Configuration of 35kV System Busbar Protection

A 35kV busbar is best protected using a combination of differential protection, sectionalized busbar zones, and appropriately configured relays to ensure fast, selective fault clearance.

Key Protection Principles

Busbar protection is designed to detect and clear faults on the bus quickly, minimizing damage and maintaining system stability. For a 35kV busbar, the protection scheme should cover all possible fault types, including phase-to-phase, phase-to-ground, and interphase faults, while ensuring selective isolation of only the affected section .

Differential Protection

Current differential protection is the most common and effective method. It compares the sum of currents entering and leaving the bus using CTs, based on Kirchhoff's current law. Any difference indicates a fault within the bus zone . Modern low-impedance or high-impedance differential relays can operate within one cycle, providing high-speed fault clearance . Adaptive trip logic in numerical relays ensures security against CT saturation during external faults .

Sectionalized Busbar Protection

For complex bus configurations, such as double-bus, single-bus with sections, or 3/2 wiring, sectionalized protection is recommended. Each bus section has its own protection relay, allowing isolation of only the faulty section while keeping the rest of the bus in service . This approach reduces outage impact and enhances system reliability.

Relay and Equipment Configuration

• Bus differential relays (e.g., GE Multilin B90) are suitable for 35kV busbars, supporting multiple segments and providing phase-segregated protection .
• CT arrangements should ensure accurate measurement and avoid misoperation due to saturation. Parallel connection of CT secondaries is typical for current differential schemes .
• Bus potential transformers may be included for voltage-based protection or backup schemes .
• Boundary points are defined at main transformer branches or bus sectional breakers to divide the bus into manageable protection zones .

Practical Implementation Steps

1. Identify main transformer branches and bus sections.
2. Divide the bus into zones, ensuring no more than three connected sections per zone for simplicity and reliability .
3. Configure bus differential relays for each zone, including CTs, bus potential transformers, and control wiring.
4. Implement adaptive trip logic to handle CT saturation and ensure selective operation.
5. Test the system for both internal and external faults to verify speed, sensitivity, and security.

Advantages of Proper Configuration

• High-speed fault clearance reduces arc-flash hazards and equipment damage .
• Selective isolation maintains supply continuity for unaffected sections .
• Reduced maintenance and cost by optimizing the number of relays and cubicles .
• Scalability for future expansion or complex bus arrangements. In summary, a 35kV busbar should be protected using numerical differential relays with sectionalized zones, carefully configured CTs, and adaptive logic to ensure fast, secure, and selective fault clearance while minimizing operational and maintenance costs .