Transformer Voltage Drop TRANSFORMER VOLTAGE DROP Primary Voltage 12.7 KV Secondary Voltage: 120/240 1∅ From Transformer Manufacturer: % Resistance, % R = 2.24% % Reactance, % XL = 3.44% R actual = 10(%R) (KV Secondary) ^2 KVA Transformer XL actual = 10(%X) (KV Secondary) ^2 KVA Transformer 1∅ Voltage Drop = I (Rcos θ + Xsin θ) 3∅ Voltage Drop = √3 I ( Rcos θ + Xsin θ) I = Actual load current R = Actual Resistance X = Actual Reactance COSθ = PF = Power Factor of load Pure Resistance PF = 1, MOTORS PF .65-.9 depending on size Single Phase Example

6/20/02 Chapter 9: Transformer Voltage Drop 1/3

Transformer 100 KVA, 1∅, 12.4 KV to 115/230V % R = 2.24% % X = 3.44% Find voltage drop across transformer assuming 230 V base at no load Load: 75 KVA @ .85 OF, Cosθ = .85, Sinθ = .527 I = KVA = 75KVA = 362 Amp KV .23 KV R actual = 10 (2.24 %) (.23KV)^2 = .01185Ω 100 KVA XL actual = 10 (3.44%) (.23KV) ^2 = .0182Ω 100KVA Volts Drop = I [R Cosθ + Sinθ] Volts Drop = 362 A [.01185 (.85) + .0182 (.527) ] = 362 A [.01966] Ω = 7.116 Volts Vd= (7.116 Volts) 100 = 3.09% Voltage drop 230 Volt base Voltage drop on 3∅ is same except for √3 in formula. Current I = KVA 3∅ KV √3 Three Phase Example 3 ∅ Voltage drop = Vd = √3 I [ R Cosθ + X Sinθ ] Load 150 KVA, P.F. = .8 = Cos θ, θ = 36°, Sinθ = .6

6/20/02 Chapter 9: Transformer Voltage Drop 2/3

6/20/02 Chapter 9: Transformer Voltage Drop 3/3