Current on Transformer Primary Side

Assumptions:

1. The transformer is connected to the source of zero impedance (infinite fault level). Practically it is not the case; every source has a finite fault MVA.
2. The cable reactance is ignored
3. The transformer secondary neutral is solidly grounded

Transformer Secondary current = 1000 / (1.732 x 0.48) = 1203 A

Transformer Primary current = 1000 / (1.732 x 13.8) = 41.8 A

The maximum secondary side fault current = 1203 / (5.75/100) = 20,921 A = 21 kA

The maximum primary side fault current = 41.8 / (5.75/100) = 727 A

These are the maximum fault currents. With a finite MVA level of the transformer source (or utility), the fault current will be less than this.

Because of the 3^rd assumption, the fault current will be same for all three cases.

- MS

A correction:

For case 2 (phase to phase fault), the secondary fault current will be 86.7% (1.732/2) of the above calculated fault current. In this case, the L-L voltage is 1.732 times of L-N voltage but the reactance will be doubled (the two phase reactance's will be in series). The primary fault current depends on the transformer configuration. If the transformer is Y-Y connected, the primary fault current also will be 86.7%.

- MS

The transformer is D/y and yes it is solidly grounded ....

samad shahab actually it is given in c37.91 but they just mention the configuration

can u explain further or any link how i can analyses..

1 single phase fault on sec side

2 phase to phase fault on sec side

3 three phase fault on sec side

the effect of solid grounding when ground fault occur the ground fault current will travel to neutral in from ground ?

There are a few methods available for short circuit calculation.

They are:

(1) Per Unit method

(2) MVA method

(3) Point-to-Point method

(4) Ohmic Method

I suggest you follow one method first (don't mix up the methods). Here are some links that have step by step approaches for various methods:

https://ecmweb.com/mag/electric_shortcircuit_calculation_methods

https://www.arcadvisor.com/faq/about_mva_method.html

https://www.mikeholt.com/freestuff.php?id=freegeneral

The good software for short circuit calculations is through ETAP (https://www.etap.com), EasyPower (https://www.easypower.com), etc. You can download the trial version.

-MS

Yesterday i just found the very good website specifically for these type of conditions

http://www.skm.com/Shifting%20Transformer%20Damage%20Curves%20for%20Through-Fault%20Current%20Protection.shtml

they have explain the different conditions and differenct type faults here and how much will be the expected primary fault current when there is a fault at secondary side ...