Wye Delta for 120V/240V Service Fault Analysis

[DistCoop]

All,

I've searched the forums and was surprised to see this hadn't been addressed, so if I've overlooked something, please let me know.

I have a three phase wye-delta 12.47kV to 240V with a tap on one delta leg for 120V. The wye is ungrounded, and the tap on the delta is grounded.

How would one calculate a LN fault between either of the 120V phase-neutral legs? How is the fault calculated for the 208V to neutral phase? Let's say that the neutral is on the AB winding, so Van and Vbn are 120V, Vcn is 208V, and the line-line voltage between any phase is 240V.

Thanks

 

[magoo2]

I have a computer program that GE developed some years ago under DSTAR, a consortium of electric utilities that funded research in the power distribution area. It enables you to calculate the fault currents with different transformer sizes, different conductor sizes and you could also vary the source impedance. It handles other three phase transformer connections but the 4-wire delta is what you asked about.

If you are working for an electric cooperative as your handle implies, then NRECA is a member of DSTAR so you should be able to get an executable copy of the program along with instructions for its use.

Otherwise, you can start writing out the equations for the 4-wire delta transformer connection!

 

[waross]

Considering three similar transformers in a delta bank:
The available fault current in a line to line 240 Volt single phase fault will be twice the available fault current of one of the transformers in a single phase circuit.
I'm not sure about a line to neutral fault.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[DistCoop]

I may have mislead you with the handle... I work for a consulting firm serving cooperatives, so I don't get access to NRECA benefits.

Any chance you can point me to a reference for developing these equations?

 

[magoo2]

Your best bet would be to contact GE Energy in Schenectady. The program was called the Graphical Secondary Voltage Imbalance Calculator.