SOLAR TODO delivered a high-reliability Power Transmission Tower line modernization in Sofia, Bulgaria, focused on replacing and upgrading critical 35kV corridors with steel tubular transmission poles engineered for long service life, strong mechanical performance, and compliance with European design rules.
Answer Capsule (TL;DR): In Sofia, SOLAR TODO installed 266 × 18m 35kV double-circuit tubular poles (≈18t/pole) over ~16km with 60m spans, designed to IEC 60826 / GB 50545 wind conditions and ACSR loading.
Project Overview: Power Transmission Tower Upgrade in Sofia
Sofia’s power network requires frequent reinforcement to maintain stable voltage and withstand dynamic weather patterns common across the Balkans—especially gust-driven wind and winter icing conditions. For this project, SOLAR TODO supplied and deployed steel tubular transmission poles (NOT lattice, NOT FRP) as the primary structural element for a 35kV double-circuit line.
The engineering intent was straightforward: use a mechanically robust, corrosion-resistant monopole structure with predictable installation behavior, then configure conductor spacing, insulator geometry, and grounding accessories to match Sofia’s operational requirements. The resulting line provides consistent mechanical margins against wind actions per IEC 60826, while maintaining safe ground clearance (5.5m) and controlled phase spacing (1.5m).
Why the Steel Tubular Pole Matters (Product-Centered)
Unlike lattice structures that can introduce complex maintenance and corrosion interfaces, SOLAR TODO’s steel tubular transmission pole is a tapered round or dodecagonal monopole made from hot-dip galvanized Q345 steel. The tubular geometry offers favorable stiffness-to-weight behavior, which is particularly valuable for mid-voltage corridors where cost-effective strength and long-term durability are both required.
This project used a configuration designed for real-world line tension and wind loading: the poles are 18m tall, each weighing approximately 18t/pole (1000kg/m), and the line is designed with 60m spans across a total length of ~16km. SOLAR TODO also standardized the hardware interfaces—cross-arm brackets for insulator strings + ACSR conductors, flanged bolt sections, and a concrete foundation with anchor cage—to reduce on-site variability and speed up erection.
Deployment in Sofia: Line Configuration & Installation Approach
SOLAR TODO installed 266 units of the 18m tapered steel tubular poles for the 35kV double-circuit line. The electrical/mechanical layout was defined by:
- Phase spacing: 1.5m
- Ground clearance: 5.5m
- Span length: 60m
- Total line length: ~16km
- Conductor: ACSR 400 (rated mass 1520kg/km, max tension 110kN)
- Insulator length: 0.8m
- Wind class: 2 (30 m/s) per IEC 60826
Because Sofia experiences strong seasonal winds and rapid temperature shifts, the project’s mechanical design emphasized reliable load paths from conductor forces to the monopole body and into the foundation. Each pole is anchored via the anchor_bolt foundation system, with the concrete designed to work with the pole’s flanged bolt sections and to maintain stable alignment under load.
Technical Specifications
- Product: Steel Tubular Transmission Pole (NOT lattice, NOT FRP)
- Quantity: 266 units
- Voltage class: 35kV
- Circuit: Double circuit
- Pole height: 18m tapered steel tubular pole
- Pole material: Hot-dip galvanized Q345 steel
- Pole weight: ~18t/pole (1000kg/m)
- Geometry: Tapered round or dodecagonal steel monopole (project configuration selected for line design)
- Cross-arm interface: Cross-arm brackets for insulator strings + ACSR conductors
- Phase spacing: 1.5m
- Ground clearance: 5.5m
- Conductor: ACSR 400 (1520kg/km, max tension 110kN)
- Insulator length: 0.8m
- Span: 60m
- Total line length: ~16km
- Wind class: 2 (30 m/s) per IEC 60826
- Foundation: concrete foundation with anchor cage; anchor system: anchor_bolt
- Accessories: climbing steps + cross arm + grounding + bird guard + vibration damper
- Standards compliance: IEC 60826 / GB 50545
Engineering Compliance: IEC 60826 / GB 50545 Design Basis
SOLAR TODO engineered the Sofia line to reflect recognized transmission design practices for wind and structural effects. IEC 60826 provides the framework for calculating weather and mechanical loads on overhead lines, including wind actions relevant to wind class 2 (30 m/s) used in this project. GB 50545 supports structural design verification for overhead line structures.
This matters in Sofia because the line must remain safe and operational during high-wind events while also ensuring that phase spacing and ground clearance remain within safe envelopes throughout the service life. The tubular pole’s stiffness, combined with the specified conductor tension (110kN max) and conductor mass (1520kg/km), supports stable sag behavior across the 60m spans.
Product Hardware Package Installed on Each Pole
To ensure the poles were not just structurally capable but also serviceable and resilient, SOLAR TODO included a complete hardware set per the project’s accessory package:
- Climbing steps: Facilitates inspection and maintenance access along the monopole body.
- Cross arm: Supports insulator strings and conductor positions for the double-circuit 35kV arrangement.
- Grounding system: Provides protective grounding continuity as part of the overall line safety design.
- Bird guard: Reduces risk of avian interference at conductor/insulator zones.
