transformer and a tripping cb...

[emahler]

have a situation where a 125kw generator was installed in a facility...it feeds a 400A panel with a Siemens Sentron 400A main...480/277V

in the panel is a 200A sentron breaker feeding a 480/277V sub panel
a 125A sentron breaker feeding a UPS system

and a 100A 3P Siemens NGB feeding a 75kVA 480V Delta to 208/120V wye transformer.

The problem apparently occurs when switching from generator power to utility power, not vice versa...

switches to generator without a hitch...when it switches back to utility, it trips the 100A NGB breaker...

any thoughts?

 

[zog]

What are the settings? Done any power monitoring?

 

[hillbilly1]

This is very common, usually can be fixed by turning up the magnetic trip adjustment. Your problem though, is the 100 amp frame breaker probably does not have an adjustable trip. It is usually on 200 amp and larger frames.

 

[Jraef]

Siemens NGB 125AF has a fixed mag trip @ 9x In, +- 20%

Stiffer utility system feeds the transformer magnetizing inrush fully, whereas the genset supply self-limits just enough to keep from tripping the breaker.

If it's a manual change over, tell them to wait a second or two longer to make sure the transformer field is collapsed first. If it's on an ATS, see if you can increase the transfer delay. If you can't for other reasons, put a contactor on the transformer primary and close is a little later.

 

[emahler]

no settings on this breaker...and yes, turning up the trip settings on the 400, 200 and 125 amp breakers did stop the problem...

just spoke to a good electrician who suggested it may have something to do with the synchronization of the generator to the utility power in the transfer switch...would explain why there is no issue going from utility to generator, but only from generator to utility...

 

[emahler]

along the same lines as jraef...

 

[SG-1]

If the return to Utility is OPEN Transition, then synchronization would not be an issue, only inrush.

If the return to the Utility is CLOSED Transition, then synchronization is an issue. The difference between the two systems may need to be closer before the transfer is allowed.

 

[hurk27]

no settings on this breaker...and yes, turning up the trip settings on the 400, 200 and 125 amp breakers did stop the problem...

just spoke to a good electrician who suggested it may have something to do with the synchronization of the generator to the utility power in the transfer switch...would explain why there is no issue going from utility to generator, but only from generator to utility...

Ok i'm a little confused, you stated in your OP that this 75 KVA transformer is fed from a 100 amp breaker which is tripping when going back to utility right?

So tell me how turning up a 400, 200, and a 125 amp breakers solved this problem? makes no sense?

also a 75kva transformer is 90.21 amps + 25% = 112.76 amps, way to much for a 100 amp breaker, and if secondary protection is required you can go 6 time or 250% for the OCPD, Table 450.3(B), if not you need at least a 125 (next size up see table 450.3(B) note 1) amp breaker to be compliant.

 

[zog]

I can't belive "cramk up the settings" is ever seen as a acceptable solution. I shocks me every time zi see that. What you have done is mess up the coordination for this entire system.

How about finding out what the cause is and fixing that?

 

[hurk27]

I shocks me every time zi see that.

Arn't you waring your PPE's when you read these post?


kidding, but I agree, check the installation and code for the proper size breaker and type for a transformer, those little dials are not a "just turn it up if it blows wheel" good way to also burn something up, kind of like the "just put a bigger fuse in" back when we had fuses in houses. when we had a GFP for a theater tripping on a low level ground fault, we strickly told the manager we would not touch it until we had a setting number from an engineer to set it to.

 

[david luchini]

also a 75kva transformer is 90.21 amps + 25% = 112.76 amps, way to much for a 100 amp breaker, and if secondary protection is required you can go 6 time or 250% for the OCPD, Table 450.3(B), if not you need at least a 125 (next size up see table 450.3(B) note 1) amp breaker to be compliant.

I don't see the 100A c/b as being non-compliant. The 125% in table 450.3(B) is the MAXIMUM rating or setting. Nothing says it can't be smaller.

 

[emahler]

ok, I should be a little clearer -

1) we did not perform this installation
2) we have not touched this installation
3) we are doing a build out for this customer at another facility, and they asked that we look at the installation (apparently, they feel something is wrong)...

that being said - we did not do anything to this installation yet...

Ok i'm a little confused, you stated in your OP that this 75 KVA transformer is fed from a 100 amp breaker which is tripping when going back to utility right?

So tell me how turning up a 400, 200, and a 125 amp breakers solved this problem? makes no sense?

also a 75kva transformer is 90.21 amps + 25% = 112.76 amps, way to much for a 100 amp breaker, and if secondary protection is required you can go 6 time or 250% for the OCPD, Table 450.3(B), if not you need at least a 125 (next size up see table 450.3(B) note 1) amp breaker to be compliant.

I thought I wrote it, but apparently I missed it...the 400A and 200A also tripped...the only breaker that didn't was the 125A...so the customer's internal maintenance simply turned up the dials....

and I agree with David, 450.3(B) is MAXIMUM OCP, not Minimum...
case in point, build out we are doing, engineer has 3 - 75kVA xfrmrs going in, all primary protected by 90A cb...

I can't belive "cramk up the settings" is ever seen as a acceptable solution. I shocks me every time zi see that. What you have done is mess up the coordination for this entire system.

