North South University
EEE 362 Electrical Transmission and Distribution System
Spring 2020
Final Assignment (Section # 1)
Issue Date: Monday, May 18, 2020
Due Date: Thursday, June 4, 2020
This assignment contains six questions. All questions have to be answered.
Total point of this assignment is 260. It counts to 60% of your course grade.
Name:
Student ID #:
Mobile Ph #:
Answering guideline:
• Print this file and write your answer on the printed paper, scan the answer script and
submit the scanned version. Add extra pages when the printed space is not sufficient.
• You may consult textbooks, class notes and online sources. However, for online
sources, make sure they are authentic sources, such as they are from journal papers,
conference papers or technical notes from well-known companies (GE, ABB, etc.). If
you give online references, please mention the sources.
• It is not allowed to consult with any person who has knowledge of this subject,
including other students of this course. You may ask question to the instructor if you
do not understand a question, but not more than that. All solutions have to be your
own work.
• You must show all work for each problem to receive full credit.
CODE OF HONOR PLEDGE
I pledge on my honor that I have not given or received any unauthorized assistance on this
assignment.
Signature: ____________________________
Date: _______________
1. Consider a single-phase electric system shown in the following figure.
Transformers are rated as follows:
X–Y 20 MVA, 13.8/138 kV, leakage reactance 12%
Y–Z 20 MVA, 138/69 kV, leakage reactance 15%
With the base in circuit Y chosen as 15 MVA, 138 kV, determine the per-unit
impedance of the 500 Ω resistive load in circuit Z, referred to circuits Z, Y, and X
separately. Neglecting line impedances, draw the impedance diagram of the system in
per unit. (5+5+5+10 = 25)
2. Following figure shows the one-line diagram of a three-phase power system. Select
system power base 100 MVA and voltage base 25 kV on the generator side. The data
are given as follows:
G:
T1:
T2:
T3:
T4:
M:
100 MVA
50 MVA
40 MVA
40 MVA
40 MVA
60 MVA
25 kV
25/220 kV
220/11 kV
25/110 kV
110/11 kV
10.45 kV
x = 0.12 per unit
x = 0.10 per unit
x = 0.15 per unit
x = 0.15 per unit
x = 0.10 per unit
x = 0.2 per unit
Lines 1 and 2 have series reactances of 50 Ω and 70 Ω, respectively. At bus 4, the
three-phase load (external system) absorbs 70 MVA at 10.45 kV and 0.6 power factor
lagging. A three-phase line to ground fault occurs at point F (between bus 4 and the
load). Draw a per-unit impedance diagram showing all impedances including the load
impedance before fault. After the fault, from the impedance diagram draw Thevenin
equivalent circuit step-by-step and find fault current in ampere flowing at point F. (30
+ 20 = 50)
3. Consider the following power transmission system:
Given, System Base: 100 MVA, 115 kV. Bus 1 is voltage regulated (i.e. generator can
adjust bus voltage at the desired value). Bus 2 has a load PL+ jQL and has a switchable
capacitor bank jQC. For all operating states, it is desired to maintain the voltage at Bus 2 at
V2 =1.0∠00 pu
Using circle diagram answer the following questions:
a. Operating condition 1: PL = 180 MW; pf = 0.9 lagging. Determine V1∠ δ
b. Operating condition 2: Maintain the load power factor at the value of state 1, but
increase load’s real power PL = 400 MW. Determine new V1∠δ
c. Operating condition 3: The load remains the same as in the state 2. But it is desired to
bring down the voltage at Bus 1 to the value of the state 1 by connecting a bank of
capacitors in parallel with the load. If the capacitor bank is connected in Y, calculate
the value of the capacitance in μF (assume f = 60hz).
(25 + 25 + 25) = 75
4. One circuit of a single-phase transmission line is composed of three solid 0.5cm radius
wires (conductor X). The return circuit is composed of two solid 2.5cm radius wires
(conductor Y). The arrangement of conductors is shown in the following figure. Find
the inductance due to the current in each side of the line and the inductance of the
complete line in henrys per meter (and in millihenrys per mile). (30)
5. Balanced three-phase voltage of 100 V line to line are applied to a Y-connected load
consisting of three resistors. The neutral of the load is not grounded. The resistance in
phase a is 15 Ω, in phase b is 25 Ω and in phase c is 40 Ω. Select voltage to neutral of
the three-phase line as reference. Draw the circuit diagram and determine the current
in phase a and voltage Van. (30)
6. Short Questions (10 x 5 = 50)
a. Why 3φ AC system is used in power transmission, why not 1φ, 2φ, 4φ, 5φ, 6φ or
any other combination of AC systems?
b. Mention four factors to control power flow to the load bus. Why two of these are
controllable and the other two are not?
c. Suppose you need to transmit 300 MW of power between locations A and B,
which are 150 km apart. What level of transmission voltage would you recommend
and why?
d. Draw a circle diagram and show and explain briefly the different factors that limit
maximum power that can be transferred to a load.
e. Explain the concept of decomposing an unbalanced system voltage into three sets:
positive, negative and zero sequence.