Description of Operation
GARD 8000 Distance Relay
RFL Electronics Inc.
SECTION 9.
DESCRIPTION OF OPERATION
9.1
DISTANCE ELEMENTS
The GARD 8000 Distance Protection Module is provided with four independent protection zones. The
operating direction for each zone, forward or reverse, is selected using the ‘Zone n direction’ setting on
web page Distance Elements, Zone1 to Zone4.
The Zone 4 direction can be forced reverse, depending on what pilot scheme is selected. When the
selected protection scheme is either Directional Comparison Blocking or when Weak Infeed Logic or
Transient Block Logic is used, Zone 4 will operate as reverse-looking even if the setting is forward.
Therefore, when either of these schemes is in use, it is not necessary to adjust the Zone 4 direction.
Each zone has six measuring elements, one for each type of fault. The measuring element operates
based on a phase angle comparison of an operate phasor and a polarization phasor. The two phasors are
derived from the voltage and current phasors, and the specific settings for the zone.
The ground fault elements use a compensation factor for the ground return in order to measure an
impedance proportional to the line’s positive sequence impedance. This compensation factor is the
setting K0, defined as:
K0=ZL0/ZL1, where ZL0 and ZL1 are the zero sequence and positive sequence impedances of the
line.
Each zone has an individual reach setting (positive sequence impedance) and individual setting of the
zero sequence compensation factor (K0=ZL0/ZL1), as well as individual phase angle setting. This
enables a more exact setting for applications with mixed line properties, such as an overhead section
followed by a cable section.
Fault detection is made by use of mho and/or quadrilateral characteristic, with a setting for
Quadrilateral characteristic, Mho characteristic, or both of them simultaneously, operating in an OR or
AND logic. The reactive reach setting is common for both the Quadrilateral and Mho characteristics.
Mho elements have a closed circular and directional characteristic, while the Quadrilateral elements
have open non-directional characteristics. Hence, Quadrilateral elements are complemented with a
directional unit and a resistive limiter with adjustable reach.
9.1.1
MHO CHARACTERISTIC
The GARD 8000 Mho characteristic is polarized by the positive sequence voltage of the corresponding
phase. This polarization gives the following characteristic, similar to cross-polarization:
•
Variable: The Mho circle always includes the origin, and the diameter is a function of
the source impedance.
•
Dynamic: The Mho circle diameter varies from the inception of the fault to a return to
the steady state.
August 1, 2009
9-1
973.334.3100
