NORTHERN TECHNICAL COLLEGE
DIPLOMA IN ELECTRICAL ENGINEERING
PROTECTION
MOCK EXAM (OCT) 2022
INSTRUCTIONS TO CANDIDATES
1.
2.
3.
4.
5.
Time allowed is (3) Three Hours.
There are Eight (8) questions in this paper. ATTEMPT FIVE (5).
Total marks (100)
No Cell phones and programmable calculators are allowed in the examination room
No candidate shall be allowed to leave the examination room the last thirty (30) minutes
before the end of examination.
DO NOT TURN THIS PAGE BEFORE YOU ARE TOLD TO DO
QUESTION 1
a) List at least five 5 Abnormal Operating Conditions of an induction motor. [5 Marks]
b) What does the term single phasing mean in relation to a 3 phase motor? [3 marks]
c) Describe what happens to the motor operation when it is subjected to single phase
condition
[5 Marks]
d) With reference to the figure and the table shown below, calculate the actual time of
operation of the standard I.D.M.T.L relay situated at sub-stations A, B and C using the data in
the given table. Also assume a selectivity factor of 0.5 and make use of the attached p.s.m /
time curve.
[7 Marks]
SUB- STATTION
A
B
C
Fault current
600A
5000A
3500A
CT ratio
400/5A
300/5A
300/5A
Current setting
150%
100%
75%
T.S.M
?
?
0.1
CT class
P
P
P
QUESTION 2
a) Describe the high-rupturing capacity (HRC) type of fuse. [ 5 Marks]
b) Derive the formula for calculating the R.R.R.V maximum [ 5 Marks]
c) A 50 Hz 3-phase alternator with grounded neutral has inductance of 1.6mH per phase and
is connected to the bus through a circuit breaker. The capacitance to earth between the
alternator and the C.B is 0.003µF per phase. The circuit breaker opens when r.m.s value of
current is 7500A. Determine the following:
(i) Frequency of oscillations. [ 2 Marks]
(ii) Time for maximum rate of rise of re-striking voltage. [ 3 Marks]
(iii) Maximum rate of rise of re-striking voltage. [ 5 Marks]
QUESTION 3
a)
Describe three (3) ways in which Lightning may strike the power
system [5 Marks]
b) What is surge absorber, how do they differ from surge diverter?
c)
[5 Marks]
What are the protective measures that taken against lightning over
voltage? [5 Marks]
a)
What is meant by arcing grounds and how can they be eliminated?
[5 Marks]
QUESTION 4
a) With the help of a schematic drawing describe the construction and principle operation
of a definite –distance type impedance relay
b)
Derive the formula of the torque equation of a definite –distance type
impedance relay
c)
A long transmission line divided into three zones has the following
details:
Z L1  (2 j 5) Ohms primary (to be protected) CT ratio = 400/1A and
d)
VT ratio 33000/110V. Line impedance of the next line section = (3 j 7.5)ohms .
Assuming MHO relay characteristics angle at 45 0 , Determine the zone settings
QUESTION 5
a)
Describe the construction, working and application of a Buchholz
Relay
b)
A three phase power transformer having a line voltage ratio of 400 V
to 33 kV is connected in star-delta. The C.T.s on 400 V side have current ratio as
1000/5. Assume current on 400 V side of transformer to be 1000 A
(i)
Draw the connections of the power transformers and Current
transformers
(ii)
What must be the C.T. ratio on 33 kV side.
(iii) What is an over fluxing protection in a transformer?
QUESTION 6
a) Draw the circuit diagram of a „Restricted Earth Fault‟ protection scheme of a
generator? Why it is named as „Restricted‟?
b)
Explain the objective of the following
(i) Neutral Earthing
(ii) Equipment Earthing
c)
A 230kV, 3 phase, 50Hz, 200km transmission line has a capacitance
to earth of 0.02µF/km per phase. Calculate the inductance and KVA rating of the
Peterson coil used for earthing the above system
d) A star connected 3 phase, 12 MVA, 11 kV alternator has a phase reactance of 10%. It is
protected by Merz-Price circulating current scheme which is set to operate for fault
current not less than 200 A. Calculate the value of earthing resistance to be provided in
order to ensure that only 15% of the alternator winding remains unprotected.
QUESTION 7
a)
A balanced beam has a pick up current of 0.1A and a 10% slop. If
the restrain coil currents are Is1= 4.2A and Is2 = 4A, would the relay operate?
b) A Merz – Price circulating current is used to protect a 3 phase alternator having a full
load current of 400A, the CT ratio is 1000/5A and the current transformer at the
opposite end of the alternator is 166m. The pilot wire has a resistance of 5 ohms per
835m. Under straight through fault conditions of 10 times full load current the CT s at
the other end have a voltage of 80% of that of the other end. A protective relay having
an impedance of 100 ohms is connected across the physical midpoint of the pilot wire
i.
Determine from the physical mid-point where the zero voltage will be located
ii.
The current the relay will have to be set to give a stability factor of 3.
QUESTION 8
a)
Draw an equivalent circuit of a CT
b)
Why the CT secondary must never be left open circuited when the
primary is energized?
c)
A relay is connected to 400/5 ratio current transformer with a current
setting of 150%. Calculate the plug setting multiplier when the circuit carries a fault
current of 4000 A.
d)
What are unit system and non-unit system?