- Backup Protection Definition: Backup protection of a transformer is a secondary safeguard that protects against external short circuits and overloads.
- Overcurrent Protection: Overcurrent protection relays detect excessive current flow due to faults or overloads and are critical for transformer safety.
- Earth Fault Protection: Earth fault protection relays detect ground faults, helping to prevent damage from ground currents.
- Relay Settings: The operation of overcurrent and earth fault relays depends on precise current and time settings to ensure proper coordination and avoid false trips.
- Relay Configuration: Backup protection involves three overcurrent relays for each phase and one earth fault relay connected to their common point.
Over Current and Earth Fault Protection of Transformer
Backup protection of electrical transformer is simple Over Current and Earth Fault protection are applied against external short circuit and excessive over loads. These over current and earth Fault relays may be of Inverse Definite Minimum Time (IDMT) or Definite Time type relays (DMT). Generally IDMT relays are connected to the in-feed side of the transformer.
Overcurrent relays cannot differentiate between external short circuits, overloads, and internal faults of the transformer. Backup protection, using overcurrent and earth fault protection on the in-feed side, will activate for any of these faults.
Backup protection is usually installed on the in-feed side of the transformer, but it should trip both the primary and secondary circuit breakers.
Overcurrent and earth fault protection relays can also be placed on the load side of the transformer. However, they should not trip the primary side circuit breaker like the backup protection on the in-feed side.
The operation of these relays is governed by current and time settings, along with the relay’s characteristic curve. This allows the use of the transformer overload capacity and coordination with other relays at about 125% to 150% of the full load current, but below the minimum short circuit current.
Backup protection of transformer has four elements; three over current relays connected each in each phase and one earth fault relay connected to the common point of three over current relays as shown in the figure. The normal range of current settings available on IDMT over current relays are 50% to 200% and on earth fault relay 20 to 80%.
Another range of setting on earth fault relay is also available and may be selected where the earth fault current is restricted due to insertion of impedance in the neutral grounding. In the case of transformer winding with neutral earthed, unrestricted earth fault protection is obtained by connecting an ordinary earth fault relay across a neutral current transformer.
The unrestricted over current and earth fault relays should have proper time lag to co-ordinate with the protective relays of other circuit to avoid indiscriminate tripping.
If you’d like to learn more about transformers, you can study our free MCQs on transformers.
