A 120 kVA, 7000/277 V (What does this rating mean) distribution transformer has the following resistances and reactances: Rp 5.5 ohms Xp 6.5 ohms Rs 0.007 ohms Xs 0.008 ohms Rc 55 kohms Xm 15 kohms The excitation branch impedance are given referred to the low voltage side of the transformer: a) What's the equivalent circuit of the transformer referred to the low voltage side b) What's the the per unit equivalent circuit c) Assume that this is supplying rated load at 277V and 0.89 lagging power factor, What is the transformer input
120 kVA, 7000/277 V (What does this rating mean) Primary voltage rating: 7000 V, secondary voltage rating: 277 V, rated load: 120 kVA It is unclear whether this is a single-phase or three-phase transformer. You probably need to assume it is single-phase. The equivalent circuit of a 3-phase transformer is analyzed as one of three single-phase transformers that could be connected to make the equivalent Y-Y three-phase transformer. The secondary voltage, 277 V, is the line to neutral voltage for a 480 V, wye distribution system. That is a USA standard system voltage. The primary would be 12,124 V L-L, 7000 V L-N. That would a reasonable primary distribution system voltage. Referring the circuit to the low side means changing the primary component values to the equivalent secondary values and moving the ideal transformer to the primary side of the circuit as shown below. To change the primary impedance values, multiply by (Sec V/Pri V)^2.
