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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...

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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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005_fedo95k-zg5faliq.pdf
Answered Same Day Dec 21, 2021

Solution

Robert answered on Dec 21 2021
115 Votes
Solution 1:
a)
The basic two advantages of per unit system are:
1) Circuit analysis can be performed without having to convert cu
ent, voltage, and impedances at
every stage of voltage transformation (at every transformer).
2) The use of per unit calculations eliminated the complexity associate with conversions across
transformers. In a per unit calculation, transformers simply to impedances (typically, only the
leakage impedance - normally it's possible to ignore the magnetizing
anch in the transformer
equivalent circuit. You can then perform the calculations to determine per unit voltage and
cu
ents, and then convert those back to actual values by multiplying by the appropriate base
quantities for the respectively locations in the system
.
) Base voltage in transmission line is 161 kV
Base MVA is 20 MVA
Base voltage at generator end is:




Per unit value of generator impedance:
(


)



Eg
Xg
Xt1 Xt2
ZL
L
o
a
d
Xm
(


)




Per unit value of transformer T1 impedance:
(


)



(


)




Per unit impedance of transmission line:
Base impedance is



Hence per unit impedance is:



Per unit value of transformer T2 impedance:
(


)



(


)




Base voltage at motor side:




Hence per unit impedance of synchronous motor is:
(


)



(


)




Impedance at load is:










Base impedance is:




Per unit impedance of load:



Solution 3:
a) Considering a simple power system:
The power system diagram is as shown
Hence the terminal voltage is:
( )
Where represents the phase shift between voltage and cu
ent.
Hence we see that the output voltage is non linear.
) In load flow analysis we define various buses:
1) Load Bus
2) Generator Bus
3) Slack Bus
Slack bus is the reference bus whose voltage and phase is kept fixed. In generator bus
voltage and real...
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