PS 1 – Transformer Protection, Monitoring & Control Q8 – What is the present practice of utilities when multifunction relays are applied for protecting transformers? By Richard J Adams VA TECH T&D Ltd, UK

UK practice for the protection of supergrid(400 or 275kV) to 132kV is generally as follows (apart from mechanical protections): Main protection – one numeric protection with two protection functions (both unit type). One must be responsive to phase-to-phase and phase-to-earth faults, the other must at least be sensitive to earth faults. Solution tends to be overall biased differential protection and restricted earth fault. Backup protection – HV 2-stage 3 phase overcurrent and highset overcurrent. LV protection – Residually connected earth fault protection.

HV 2-stage 3 phase overcurrent set to protect against faults, not overload (winding temperature caters for this). First stage SI curve, trips LV CB only to act as backup for LV system faults. Second stage trips HV CB after a fixed time delay to act as backup to transformer main protection. Highset overcurrent set to protect for faults on the HV bushings and not operate for LV faults. LV earth fault generally set to trip LV CB only to clear any faults that are too sensitive for the HV stage 1 protection.

Transformer protection is achieved by three protection devices. Main Protection Backup Protection LV EF Protection HV CB LV CB