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IEC TS 62738-2018: Ground-Mounted Photovoltaic Power Plants Design Guidelines
Design Guidelines and Recommendations for Utility-Scale Solar PV Installations
1. Introduction and Scope
IEC TS 62738-2018, prepared by IEC TC 82 (Solar photovoltaic energy systems), provides comprehensive design guidelines for utility-scale ground-mounted photovoltaic (PV) power plants. As the PV market has matured, utility-scale plants have become a dominant segment, yet design practices have varied considerably due to the lack of consolidated guidelines for systems not accessible to the general public. This Technical Specification addresses the unique aspects of large-scale PV plants that distinguish them from residential and commercial systems, including: ground-mounted array configurations, cable routing methods, cable selection, overcurrent protection strategies, equipotential bonding over large geographical areas, and equipment specifications for medium-voltage collection systems.
Tip: The standard applies to grid-connected systems with direct interconnection to utility medium-voltage or high-voltage grids, where access is restricted to qualified personnel only, and continuous monitoring (on-site or remote) is maintained. Rooftop, BIPV, and BAPV systems are explicitly excluded.
2. PV Array System Configuration
2.1 Array Physical Configurations
The standard describes several physical mounting configurations for PV arrays:
| Configuration | Description | Energy Yield | Capital Cost |
|---|---|---|---|
| Fixed tilt | Structures at a fixed optimal angle | Baseline | Lowest |
| Adjustable tilt | Seasonal manual angle adjustment | +3-5% | Low |
| Single-axis tracking | Rotation about one axis (typically horizontal N-S) | +15-25% | Medium |
| Two-axis tracking | Full sun tracking in two axes | +25-40% | Highest |
| Concentrating PV (CPV) | Lenses/mirrors focus sunlight on small high-efficiency cells | Highest per area | Very high |
2.2 Earthing Configurations
The standard addresses four earthing configurations for PV array DC circuits:
- Unearthed DC circuits — most common for modern transformerless inverters; requires insulation monitoring devices (IMD)
- High-ohmic earthed DC circuits — used where continuous operation is required despite first fault
- Functionally earthed DC circuits — one pole connected to earth through a defined impedance for EMC or safety reasons
- Solidly earthed DC circuits — traditional approach, less common with transformerless topologies
Warning: The choice of earthing configuration significantly affects fault current levels, protection coordination, and personnel safety. Unearthed systems require mandatory insulation monitoring per IEC 61557-8. The standard emphasizes that the earthing strategy must be documented in the system design and coordinated with the inverter manufacturer’s requirements.
3. Safety and Protection Systems
3.1 Overcurrent Protection
The standard provides detailed guidance on DC overcurrent protection for PV arrays. Key requirements include:
- String overcurrent protection is required when the maximum reverse current exceeds the module’s rated fuse current
- Sub-array and array overcurrent protection must be provided at combiner boxes and recombiner boxes
- Protection devices must be rated for DC voltage and current, with appropriate interrupting rating
- Coordination between string, combiner, and main array protection must be verified by calculation
3.2 Lightning and Surge Protection
Given the large geographical area of PV power plants (often spanning 10-100+ hectares), lightning protection presents unique challenges. The standard provides guidelines for:
- External lightning protection system (air terminals, down conductors, earthing)
- Equipotential bonding across the entire plant area, including bonding of metallic structures, cable trays, and equipment enclosures
- Surge protective device (SPD) selection and placement at the array, combiner box, inverter, and LV/MV interface levels
- Separation distance calculations between lightning protection system and PV structures
Engineering Insight: Equipotential bonding for a 100-hectare solar farm requires careful design to avoid ground loops and circulating currents. The standard recommends a grid-type earthing system with horizontal conductors buried at 0.5-1 m depth, spaced at regular intervals (typically 20-40 m), interconnected at multiple points. Each PV mounting structure, tracker, combiner box, and inverter must be bonded to this grid. Use welded or crimped connections (not bolted) for buried bonds to ensure long-term corrosion resistance. The total earthing resistance should be less than 10 Ω.
4. Cable Selection and Installation
The standard dedicates substantial attention to cable systems, which represent a significant cost and failure risk in large PV plants. Key recommendations include:
- Cable types: Use PV1-F or H1Z2Z2-K cables for DC circuits (double-insulated, UV-resistant, halogen-free)
- Cable sizing: Voltage drop should not exceed 2% for DC circuits; ampacity must consider derating for ambient temperature, grouping, and burial depth
- Cable routing: Detailed guidance for underground trench configurations (direct burial, ducts, cable trays) with separation between DC, AC, and communication cables
- Armoured cables: Required for direct burial in areas with heavy machinery or vermin activity
Critical: One of the most common causes of fires in PV power plants is DC arc faults in connectors and cables. The standard references the requirements for arc-fault detection (per IEC 63027) and recommends using qualified installers, factory-assembled harnesses where possible, and regular thermal imaging surveys of all DC connections. Never mix connector manufacturers — using connectors from different brands on the same mating pair is a known cause of connector overheating and arcing.
5. Acceptance Testing and Maintenance
The standard specifies two levels of performance acceptance testing:
- Preliminary performance acceptance test — performed within 30 days of commissioning, verifies that the plant meets the minimum performance requirements specified in the power purchase agreement (PPA)
- Final performance acceptance test — performed after 12 months of operation, accounting for seasonal variations, verifies the annual energy yield meets specifications
For maintenance, the standard recommends periodic inspections of mechanical structures (torque checks on bolts, corrosion inspection), electrical connections (thermal imaging), and cleaning schedules based on soiling rates at the specific site location.
6. Frequently Asked Questions
Q1: Does IEC TS 62738 apply to residential rooftop solar systems?
A: No. The standard explicitly excludes rooftop, BIPV, and BAPV systems. It is specifically for ground-mounted utility-scale plants with restricted access, continuous monitoring, and direct interconnection to medium or high voltage grids. For residential systems, refer to IEC 62548 and relevant national codes.
Q2: What is the recommended distance between the DC array and the inverter?
A: The standard does not specify a fixed distance but recommends that DC cable runs be minimized to reduce voltage drop and cable costs. For central inverter configurations, the distance is determined by the plant layout and may be several hundred meters. Voltage drop calculations must confirm that the total DC circuit voltage drop does not exceed 2% at rated current.
Q3: How does the standard address energy storage systems?
A: Clause 5.4 addresses energy storage integration, recognizing that battery storage is increasingly co-located with PV power plants. The standard provides guidance on DC-coupled (battery connected to DC bus) and AC-coupled (battery connected via its own inverter) configurations. However, detailed battery system requirements are referenced to other IEC standards such as IEC 62619 and IEC 62933.
Q4: What monitoring is required for a PV power plant?
A: The standard requires continuous monitoring of: DC string currents (for string-level fault detection), inverter operating parameters, AC power output, meteorological data (irradiance, temperature, wind speed), and insulation resistance (for unearthed DC systems). Clause 13 specifies communication system requirements including data sampling speed and protocol considerations for integrating with plant SCADA systems.