How about finding out what the cause is and fixing that?

How about that is what we are doing?....we haven't touched the system yet...now, any helpful ideas on why the breaker is only tripping on a transfer from generator to utility?

 

[markstg]

ok, I should be a little clearer -

1) we did not perform this installation
2) we have not touched this installation
3) we are doing a build out for this customer at another facility, and they asked that we look at the installation (apparently, they feel something is wrong)...

that being said - we did not do anything to this installation yet...




I thought I wrote it, but apparently I missed it...the 400A and 200A also tripped...the only breaker that didn't was the 125A...so the customer's internal maintenance simply turned up the dials....


How about that is what we are doing?....we haven't touched the system yet...now, any helpful ideas on why the breaker is only tripping on a transfer from generator to utility?

SG-1 in post #7 is where to start. Is it open transiton or closed transition? Or is it manual?

 

[hillbilly1]

I can't belive "cramk up the settings" is ever seen as a acceptable solution. I shocks me every time zi see that. What you have done is mess up the coordination for this entire system.

How about finding out what the cause is and fixing that?

We already know the cause, fixing it requires turning up the magnetic trip setting or Installing a larger compliant breaker. The magnetic trip adjustment on those breakers are not like turning it up on larger breakers where you know exactly the amount that it is increased. These have a dial that just has minimum and maximum, with no definete settings in between. How are you going to have an engineer determine exactly where the dial should be set? A smidge? A lot?

 

[emahler]

SG-1 in post #7 is where to start. Is it open transiton or closed transition? Or is it manual?

at this point, I do not know...It's automatic...but open or closed, it's unknown...

we'll be onsite to inspect it in a week or so, at that point we'll be thoroughly going over the system to try to find the cause of the problem...

 

[templdl]

You don't have a problem switching from normal to emergancy because you have made the transfer due to the loss of the normal sourse. All of the loads the generate counter EMF have decayed befire the transformer.
Not so when transfering back to the normal sorce from emergancy. You have a slightly out of phase transfer when you do so which will trip a breaker magnetically.
All that you need to do is to add a time delay neutral which will provide a short delay in the neutral position to allow the counter EMF to decay.

 

[emahler]

You don't have a problem switching from normal to emergancy because you have made the transfer due to the loss of the normal sourse. All of the loads the generate counter EMF have decayed befire the transformer.
Not so when transfering back to the normal sorce from emergancy. You have a slightly out of phase transfer when you do so which will trip a breaker magnetically.
All that you need to do is to add a time delay neutral which will provide a short delay in the neutral position to allow the counter EMF to decay.

at this point, i'm not positive if the neutral is switched in the transfer switch or not...or does it not matter?

 

[SG-1]

at this point, i'm not positive if the neutral is switched in the transfer switch or not...or does it not matter?

The Neutral Position templdl refers to is the transfer switch not being connected to either source. The transfer switch would have 3 positions, Normal, Emergency, Neutral.

Motor loads would generate back EMF while spinning down.

The neutral being switched or not is unimportant, provided it is to code.

 

[jghrist]

and I agree with David, 450.3(B) is MAXIMUM OCP, not Minimum...
case in point, build out we are doing, engineer has 3 - 75kVA xfrmrs going in, all primary protected by 90A cb...

True, it is a code maximum (if there is no secondary protection), but it is probably also a practical minimum if you want to avoid tripping on inrush. Chances are, the three new installations with 90A cbs will have inrush tripping problems.

I agree with jraef. The reason you have no inrush tripping on the generator is the higher impedance of the system.

 

[hurk27]

Instantaneous trip levels can be a trick to coordinate for a transformer inrush, an engineer should be consulted with to determine the ITL's of all these breakers that have had their settings modified, the last is the rating of the 100 amp breaker for this transformer, I think it may have a low ITL and needs to be changed to one with a higher ITL, or one thats not instantaneous trip, and or a higher rating, question is did this set up ever work ok at one time? if your secondary has OCPD for it, you can go as high as 250% this would have to be determined along with the other breaker settings.
One question, has anyone ever tried to manually transfer or just turn off the main then turn it back on to see if the same happens without the generator running? that would eliminate the zero sequence problem some have brought up?

As was said table 450.3(B) is the max allowed but not required, (my mistake) and if you do have over current protection see also note 3 to the table above.

If this has been a problem since this transformer was first installed, then maybe no coordinated study was ever done, in which one should be done by an electrical engineer who understand these types of studies. AS Zog has pointed out a very big concern, their maintenance personnel should have not just started turning up the ITL's to get the breakers to stop tripping as they could be putting conductors and equipment in jeopardy if not sized for this new setting.

I'm not by any means any kind of expert on these types of problems, and would love to learn more about it one day, but I have run into very similar types of problems with trying to fire up a cold transformer, with the same results, but in our cases we consulted with the manufactures of the transformers engineers who knew the particular transformer, which might be another road to take also.

In one case we had a 750 kva 480 D X 208/120 Y that was tripping a 900 amp on the 480 every time we had a power failure and power was restored, the transformer manufacture sent out an engineer who had us change the breaker ITL setting to .9 from the .4 it was set at and trip time to 1 second from the .6 it was set for, on a GE series 2k MDP but where those numbers came from I have no idea, well I have a dangerous inkling