Guodian Nanzi PSL 691U Line Protection and Control Device
1 Functions
Three-stage overcurrent protection (can be initiated by composite voltage, directional option available)
Three-stage zero-sequence overcurrent protection (directional option available)
Overload alarm
Reclosing
Manual closing with synchronization check
Closing acceleration protection
Under-frequency load shedding
Under-voltage load shedding
Small current grounding line selection
 I, U, P, Q, Cosφ, active energy, reactive energy, 14 channels of switch status acquisition
 GPS time synchronization (minute pulse, second pulse, or IRIG-B method)

2 Principle Description
2.1 Three-stage Overcurrent Protection
When the current of any phase exceeds the set value, after a time delay, the device trips.
Each stage of the overcurrent protection can be independently enabled or disabled via control word settings, with options for composite voltage initiation and directional control.
When composite voltage initiation is selected: Phase A current is initiated by Uab, Uca, or negative sequence voltage; Phase B current by Uab, Ubc, or negative sequence voltage; Phase C current by Ubc, Uca, or negative sequence voltage.
When directional control is selected: Ia and Ubc form the Phase A directional element; Ib and Uca form the Phase B directional element; Ic and Uab form the Phase C directional element. The directional element considers the current to be in the positive direction when the phase angle relative to the voltage is between -30° and +90°. The directional element includes a memory function to eliminate dead zones during close-range three-phase faults.