- 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 against external short circuits and excessive overloads. These overcurrent and earth-fault relays may be Inverse Definite Minimum Time (IDMT) or Definite Time (DMT) types. IDMT relays are usually connected to the in-feed side of the transformer.
Overcurrent relays cannot tell an external short circuit from an overload or from internal faults of the transformer. Backup protection using overcurrent and earth-fault protection on the in-feed side will operate 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. They should not trip the primary-side circuit breaker the way backup protection on the in-feed side does.
Operation of these relays is set by current and time settings, along with the relay’s characteristic curve. That lets the scheme use the transformer overload capacity and grade with other relays at about 125% to 150% of full-load current, but below the minimum short-circuit current.
Backup protection of transformer has four elements: three overcurrent relays (one in each phase) and one earth-fault relay connected to the common point of the three overcurrent relays as shown in the figure. The normal range of current settings on IDMT overcurrent relays is 50% to 200% and on earth-fault relays 20 to 80%.
Another range of setting on the earth-fault relay is also available and may be selected where earth-fault current is restricted by impedance in the neutral grounding. For a transformer winding with the neutral earthed, unrestricted earth-fault protection is obtained by connecting an ordinary earth-fault relay across a neutral current transformer.
The unrestricted overcurrent and earth-fault relays should have enough time lag to grade with the protective relays of other circuits and avoid indiscriminate tripping.
For more transformer questions, see the free MCQs on transformers.
