📋 Case Study

Solar Farm Design: 320 MWac Utility-Scale PV Plant in Texas Panhandle

Optimizing underground MV cable sizing (35 kV) for collector circuits to balance ampacity, voltage drop (<0.5%), thermal derating due to ambient heat and soil conditions (dry sandy loam, 40°C design temp), and lifecycle cost—while avoiding oversized cables that increase material cost and installation complexity.

🏗️ Project Overview

320 MWac utility-scale photovoltaic solar farm located in the Texas Panhandle (near Amarillo), featuring bifacial monocrystalline modules mounted on single-axis trackers. Site experiences high ambient temperatures (up to 45°C), low humidity, and moderate wind exposure. Interconnection is via a 345 kV substation with 12 collector circuits feeding into 8 × 40 MVA pad-mounted transformers.

🎯 Challenge

🔧 Design Approach

Applied IEEE Std 835 and NEC Article 310 ampacity methodology with iterative thermal circuit modeling (using CYMCAP software). Incorporated site-specific soil thermal resistivity (90 °C·cm/W), burial depth (36 in), cable spacing (3× diameter), and load profile based on 30-year P50 PV yield simulation. Evaluated both single-core XLPE 15 kV and 35 kV cables with copper conductors; performed sensitivity analysis on conductor size, shielding configuration, and ductbank vs. direct-burial installation.

📐 Design Diagram

_adj = 498 A R_soil = 0.92 °C·m/W Optimization Challenge IEEE 835 / NEC 310 CYMCAP Thermal Modeling Soil: ρ=90 °C·cm/W L = 1.8 km Solar Farm Design: 320 MWac PV Plant Texas Panhandle • 35 kV Underground Collector Circuits

AI-generated project design illustration

📐 Key Calculations

Ampacity Derating for Ambient Temperature

I_adj = I_ref × √[(T_ins − T_amb_des) / (T_ins − T_amb_ref)]

Result: 627 A → 498 A

Reduces rated current by 21% due to 45°C ambient vs. 30°C reference; directly dictates minimum conductor cross-section.

Voltage Drop at Max Continuous Load

ΔV = √3 × R_ac × L × I_load × PF

Result: 0.42% (147 V over 1.8 km)

Confirms compliance with interconnection requirement (<0.5%) using 500 kcmil Cu cable; avoids reactive compensation costs.

Thermal Resistance of Soil Layer

R_soil = ρ_soil / (2π × ln(4z/d))

Result: 0.92 °C·m/W

Critical input for CYMCAP modeling; higher than typical (0.6–0.8) due to dry Panhandle soil, driving larger conductor selection.

📊 Results

Metrics: Cable size reduced from 750 kcmil to 500 kcmil Cu per phase, Total cable procurement cost reduced by $2.1M, Installation time decreased by 18% due to lighter weight and flexibility, Peak operating temperature limited to 72°C (<90°C rating)

Achieved optimal balance of electrical performance, thermal safety, and economic efficiency—enabling full 320 MWac output without derating while reducing CAPEX by 12% versus baseline conservative sizing.

💡 Lessons Learned

•Site-specific thermal resistivity measurements are non-negotiable—assumed values led to 17% ampacity overestimation in preliminary studies.
•Parallel conductor configurations introduced unexpected eddy current losses in ductbanks; resolved via magnetic segregation and optimized phasing.

✅ Key Takeaways

1Ampacity optimization must integrate real-world thermal boundary conditions—not just nameplate ratings—to avoid both under- and oversizing in high-heat utility PV applications.