What type of protection system is specifically designed to detect internal faults within a generator stator winding, and how does it operate?

The type of protection system specifically designed to detect internal faults within a generator stator winding is differential protection. Differential protection operates on the principle of Kirchhoff's Current Law, which states that the sum of currents entering a node (or winding) must equal the sum of currents leaving the node. In a generator, current transformers (CTs) are installed at both ends of each stator winding phase. These CTs provide a scaled-down representation of the current flowing into and out of the winding. Under normal operating conditions, the currents at both ends of the winding are nearly equal, and the differential current (the difference between the two currents) is close to zero. However, if an internal fault occurs within the stator winding, such as a short circuit between phases or a ground fault, a large fault current will flow. This fault current will cause a significant difference between the currents measured by the CTs at each end of the winding, resulting in a substantial differential current. The differential relay is set to trip when the differential current exceeds a predetermined threshold, quickly disconnecting the generator from the system to prevent further damage. Percentage differential relays are commonly used to account for CT errors and avoid spurious tripping. These relays trip only when the differential current exceeds a certain percentage of the through current. This provides enhanced sensitivity and stability.