KING FAHD UNIVERSITY OF PETROLEUM AND MINERALS
Electrical Engineering Department
_______________________________________________________________
EE 466
Mid Term Examination
Term 121
Instructors:
Course
EE 466
Date
November 13, 2012
Time
17:30- 18:45
Place
59-1001
Dr. Ibrahim El-Amin
Student Name:_______________________________________
Student Id # ____________________________
Section No. ____________________________
Student Sequence #------------------------------
Question #
Q 1 (30)
Q 2 (30)
Q 3 (20)
Q4 (20)
Total
Score
Q1
A) Draw an impedance diagram for the electric power system shown in Figure
1. Showing all impedances in per unit on a 100-MVA base. Choose 20-kV
as the voltage base for generator. The three-phase power and line-line
ratings are given below.
Gl :
Tl :
T2 :
G2:
Line:
Load:
90 MVA
80 MVA
80 MVA
90 MVA
20 kV
20/200 kV
200/20 kV
18 kV
200 kV
200 kV
X = 9%
X = 16%
X = 20%
X = 9%
X = 120 
S = 48 MW +j64 Mvar
Figure 1
B) The reactance data for the power system shown in Figure 2 in per unit on a common
base is as follows:
Obtain the Thevenin sequence impedances for the fault at bus 1 and compute the fault
current I1012 and I1abc in per unit for a bolted single line-to-ground fault at bus 1.
Figure 2
Q2
Select the correct answer
1. A three phase balanced Y-connected generator has a phase voltage of 265.9 V. The
sequence voltages of the generator are found to be:
a. V0=265.9.5 V ; V1=265.9 V; V2= 265.9 V
b. V0=0.0 V ; V1=265.9 V; V2= 265.9 V
c. V0=0.0 V ; V1=265.9 V; V2= 0.0
d. V0=265.9.5 V ; V1=265.9 V; V2= 0.0 V
2. A three phase unbalanced system has phasor currents as follows:
Ia=
;
Ib=
; Ic=
The sequence currents are found to be:
a. I0= 0.195
I1= 1.311
b. I0= 1.311
I1= 0.494
; I2= 0.494
; I2= 0.195
c. I0= 1.311
I1= 0.494
; I2= 0.195
d. I0= 0.195
I1= 1.311
;
I2= 0.494
3. The equivalent p.u. values of positive , negative and zero reactnces at a fault
location are X1= 0.08; X2=j0.07 p.u. ; and X0 =0.05 respectively. The value of the
fault current following a line-line- ground fault is
a. IF = -j8.0.0 p.u
b. IF = -j16.0. p.u
c. IF = -j20.0 p..u
d. IF = -j12.5 p.u
4. A grounding impedance in the neutral of a generators is represented by :
a.
b.
c.
d.
Three
Three
Three
Three
times its value in the positive sequence network only.
times its value in the negative sequence network only.
times its value in the zero sequence network only.
times its value in the positive and negative sequence networks.
5. A 3-phase 20 MVA 10 kV generator has a reactance of 0.05 p.u. An external
reactance is connected in series with the generator to limit the fault current to 8 times
the load current. The value of this reactance in p.u is :
a.
b.
c.
d.
0.10
0.05
0.075
0.025
Q3
a. A 1200:5 CT has is of class 06C240. The primary fault current is 10000 A.
Determine
(i) Magnetizing impedance
(ii) The magnetizing current under the fault primary current
(iii) The maximum burden impedance of the CT.
b. Two CT of CTs are connected to provide a differential current of zero value
under normal condition .
CT1 is
rated as 500/5
CT2 is
rated as 800/5
i. Specify an auxiliary CT to assure zero current across the relay burden , Zb,
for a primary current of 1000 A.
ii. What is burden of each CT after the auxiliary CT is connected.
c. What is the turns ratio of VT that connects a 230 kV system to 70- V relay
Q4
a. Define the following terms
i. Relay Dependability
ii. Relay Security
iii. Duplicate protection
iv. Backup protection
v. Relay speed
vi. Auxiliary CT
b. Describe in details the operation of :
a. An air circuit breaker
b. A vacuum circuit breaker.