Cumulative transformer inrush

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mayanees

Senior Member
Location
Westminster, MD
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Electrical Engineer and Master Electrician
We're doing a design for a 13.2 kV campus system. Each distribution breaker will energize multiple padmount transformers.
Can anybody direct me to data that shows the amount of inrush that a group of simultaneously energized transformers generates?
The safe choice is to sum up the capacity and use a 12X inrush factor, and adjust for that, but I'd love to see historical data from a real-world installation if possible.
Thanks for any attention.
John M
 

kingpb

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SE USA as far as you can go
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Engineer, Registered
The inrush of each will depend on the switching angle between the source and residual flux in each of the transformers, so, not sure if the "safe" way is to simply add the rated current and assume 12X. Will have to give that some thought.


The inrush value could be significant. Calculated versus real world dat may be hard to obtain.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
121108-1906 EDT

My guess is that your 12x factor is probably the least to use unless you have a special breaker designed to avoid this problem. With new high efficiency transformers the problem may be worse than with older transformers. This will be very much a function of the trip-time characteristics of the breaker and its instantaneous response.

With different loads and phase angles of the loads at power turn-off there will be different values of residual flux. Thus, all transformers may not be in the same state as indicated by kingpb. Heavy resistive load at turn-off should produce a lower residual than an unloaded transformer.

See my photo P6 at http://beta-a2.com/EE-photos.html . This is from standard transformer iron of a transformer using general industry standards for design of about 25 years ago.

I am not at all close to transformer or breaker design today so my comments are broad and general. Some long time ago I designed a breaker concept to circumvent the problem of inrush current. This is described in patent number 3,299,322 issued Jan. 17, 1967 and initially filed July 23, 1962.

A broad claim read as follows:
" 1. In an electrical circuit comprising a source of electrical energy and a load, an electronic circuit protector for interrupting the electrical circuit upon the occurrence of a predetermined condition therein, said electronic circuit protector comprising a signal responsive switch electrically connected to the source and load, means for sensing electrical conditions in the circuit having an output signal, a switch control circuit normally responsive to the output from said means for controlling the condition of said switch, an inhibiting circuit for inhibiting the output of said sensing means for a predetermined initial period subsequent to turn-on of said electronic circuit protector, and a desensitizing circuit for desensitizing said switch control circuit for a predetermined period of time subsequent to the inhibited period."

The intent of this circuit and patent was to overcome the inrush problem and yet provide very tight overload detection after the initial inrush. We could sense a 1% overload after the large inrush current had passed.

Do any of the electronic breakers perform this function today? I do not know, but they should.

.
 

ron

Senior Member
121108-1906 EDT

The intent of this circuit and patent was to overcome the inrush problem and yet provide very tight overload detection after the initial inrush. We could sense a 1% overload after the large inrush current had passed.

Do any of the electronic breakers perform this function today? I do not know, but they should.

.

In MV relays, you can enable "cold load pickup" which can overide inrush.
http://www.geindustrial.com/publibrary/checkout/GET-6450?TNR=White Papers|GET-6450|generic
 

mayanees

Senior Member
Location
Westminster, MD
Occupation
Electrical Engineer and Master Electrician
Wow Ron, that's an old GE publication, but the principles still apply. The pub gives cause for lowered expectations of inrush, but to GAR's point, the new transformers will negate that!
As far as breaker response capabilities, it's a C-H DT-3100 with reasonably flexible adjustability and ZSI with the mains, so I should be able to protect and still tolerate the inrush for the 100 ms duration. Thanks for the responses.
I'll know soon whether use of the 12X value works, because one of the four 13.2 kV distribution circuits, when operated in a certain scenario, has enough capacity to test it.

John M
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
I'll know soon whether use of the 12X value works, because one of the four 13.2 kV distribution circuits, when operated in a certain scenario, has enough capacity to test it.

John M

Because the inrush depends on the phase angle at which the xformer is engergized, and because transformers are seldom switched on or off, it can take years for a inrush problem to actually show up and trip a breaker.
 
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