- 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
Restricted earth fault (REF) protection detects internal earth faults on a star winding. An external fault on the star side produces current in the line current transformer of the affected phase and a balancing current in the neutral current transformer, so the resultant current in the relay is zero. The REF relay will not operate for that external earth fault. During an internal fault, the neutral current transformer only carries the unbalance fault current and the Restricted Earth Fault relay operates. This restricted earth fault protection is sensitive to internal earth faults of an electrical power transformer. The scheme costs less than a full differential 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. Those 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 the three phases of the transformer are unbalanced, an unbalanced current flows through the closed path connected to the common terminals of the CT secondaries. That unbalanced current also flows through the transformer neutral and produces 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 those 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 unbalance in the transformer.
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