Primary Transformer Circuit Breaker Tripping on in Rush

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Our telecom client has an inverter with an internal 40kVA, 480V delta to 120/208V wye transformer supplying a bypass circuit for the inverter. The only time the transformer has a load on it is when the inverter is in bypass. Normally DC power is converted to AC power to feed the critical loads. The client tests the generators every Thursday and about 25% of the time when the building returns from generator power to utility power, the primary circuit breaker trips.

As I read it the NEC allows the primary protection to be sized for 250% of the transformer rated current if secondary protection is also provided (Table 450.3(B) which the inverter has. A 40 kVA transformer draws 48A on the primary side so the largest circuit breaker allowed by the NEC is 125A, the next standard circuit breaker size up from 120A (2.5*48).

The panelboard where the 125A circuit breaker is installed is a Square D, NF Series, which will not accept a circuit breaker with adjustable time delay.

Is there a commercial device available that could be added to the circuit to dampen the in rush current?
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
Our telecom client has an inverter with an internal 40kVA, 480V delta to 120/208V wye transformer supplying a bypass circuit for the inverter. The only time the transformer has a load on it is when the inverter is in bypass. Normally DC power is converted to AC power to feed the critical loads. The client tests the generators every Thursday and about 25% of the time when the building returns from generator power to utility power, the primary circuit breaker trips.

As I read it the NEC allows the primary protection to be sized for 250% of the transformer rated current if secondary protection is also provided (Table 450.3(B) which the inverter has. A 40 kVA transformer draws 48A on the primary side so the largest circuit breaker allowed by the NEC is 125A, the next standard circuit breaker size up from 120A (2.5*48).

The panelboard where the 125A circuit breaker is installed is a Square D, NF Series, which will not accept a circuit breaker with adjustable time delay.

Is there a commercial device available that could be added to the circuit to dampen the in rush current?

I would guess that this is more related the sources being out of phase when the transfer occurs. Do you have the option of an in phase monitor for the ATS? Or if you tolerate it just program a delay in transfer from genset to utility.
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
It has to do with the inrush, which is not going to be fixed by a higher rated breaker, per se. Since it only trips 25% of the time most likely what is happening is that the primary winding is being connected at around the zero crossing of the primary voltage. This can cause extremely high inrush currents that can last for a few seconds. A device with an adjustable trip setting is going to be one solution.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
It has to do with the inrush, which is not going to be fixed by a higher rated breaker, per se. Since it only trips 25% of the time most likely what is happening is that the primary winding is being connected at around the zero crossing of the primary voltage. This can cause extremely high inrush currents that can last for a few seconds. A device with an adjustable trip setting is going to be one solution.
I thought that the residual flux and zero-crossing-enhanced inrush current lasted for only a few cycles of 50/60 Hertz, not for a few seconds. Do you have a reference I should look at?
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
I presume that the transfer from generator to utility is 'open' since the transformer is being re-energized.

What is the transition time? Could the delay be increased?

My thought is that the inrush is being made particularly bad because there is still lot of residual flux in the core, getting added to the normal problems caused by timing of the AC cycle when the transformer is energized.

-Jon
 

beanland

Senior Member
Location
Vancouver, WA
Inrush

Inrush

Transformer inrush is quite sensitive to the timing of energization on the cycle. But is also affected by the residual magnetism in the core when being energized. If you can get a thermal-only circuit breaker, no magnetic element, you still provide overcurrent protection as required but will avoid tripping on inrush.
 

topgone

Senior Member
Transformer inrush is quite sensitive to the timing of energization on the cycle. But is also affected by the residual magnetism in the core when being energized. If you can get a thermal-only circuit breaker, no magnetic element, you still provide overcurrent protection as required but will avoid tripping on inrush.

Or, you could tweak the short time setting to the highest possible setting --> if your breaker has an electronic trip unit.
@Paul,
What type of breaker do you have?
 

kwired

Electron manager
Location
NE Nebraska
Or, you could tweak the short time setting to the highest possible setting --> if your breaker has an electronic trip unit.
@Paul,
What type of breaker do you have?
I believe he said Square D NF breaker -not a lot of options for them, basically like the QO line but rated 277/480 instead of 120/240. Also fairly sure that 125 amp is the highest amp rating available for this particular line. If it won't hold one needs to either find a way to lessen the inrush or use some other line of breaker
 

Phil Corso

Senior Member
Paul (tBF)...

1) Since the Xfmr is already energized before load is applied, then, discussion about residual-magnetism should can be ignored!

2) The simplest solution, if the nature of the load is known, is to set the protective device's tripping-point to about 2x the load's inrush. The inrush-current can be easily determined wiith a clamp-on!

3) A more complex solution is to sync the inverter to the Xfmr!

Regard, Phil Corso!
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Paul (tBF)...

1) Since the Xfmr is already energized before load is applied, then, discussion about residual-magnetism should can be ignored!

2) The simplest solution, if the nature of the load is known, is to set the protective device's tripping-point to about 2x the load's inrush. The inrush-current can be easily determined wiith a clamp-on!

3) A more complex solution is to sync the inverter to the Xfmr!

Regard, Phil Corso!
Phil, the way I understand the description the ATS that switches between POCO and generator is connected to the primary of the bypass transformer. That means that residual flux in the primary is indeed a factor unless the generator syncs to POCO before the reverse transfer.
The inverter is supplying power to the load continuously through both transfers as well as any momentary outage and so its synchronization or lack of it is not an issue.
I suspect that the problem occurs only during the reverse transfer because there is a delay before the generator picks up the load at the beginning of the test and a large portion of the instantaneous flux has decayed. A simple time delay with the generator turned off before the back transfer to POCO should, I believe, resolve the problem.

Sent from my XT1585 using Tapatalk
 

ATSman

ATSman
Location
San Francisco Bay Area
Occupation
Electrical Engineer/ Electrical Testing & Controls
Breaker Frame Type

Breaker Frame Type

Paul
You said the tripping breaker is in a NF series panelboard.
What is the breaker frame? EDB, EGB, EJB etc........?
Can you remove the panel cover and take a closeup pic of breaker front?
We need to find out if the inst (mag trip) setting is fixed or adjustable and
what that value is.
 

Phil Corso

Senior Member
GoldDigger...

Your interpretation is correct. I misinterpreted the layout!

However, in my 64 year career, I always recommended the utility-fed Xfmr be continuously energized, so that load transfer time-delay was limited to just that of the ATS!

Phil
 

kwired

Electron manager
Location
NE Nebraska
Paul
You said the tripping breaker is in a NF series panelboard.
What is the breaker frame? EDB, EGB, EJB etc........?
Can you remove the panel cover and take a closeup pic of breaker front?
We need to find out if the inst (mag trip) setting is fixed or adjustable and
what that value is.
AKAIK all NF breakers have fixed magnetic trip.
 

kwired

Electron manager
Location
NE Nebraska
Methinks @ATSman is asking what the circuit breaker interrupting ratings are. Type EDB has an interrupting rating of 18kA, EGB = 35kA and EJB = 65kA.
That is how much the breaker can interrupt and is supposed to not be destroyed in the process. Magnetic trip setting is what level does it trip at when there is a rapid rise as in a short circuit, ground fault, or when an inductive load is turned on and has a temporary surge of high current. I know it is not adjustable on the EDB breakers, don't know that I have ever seen EGB or EJB so don't know if they have field adjustable settings.
 
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