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The per unit impedance of a synchronous machine is 0.242 ┬╖ If the base voltage is increased by 1.1 times, the per unit value will be
266
242
22
Given: Initial per unit impedance Z_{pu(old)} = 0.242 ┬╖ Base voltage V_{new} = 1.1 * V_{old}. Base MVA (SbтАЛ) remains constant.
Initial per unit impedance Z_{pu(old)} = 0.242 ┬╖ Base voltage V_{new} = 1.1 * V_{old}. Base MVA (SbтАЛ) remains constant.
Zpu(new)тАЛ=Zpu(old)тАЛ├Ч(Sb(old)тАЛSb(new)тАЛтАЛ)├Ч(Vb(new)тАЛVb(old)тАЛтАЛ)2
Identify the base impedance relationship
Per unit impedance is defined as ZpuтАЛ=ZbaseтАЛZactualтАЛтАЛ, where ZbaseтАЛ=SbтАЛVb2тАЛтАЛ. Substituting this, we get ZpuтАЛ=Vb2тАЛZactualтАЛ├ЧSbтАЛтАЛ.
ZpuтАЛтИЭVb2тАЛ1тАЛ
Apply the scaling factor
Given VnewтАЛ=1.1VoldтАЛ and keeping the actual impedance ZactualтАЛ and base MVA SbтАЛ constant, the new per unit impedance relates to the old one as Zpu(new)тАЛ=Zpu(old)тАЛ├Ч(VoldтАЛ/VnewтАЛ)2.
Zpu(new)тАЛ=0.242├Ч(1.11тАЛ)2
Calculate the result
Calculating the square of the ratio: $1.1^2 = 1.21.Then,Z_{pu(new)} = 0.242 / 1.21 = 0.2$.
Zpu(new)тАЛ=0.2
D is correct because the new per unit impedance is calculated as $0.242 / 1.1^2 = 0.2$, which is not listed in the provided options A, B, or C.
Always remember that base change formulas in per-unit systems rely on the square of the voltage ratio ┬╖ This is a common pitfall in Power Systems analysis during fault calculations.