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CORK INSTITUTE OF TECHNOLOGY
INSTITIÚID TEICNEOLAÍOCHTA CHORCAÍ
Autumn Examinations 2010/
Module Title: Advanced Power Systems
Module Code: ELEC
School: School of Electrical & Electronic Engineering
Programme Title: Bachelor of Science (Honours) in Electrical Power Systems
Programme Code: EELPS_8_Y
External Examiner(s): Mr. G. Beecher, Dr. M. Duffy Internal Examiner(s): Mr. S. Boljevic
Instructions: Answer any FOUR questions.
Duration: 2 Hours Sitting: Autumn 2011
Requirements for this examination:
Note to Candidates: Please check the Programme Title and the Module Title to ensure that you have received the correct examination. If in doubt please contact an Invigilator.
Q1. For the circuit shown in Fig 1, find the following:
(a) Current in each branch (3 marks) (b) Voltage across each branch (3 marks) (c) Total amounts of Watts (w) VAv , VA consumed by the circuit (6 marks) (d) The circuit power factor (3 marks) (e) Draw the phasor diagram showing: current across each branch, voltage across each branch (10 marks)
~
Z
Z
Z
Z
Z
Z
Z
230V
Fig 1.
Q3. For the circuit shows in Fig 3 find:
(a) The complex power absorbed by each branch (5 marks)
(b) The total complex power absorbed by the circuit (5 marks)
(c) Draw the phasor diagram showing the phasors of currents in each branch and total current. (5 marks)
(d) Draw the phasor diagram showing the phasors of complex power absorb by each branch and total complex power absorb by the circuit. (10 marks)
1.6 (^) j7.2
4 (^) j3
6 (^) -j8
~
IT
a
c
b
e
d
g
f
(100 + j0)V
Fig 3.
Q4.
Diagram below shows on one-line diagram of a three-phase system. Assume that the line length between two transformer is negligible and the three-phase generator is rated at 4169kVA; 2.4kV; and 1000A and that it supplies a purely inductive load of Ipu = 2.08∠-90^0 pu. The three-phase transformer T1 is rated 6000kVA; 2.4Y kV-24Y kV, with leakage reactance of 0.04pu. Transformer T2 is made up of three single-phase transformer and is rated 4000kVA ; 24Y kV -12Y kV, with leakage reactance of 0.04 pu. Determine the following for all three circuits, 2.4kV ; 24kV and 12kV circuit. (a) Base kilovoltamperes values, (2 marks) (b) Base line-to-line kilovolt values, (2 marks) (c) Base impedance value, (2 marks) (d) Base current value, (2 marks) (e) Physical current values (neglect magnetizing currents in transformers and charging current in lines), (3 marks) (f) Currents per units values, (3 marks) (g) New transformer reactance based on their new bases, (3 marks) (h) Per-unit voltage values at buses 1,2 and 4, (4 marks) (i) Per-unit apparent power values at buses 1,2 and 4 (4 marks) Note: base kVA = 2080kVA ; base voltage for the 2.4 kV circuit is 2.5 kV
Fig 4.
Q6.
(a) The current in a three- phase unbalanced system are:
⃗ R = (12 + j6)A; ⃗y = (12 – j12)A; IB = (-15 + j10)A The phase sequence are RYB. Calculate the zero, positive and negative sequence components of the currents. (15 marks)
(b) Discuss the ‘symmetrical component method’ to analyse an unbalanced three-phase system. (10 marks)
Q7.
(a) For the diagram shown in Figure 7 where the percentage reactance of each alteration is based on its own capacity, find the short circuit current that will flow into a complete three-phase short-circuit at F. (Note: base kVA is 35000 kVA) (10 marks)
(b) What is the importance of short-circuit calculations? (5 marks)
(c) What is the importance of base kVA in short-circuit calculations? (5 marks)
F
11000V
A B
15000kVA 20000kVA
Fig 7
