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

 

[Andy32821]

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

 

[dpc]

Andy,

You can change from 500 A to 600 A rating plug only if the conductor is adequately sized for a 600 A breaker.

 

[waross]

slavag
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

 

[davidbeach]

Where are the breakers in question located? The assumption seems to be that they are at the source of the circuit, in which case the concerns about conductor sizing would apply. If the breaker is mounted on the transformer as a local disconnect/protection and the cables are adequately protected by something at the source end of the conductors, you can go as high as you want while still complying with the transformer protection requirements of the code. If the source breaker meets those requirements, the local breaker could be most anything. If these are at the transformer, my guess is that someone specified the current limiting as an attempt to provide coordination with the upstream device, but in doing so introduced a bit of over tripping.

 

[slavag]

Exactly, Bill, exactly.
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

 

[Andy32821]

Hi Guys


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

 

[dpc]

Your 264 kW of losses works out to 3.5 kW per transformer. This seems a bit high for core loss only on a 300 kVA unit, but I assume you have the core loss data.

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.

 

[stevenal]

DPC,

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.

 

[dpc]

By definition, any interrupting device considered "current-limiting" by UL must CLEAR a fault prior to the first peak current. This will be within 1/2 cycle. The only way a fuse can be current-limiting is by melting.

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.

 

[moreause]

ok will make it short

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)

 

[Bronzeado]

Hi folks,

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

 

[itsmoked]

Welcome to Eng-tips bronzeado.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Bronzeado]

Thanks, Keith!

H. Bronzeado