Description of Operation
GARD 8000 Distance Relay
RFL Electronics Inc.
9.8
LOAD ENCHROACHMENT
The GARD 8000 Distance Protection has dynamic mho characteristic which will expand with the
source impedance. As it is not possible to control the resistive reach of the dynamic mho, there is a risk
that the characteristic will fall into the load impedance on long, heavily loaded lines. To prevent this, a
load encroachment characteristic is provided. The characteristic has individual settings for forward and
reverse load, which enables fine tuning for the actual application. Generally, maximum forward and
reverse load differs and the blocking zones should not be larger than necessary. The load encroachment
characteristic provides blocking zones that will prevent an expanding mho characteristic from
operation within the set zone
.
Blocking zone
α1
α2
R1
R2
X
R
LOAD AREA
LOAD AREA
Blocking zone
α1
α2
R1
R2
X
R
LOAD AREA
LOAD AREA
Figure 9-31. Load Encroachment Characteristic
9.9
POWER SWING BLOCK UNIT
Power swing conditions are disturbances produced by imbalances between generation and demand,
which may originate from changes in the topology of the network, load variations, system faults, etc.
Current and voltage variations, in magnitude as well as angle, during a power swing may result in
distance relays measuring an impedance that falls within its operating characteristic.
Power swings may be stable (dampened until reaching a new balance situation) or unstable (balance
not recovered). In case of unstable power swings, it is necessary to separate the system, creating
islands in which there is balance between generation and demand.
For any power swing, it is necessary to block the trip of the distance units: if the swing is stable, a trip
may convert this to unstable and if the swing is unstable, a controlled islanding should take place by
trips in pre-determined locations rather than the line protection.
The GARD 8000 distance relay includes a power swing unit to prevent tripping of the distance
elements on stable power swings (block) and allow controlled tripping on unstable power swings (trip).
August 1, 2009
9-33
973.334.3100
