Breaker sometimes trips on transformer inrush current
Breaker sometimes trips on transformer inrush current
(OP)
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
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






RE: Breaker sometimes trips on transformer inrush current
For my pinion, no problem (for both Q's).
Problem only, why CB triped?
Regards.
Slava
RE: Breaker sometimes trips on transformer inrush current
RE: Breaker sometimes trips on transformer inrush current
RE: Breaker sometimes trips on transformer inrush current
You also did not state the type of protection, but I would expect time and inst. over current. It is possible that they could not be set properly for inrush.
Your other question about closing and tripping breakers several times, there is a wear and tear factor anytime you open and close breakers, no matter what type of breakers. So to operate them more will require more maintenance or replacments as time goes.
My thoughts are you should see what is behind the difficulties to close issue, and the $500 a day seems significent, so investigate the cost of maintenance, and make a compairson. In any event, operating the breakers like that will require more maintenance, and eventual replacments, so the maintenance group needs to be up to the task.
RE: Breaker sometimes trips on transformer inrush current
I learned something about MCCB and IEEE requerements.
Regards.
Slava
RE: Breaker sometimes trips on transformer inrush current
RE: Breaker sometimes trips on transformer inrush current
The transformers can get disconnected while on, say, the peak negative of the cycle. This leaves the core magnetized in that state. Then later someone closes the breaker while the negative part of the cycle is in effect. Now the flux continues to build and the transformer promptly saturates. Current skyrockets 12X is not uncommon. Breaker trips.
If you can monitor or control when you turn OFF the breakers so the transformer shuts down unmagnetized this wouldn't be an issue.
Anyone heard of a device for this?
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Breaker sometimes trips on transformer inrush current
It's done with large high voltage breakers in some situations, but it requires single-pole switching. I've never seen it done for an application like this.
For this voltage and breaker type, the practical solution is to size the breaker's instantaneous trip to ride through the maximum inrush. These types of problems are errors in design or in establishing proper breaker settings.
RE: Breaker sometimes trips on transformer inrush current
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Breaker sometimes trips on transformer inrush current
The transformer voltages aren't stated, but assuming 480V, I'd have sized the breaker at 800A, with an instantaneous setting of 8000A, that 500A is rather small. I'd expect peak inrush currents between 4300A and 5400A and that 500A breaker won't let 5400A through without tripping.
RE: Breaker sometimes trips on transformer inrush current
Transformer in rush current always contain 10-60 % second harmonic component depending on the angle of voltage at which switching is done.In differential relays, harmonic restraint feature is provided( ie relay will not respond when there is second harmonics in the over current )to avoid maltripping during energisation.I dont think overcurrent relays can incorporate that feature.For such small breakers the only solution seems to go for time delay with higher instantaneous ratings as David mentioned.
RE: Breaker sometimes trips on transformer inrush current
RE: Breaker sometimes trips on transformer inrush current
I heard about this device ( I think made by Sweden).
Could you please send any link.
Regards.
Slava
RE: Breaker sometimes trips on transformer inrush current
RE: Breaker sometimes trips on transformer inrush current
I found.
Regards.
Slava
RE: Breaker sometimes trips on transformer inrush current
This breaker is a GE sprctra RMS electronic breaker but does not have the digital readout. Only an inst pot that is at max.
My main concern is damaging the breaker with resets. Since this is an electronic breaker I am hoping that resets are not a problem.
I assume that cycling the breaker 50 time a year to turn the transformer off when not in use will have little impact on the breaker's life. But I get myself in trouble assuming things.
As far as the tripping goes, I can change the 500 amp plug to a 600 amp plug if this will help. The transformers have primary and secondary protection. It's probably not cost effective to add any other hardware.
I will do more studies on whats causing the trips but I need to come up with a procedure on what happens when a breaker does trip. (A rare event).
The whole idea is to save money and at $500 a day a $3000 breaker is not a big deal. (However a $20,000 transformer might get the maintenance manager irked).
