Assignment 2024S1
ELEC9713 Industrial and Commercial Power System
Lecturer: Dr. Daming Zhang, Email: daming.zhang@unsw.edu.au.
This assignment is due on Tuesday midnight of week 10 (16/April/2024). You need to TYPE your answers in word
file, convert it to a pdf file, and submit it via Moodle. It is allowable to have sketched and clear figures and drawings.
The assignment must be submitted individually and must be your own work. The UNSW policy on student plagiarism
can be found on the www.unsw.edu.au website. Note that UNSW uses automated plagiarism-checking software.
The assignment will be marked out of 20. Note that late submission without good reason will see you lose significant
marks.
Part A
A power supply system is designed for four groups of loads as shown in Fig.1, where there is one equivalent source at
bus 1.
T1
CB3A
CB4B
Source
1
2
3
Line1
CB1A
CB1B
CB3B
Line2
CB2A
CB2B
CB4A CB5A
CB5B
CB6A
CB6B
4
5
6
T2 T3 T4
CB7
Lo
ad
gro
u
p
1
Lo
ad
gro
u
p
2
Line3 Line4
Lo
ad
gro
u
p
3
Lo
ad
gro
u
p
4
Line5 Line6
7
8
9
10
SW1 SW2
SW3 SW4
Figure 1 Power supply system for four groups of loads
The ratings of each of two identical lines 1 and 2 are 7.5MVA, 11kV.
The ratings of each of four transformers are as follows:
Transformer T1: 2.50MVA, 11kV ∆/415V Y;
Transformer T2: 2.50MVA, 11kV ∆/415V Y;
Transformer T3: 1.25MVA, 11kV ∆/415V Y;
Transformer T4: 1.25MVA, 11kV ∆/415V Y.
Each of the four transformers is in wye-connection with neutral conductor at low-voltage side for supplying power to
both three-phase loads and single-phase loads.
Transformers T1 and T2 are identical and have the same series impedances.
The ratings of two identical lines 3 and 4 are the same and each of them has a power rating of 2.5MVA and line-line
voltage rating of 415V.
The ratings of balanced three-phase loads for each of four groups are given below:
Load group 1: 1.25MVA, 415V;
Load group 2: 1.15MVA, 415V;
Load group 3: 1.10MVA, 415V;
Load group 4: 1.10MVA, 415V.
Besides balanced three-phase loads, there are also single-phase loads for each of four groups.
Each of the lines 3, 4, 5 and 6 is chosen in such a way that the maximum current flowing through each phase is less
than that the line can bear.
a) Under the first operating condition, CB3A, CB3B, CB4A, CB4B and CB7 are closed. The loading conditions for
each of load groups 1 and 2 are as follows:
Load group 1: 0AI 1250 10 (A)  ,
0
BI 1200 122 (A)  , and
0
CI 1150 108 (A)  ;
Load group 2: 0AI 920 8 (A)  ,
0
BI 900 125 (A)  , and
0
CI 945 109 (A)  ;
Calculate the currents flowing through each of Line 3 and Line 4.
(Marks: 8%)
b) Under the second operating condition, CB3A, CB3B, CB4A, and CB4B are closed while CB7 is open. The loading
condition for Load group 1 is as follows: three-phase loads 1.0MW with a power factor of 0.95 lagging; Single-phase
loads: 50kVA with a power factor of 0.98 lagging between phase A and neutral; 80kW with a power factor of 0.975
lagging between phase B and neutral.
Calculate the currents flowing through Line 3. Also calculate currents flowing from bus 2 to the delta side of T1.
Assume that the voltages at bus 4 are kept approximately at rated one for each phase and phase-A voltage is taken as
reference phasor with an angle of zero degree.
(Marks: 8%)
c) Under the third operating condition, CB3A and CB3B are open while CB4A, CB4B and CB7 are closed. The
loading condition for each of load groups 1 and 2 are as follows:
Load group 1: 0AI 1340 5 (A)  ,
0
BI 1300 129 (A)  , and
0
CI 1410 112 (A)  ;
Load group 2: 0AI 925 6 (A)  ,
0
BI 842 128 (A)  , and
0
CI 900 114 (A)  ;
Calculate the currents flowing through Line 4 and CB4A respectively. Also calculate the sequence components of
these two groups of currents.
(Marks: 10%)
d) Describe lightning protection system for the overhead transmission lines between bus 1 and bus 2. Sketch a diagram
to facilitate your description. Assume that line 1 is separated from line 2 and each of them is supported by different
poles. They only join together at bus 1 and bus 2.
(Marks: 8%)
e) Design a surge protection at the primary side of each of four transformers and explain how it works. Also add a
protective device to the surge protection against potential danger of frequent overvoltage in the system. Sketch a circuit
diagram to facilitate your explanation.
(Marks: 6%)
Part B
The single line diagram of a power system supplying power to two groups of induction motors is shown in Figure 2
below. The cable data are given in Table I below.

