The current with the same magnitude and the same magnitude at both ends of the differential protection device of the high voltage motor is referred to as the crossing current, while the current in the opposite direction is called non-crossing current. As the main protection, generator ratio brake differential protection based on non cross current as the amount of action, to cross the current braking quantity, as to distinguish the normal state of protected component, fault status and abnormal running state.
In normal operation, the crossing current is the load current, and the non-crossing current theory is zero. Interphase short circuit, non - crossing current surge. When an external fault occurs, the traversing current increases dramatically.
In the above three states, the protection can be sensitive to the internal and interphase short-circuit state action exit, so as to achieve the purpose of protecting the element, while in normal operation and out-of-area fault, reliable and not action.