What is the principle of the rotor second-harmonic current protection with negative-sequence power blocking?

Negative sequence power is generated internally in the generator; when there is an asymmetric short circuit or asymmetric overload in the external system, negative sequence power flows into the generator from the outside. Therefore, the direction of negative-sequence power is an exact criterion for determining whether a fault has occurred inside the generator. In the event of an internal fault in the generator, there is a second-harmonic current component in the rotor current; therefore, the criterion based on the rotor’s second-harmonic current can serve as a necessary condition for triggering protection actions, and this criterion can be enabled or disabled as needed. Furthermore, considering that the negative sequence power direction element and the rotor second harmonic current element, when used as sudden change elements for a prolonged period, may experience calculation errors due to differences in system frequency and sampling frequency, the program activates these sudden change elements only for a short time before automatically switching them back to normal elements. The protection action criteria are as follows: 1. (I2 > I2s) ∩ (U2 > U2s) ∩ (P2 > 0) ∩ (If2 > If2s); 2. (IA = IB = IC = 0) ∩ (U2 > U2s) ∩ (If2 > If2s); 3. (I2 > I2s) ∩ (U2 > U2s) ∩ (P2 > 0). Among these, criterion 1 is the action criterion when the rotor second harmonic is activated under non-no-load conditions of the system, criterion 2 is the action criterion under no-load conditions when the rotor second harmonic is activated, and criterion 3 is the action criterion when the rotor second harmonic is deactivated. I2s and U2s represent the threshold values for negative-sequence current and voltage at the machine side, while If2s is the threshold value for the rotor second-harmonic current