• Zhejiang Zhilu Transmission and Distribution Equipment Co., Ltd

Research on Foundation Construction and Grounding Grid Installation Standards for Photovoltaic Dedicated Transformer Substations

1. Foundation Construction Standards for Photovoltaic Dedicated Transformer Substations

1.1 Design Principles and Technical Requirements

• Load-bearing Capacity: Must withstand the total weight of the transformer substation (including equipment, cables, and auxiliary facilities), with a safety factor ≥ 1.5.

• Geotechnical Conditions:

• Soil bearing capacity ≥ 100 kPa (adjusted based on geological survey reports).

• Anti-seismic design should comply with local seismic intensity requirements (e.g., seismic fortification intensity ≥ 7 degrees).

• Dimension Specifications:

• Length and width tolerance: ±5 mm; flatness error ≤ 3 mm/m.

• Foundation elevation deviation: ≤ 10 mm across the entire area.

1.2 Construction Process and Quality Control

• Site Preparation:

• Remove topsoil and soft layers; backfill with compacted gravel (compaction coefficient ≥ 0.94).

• Set out the foundation boundaries using total station surveying, with positioning accuracy ≤ 10 mm.

• Reinforcement Works:

• Steel grade: HRB400, with lap length ≥ 35d (d = bar diameter) and protective layer thickness ≥ 50 mm.

• Welding quality: Joint strength ≥ 90% of the parent material, with no cracks or porosity.

• Concrete Construction:

• Concrete grade: C30 or above, with slump control at 120-160 mm.

• Curing period: ≥ 14 days, with surface moisture retention to prevent cracking.

• Anchor Bolt Installation:

• Positioning error: ≤ 2 mm in horizontal direction; verticality deviation ≤ 1/1000.

• Tightening torque: Compliant with manufacturer’s requirements (e.g., M24 bolts: 800-1000 N·m).

1.3 Reference Standards

• IEC 61724-1: Photovoltaic (PV) System Performance Monitoring – Part 1: Measurement, Data Exchange and Performance Evaluation

• GB 50007-2011: Code for Design of Building Foundations (China)

• ASTM D1120: Standard Test Method for Moisture in Soil

2. Grounding Grid Installation Standards for Photovoltaic Dedicated Transformer Substations

2.1 Grounding System Design Requirements

• Grounding Type: Combined grounding system (working ground, protective ground, and lightning protection ground shared), with total grounding resistance ≤ 4 Ω.

• Grounding Material Specifications:

  Component	Material	Cross-Sectional Area	Anti-Corrosion Treatment
  Horizontal 导体	Galvanized steel flat bar	40 mm × 4 mm	Hot-dip galvanizing (thickness ≥ 85 μm)
  Vertical electrode	Galvanized steel pipe	Φ50 mm × 2.5 m	Hot-dip galvanizing
  Connection	Copper-clad steel cable	≥ 70 mm² (flexible sections)	Tinned copper terminals

2.2 Installation Process and Technical Specifications

• Grid Layout:

• Horizontal grounding conductors arranged in a grid pattern, with spacing ≤ 5 m; buried depth ≥ 0.8 m.

• Vertical electrodes (ground rods) installed at grid intersections, with spacing ≥ 5 m (≥ 2 times rod length).

• Connection Requirements:

• Welding: Lap length of flat bars ≥ 2 times width (minimum 100 mm), with full circumferential welding.

• Exothermic welding: Joints must be free of porosity, with solder filling ≥ 95% of the joint volume.

• Anti-Corrosion Treatment:

• Welded joints: Apply zinc-rich primer (zinc content ≥ 95%) + two coats of epoxy resin paint.

• Buried parts: Additional asphalt wrapping for soil with pH < 4 or > 9.

• Grounding Resistance Testing:

• Method: Three-point method (fall-of-potential), with test current ≥ 10 A.

• Test conditions: Soil moisture ≥ 15%, ambient temperature 10-30°C.

2.3 Safety and Environmental Standards

• Lightning Protection:

• Install lightning arresters at the transformer substation inlet, with protection level ≤ 1.5 kV (10 kA lightning current).

• Grounding leads for lightning arresters: Cross-sectional area ≥ 25 mm² (copper) or ≥ 50 mm² (aluminum).

• Environmental Protection:

• Grounding materials must comply with RoHS standards (restriction of hazardous substances).

• Soil backfill around the grounding grid: Free of corrosive substances (chloride content ≤ 0.1%).

2.4 Reference Standards

• IEC 62305-3: Lightning Protection – Part 3: Physical Damage to Structures and Life Hazard

• IEEE Std 80: Guide for Safety in AC Substation Grounding

• GB 50169-2016: Code for Construction and Acceptance of Electrical Equipment Grounding (China)

3. Key Inspection Points for Construction Acceptance

• Foundation: Flatness, load-bearing capacity test reports, and anchor bolt positioning accuracy.

• Grounding Grid: Welding quality, grounding resistance test results, and anti-corrosion layer thickness (measured by eddy current gauge).

• Documentation: Construction records, material certification, and non-destructive testing reports must be archived.