Nuisance trip CB at transformer primary when transfer to generator 1

[UniversalTraveller]

Description: We have just recently installed a 480V, 560kW/700kVA diesel generator as an optional standby backup power for an existing building (single-line shown in the link below). The generator has a built-in 800A main CB, feeding a new Generator Dist. Board (main lug only). Three outgoing feeders are distributed in the bldg. feeding generator side of 3 new service-entrance rated ATSs. Each ATS has a built-in CB on its utility power side, because the existing dist. boards (on the secondary of existing 4160:480 or 4160:208 transformers in the bldg) were installed long time ago and didn't have a main, so we needed to provide them with a main for code-compliance. The ATSs are break-before-make (no paralleling with utility is allowed, and the 1-2 min. outage due to delay in transfer is acceptable for our loads).

Problem: Upon transferring the power from the utility to generator, one of the CBs in the Generator Dist. Board trips immediately. This 250AF/225AT CB is at the primary side of a new 150kVA, 480:208Y/120V transformer. It's a nuisance trip. The other breakers don't trip.

Suspected Root causes: (1) Transformer inrush: I verified the coordination study (TCC) and the inrush is 2200A @0.1Sec. The breaker has an instantaneous over-ride at 4000A @0.1Sec. So, it doesn't look like it trips due to inrush. (2) Governor poor frequency and/or voltage regulation causing surge current: The generator vendor strongly believes that this is not an issue with governor setting which has an electronic isochronous governor (we cannot change the settings to a higher voltage or frequency from generator control panel).

Other hints: The breaker did not trip without load on the secondary of the transformer downstream.

Any suggestions?

 

[jraef]

It's not likely that your transformer inrush is going to be that low, there are a lot of mitigating factors that can push it much higher, i.e. 20X FLA so upward of 3600A.

Not sure what you mean by an instantaneous override of 4000A. If it was a 400AF breaker I would buy that, but not on a 250AF breaker, I would expect to see the instantaneous override at 10X I_r, so 2500A. At any rate your peak instantaneous setting on a 225A LSI trip will be 2250A anyway.

2250A trip, 3600A potential spike, it's actually a common problem. Generally the magnetizing inrush is of short enough duration that the breaker trips will not react, but it can also depend on the point in the waveform when the breaker closes, the residual magnetism in the transformer etc. A 400AF 250AT breaker might be a better solution.


"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[UniversalTraveller]

@jraef:

Thanks for the reply. As for the XFMR inrush current, I myself was really suspicious that this is most likely the root cause. However, after I verified the coordination study, the inrush current is not in the tripping zone of that CB. (see attached TCC - The breaker with the nuisance tripping is CB TX2 (purple curve). The override (instantaneous pickup current) is fixed (non-adjustable) at 4000A @0.1Sec.

Another hint: TX2 transformer is de-energized prior to the transfer to generator power.

I'm also including a photo of the CB trip unit in another reply to this thread.

Considering I have a deadline to complete this project this Friday and no funds to replace the CB with a 400AT/250AF, do you think changing the setting to 250AT on the existing CB would resolve this? We've already tried increasing the Short-Time Pickup and Delay, but nuisance trip still persists.

Thanks again for your feedback.

 

[UniversalTraveller]

Here's a Photo of the CB trip unit.

 

[waross]

I'm with jraef all the way on this one. I agree with the vendor also.
Transformer inrush exceeds the maximum instantaneous rating of the breaker.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Zogzog]

I am confused, looks like you have a 400AF/250AT from the photo, can you change the rating plug to something larger?

 

[UniversalTraveller]

The CB is 400AF/225AT (sorry for saying 250AF in the beginning...that was what specified on single-line. The furnished CB is 400AF).

The plug rating is adjustible for Short-Time Pickup and Short-Time Delay. The time delay can be either LS (if set to 100, 200 or 300ms) or LI (if set to instantaneous), but not both at the same time. The instantaneous pick up (override) is fixed to 4000A @0.1sec which should be above the transfomer (150kVA, 480:208Y/120V) inrush. We tried all values on both dial settings (all pickup and delay values), but the niusance trip still persists. Interestingly, the other two CBs which don't have a transformer on their downstream don't trip.

