Fault Contribution from Radially Fed Distribution Transformers

[distributionPlanner]

I'm having some trouble understanding why a radially fed transformer can be a source of fault current during an upstream fault.

SYSTEM:
[TRANSMISSION SYSTEM 25kV BUS]---[25kV BKR]-----------------------[FAULT]--------------------------------[TRANSFORMER]

I've heard that, for certain faults, the downstream radially fed transformer can act as a source (even without generation) and contribute to the fault.
However, I am having some trouble understanding this. I believe that it also depends on the configuration of the transformer and the type of fault. So, for upstream ground faults, any downstream transformers that are delta primary would not contribute to the fault.

Could anyone either shed some light on this, or link to a paper that explains this fault contribution from a downstream radially fed transformer?

I should note that there are no sources other than the transmission system in this scenario.

Thank you!

 

[mgtrp]

Downstream transformers can provide fault current in two ways:
1/ If there are any motors connected as loads, these will contribute to the fault (capacitance could as well, but it's generally not as much of a concern). On a distribution system, this would be the most common way.
2/ If the transformer is connected as grounded Y on the HV side and has a delta connection as either the secondary or tertiary, this will be a return path during ground faults, making the transformer a zero sequence source. This is not so common on a distribution system since it would be an unusual transformer connection.

 

[ijl]

A similar problem is discussed in thread238-376120

 

[waross]

The issue arises with one or two single phase to ground faults.
The feed back from a wye delta transformer is of most concern if the primary wye point is connected to the system neutral. In most cases grounding the primary wye point will effectively be a connection to the system neutral.
If the primary wye point is floating in relation to the system ground the feed back effect is very much less.
A transformer with a three legged core will develop a "Phanton delta". The comments as to the wye point connection to the system neutral still apply.
In the worst case the back feed current will be limited by three times the transformer impedance, (plus asymetrical effects).

Bill
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"Why not the best?"
Jimmy Carter