4. Calculation of short circuit happened on Node 5
Because the 3-phase short circuit is very rare on Node 5, we just take consideration of the one-line ground fault.The short circuit could be calculated from equation (10.19)
Home Work for ELEG 5503
#4 Homework on Design of Advanced Power Distribution
Instructor: Dr. Juan Balda
University ID:
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Fig.1 The one-line diagram of the system
5. Selection of the fuses when load current through Fuse A is 140A.
We assume that the type of the system is overurrent of fuse B is 40A. The short circuit current on the node 5 is 2129A and the voltage is 7.2 kV.
Obviously, 9940 A>2129 A. Therefore, the two fuses could coordinate well.
Additionally, we can use the coordination tables from the S&C Website. It is very smart because a on-line tool is available to draw the TCC. The curves are as shown in Fig.3.
Fuse A:
The peak load current of fuse A is 140A and the short circuit current of node 4 is 4.7kA.
According to the calculation above, we can choose the“K”SpeedPistrolFuse Linkof S&C Electric Company,40K for Fuse B and 140K for Fuse A. The parameters of the two fuses are as shown in Table 1.
7. Selection of the fuses when load current through Fuse A is 90A.
Fig.3 The TCCs curve of the fuses when Ia is 140A
Table 2The parameters of S&C Pistrol fuses link
Fuse
Type
Rating current
Nominal Voltage
B(Protecting)
40K
51A
1. Calculation of the transformer parameters
A
2. Calculation of the line parameters
3. Calculation of short circuit happened on Node 4
Becausethree-phase faultis the most serious fault, we just take consideration of three-phase fault on Node 4. Because that the connection of XFMR is delta-wye-grounded, the short circuit could
Fig.2 The TCCs curve of the fuses
As is shown in Fig.2, the coordination limit point of the two fuses is (9940, -1.718). That means, the limit point is 9940A and 0.019s.
6. Time-Current Characteristic of fuses
The TCCs of the fuses could be downloaded on the website of S&C. Therefore, we can draw the TCCs curves of the fuses through MATLAB, which are as shown in Fig.2, and find the coordination limit.
Table 1 The parameters of S&C SMU-20 fuses
Fuse
Type
Rating current
Nominal Voltage
B(Protecting)
40K
51A
7.2 and 14.4 kV
A(Protected)
140K
210A
7.2 and 14.4 kV
becalculated from equation (10.8).Generally, negative (-) sequence impedance is equal to positive (+) sequence impedance.
7.2 and 14.4 kV
A(Protected)
100K
115A
7.2 and 14.4 kV
From Fig.3, the coordination limit is (6882 A, 0.014 s). Therefore, the selection of the fuses is reasonable.