-1University of Saskatchewan
Department of Electrical & Computer Engineering
EE 868.3 Digital Techniques for Power System Protection (Spring & Summer, 2012)
Midterm Examination (In Classroom)
Examination Paper Hand-In Time: June 18, ‘12
Instructor: Dr. Rama Gokaraju
Time: 9:00am-1:00pm
Total Marks: 25
Instructions:
1) This examination paper consists of 6 problems and 4 pages in total.
2) You are not permitted to discuss or take assistance of others to solve the examination problems.
You would be severely penalized if your solutions appear to have been copied from others.
3) Your solutions should be methodical. You would be penalized if your solutions are illegible.
4) Mark allotted for each problem is shown on the right margin.
Problem 1 (Design Problem)
A portion of a 765 kV network is shown in Figure 1. The positive and zero sequence impedances of the
transmission lines are 0.02 + j0.6 Ω and 0.1 + j 1.8 Ω per mile, respectively. The negative sequence
impedance is same as the positive sequence impedance for the transmission line. Assume that the
generator impedance is j10 Ω for positive and negative sequence, and j20 Ω for the zero sequence.
(a) Relays R12, R23, and R34 use directional type IEC standard inverse current characteristics for
single-line-to-ground fault protection of this system. Determine the fault current needed for setting
the pickup value of R12 ground relay. You may neglect line resistances for this calculation.
(b) Select CT and CVT ratios for phase distance relays at bus 1. You may assume that the relay
current coils can carry 10 A continuously, and that the emergency line loading limit is 3000 MVA.
Use standard CT ratios.
(c) Determine and show on the (secondary) R-X diagram the three zones of a directional impedance
realy at bus 1 for phase fault protection.
(d) Also show the location of the equivalent impedance of the emergency load on the R-X diagram.
Do you see any problems with line operating at emergency load? What solution would you
propose?
The tap settings are as follows: 1.0, 1.2, 1.5, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 12.0 A.
The standard CT ratios are listed in Table 1. The IEC standard inverse over-current characteristic curves
are given in Figure 2.
Figure 1: One-line diagram of the 765 kV network.
-2Current ratio
50:5
100:5
150:5
200:5
250:5
Table I: Standard CT ratios
Current ratio
300:5
400:5
450:5
500:5
600:5
Figure 2: Characteristic curves of type IEC standard inverse overcurrent relays.
Current ratio
800:5
900:5
1000:5
1200:5
7 Marks
-3Problem 2
A 100 MVA, 18 kV turbogenerator having X d''  X 1  X 2  20% and X 0 =5% is about to be connected to
a power system. The generator has a current-limiting reactor of 0.162 Ω in the neutral. Before the
generator is connected to the system, its voltage is adjusted to 16 kV when a double line-to-ground fault
develops at terminals b and c. Find the initial symmetrical rms current in the ground and in line b.
4 Marks
Problem 3
Consider the multi-terminal line in the system shown in Figure 3. Each of the buses C, D, G, H and J has
a source of power behind it. For a three-phase fault on bus B, the contributions from each of the sources
are as follows:
Figure 3 System diagram for Problem 3.
You may assume that the fault current contributions from each of these sources remain unchanged as the
fault is moved around throughout the system shown. Determine the zones 1, 2 and 3 settings for the
distance relays at buses A and B.
Take into account the effect of the infeed for determining the zones 2 and 3 settings, while no infeeds are
to be considered for the zone 1 settings.
7 Marks
-4-
Problem 4
A blown fuse or broken conductor that opens one of the three phases represents a series unbalance and is
referred to as one-open conductor fault. Consider phase a open on the 34.5 kV line at bus H that is given
in Fig. 4. All constants are in per unit on a 30 MVA base.
Fig 4: Single-line diagram of the system.
Consider a 30 MVA, 90% pf load as shown in the figure. Assume voltage at the load to be 1.0∟00.
Determine the change in bus voltage at bus H. Find the currents flowing in the system. From the
calculated values of the currents, comment whether there would be any difficulty in providing protection
for open-conductor faults in the system.
If there is no load connected to the system, would an open-conductor fault have any difference on the
system.
7 Marks
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