Dr. Ömer Cihan Kıvanç
EEE458
ELECTRICAL DISTRIBUTION SYSTEMS
MID-TERM EXAM
13.04.2020
1. A power plant single-line diagram is shown in Fig. 1. A breaker is connected to 10 kV bus. Calculate
the trip short-circuit current and power (𝐼𝑎 , 𝑆𝑎 ). [𝜇 = 0.8] (20 PTS)
𝑆𝑘" = 3000 𝑀𝑉𝐴
110 𝑘𝑉
30 𝑀𝑉𝐴
𝑢𝑘 = 12%
25 𝑀𝑉𝐴
30 𝑀𝑉𝐴
𝑥𝑑" = 12.5%
𝑢𝑘 = 12%
10 𝑘𝑉
Fig. 1 Power plant single-line diagram.
2. A 10 kV medium voltage distribution line is shown in Fig. 2. Calculate rated cable cross sections.
[𝑣 ≤ 10%, 𝑥 " = 0.4 Ω/𝑘𝑚, k = 56 m/Ω𝑚𝑚2 ] (15 PTS)
10 𝑘𝑉
6 𝑘𝑚
4 𝑘𝑚
500 𝑘𝑉𝐴
20 𝐴
cos 𝜑 = 0.8
cos 𝜑 = 0.6
Fig. 2 Medium voltage distribution line.
3. A low voltage distribution line is shown in Fig. 3. Calculate the end-line voltage value, rated cable
cross sections and total power losses. [𝑣 ≤ 3%, 𝑥 " = 0.4 Ω/𝑘𝑚, k = 56 m/Ω𝑚𝑚2 ] [Rated Cable Cross
Sections: 2.5; 4; 6; 10; 16; 25; 35; 50; 70; 95 (𝑚𝑚2 )] (15 PTS)
220
380 𝑉
50 𝑚
𝐴
30 𝑚
𝐵
20 𝑚
𝐶
M
25 𝐴
15 𝐴
Fig. 3 Low voltage distribution line.
13 𝑘𝑉𝐴
cos 𝜑 = 0.8
4. Calculate the starting 3Ø short-circuit current value at 34.5 kV bus (B). Simulate the power system
shown in Fig. 4, using Matlab/Simulink and show the current and power flow in scopes (assuming
that there is no short-circuit anywhere). (35 PTS)
𝑆𝑘" = 2500 𝑀𝑉𝐴
154 𝑘𝑉
𝑆𝐺 = 25 𝑀𝑉𝐴
2𝑥
𝜀𝑑" = 12.5%
𝑆𝑇𝑅 = 30 𝑀𝑉𝐴
𝑢𝑘 = 12%
𝐴
10 𝑘𝑉
3 𝑥 31.14 𝑚𝑚 2
3 𝑥 62.44 𝑚𝑚 2
𝑥0 = 0.4 Ω/𝑘𝑚
𝑆𝐺 = 10 𝑀𝑉𝐴
𝜀𝑑" = 12.5%
𝑥0 = 0.4 Ω/𝑘𝑚
8 𝑘𝑚
𝜘𝐴𝐿 = 27.77 𝑚/Ω𝑚𝑚 2
34.5 𝑘𝑉
𝐵
M
𝐼𝑁 = 176 𝐴
𝐼𝐼𝐶 = 5𝑥𝐼𝑁
Fig. 4 Distribution system single-line diagram.
5. Calculate maximum voltage drop point & value at Fig. 5. [k = 56 m/Ω𝑚𝑚2 , 𝑞𝑛 = 25 𝑚𝑚2 , 𝑥𝑙𝑖𝑛𝑒 =
0.1 Ω/𝑘𝑚] Simulate the power system shown in Fig. 5, using Matlab/Simulink and show the
current and power flow in scopes. (35 PTS)
500 𝑘𝑊
cos 𝜑 = 0.8
630 𝑘𝑉𝐴
400 𝑚
cos 𝜑 = 0.6
500 𝑚
800 𝑚
10 𝑘𝑉
600 𝑚
500 𝑚
380 𝑘𝑊
500 𝑘𝑉𝐴
36 𝐴
cos 𝜑 = 0.8
Fig. 5 A distribution system.