Q: I need to test a 220 kV cable, 8 km long. What SR rating do I need?
A: 7-step sizing procedure: determine test requirements → calculate C → estimate L → compute f_res → verify ratings → iterate → finalize.
Step-by-step - 220 kV cable, 8 km:
1. IEC 60840: V_test = 2.0 × 127 kV = 254 kV. Duration 60 min.
2. C ≈ 8 km × 0.2 μF/km = 1.6 μF.
3. Consider 3× 100 kV/2 A reactors in parallel: L=26.7 H, f=24.4 Hz.
4. Current at 254 kV: I = V × 2πf × C = 6.9 A. But 3×2A → 6A total → NOT OK.
5. Upgrade to 3× 100 kV/4 A reactors: total I = 12 A → OK. P_in = 254×6.9/35 = 50 kW.
6. Total weight: 3×450 kg + 200 + 150 + 100 = ~1,800 kg.
Sizing summary for common scenarios:
|
Scenario |
V_test (kV) |
C (μF) |
f (Hz) |
Recommended System |
|
33 kV cable, 3 km |
48 |
0.6 |
45 |
60 kV/2A ×1, 5 kW |
|
110 kV cable, 5 km |
160 |
1.0 |
24 |
200 kV/4A ×2, 20 kW |
|
220 kV cable, 8 km |
254 |
1.6 |
24 |
300 kV/4A ×3, 50 kW |
|
400 kV cable, 5 km |
330 |
1.0 |
48 |
400 kV/2A ×2, 25 kW |
|
GIS 245 kV, 10 bays |
230 |
0.05 |
80 |
300 kV/1A ×2, 5 kW |
|
Generator 100 MVA |
28 |
2.5 |
45 |
30 kV/5A, 10 kW |
Critical reminders:
• +20% margin on reactor voltage rating. +30% margin on current rating.
• If P_in > 100 kW, consider sectional testing (permitted by most standards).
• Actual Q may be lower than estimated - conservative planning avoids field failures.