2 What is the best autotransformer to use for the following

2) What is the best autotransformer to use for the following? It is expected to use an auto to regulate a 30 MVA, 69 kV to 12.47 kV transformer’s secondary on a phase basis to plus or minus approximately 10 percent of rated secondary voltage. The choices are: i) one three-phase auto-regulators, rated 30 MVA, 12.47 kV to 1.25/1.25 kV with 1.5 percent impedance and an LTC with a +/- 10 percent range that operates on the duel 1.25 kV secondaries in accordance with the standards. ii) three single-phase auto-regulators, each rated 10 MVA, 12.47 kV to 1.3/1.3 kV with 1.5 percent impedance and an LTC with a +/- 10 percent range that operates on the duel 1.3 kV secondaries in accordance with the standards. iii) three single-phase auto-regulators, each rated 10 MVA, 12.47 kV to 1.3/1.3 kV with 1.5 percent impedance and an LTC with a +/- 10 percent range that operates on the duel 1.3 kV secondaries in accordance with the standards. iv) three single-phase auto-regulators, each rated 1.1 MVA, 12.47 kV to 1.3/1.3 kV with 1.5 percent impedance and an LTC with a +/- 10 percent range that operates on the duel 1.3 kV secondaries in accordance with the standards. v) three single-phase auto-regulators, each rated 10 MVA, 7.2 kV to 0.75/0.75 kV with 1.5 percent impedance and an LTC with a +/- 10 percent range that operates on the duel 0.75 kV secondaries in accordance with the standards. vi) three single-phase auto-regulators, each rated 1.1 MVA, 7.2 kV to 0.75/0.75 kV with 1.5 percent impedance and an LTC with a +/- 10 percent range that operates on the duel 0.75 kV secondaries in accordance with the standards.

Solution

les consider

30 MAv is 3 phase

now if we convert this MVA for single phase

30 mva , 69 kv to 12.47

69 Kv = 69,000 volt

now maximum allowable current for this MVA will be

from Equation

Power =   3 V x I x Cos ( fi)

va = 3 V I

so , I= va /   3 V

from equation

I = 30 x 10 ⁶   /   3  ( 69 x 10³⁾

I= 251.31  amp...........................................1)

now ,

to calculate ragulation for this in single phase

lets convert in single phase

69 KV , 3phase

for single phase

V line = Vphase / 3   = 69 /   3 = 39.88 Kv

I will be as per equation ( 1 ) = 251. 31

VA in single phase =  V x I     = (39.88 x 10³ ) x ( 251.31 ) = 10022.24 x 10³  Va = 10.02 MVA

now lets find its required secondry

12.47 KV ,    3 phae

V line =     V phase /  3    = 12.47 /   3 = 7.2 K V

so ,

Option V ) will be sutable for this as its having 3 single phase regulator with 10 MVA with Voltage of 7.2 V

three single-phase auto-regulators, each rated 10 MVA, 7.2 kV to 0.75/0.75 kV with 1.5 percent impedance and an LTC with a +/- 10 percent range that operates on the duel 0.75 kV secondaries in accordance with the standards