We are trying to see if the trip unit is defective today and replace it if needed. And, possibly will also record the events on the CB with a Fluke Power Analyzer to see what's going on. Maybe there's really a short in downstream and the CB is doing its job. Still no clue about what really causing this.

Appreciate your feedback.

 

[jraef]

Looking at your coordination study chart, my feeling is still that this is transformer magnetizing inrush. That study seems to be using a "standard" model of normal transformer inrush, namely 2000A which is roughly 1100% of FLA (you don't say what FLA is, I'm going from a chart). But from a transient standpoint it can be as high as 2000% and maybe even as high as 2500% according to some studies I've read. 2500% of a 180A transformer is 4500A, which is above your Instantaneous override threshold. When you take your measurements, make sure your instrument can capture fast transients, i.e. 0.1sec or less. You can try using a different rating plug and different (hiogher) settings, but if it is going above the Inst threshold, which will remain the same in that frame size, I'm affraid that won't help.

One factor that can affect that peak level is source impedence and given that this only happens when you energize from the generator supply, that follows. Do some reading on transformer inrush nuisance tripping, there is a lot more known about it now than there was even 10 years ago when a lot of protection relaying and coordination data was compiled.



"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[living2learn]

Jraef,

Wouldn't the source impedance be much higher when on Generator meaning the inrush current to the transformer would be much lower than when on Utility? I would think it would trip the second they energ

I always thought this would happen when going from Generator back to Utility? I agree that the inrush could be very high. I am attaching a SQD cut sheet showing inrush x rated primary current because I would always assume 12x in my studies until...I found out it was 2500%!!! You know what they say about assumptions. Side note - I review a lot of studies and people always use the standard skm library data and IMO makes the study a little irrelevant. I am getting called out more and more to different job sites for this type of problem.

 

[waross]

Try dropping the voltage setting on the generator about 5% at a time. Leave the other circuits on the grid and just work on the transformer circuit.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[TLCPA]

The 225A 2KES225T rating plug is too small for a 150 kVA 480-208Y/120 transformer. The full load current on a 150 kVA, 480V transformer is 180A. Per NEC, with primary and seconday overcurrent protection on the transformer, the 480V CB-TX2 could be 180A x 250% = 450A. Like Zogzog said, change the rating plug.

 

[Zogzog]

The max plug you can put in that trip unit is 250A, anything higher than that and you have to replace the whole trip unit.

 

[pwrtran]

UniversalTraveller - while focusing on the transformer inrush and the breaker trip settings, have you checked if there are some issue with the ATS itself and power cable from the breaker to the the ATS?

 

[UniversalTraveller]

Last week, we upgraded the rating plug and the trip unit with a 300A one and this resolved the issue. Luckily, I didn't have to upgrade the cable sizes. It's interesting that on the coordination study, the inrush current was not in the tripping zone of the 225A unit, but it did have this nuisance trip issue in reality as jraef and zogzog said. Lesson that I learned is never size your equipment "marginally" to save money, because it's not worth a possible headache down the road.

 

[RonShap]

Does the the breaker trip if there is a longer delay between the two sources being energized to the load? Maybe there is spinning load that hasn't slowed down enough? Does it happen when the utility is still on (test conditions) and the two sources are out of synch and you don't have the ATS's set to wait for synchronization?

 

[jraef]

Lesson that I learned is never size your equipment "marginally" to save money, because it's not worth a possible headache down the road.

It's a lesson we all learn in the same manner it seems...

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[stevenal]

Transformer inrush is not a single point on a TC diagram,it is a line. I think if you enter a few other commonly used points, you'll see the inrush line will cross the protection curve. 25XFLA at 0.01s, 12XFLA at 0.1, 6XFLA at 1s, 3XFLA at 10s, and 2XFLA at 100s are the points suggested by Cooper to account for both hot load and cold load inrush.