Selection of 35kV busbars

For 35 kV systems, the busbar type selection depends on current rating, fault withstand, material, configuration, and switchgear design, with copper or aluminum busbars in single, double, or sectionalized arrangements being common.

Busbar Material

Copper and aluminum are the primary materials for 35 kV busbars. Copper offers higher conductivity and better thermal performance, making it suitable for high-current or compact installations, while aluminum is lighter and more cost-effective but requires a larger cross-sectional area to carry the same current .

Busbar Configurations

• Single Busbar: Simple and cost-effective, suitable for non-critical systems but lacks redundancy.
• Double Busbar: Provides redundancy; if one bus fails, the other can supply power, enhancing reliability .
• Sectionalized Bus: Divides a single bus into sections for easier isolation during maintenance or faults.
• Ring Main: Looped configuration allowing bidirectional power flow, commonly used in distribution networks.
• Double Breaker: Each circuit has two breakers for critical systems, improving control and reliability .

Sizing Criteria

Busbar sizing for 35 kV systems must consider:

• Continuous Current Rating (Ir): Determined by cross-sectional area and current density (typically 1.2–2.5 A/mm² for copper in enclosed switchgear), .
• Short-Circuit Withstand (Isc): Thermal and electrodynamic forces during faults must be within material and support limits .
• Temperature Rise: IEC 61439 specifies limits (70°C for copper, 55°C for aluminum above ambient) to prevent overheating .
• Mechanical Strength: Busbar supports must handle electromagnetic forces during short circuits without excessive deflection .

Switchgear Considerations

For 35 kV medium-voltage switchgear, gas-insulated switchgear (GIS) or metal-enclosed switchgear is commonly used. GIS allows compact dimensions, hermetically sealed enclosures, and high reliability, suitable for urban or space-constrained installations . Busbars in GIS are typically single-pole metal-enclosed, with options for single or double busbar arrangements.

Practical Example

A typical 35 kV busbar selection might include:

• Main Bus: 2×80×10 mm copper (1600 mm²) for high current capacity
• Neutral Bus: 2×100×10 mm copper (2000 mm²)
• Branch Risers: 40×10 mm copper, rated for 50 kA short-circuit This ensures a safe, compact, and future-proof design, balancing current capacity, fault withstand, and maintainability.

Standards Compliance

Busbar selection must comply with:

• IEC 61439: Low-voltage and medium-voltage switchgear assembly standards
• IEC 62271: High-voltage switchgear and controlgear standards
• IEC 60865-1: Electrodynamic withstand calculations for busbars

Key Takeaways

1. Choose copper for high current and compact designs, aluminum for cost-sensitive or lightweight applications.
2. Select a busbar configuration based on reliability and redundancy requirements.
3. Size busbars according to continuous current, short-circuit withstand, temperature rise, and mechanical strength.
4. Ensure compliance with IEC standards and consider switchgear type (GIS or metal-enclosed) for environmental and space constraints. This approach ensures safe, reliable, and efficient operation of 35 kV power distribution systems.