- REF Protection Definition: REF protection is a scheme used in transformers to detect internal earth faults by monitoring unbalanced currents in the transformer phases and neutral.
- External Fault Current Flow: During an external fault, balanced currents in the line CT and neutral CT result in zero net current in the relay, preventing activation.
- Internal Fault Detection: Internal faults cause unbalanced current in the neutral CT, leading to the operation of the REF relay.
- Cost and Sensitivity: REF protection is highly sensitive to internal faults and more cost-effective compared to differential protection schemes.
- CTs Connection and Opposition: Phase CT secondaries and the NCT are connected in a way that their unbalanced currents oppose each other, ensuring the relay activates only for internal faults.
Restricted Earth Fault Protection of Transformer
An external fault in the star side will result in current flowing in the line current transformer of the affected phase and at the same time a balancing current flows in the neutral current transformer, hence the resultant current in the relay is therefore zero. So this REF relay will not be actuated for external earth fault. But during an internal fault, the neutral current transformer only carries the unbalance fault current and operation of Restricted Earth Fault Relay takes place. This scheme of restricted earth fault protection is very sensitive for internal earth fault of electrical power transformer. The protection scheme is comparatively cheaper than differential protection scheme.
Restricted earth fault protection is used in transformers to detect internal earth faults. In this scheme, the CT secondaries of each transformer phase are connected together. These common terminals are then connected to the secondary of a Neutral Current Transformer (NCT).
The CT connected to the neutral of a power transformer is called a Neutral Current Transformer (NCT). When there is an unbalance between the three phases of the transformer, an unbalanced current flows through the closed path connected to the CT secondaries’ common terminals. This unbalanced current also flows through the transformer’s neutral, causing a secondary current in the NCT.
In the Restricted Earth Fault scheme, the common terminals of the phase CTs are connected to the secondary of the NCT so that the secondary unbalanced current of the phase CTs and the secondary current of the NCT oppose each other. If these currents are equal in amplitude, no resultant current circulates through the closed path. The REF relay is connected in this path, so it does not respond to phase current unbalances in the transformer.
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