ABB Modures LZ92-1 User Manual

Should (I /I ) . be less than (I/I ) . according to the above relationship,
then a re!ayNwTtR underimpendance sW.a~E~ng, e.g. LZ92 or LZ92-l, must be used
instead of a distance relay LZ91.
When used in ungrounded or impedance grounded systems (mode L. ) , the current
measuring elements of unit KI91 are connected to R, T and E. The sensitivity
of the neutral element in the standard versions is twice that of the phase
elements (I,r/IPh = 0.5, I/ ••• / in the code of the KI91). If some other
sensitivity is specified, it can be seen from the relay ordering code:
If ... /sensitivity factor
Should it be impossible to set the phase elements such that they pick up
reliably at the lowest phase fault level, but do not pick up at the maximum
earth fault current, then a distance relay LZ92 or LZ92-1 must be used.
3.2.2 Underimpedance starting
The following two frontplate (KZ91 or KZ91-l) settings apply to the underimpe-
dance starting units:
- the pick-up of the neutral current element
- the reach of the underimpedance elements.
3.2.2.l Neutral current earth fault detector
The criterion for the highest setting is:
- The earth fault detector must pick up for all earth faults in grounded
systems, respectively all cross-country faults in ungrounded or impedance
grounded systems which lie within the set reach of the underimpedance
units.
The criterion for the lowest setting is:
- The earth fault detector may not pick up for an earth fault on a single
conductor of an ungrounded or impdance grounded system.
- The earth fault detector may not pick up during heavy phase faults due to
spurious neutral currents caused by c.t. errors.
A typically recommended value is I,r
(standard version of the LZ91: I,r/IN
0.8 to 1.0 x IN for LZ92 and LZ92-l
0. 5) •
If there is no setting which satisfies both these limits, a neutral voltage
polarising ancillary must be added to operate in conjunction with the
neutral current element:
- This is available as a separate voltage relay to be inserted into the
equipment rack of the distance relay.
7

ABB Modures LZ92-1 Specifications

General

Brand	ABB
Model	Modures LZ92-1
Category	Relays
Language	English