- 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 is typically employed for electrical power transformers rated above 5MVA.
Differential protection offers several advantages compared to other transformer protection schemes.
- 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
Principle of Differential Protection scheme is one simple conceptual technique. The differential relay actually compares between primary current and secondary current of power transformer, if any unbalance found in between primary and secondary currents the relay will actuate and inter trip 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 CT of ratio Ip/1A at the primary side and similarly, CT of ratio Is/1A at the secondary side of the transformer. The secondaries of these both CTs are connected together in such a manner that secondary currents of both CTs will oppose each other.
In other words, the secondaries of both CTs should be connected to the same current coil of a differential relay in such an opposite manner that there will be no resultant current in that coil in a normal working condition of the transformer. But if any major fault occurs inside the transformer due to which the normal ratio of the transformer disturbed then the secondary current of both transformers will not remain the same and one resultant current will flow through the current coil of the differential relay, which will actuate the relay and inter trip both the primary and secondary circuit breakers. To correct phase shift of current because of star-delta connection of transformer winding in the case of 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 the small percentage unbalance may be substantial. Therefore, differential protection of transformer should be provided with a proportional bias of an amount which exceeds in effect the maximum ratio deviation.
