows, it is common for the exhibitors to tear down prior to de-energizing the breakers.

Am I off base in describing how your facility operates? I have not seen these temporary power feeds used very often to energize or de-energize a fully loaded convention floor. The "down-time" you discribed in your OP I am issuming is between shows. Or are you talking about switching these off every night during a show's run?

In any case the previous posters have provided very good insights as to what may be causing the tripping if in fact the breakers are switching on a maximally loaded transformer.

 

[itsmoked]

EEJaime I suspect that they would be shut down whenever there are multi day gaps between shows. When no one is tearing down or setting up. Hence the 50 times a year.

So I suspect that these transformers are rarely loaded up when they get turned on because they would be turned as any setup commences.

I thought that the inrush was greatly exacerbated by a specific loading. Like unloaded is a bigger problems than loaded. Can someone enlighten me?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Skogsgurra]

We had a discussion on this one or two years back. I was assuming that the switch-on time was the only parameter causing the problem with high in-rush current.A German guy told me that remanent magnetism also was important. I did some measurements and realized that he was right. I then tried a simple degaussing method (adding a capacitor to the transformer secondary so that the damped oscillation carried flux down to zero) and then was able to demonstrate that my former very high in-rush current was non-existant.

If your transformers are single-phase, it could be an easy answer to the problem. Add a suitable capacitor - probably in the 10 - 100 uF region if the transformers are 300 kVA and see if that helps. Try it out on one transformer before ordering for all of them

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Skogsgurra]

Found it! Have a look at

http://www.eng-tips.com/viewthread.cfm?qid=144465&page=1

There are recordings showing inrush without and with the degaussing capacitor.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[stevenal]

"Is it ok to use this breaker to turn this unloaded transformer on/off a few times a week?"

I think he made it clear there was no load.

I never heard that unloaded may be worse. Load adds to the total. The 12X FLA rule includes (cold) load, so that you may be able to get away with 11X if energization is always done without load. Since always is tough to guarantee, stick with 12X or more.

 

[davidbeach]

Load will in general have less impact on inrush than the exact phase angle when the transformer is closed into. What load will do, though, is to provide damping to reduce the inrush sooner. An unloaded transformer will ring far longer than a loaded transformer, but if the transformers at issue are tripping on instantaneous then load probably doesn't matter, and with out load you do avoid any through current adding to the inrush current.

 

[stevenal]

OP says this is a current limiting breaker. Regarding current limiting fuses, Cooper says to consider 25X at 0.01s in addition to the 12X at 0.1s due to their speed. This breaker must also be very fast to limit the current.

 

[davidbeach]

Good point, stevenal. I'd missed that; I'm surprised they only trip occasionally. Time to replace the breakers with non-current limiting as this is not a good application for current limiting circuit breakers (or current limiting fuses for that matter).

 

[waross]

Hello davidbeach.
I understand that the breaker ratings are
Current limit x 25
Instantaneous x 10
I understand that the breaker is going out on instantaneous.
If it goes on current limit, then it may be necessary to replace internal fuses.
If the breaker uses a rating plug, it is probably a 600 amp breaker with a 500 amp plug.
The rating plugs I am familiar with are changed from the front without dismounting the breaker. Similar to changing an E-Prom.
See

http://www.geindustrial.com/cwc/Dispatcher?REQUEST=PRODUCTS&pnlid=3&famid=8&catid=9993&id=cb-pd-et

 

[waross]

A star Gunnar, and a thank you for this and all the other tips I have learned from you on the forum.
Thanks
Bill

 

[dpc]

I believe these are non-fused current-limiting circuit breakers. At these current levels, I doubt there is much current limiting going on.

600 A trip would be about right for a 300 kVA 480 V primary. 500 A is just too small. The extra 100 A of trip rating would provide another 1000 A of instantaneous setting range.

 

[slavag]

Hello All.
I miss something in this thread.
Pleas don't LOL on me, MCCB isn't my field.
SGLA36AT0600 type, I assume 600 is mean 600A, OP state is 500A,why?
Second, why we decide that was inst. trip, Andy wrote only about some trip not about kind of trip.
Next, I assume that this CB have some regulation of inst. trip setting, may be now it's on low level position.
I remember KM NZTM series, it was range from 2 up to 10 In. ( or something like to this)
I don't understand where installed this CB, on primary side or on secondary side, what is voltages on primary/secondary side?
What is type of load connected to this trafo?
Sounds me, that Andy open only secondary side.
For my curios only, 500$ per day saving on non loaded transformer 300kVA. non loaded transformer have only losses.
How, 1%, 5%, 10% ( it depend)?
What is a price of 1kW- half $, one $?
20.8$ saving per hour.
Regards.
Slava

 

[slavag]

Sorry, or it's 20.8$/75 trafos.