Table I Per-meter resistance and reactance for three-phase four-wire Cu/pvc/pvc cables with different sizes
Cable Values in mΩ/m (Referred to one phase)
r
pm
x
pm

4X70 mm
2
0.3175 0.09238
4X 300 mm
2
0.0751 0.0808
4X 400 mm
2
0.0606 0.0808

The rated line-line voltage at bus 2 and bus 3 through bus 5 is 400V.
The motors in group M1 are identical, so are motors in group M2. Use the average sub-transient reactance of 25% and
a typical X/R ratio of 6 to calculate the impedance of each of the two groups of motors. Treat main DB as open circuit
in the fault analysis.
22kV/400V
1000 kVA
R=1.21%
X=5.62%
22kV
400V
1
2
MCCB1 24 300mm Cu/pvc/pvc
(120m) on cable tray
MCCB2 Motor group
M1: 400 kVA
3
MCCB3 24 400mm Cu/pvc/pvc
(80m) on cable tray
MCCB4
Motor group
M2: 500 kVA
Source:
Fault level =600 MVA
X/R=15
4
F
MCCB5 Main DB
(distribution board)
24 70mm Cu/pvc/pvc
(45m) on cable tray
CB1
5

Figure 2 Power system under study
a) Use a power base of 2MVA and a line-line voltage base of 415V at load side to calculate the per-unit Thevenin
impedance seen between bus 4 and ground;
(Marks: 8%)
b) Use a power base of 1MVA and a line-line voltage base of 400V at load side to calculate the per-unit Thevenin
impedance seen between bus 4 and ground;
(Marks: 8%)
c) Calculate the Thevenin impedances in ohms for both a) and b). Comment on the results;
(Marks: 6%)
d) List down basic criteria when choosing circuit breakers and MCCBs at two sides of bus 2 assembled in a large
switchboard or switchgear assembly.
(Marks: 8%)


Part C
1. Figure 3 shows a substation sitting on ground surface which is simplified as a rectangular cuboid with top and side
views shown. Four identical rods are adopted for its lightning protection. They are placed at four corners of an
isosceles trapezium as shown in Fig. 3. Such a design is also for protecting other devices peripheral to the
substation. The rectangular cuboid is placed within the trapezium symmetrically with its two sides overlapping
with two sides of the trapezium. The dimensions are as follows: a=12.5m, b=16m, c=8.0m, h1=7.5m, w1=6.0m.

To meet lightning protection level-II requirement, determine the minimum height h of each of four identical rods
above the ground using rolling sphere method to protect the substation. Also list down other forming parts in the
lightning protection system for the substation.

a
b
c
h
h1
w1
Top view
Side view
A B
CD
w1
A BCD

Figure 3
(Marks: 20%)
2. List down the forming components of a roof-top lightning protection system and discuss the function of each
component. Discuss how to design an effective grounding system and its role in lightning protection and also
discuss its other roles.
(Marks: 10%)