High-Altitude Desert Solar Farm in Ladakh, India

Case Study 1: High-Altitude Desert Solar Farm in Ladakh, India

Scenario

A 5 MW utility-scale PV plant is being designed for a remote site near Leh, Ladakh (elevation ~3,500 m). Extreme diurnal temperature swings and low ambient pressure pose significant voltage derating risks. The project must comply with local grid interconnection requirements mandating strict MPPT window adherence and avoid winter under-voltage shutdowns. Space is abundant, but logistics limit module handling to ≤20 modules per string for manual installation.

Given Data

• voltage_mppt_min = 280 V (inverter’s cold-start MPPT lower bound)
• voltage_mppt_max = 550 V (inverter’s absolute max input voltage — not MPPT upper limit)
• voltage_module_voc_stc = 42.8 V
• voltage_module_vmp_stc = 34.2 V
• temp_coeff_voc = −0.0042 /°C
• temp_coeff_vmp = −0.0040 /°C
• temperature_min = −25 °C (recorded winter minimum at site)
• temperature_max = 65 °C (summer daytime ambient, elevated by altitude & albedo)

Calculation

Step 1: Cold-condition Voc correction Voc at −25°C = Voc_STC × [1 + temp_coeff_voc × (T_min − 25)]
= 42.8 × [1 + (−0.0042) × (−25 − 25)]
= 42.8 × [1 + (−0.0042) × (−50)]
= 42.8 × [1 + 0.21] = 42.8 × 1.21 = 51.79 V

Step 2: Hot-condition Vmp correction Vmp at 65°C = Vmp_STC × [1 + temp_coeff_vmp × (T_max − 25)]
= 34.2 × [1 + (−0.0040) × (65 − 25)]
= 34.2 × [1 − 0.0040 × 40] = 34.2 × [1 − 0.16] = 34.2 × 0.84 = 28.73 V

Step 3: Minimum string length Ensures Vmp_string ≥ MPPT min at hottest condition: string_length_min = ceil(280 V ÷ 28.73 V) = ceil(9.75) = 10

Step 4: Maximum string length Ensures Voc_string ≤ inverter max input (550 V) at coldest condition: string_length_max = floor(550 V ÷ 51.79 V) = floor(10.62) = 10

Result and Decision

Both calculations yield exactly 10 modules per string. This tight constraint confirms the design is temperature-critical: any deviation (e.g., using a module with higher Voc or installing at lower elevation) would violate either cold overvoltage or hot undervoltage limits. The team selected 10-module strings, validated with manufacturer’s low-temp Voc curve and confirmed via inverter firmware log testing during commissioning.

Lesson

At high-altitude sites, extreme cold increases Voc more than standard models assume — always use site-specific min temperature (not “standard” −10°C) and verify with actual module datasheet low-temp Voc curves; generic coefficients may underestimate derating.