Problem One: A three-phase 1 MVA 2400:415 V 50 Hz distribution transformer is constructed by Fig.P1a below. To determine the per phase T equivalent circuit parameters, the following tests were carried out:
The primary phase winding resistance test was conducted by applying a DC voltage of 60 V to terminal pairs AB, BC, and CA separately, and the corresponding readings of ammeters connected in lines A, B, and C were 240 A, 241 A, and 239.5 A, respectively.
The open circuit test was conducted on the secondary side by applying three-phase voltage of 415 V (line to line) to the secondary side with the primary side open-circuited, and the measured secondary line current and the total input power from the secondary side were 156 A and 15 kW, respectively.
The short circuit test was also conducted on the secondary side by adjusting the applied three-phase secondary voltage until the secondary side line current reached the rated value with the primary side short-circuited, and the measured secondary side line to line voltage and the total input power from the secondary side were 73 V and 85 kW, respectively.
(a) Sketch the corresponding phasor diagram of the primary and secondary voltages for the Dyn1 vector group;
(b) Determine the parameters of per phase T equivalent circuit referred to the primary side;
(c) Calculate the voltage regulation for the rated load with a power factor of 0.70 lagging; and
(d) If the transformer is supplying the full load of power factor 0.70 lagging as in (c), and the primary side is connected to an infinite power grid, i.e. the primary line to line voltage is fixed at 2400 V line to line, to regulate the secondary voltage to the rated value, i.e. to achieve zero voltage regulation, a three-phase star-connected capacitor bank is connected to the secondary in parallel to the load (Zi) as shown in Fig.P1(b), calculate the capacitance C per phase. (20 marks)
A a B b C c Zi Z
T. m
r
b c
In
In
(a) (b) Fig.P1 (a) Three-phase transformer connected in Dyn1 1 vector group, and (b) Three capacitors connected in star and parallel to the three-phase load
