Control Power Transformer (CPT) Secondary Short Circuit Current

[brainsalad]

Greetings:

I am wondering if someone can please confirm a small calculation to determine the maximum secondary short circuit current (i.e bolted fault) on a CPT. The application is for secondary fuse protection of a CPT.

CPT Data:

150-VA
N_turns = 3.7978
480-V primary
480/3.7978 = 126.389-V secondary
%IR = 3.472
%IX = 0.892

Calculation:

%Z = sqrt(%IR^2 + %IX^2) = 3.5848

I_sc = (1.0 pu V / 0.0358 pu Z) * (150-VA / 126.389-V) = 33.152-A


Do you agree with this method?

Does it make sense to account for the available short circuit kVA at the CPT's primary? For example, if I assumed that at 480-V, 10000-A are available, then this fault Z would actually reduce the I_sc which I have calculated above?

Aside: There seems to be a "rule of thumb" in various literature which states that the secondary fuses on such CPT's be able to interrupt the maximum secondary short circuit current in one second. What basis, besides the "engineering judgment" of using the magical number 1, could there be for such a rule?


Thanks very much!

brainsalad

 

[Skogsgurra]

Simply put, your Isc is equal to rated current divided by regulation. So, if rated current is 1.2 A and regulation is around 3.6 percent, your Isc will be 1.2/0.036=33 A, which is your result.

The primary prospective Isc is of very little interest in control power applications and you can forget about it. The calculation you show is used for power transformers and very seldom for control transformers (exept for school).

The 1 second is what is used. And that is a convenient number that makes sure that a fuse blows fast enough so a person will not be killed and slow enough to prevent nuisance tripping. ten seconds would be unnecessary long and 100 milliseconds would give you lots of nuisance tripping when pulling in large AC contactors and such.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

In this instance the difference may be negligible but for distribution transformers this may not be true.
The current in a short circuited transformer is limited by the impedance. The power factor or current angle is determined by the X:R ratio of the transformer.
The regulation or voltage drop under load is dependent on the impedances of the transformer and of the load. With a resistive load, and a high X:R ratio the resistance may dominate. I have seen transformer specs where the impedance was a few percent higher than the regulation with a unity power factor load.
I have to admit that I learned this on Eng-Tips.

Bill
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"Why not the best?"
Jimmy Carter

 

[dpc]

The issue with these small CPTs is that the secondary fault current is quite low, as you have calculated. As Gunnar state, the secondary fuse is sized to reliably operate for dead short circuit, but not so small as to result in nuisance tripping. It also shows why relying on primary fuses doesn't work too well for secondary faults.

I'm curious where you got the impedance data for this little transformer. It's usually easier to measure the impedance voltage than to find any data from the manufacturer.

 

[brainsalad]

Skogsgurra, waross, dpc: thank you for your thoughts.

dpc: that is an old Allen-Bradley CPT, data obtained from vendor.

I am a recent EE graduate and am presently lucky to be working in the nuclear industry. Thanks for your contributions to this great body of knowledge that is Eng-Tips - you are helping me and others learn to walk the rice paper!

The nuclear plant wishes to remain connected to offsite power during an accident, switching over to emergency diesel generators as a last resort. If the source of offsite power (the grid) went into a sustained degraded voltage condition (that is analytically the degraded voltage relay limit, i.e the grid/4kV bus voltage which causes the plant to separate itself from offsite and start EDGs), the depressed 480V MCC voltages could cause starters, for example, to draw excessive amounts of current, and potentially blow the CPT secondary fuse, rendering the device useless in its time of most need. This is an example of the need for such a critical analysis of fuse sizing.

 

[waross]

Typically the contactor drops out on under-voltage long before it overheats. You may be causing problems by overprotecting. An under-voltage condition which will cause motor burn-outs long before it will cause coil and or control transformer issues.


Bill
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"Why not the best?"
Jimmy Carter