1. (a) The per-unit system is frequently used when performing power system calculations. Briefly describe two of the principal advantages of such a calculation system.
T1
L
12
A Y
Yo
®
Load
Generator G: 15 MVA, 13.8 kV, X = 0.15 pu Synchronous motor M: 10 MVA, 13.2 kV, X= 0.15 pu Ti: 25 MVA, 13.2 to 161 kV, X= 0.10 pu T2: 15 MVA, 13.8 to 161 kV, X= 0.10 pu Line L: (50 + j100) Q Load: 4 MVA at 0.8 pf lag
Figure 1
(b) Develop a single-phase circuit diagram for the system shown in Fig. 1. All impedance values should be in per-unit, using a system base of 20 MVA and a base voltage of 161 kV for the transmission line.
2. A 3-phase, 8-pole, star-connected synchronous generator has a synchronous reactance of 6.6 Q and negligible losses. It is directly connected to a 50 Hz, 6.6 kV (line) infinite busbar. When the field current is set to give an open-circuit voltage of 4.5 kV per phase, the generator gives an output power of 2.5 MW.
[6 marks]
[14 marks]
(a) Draw an equivalent circuit for the generator. [3 marks] (b) State whether the machine is underexcited or overexcited. [2 marks] (c) Draw a phasor diagram for the machine. [3 marks] (d) Calculate the output current, the power-factor, the load angle and the electromagnetic torque. [12 marks]
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