I am thinking of this procedure for the switching technician:
If the breaker trips when turning the transformer on then open the secondary breaker and try the primary breaker again.
If the primary breaker trips again wait 5 minutes and try once more.
If the breaker trips again leave it off and call an electrician.
Is this asking too much of the breaker or is this in the normal design duty of this style breaker?
Thanks for any help you can offer.
Andy
RE: Breaker sometimes trips on transformer inrush current
Try the 600 amp trips on one or two breakers. Knowing that there are inrush problems, I would try two or three resets before calling the electrician, however you are on site and it is ultimately your call as to how many times to retry.
The inrush is a fact of life and cannot be reasonably avoided. The breaker tripping is detrimental to the breaker and should not be happening. Step one is checking the settings. Step two is changing the rating plug. Step three is changeing the breaker. All transformers have inrush current. Transformer breakers should withstand the inrush without tripping.
Switching with no load on the transformer is probably a good idea.
respectfully
RE: Breaker sometimes trips on transformer inrush current
If the 500 A trips are at max instantaneous, you will need to look at larger breakers - since it is a 600 A frame, they can probably be changed out to 600 A, but only if the feeder ampacity is adequate.
Frequent tripping of a 500 A breaker on xfmr inrush would not be an acceptable situation for me.
RE: Breaker sometimes trips on transformer inrush current
I hadn't looked closely at the specs on that breaker before, but now I see that the 500A rating plug only allows adjustment up to 5000A; 10X the rating. A 300kVA transformer is probably around 361A FLC at 480V (you never said) so if the inrush is 13X, then the 5000A setting should have sufficed. But those "K-Factor" transformers may have different magnetic characteristics that end up pushing the worst-case inrush over the edge of that 5000A limit.
RE: Breaker sometimes trips on transformer inrush current
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.
RE: Breaker sometimes trips on transformer inrush current
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
RE: Breaker sometimes trips on transformer inrush current
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
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Breaker sometimes trips on transformer inrush current
There are recordings showing inrush without and with the degaussing capacitor.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Breaker sometimes trips on transformer inrush current
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.
RE: Breaker sometimes trips on transformer inrush current
RE: Breaker sometimes trips on transformer inrush current
RE: Breaker sometimes trips on transformer inrush current
RE: Breaker sometimes trips on transformer inrush current
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
RE: Breaker sometimes trips on transformer inrush current
Thanks
Bill
RE: Breaker sometimes trips on transformer inrush current
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.
RE: Breaker sometimes trips on transformer inrush current
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
RE: Breaker sometimes trips on transformer inrush current
RE: Breaker sometimes trips on transformer inrush current
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.
RE: Breaker sometimes trips on transformer inrush current
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!
RE: Breaker sometimes trips on transformer inrush current
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.
RE: Breaker sometimes trips on transformer inrush current
Is this correct? Not sure I'm following this..
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Breaker sometimes trips on transformer inrush current
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)
RE: Breaker sometimes trips on transformer inrush current
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
RE: Breaker sometimes trips on transformer inrush current
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
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Breaker sometimes trips on transformer inrush current
Thanks for the help everyone.
Sounds like if I prevent the trips the breakers should be ok.
I will try the 600 amp plugs where needed.
If I can capture a trip on a pqm I will post the results.
Thanks again,
Andy
RE: Breaker sometimes trips on transformer inrush current
You can change from 500 A to 600 A rating plug only if the conductor is adequately sized for a 600 A breaker.
RE: Breaker sometimes trips on transformer inrush current
Remember that these transformer tunnels are air conditioned. I assumed that $500 per day included the energy to remove the losses as well as the transformer losses.
respectfully
RE: Breaker sometimes trips on transformer inrush current
RE: Breaker sometimes trips on transformer inrush current
What I think about. That problem isn't transformer inrush, problem is load on transformers( for examples restart of AIR cond., chillrers, etc..). From time to time we have same problems on the MV side, solution is reconnection load
step by step.
