Siemens 7SA511 manual Overcurrent Fault Detection

7SA511 Line Protection Relay Reference A

IE>

IN

1.0

IE pick-up

0.8

0.6

0.4

0.2

10%

0

Pickup

IE reset

0 1 2 3 4 5 6 7

Iph max

IN

Figure A-2. Ground Current Detector Characteristic

The displacement voltage detector monitors the fundamental wave of the displacement voltage (VE = 3 V0) in comparison with the set threshold value. The reset value is 95% of the pickup value.

In grounded systems (solidly or resistance grounded), the pickup of the ground current monitor or the displacement voltage monitor lead to pickup of the ground fault detector. Detection of a ground fault on its own is not announced, and it does not lead to a general fault detection signal of the distance protection. It does, however, control the other fault detection elements.

In ungrounded systems (isolated or compensated starpoint), the displacement voltage monitor is not used for ground fault detection. When a single-phase fault detection occurs in these systems, a single-phase ground fault is initially assumed and detection is suppressed to prevent faulty pickup by the oscillation transients on the occurrence of a simple ground fault. Pickup is enabled again following an adjustable delay time (address 1801); this is necessary so that the distance protection is still able to recognize a double ground fault with one base point on a spur feeder. A ground fault that is already present in the system is detected by the displacement voltage monitor, so the delay is not effective—a fault which now occurs in another phase can only be a double-phase ground fault.

A.2.2 Overcurrent Fault Detection

Overcurrent fault detection in the 7SA511 relay is a phase-dedicated fault detection procedure. Following digital filtering, the currents in each phase are monitored in comparison with a set threshold value. A pickup signal is output for the phase(s) for which the set threshold has been exceeded.

To enable measured value selection, the phase-dedicated fault detection signals are converted into phase-loop information (see section A.3). In grounded systems, this process is carried out accord- ing to the ground fault detection function and the parameter, 1PH FAULTS, selected for single-

July 27, 1995 A-9