short circui at primary utility xfmr

[anbm]

Utility power company gave me these short circuit data at their xfmr primary (12.5kV pri/480V sec xfmr)

LLL = 5,100A
LLG = 4,905A
LL= 4,560A
LG = 3,100A

I assume LL is short circuit between Line-Line, LG is short circuit between Line-Ground, What are LLL, LLG?

 

[mivey]

Utility power company gave me these short circuit data at their xfmr primary (12.5kV pri/480V sec xfmr)

LLL = 5,100A
LLG = 4,905A
LL= 4,560A
LG = 3,100A

I assume LL is short circuit between Line-Line, LG is short circuit between Line-Ground, What are LLL, LLG?

Three phase and double line to ground fault

 

[don_resqcapt19]

So are you going to use the available primary current along with the transfomer size and impedance to get the secondary short circuit current?

 

[anbm]

kind of

 

[skeshesh]

kind of

A response that I hear surprisingly often to engineering questions!

 

[kingpb]

Looks to me they told you to use an infinite bus.

A 3200A, 480V service with a Xfmr with Z = 5.75% is going to knock it down to under 65kA. Piece of cake!

 

[bob]

King
Where did you get this information?

 

[anbm]

A response that I hear surprisingly often to engineering questions!

They gave me those # so I can use them with SKM software to calculate short circuit at xfmr secondary and other stuffs as needed.

 

[mivey]

Looks to me they told you to use an infinite bus.

A 3200A, 480V service with a Xfmr with Z = 5.75% is going to knock it down to under 65kA. Piece of cake!

Did you mean to say "they should have told you"?

 

[kingpb]

For short circuit calculations, an infinite bus is going to be anything greater than what the transformer will let through it.

In this case, 5100A at 12.5KV equates to 110.4MVAsc
Without any impedance between the 12.5KV and 480V system, it equates to 132.8kAsc

Typical transformer impedance for a voltage less than 25kV primary and 480V secondary is 5.75%. So, 480V, x 3200A x 1.732 = 2.66MVA. Divide it by the impedance and you get 46.3MVAsc which equates to 55.6kA @ 480V, assuming infinite bus.

Even if you increase your service to 5000A at 480V, the maximum possible sc current on the 480V side (assuming 5.75% impedance) is going to be around 87kAsc. You could order a transformer with a 6.75% impedance and still get the SC current under 65kA.

The transformer is going to limit the fault current to it's own MVAsc capability on a system that assumes an infinite bus.

Actually, I have to retract the infinite bus statement, as further number crunching determined in using 3200A service, it is not. 110.4MVAsc combined with 46.3MVAsc equates to only 32.6MVAsc on the 480V side which is around 39.2kA. However, if the service is around 800A or less, it would appear to be an infinite bus.

As far as the OP, they did not provide enough information to model in SKM. You need either system impedance, i.e. R and X values, or the X/R ratio in the program.

 

[mivey]

kingpb,

My point was that if the utility is giving you the system fault data on the primary side of the transformer then they are not giving you an infinite bus calculation.

 

[kingpb]

That's probably true, but I don't recall (by age that doesn't mean it hasn't happened ) that they will tell you to use an infinite bus. Infinite bus is really just an arbitrary term used by power system engineers to describe the available fault current and short circuit impedance. What might be an infinite bus for one person on a system, may not be an infinite bus for another person on that same system, i.e. my example that at 3200A @480V it is not an infinite bus, but at 800A or less it is. Good discussion.

 

[ron]

Don't forget a multiplier to account for motor contribution.

 

[skeshesh]

They gave me those # so I can use them with SKM software to calculate short circuit at xfmr secondary and other stuffs as needed.

Well that's pretty much exactly what Don said (while not specifying a calculation tool) and with a response like "kind of" I thought you had something else in mind. My mistake.

King I think Mivey concluded correctly in that why ask for the primary side fault if you're going to assume an infinite bus.

By the way, speaking of available fault info from a utility, I was never able to figure out what the "ultimate fault current" referred to in a recent post means...

 

[mivey]

That's probably true, but I don't recall (by age that doesn't mean it hasn't happened ) that they will tell you to use an infinite bus. Infinite bus is really just an arbitrary term used by power system engineers to describe the available fault current and short circuit impedance. What might be an infinite bus for one person on a system, may not be an infinite bus for another person on that same system, i.e. my example that at 3200A @480V it is not an infinite bus, but at 800A or less it is. Good discussion.

By infinite bus we mean that we assume zero source impedance (Z=0+j0 ohms or V/Z = infinity). The bus in this case would be the transformer primary. Instead of a zero source impedance, the POCO provided values so you can calculate the modeled source impedance instead of using zero ohms.

 

[mivey]

By the way, speaking of available fault info from a utility, I was never able to figure out what the "ultimate fault current" referred to in a recent post means...

What post? I would think they meant maximum fault current. That might be the three-phase fault or the line-ground fault depending on the situation.

 

[skeshesh]

What post? I would think they meant maximum fault current. That might be the three-phase fault or the line-ground fault depending on the situation.

Didn't catch your response last time around. I was referring to this post: http://forums.mikeholt.com/showthre...hort-circuit-ratings?highlight=ultimate+fault