Regards.
Slava
RE: Breaker sometimes trips on transformer inrush current
When the new building is empty the floor box transformers use about 264 kw of power to keep the transformers energized.
So (24 hours) cost is 264 kw * 24 hour * .08 dollars/kwh = $506 a day.
The kva is almost seven times the kw so this amount includes the cable copper loss as well as the transformer heat loss.
Figure 150 days the transformers could be off and the savings would be about $76,000/year.
Since 100% of this power is turned to heat another $15,000/year could be saved on HVAC. (This is probably a low number but the convention center purchases chilled water at a very good rate so I am just guessing on the HVAC)
The two parallel cable sets between the primary breaker and the transformer primary are 250 mcm XHHW so a 600 amp breaker should be ok. (Dry air conditioned tunnel). The longest run is about 100 feet so voltage drop is not an issue.
I should have mentioned before these are 480 to 208/120 three phase transformers GE cat # 9T23B3479G13. They feed only floor pockets for exhibitor equipment (no permanent loads are connected) and are only switched off and on when the building is empty and there is zero load on the transformers.
Please let me know if it looks like I have gone astray on my number crunching.
Andy
RE: Breaker sometimes trips on transformer inrush current
The 2 x 250 kcmil (copper, I assume) is good for 510 A, so you just sneak in to a 600 A breaker. The 90 deg C ampacity in the NEC Table 310.16 generally can't be used because the breaker is rated for use only with 75 deg cable.
I'd suggest changing out one or two breakers to see if the 600 A trip is going to work. You should pick one of the transformer that has been known to trip on inrush. You'll have to try it multiple times to account for voltage phase angle at closing.
RE: Breaker sometimes trips on transformer inrush current
My source Electrical Distribution System Protection third edition, pages 90 to 93. The example shows the choice of a current limiting fuse with a melt curve well above the 25X FLA at .01s. The point is that current limiting can occur due to inrush if one does not choose the right protection.
It is not clear from the the literature how the breaker in question limits current. If by speed of operation, it would look like an instantaneous trip.
RE: Breaker sometimes trips on transformer inrush current
A current-limiting breaker utilizes the inherent inductance and arc resistance in the breaker mechanism and its "blow apart" contacts to achieve current-limiting status. All molded case breakers are somewhat current limiting, but this is generally neglected in fault calculations. The breaker must clear the fault on its instantaneous trip to be considered current-limiting. Modern MCCBs have contacts that actually open before the breaker mechanism trips for high values of fault current.
So the current limiting effect does depend on the size of the fuse or breaker.
RE: Breaker sometimes trips on transformer inrush current
1-the problem with a breaker triping time to time, is one days it will trip for real and the operator will get it face blown or blow something...so it is better to solve the tripping probleme
2-i have seen i similar probleme were i customer had large tranformer but with low load,(money was not a issue so he went big to be secure) so inrush was quite high
3-it is a patch, but maybe you could had 1,2 switch along the circuit, to start less transformer
4-i'm not 100% sure but if you need higher inst setting
you can get an higher plug, and lower the overload setting
(if it's an electronic unit)
RE: Breaker sometimes trips on transformer inrush current
That's is my first time in this forum. I am a Transmission Electrical Engineer.
Have you heard about "transformer sympathetic interaction"? This phenomenon occurs when a transformer is energized nearby others transformers, prolonging the inrush current.
In general, it happens in electric systems with very high resistances.
In the Andy's case, the inrush current may be takes a long time to decay, with the protection (I think it is overcurrent)triping properly.
To solve the questions, I suggest firt recording the currents in the transformer being enegized and, also, in the transformer already in operation to see if the phenomenon I said happens.
Regards.
H. Bronzeado
RE: Breaker sometimes trips on transformer inrush current
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Breaker sometimes trips on transformer inrush current
H. Bronzeado