- Vibration damper: Helps mitigate conductor oscillation and reduces fatigue risk from aeolian vibration under wind.
By bundling these items into the delivery scope, SOLAR TODO reduced the chance of mismatched components during field assembly and ensured that the installed line behaves as designed.
Local Infrastructure Challenges in Sofia (and How the Design Responded)
Sofia’s transmission corridors often interface with dense urban growth patterns, road networks, and variable site accessibility. For this reason, the project prioritized:
- Predictable erection behavior: The poles use flanged bolt sections, enabling controlled assembly on-site and reducing time spent aligning complex structural members.
- Corrosion resistance for long-term reliability: Hot-dip galvanized Q345 steel supports long service life in an outdoor environment where maintenance windows can be limited.
- Mechanical robustness under wind: With wind class 2 (30 m/s) explicitly considered, the tubular monopole structure is designed to handle the combined effect of conductor forces (including ACSR 400 max tension 110kN) and wind loading.
- Safety clearances: The specified 5.5m ground clearance and 1.5m phase spacing help maintain safe operating geometry.
Results and Impact
Across the Sofia corridor, SOLAR TODO deployed 266 standardized 18m tubular poles to form a 35kV double-circuit line with ~16km total length and 60m spans. The project’s product configuration achieved mechanical design alignment with IEC 60826 / GB 50545, using wind class 2 (30 m/s) and conductor loading based on ACSR 400 (1520kg/km, max tension 110kN).
Key quantified outcomes include:
- 266 poles installed with consistent structural and hardware configuration
- ~16km of upgraded transmission corridor using 60m span design
- 35kV double-circuit geometry maintained with 1.5m phase spacing and 5.5m ground clearance
- 18t/pole (~1000kg/m) tubular monopole system providing stiffness and durability under wind class 2 conditions
Why SOLAR TODO’s Power Transmission Tower Approach Works
SOLAR TODO’s pole delivery strategy is built around repeatability and compliance:
- Product-first engineering: Each pole is configured for the mechanical and electrical requirements of the line—voltage class, conductor type, span, and wind class—rather than treated as a generic structure.
- Standards-driven design: The project uses IEC 60826 / GB 50545 as the governing design basis for weather/structural actions.
- Serviceability by design: Integrated accessories (climbing steps, grounding, bird guard, vibration dampers) support inspection and long-term operational safety.
- Field installation efficiency: Flanged bolt sections and the anchor_bolt foundation approach reduce variability during erection.
Pricing & Quotation
SOLAR TODO offers three pricing tiers for this product line: FOB Supply (equipment ex-works China), CIF Delivered (including ocean freight and insurance), and EPC Turnkey (fully installed, commissioned, with 1-year warranty). Volume discounts are available for large-scale deployments. Configure your system online for an instant estimate, or request a custom quotation from our engineering team at cinn@solartodo.com.
Frequently Asked Questions
Q1: What makes SOLAR TODO’s Power Transmission Tower different from lattice towers? A1: This project uses a steel tubular transmission pole (NOT lattice, NOT FRP) made from hot-dip galvanized Q345 steel, with a tapered tubular geometry designed for robust stiffness, corrosion resistance, and service-friendly assembly.
Q2: What wind conditions were considered for the Sofia installation? A2: The line was designed under wind class 2 (30 m/s) in accordance with IEC 60826, ensuring mechanical adequacy for overhead line operation in Sofia’s weather patterns.
Q3: Which conductor and mechanical limits were used? A3: The system uses ACSR 400 with 1520kg/km mass and max tension 110kN, integrated into the pole and span design for 60m spans.
Q4: What maintenance-access and protection accessories are included? A4: Each pole package includes climbing steps + cross arm + grounding + bird guard + vibration damper, supporting safe inspection and reducing conductor oscillation risk.
References
- IEC 60826 — Overhead lines design criteria for wind loading and mechanical actions.
- GB 50545 — Design code for overhead transmission line structures.
- IEEE Std 693 — (General overhead line reliability and mechanical performance considerations, relevant to structural integrity practices).
- NREL — (Transmission and grid resilience research supporting reliability-focused design thinking).
- World Bank Grid Infrastructure guidance — (General best practices for resilient grid infrastructure planning and implementation).
For product details, visit our Power Transmission Tower product page or contact us for a project-specific structural and mechanical configuration review.
Equipment Deployed
- 266 × 18m tapered steel tubular transmission poles for 35kV double-circuit line; hot-dip galvanized Q345 steel; ~18t/pole (1000kg/m); flanged bolt sections; designed for IEC 60826 wind class 2 (30 m/s) and GB 50545; phase spacing 1.5m; ground clearance 5.5m
- Insulator string support hardware: cross-arm brackets for insulator strings + ACSR conductors; insulator length 0.8m (project configuration)
- Conductor system: ACSR 400 (1520kg/km, max tension 110kN) for 60m spans across ~16km total line
- Foundation system: concrete foundation with anchor cage; anchor_bolt arrangement for each pole
- Pole accessories package: climbing steps + cross arm + grounding + bird guard + vibration damper (included per pole)