Question 8.7: Determine all the required sections of a unit substation for......
Determine all the required sections of a unit substation for a radial distribution system shown in Figure 8.21. Then, size the electrical room assuming the unit substation is placed in the center of the room.
Using the different steps outlined in the general design procedure, the unit substation sections can be selected for the system of Figure 8.21.
1. Primary section:
I_L=\frac{1750 \ kVA}{(1.732)(13.8 \ kV)}=73.2 \ A→ Use 600 \ A, 15 \ kV, W=36 \ in., H=90 \ in., D=54 \ in. primary section
2. Transformer section:
Table 8.6 → W = 100 \ in., H = 100 \ in., D = 54 \ in.
3. Main secondary section:
I_L=\frac{1750 \ kVA}{\sqrt{3} (0.48 \ kV)}=2105 \ A \\I_{CB}=1.25\times I_L=2631 \ ATable 8.7 → use 3000 \ A CB (I_{SC}=65,000 \ A <50,000 \ A)
Table 8.8 → unit \#1,W=30 \ in.
4. Feeder section:
I_L=700 \ A ,I_{CB}=840 \ ATable 8.7 → 1600 \ A CB (I_{SC}=50,000 \ A <40,000 \ A)
Table 8.8 → use unit \#3,W=26 \ in.
Each can handle three panels → two feeder sections are needed
5. Electrical room size: Figure 8.22 shows the layout of the unit substation based on the selection of all the sections. Since the total width of the unit substation is 218 \ in. or 18.16 \ ft and its depth is 54 \ in., the size of the room with a clearance distance of 6 \ ft in all sides can be determined:
Electrical room size = 30.16 \ ft(width)\times 16.5 \ ft(depth)
| TABLE 8.6 Ratings and Dimensions of Dry-Type Transformer Sections Suitable for Unit Substations |
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| kVA Rating of the Transformer | Dimensions of the Transformer Section^a | |||||
| 5 kV Systems | 15 kV Systems | |||||
| H | W | D | H | W | D | |
| 111.5 | 90 | 30 | 54 | 90 | 30 | 54 |
| 150 | 90 | 30 | 54 | 90 | 30 | 54 |
| 225 | 90 | 30 | 54 | 90 | 42 | 54 |
| 300 | 90 | 42 | 54 | 90 | 90 | 54 |
| 500 | 90 | 42 | 54 | 90 | 90 | 54 |
| 750 | 90 | 90 | 54 | 90 | 90 | 54 |
| 1000 | 90 | 90 | 54 | 90 | 100 | 54 |
| 1500 | 90 | 100 | 54 | 90 | 100 | 54 |
| 2000 | 90 | 100 | 54 | 100 | 100 | 54 |
| 2500 | 90 | 100 | 54 | 100 | 100 | 54 |
| ^a The dimensions are expressed in inches and should be multiplied by 2.54 to express them in cm. |
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| TABLE 8.7 Frame Sizes and Typical Ratings for Low-Voltage Power Circuit Breakers |
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| Frame Size (A) | Trip Ratings^a (A) | Instantaneous Trip Interrupting Symmetrical Current Ratings (A) |
Short-Time (0.5 s Delay) Trip Interrupting Symmetrical Ratings (A) |
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| 208/240 V | 480 V | 600 V | 240 V | 480 V | 600 V | ||
| 600 | 50–600 | 42,000 | 30,000 | 30,000 | 30,000 | 30,000 | 30,000 |
| 600 | 50–800 | 42,000 | 30,000 | 30,000 | 30,000 | 30,000 | 30,000 |
| 1600 | 50–1600 | 65,000 | 50,000 | 42,000 | 50,000 | 50,000 | 42,000 |
| 2000 | 50–2000 | 65,000 | 50,000 | 50,000 | 50,000 | 50,000 | 50,000 |
| 3000 | 1200–3000 | 85,000 | 65,000 | 65,000 | 65,000 | 65,000 | 65,000 |
| 4000 | 1600–400 | 130,000 | 85,000 | 85,000 | 85,000 | 85,000 | 85,000 |
| ^a Standard trip ratings include 50, 100, 150, 200, 225, 300, 400, 600, 800, 1200, 2000, 2400, 3000, and 4000 A. | |||||||
| TABLE 8.8 Ratings and Dimensions of Main Feeder and Subfeeder Sections |
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| Unit No. | Width [in. (cm)] | Maximum Current Ratings (A) |
Number of Compartments |
Applications |
| Unit 1 | 30 (76) | 3000 | 1 | Main secondary sections |
| Unit 2 | 18 (46) | 600 | 4 | Feeder sections |
| Unit 3 | 26 (66) | 1600 | 3 | Feeder sections |
| Unit 4 | 30 (76) | 3000 | 3 | Feeder sections |
| Unit 5 | 34 (86) | 4000 | 2 | Main secondary or feeder sections |
| Unit 6 | 26 (66) | 600–1600 | 4 | Feeder sections (with different feeder ratings) |
