Siemens 7SA511 manual Ground Fault Detection and Processing

7SA511 Line Protection Relay Method of Operation

When a fault is detected the following procedures are initiated:

Trip delays

Determination of the faulted line loop(s)

Release of trip commands

￿Auxiliary functions

Identification of the faulty phase(s)

The overcurrent fault detection unit is used for high fault current detection. If there is no significant difference between normal current (including overload) and short circuit current, then the voltage controlled overcurrent or polygonal impedance fault detection unit must be used. Power systems with highly fluctuating system impedance or systems using short circuit current limiting devices also require voltage controlled overcurrent or polygonal impedance fault detection units.

A.2.1 Ground Fault Detection and Processing

An important element for all fault detection methods is the detection of a ground fault since the determination of the fault type or faulty line loop depends on whether or not it is a ground fault. The 7SA511 relay is equipped with restraint measurement of the ground current as well as detection of the displacement voltage. Fault detection is suppressed in the case of single-phase ground faults in ungrounded or reactance-grounded systems.

The ground current detector monitors the fundamental wave of the total of the phase currents (IE

=3I0) in comparison with the threshold value. It is restrained against faulty pickup caused by asymmetric operating currents or distorted currents in the secondary circuit of the current transformers (CTs) resulting from different CT saturation with ground-free short circuits. The actual pickup value is automatically increased as the phase currents increase. In Figure A-2, with an example for setting IE>/IN = 0.2, the horizontal branch is shifted according to different settings. The reset value is 95% of the pickup value.

A-8 July 27, 1995