2) What is the best autotransformer to use for the following? It is expected to

Question

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.

Explanation / Answer

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

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