Secondary unit substations design guide
VFI Transformer The VFI transformer combines a conventional liquid-filled distribution substation transformer with a
Several times the protection was set in the secondary distribution box
Repeated activation of protection in a secondary distribution box often indicates overcurrent conditions, improper breaker sizing, or inadequate coordination between transformer secondary protection and downstream loads.
Common Causes of Repeated Tripping
Overcurrent or Short Circuit: The secondary distribution box may be experiencing currents exceeding the rated capacity of the breakers due to faults, inrush currents from motors, or simultaneous high-load operation .
Breaker Sizing Issues: If the main breaker in the panelboard is not properly sized relative to the transformer's secondary full load current (FLC), it may trip unnecessarily. Proper sizing typically considers 100%–125% of the transformer FLC for long-time pickup and higher multiples for short-time or instantaneous protection .
Coordination Problems: Protection devices must be coordinated so that only the breaker closest to the fault trips. If the secondary breaker is set too low or the relay settings are not coordinated with upstream protection, nuisance tripping can occur .
Transformer Inrush Currents: Transformers draw higher currents during energization. If the secondary protection is not set to accommodate inrush, the breaker may trip repeatedly during startup .
Proper Secondary Protection Practices
Use Correct Breaker or Relay Settings: Long-time pickup is generally set at 1.0–1.1 times the transformer FLC, short-time pickup at 4–8 times FLC, and instantaneous pickup above the maximum downstream fault current but below the transformer's mechanical damage threshold .
Panelboard Main Breaker as Secondary Protection: Modern NEC guidance allows the panelboard main breaker to serve as transformer secondary protection if properly sized and located close to the transformer . Distance limitations for secondary conductors should follow NEC 240.21(C) to ensure effective protection.
Selective Coordination: Ensure that downstream breakers trip before upstream protection to prevent unnecessary outages. Ground fault protection should be set between 20%–40% of FLC depending on the earthing system .
Use of Relays: In complex secondary distribution systems, dual/self-powered protection relays can provide overcurrent, overvoltage, and undervoltage protection, with communication capabilities for monitoring and coordination .
Troubleshooting Steps
- Verify the load current and compare it with the breaker rating.
- Check for short circuits or ground faults in downstream circuits.
- Review transformer secondary protection settings and adjust long-time, short-time, and instantaneous pickups as needed.
- Ensure proper coordination between the transformer secondary protection and panelboard breakers.
- Consider adding a dedicated disconnect or relay if multiple panels are fed from the same transformer to isolate faults without affecting other loads .
By following these practices, repeated tripping in the secondary distribution box can be minimized, ensuring both safety and reliable operation of the electrical distribution system.
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