 

[prc]

Let me summarise the issues with magnetising in-rush current in transformers considering the above discussions and also the thread pointed out by Skosgurra:


1) Transformer will be switched off when current wave is at zero,including excitation current.So depending on Pf of load, voltage also will be near to that point in the wave with flux wave at near to maximum level.This flux ( 70-90 % of normal flux) will remain in core for several weeks till withering down completely.When such residual flux is there in core and if the transformer is switched at the voltage wave zero level,flux has to be at the peak level.So flux start build up over the residual flux.So this will be maximum( also excitation current) when switched at zero voltage at start of positive cycle( earlier switched off at positive cycle) or at zero of start of negative cycle( if earlier switched off at negative cycle).Inrush current ( 12-15 x FL current in case of small trfs and 6-10 times FLC in large units)magnifies from normal excitation current by 140-1000 times depending on the size of transformer for the same worst conditions of switching off and ON.So inrush current as times of FLC depends on the size of transformer too.


2) This high inrush current is generated to drive the excess flux ( as core no longer can contain it due to saturation) through the air core inductance of the winding.
So inrush current will be higher if transformer is energised from the winding nearer to core ie LV winding


3) Inrush current is not affected by the grade of silicon steel used in core.Saturation flux density is almost same for all grades of cold rolled silicon steels.
Similarly it is not affected by the flux density adopted in the core.

4) Inrush current will be increased if energised on secondary loaded condition.Increase will be to the extent of load.

5) In three phase transformers,it also depends on the connection of windings.In a YY connection, inrush will be 60 % of value during Dy connection.

6) Before the advent of controlled switching,the only way with EHV breakers to control inrush current was to use a closing resistor in the breaker.This was bypassed after voltage build up.

7) One recent problem found with large transformers is the inabilty of differential relays to stop from operating during energisation.As I said in my earlier posting,restraint in relays is incorporated by sensing the second harmoinic content in inrush current.In modern large units,this is some times less tahn 10 % compared to 15-50 % in earlier days.

 

[burnt2x]

I agree with slava on the CB sizing, it's too small for the trafo size and 600A could solve that!
prc, I have experienced the same unwanted CB trips long ago on my previous employment on a 1MVA SST. Diff prot actuates on energization and we tried "de-sensitizing" but still diff prot picks up sometimes! Like davidbeach said, I loaded up the trafo before energizing, no more trips!

 

[jraef]

SGLA36AT0600 type, I assume 600 is mean 600A, OP state is 500A,why?

The 600 in the part number means it is a 600A frame. The frame needs a rating plug to make it work which can be anything from 250A to 600A. So in the original post, he stated it was a 500A breaker, which would mean it had a 500A rating plug installed in it. In these GE breakers, the rating plug sets both the long time thermal trip (which is the 500A rating in this case) plus it also provides the setting range for the instantaneous magnetic trip. The range is essentially 3x to 10x the rating plug value. For example, on the 500A rating plug, the instantaneous magnetic trip range is 1525 to 5060A. He later mentioned that it was set to maximum, which means 5060A, but it was still tripping. That then means that (assuming a transformer FLC rating of 361A) his inrush current is occasionally in excess of 14x FLC which is unusual, but apparently possible according to the info provided by prc above.

He can change the rating plug to a 600A unit so that the the adjustment range will increase to be 1830 to 6075A, so if set to maximum it will allow up to almost 17x FLC. However, it will come with the 600A long time trip setting, which is what the cable ratings are based on. So he can only do this if his cables are rated for 600A.

 

[itsmoked]

magnifies from normal excitation current by 140-1000 times depending on the size of transformer for the same worst conditions of switching off and ON

Is this correct? Not sure I'm following this..

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[prc]

itsmoked, Iam sorry .I made a mistake.Thank you for pointing out.

In modern transformers normal excitation current is 3-0.5 % FLC for distribution transformers and 0.05-0.5 % for Power Transformers(say 400-20 MVA) So inrush enhancement for DT will be 5-30 times only ( assuming 15xFLC as inrush)and for PT 200-100 times (assuming 10x FLC as inrush)

 

[slavag]

O.K.
Andy may be need change plug to 600A of course if his cable are rated for 600A. Same DPC was recommended.
PRC is right, for small trafos recommended calculate inrush 16xFLA ( for example for the HI protection).
But actaully same Q's are important: where installed those CB's and what are rating voltages of trafos.
Regards.
Slava

 

[Skogsgurra]

It is probably better, from a physics point of view, to relate the inrush current to excitation current. As prc does.

But it is not very practical when it comes to selecting trip levels or fuses. For such purposes, the inrush current related to nominal (rated) current is more common and also more practical.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...