- Differential Protection Definition: Differential protection is defined as a system that compares the incoming and outgoing currents of a transformer to detect faults.
- Role of Differential Relays: Differential relays activate to disconnect the transformer when they detect an imbalance between the primary and secondary currents, preventing further damage.
- Current Transformers (CTs): CTs are crucial for differential protection, transforming primary and secondary currents to levels suitable for comparison by the relay.
- Fault Detection Speed: Differential protection offers rapid detection and response to internal transformer faults, faster than other protection methods like Buchholz relays.
- Bias Setting in Protection: The bias setting in differential protection ensures that the relay can handle slight current imbalances caused by external faults without tripping unnecessarily.
Differential protection compares primary and secondary currents of an electrical power transformer and trips both breakers if those currents no longer match. It is used on transformers rated above 5MVA.
It also covers faults that a Buchholz relay may miss, such as bushing flashovers outside the oil.
- While the Buchholz relay can detect faults occurring within the transformer’s insulating oil, it fails to detect faults that do not involve the oil. Buchholz relays do not adequately cover flashovers at the bushings, but differential relays can detect these types of faults. Although Buchholz relays detect internal faults in ttransformer, differential protection schemes do so more quickly.
- The differential relays normally response to those faults which occur in side the differential protection zone of transformer.
Differential Protection Scheme in a Power Transformer
Principle of Differential Protection
The principle of a differential protection scheme is a current comparison. The differential relay compares primary current and secondary current of the power transformer. If those currents are unbalanced the relay operates and inter-trips both the primary and secondary circuit breaker of the transformer.
Suppose you have one transformer which has primary rated current Ip and secondary current Is. If you install a CT of ratio Ip/1A at the primary side and similarly a CT of ratio Is/1A at the secondary side of the transformer, the secondaries of both CTs are connected so that their secondary currents oppose each other.
In other words, the secondaries of both CTs should be connected to the same current coil of a differential relay in opposite sense so that there is no resultant current in that coil in normal service. If a major fault occurs inside the transformer and the normal ratio of the transformer is disturbed, the secondary currents of both CTs will no longer match and a resultant current will flow through the current coil of the differential relay. The relay then inter-trips both the primary and secondary circuit breakers. To correct the phase shift of current caused by star-delta connection of transformer windings in a three-phase transformer, the current transformer secondaries should be connected in delta and star as shown here.
At maximum through-fault current, the spill output produced by a small percentage unbalance may be substantial. So differential protection of transformer should be provided with a proportional bias of an amount which exceeds in effect the maximum ratio deviation.
