Page 248 - Offshore Electrical Engineering Manual

Worked Example: Fault Calculation 235

connected to the LV switchboard by a 2 MVA transformer with 440 V secondary
winding. Then the sequence reactances (pu) are as follows:

X 1 X 2 X 0
At transformer rating 0.13 0.11 0.11
On the base MVA 1.1505 0.974 0.974

Summing generator and transformer reactances,
X 1 = 0.0912 + 1.1505 = 1.2417 pu
X 2 = 0.104 + 0.974 = 1.078 pu
X 0 = 0.974 = 0.974 pu
Hence,
(X 1 + X 2 ) = 1.2417 + 1.078 = 2.3197 pu
(X 1 + X 2 + X 0 ) = 1.2417 + 1.078 + 0.974 = 3.294 pu
Therefore the LV system fault currents are
17.7
Three ‐ phase fault current = = 18.7 kA
3 × 0.44 × 1.2417
17.7
Phase ‐ to ‐ phase fault current = = 17.34 kA
0.44 × 2.3197
3 × 17.7
Earth fault current = = 21.15 kA
0.44 × 3.294
The workings required for protection settings of the same scenario are continued
in PART 4 Chapter 7.