Solar Farm Interconnection Cable Sizing for Rural Grid Tie

Case Study 1: Solar Farm Interconnection Cable Sizing for Rural Grid Tie

Scenario A 2.5 MW AC solar photovoltaic (PV) plant is being commissioned in central Rajasthan, India — a hot, arid region with ambient temperatures regularly exceeding 45°C. The plant must interconnect to a nearby 33 kV substation via an overhead line segment and underground transition. Due to land acquisition constraints, the final 85 m of the connection runs as single-core copper cables in rigid PVC conduit buried just beneath a service road. Local utility regulations mandate ≤3% voltage drop at full load and require derating for ambient temperature (though the calculator does not model temperature derating directly — this was handled separately per IEC 60364-5-52). Critical constraints include limited trenching access, no opportunity for future capacity upgrade, and strict compliance with CEA (Central Electricity Authority) guidelines.

Given Data

• Voltage: 400 V (LV side of site transformer, 3-phase, 4-wire)
• Power: 20,000 W (20 kW auxiliary load — SCADA, lighting, gate power, and cooling for inverters’ LV control panels; not the main 2.5 MW export path)
• Power Factor: 0.85 (measured average for mixed electronic loads)
• Cable Length: 85 m (actual routed length including bends and conduit entry offsets)
• Cable Material: Copper
• Installation Method: In Conduit

Calculation Using the Cable Size Calculator:

1. Current calculation: ( I = \frac{P}{\sqrt{3} \times V \times \text{PF}} = \frac{20{,}000}{1.732 \times 400 \times 0.85} \approx 34.18 , \text{A} )
2. Voltage drop estimation (per calculator’s internal model, based on AC resistance & reactance for 3-phase, using IEC 60287 approximations):
   • For copper, 16 mm² in conduit: estimated R ≈ 1.15 Ω/km, X ≈ 0.08 Ω/km → total impedance ≈ 1.154 Ω/km
   • Voltage drop = ( \sqrt{3} \times I \times Z \times L = 1.732 \times 34.18 \times (1.154 \times 0.085) \approx 5.42 , \text{V} ) (1.36% of 400 V)
   • Calculator outputs: voltage_drop = 5.42 V, current = 34.18 A
3. Cable size selection: The tool recommends 16 mm², satisfying both current-carrying capacity (≥34.18 A) and voltage drop (<15 V, i.e., <3.75% — well within utility’s 3% limit for critical auxiliaries).

Result and Decision The engineering team selected 16 mm² Cu, XLPE-insulated, PVC-sheathed, single-core cables (IS 694 / IS 7098) installed in non-metallic conduit. Although 10 mm² met ampacity under standard conditions (51 A @ 30°C), it yielded 8.9 V drop (2.2%) — acceptable numerically, but rejected due to insufficient thermal margin in 45°C ambient and lack of headroom for future load growth (e.g., EV charging pilot). 16 mm² provided 12% margin on voltage drop and 55% margin on current rating after applying 0.71 derating factor (per IS 1554-1 Annex D for 45°C).

Lesson

Always validate the calculator’s output against local ambient derating factors and regulatory voltage drop thresholds — a result that passes numerically may still be non-compliant or unreliable in real-world environmental conditions.