GE 489 - Manual (Page 94)

4-44 489 Generator Management Relay GE Multilin
4.7 S6 VOLTAGE ELEMENTS 4 SETPOINTS
4
The distance protection function (ANSI device 21) implements two zones of mho phase-to-phase distance protection (six
elements total) using the conventional phase comparator approach, with the polarizing voltage derived from the pre-fault
positive sequence voltage of the protected loop. This protection is intended as backup for the primary line protection. The
elements make use of the neutral-end current signals and the generator terminal voltage signals (see figure below), thus
providing some protection for internal and unit transformer faults. In systems with a delta-wye transformer (DY330°), the
appropriate transformations of voltage and current signals are implemented internally to allow proper detection of trans-
former high-side phase-to-phase faults. The reach setting is the positive sequence impedance to be covered, per phase,
expressed in secondary ohms. The same transformation shown for the Loss of Excitation element can be used to calculate
the desired settings as functions of the primary-side impedances.
The elements have a basic operating time of 150 ms. A VT fuse failure could cause a maloperation of a distance element
unless the element is supervised by the VTFF element. In order to prevent nuisance tripping the elements require a mini-
mum phase current of 0.05 x CT.
Figure 4–8: DISTANCE ELEMENT SETUP
808740A1.CDR
Protection Zone 1
Protection Zone 2
Neutral End CT
52
Terminal VT
489
Relay