Two 25 MVA, 3 phase alternators operate in parallel to supply a load of 35 MVA at 0.8 power factor lagging. If the output of one machine is 25 MVA at 0.8 lagging, what is the output and power factor of the other machine?

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Question

This question was previously asked in

PGCIL DT Electrical 22 Aug 2021 Official Paper (SR I)

Answer 1

Calculation: Given,

Rating of Generators = 25 MVA

Rating of load = 35 MVA at 0.8 p.f lagging

= 35 MVA ∠-36.86°

If two generators are connected in parallel then both will combinedly supply the load demand.

i.e, S₁ + S₂ = S_L

∴ S₁ = 25 ∠-36.86°

S_L = 35 ∠-36.86°

∴ S₂ = S_L - S₁

= 35 ∠-36.86 - 25 ∠-36.86°

= 10 ∠-36.86°

= 10 MVA, 0.8 p.f lagging

Note:- Here '-ve' sign indicates p.f as lagging.

Therefore, option (2) is correct.