"Neutral" Transformer sizing

[HienN]

Dear all,

We have come across with below situation.

The power system is available with 3 wire/22kV. Now, we want to use the neutral wire for the using area at 22kV. Due to so long distance and mountaneous area. We force to use one another transformer 22/0.4kV_Wye/Delta connected to the system. The purpose of this transformer is nothing more than taking advantage of its primary side neutral point to run along with the original 22kV to the using point.

Question for this is: How to correctly size this transformer capacity? Take note that we will just only use its neutral point.

Any suggestion is welcome.

Regards,

Hien

 

[burnt2x]

Let me try. Here it is:

  Phase to Neutral Voltage X expected phase to ground fault current X "K" = 3-phase kVA

K is a constant that depends on the allowed fault duration:

time          K
10 sec      0.064
1 min       0.104
2 min       0.139
3 min       0.170
4 min       0.196
5 min       0.220

This information is taken form the Westinghouse "Electrical Transmission and Distribution Reference Book" published in 1964.

 

[waross]

If you are sizing the transformer for steady state single phase loads on the 22 kV system, then the transformer must be three times the maximum single phase load.
Example: With a 10A load on "A" phase the 10A must return through the "A" phase transformer winding. The voltage will be 22kV / 1.72 = 12.7kV
Single phase load = 10A x 12.7kV = 120 KVA
Transformer bank = x 120 KVA
A corresponding current will circulate in the delta secondary. 10A x (22kV/.4kV) = 550A
Phase angle errors may cause a neutral displacement.
Phase angle errors may cause circulating currents in the delta winding.

Bill
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