RFL Electronics GARD 8000 - Page 556

Current Differential Relay
GARD 8000 SYS RFL Electronics
June 20, 2014 13-61 973.334.3100
13.5.1.5.3 Delay Timer and Action
Once the phase loss is detected the phase imbalance must continue for a user specified duration in
order to satisfy the algorithm (at which point the fault detectors will most likely be inactive as will the
prefault condition).
The user can specify if they want the open conductor algorithm to cause a trip, an alarm, or be ignored.
13.5.1.6 BACKUP PROTECTION
If enabled the relay will enter the backup protection mode when the communications channel has
failed (section 13.5.2.6.3) or the communications channel is OK but the primary guard before trip
criteria has not been satisfied (and it is enabled).
13.5.1.6.1 Instantaneous and Definite Time Overcurrent elements (50)
The overcurrent elements work on the measured RMS values of the measured currents. The
measurement must exceed the user setpoint for a time period exceeding the user specified duration for
the element to become active, there must also be an active fault detector in the phase calculating the
trip and at least one additional fault detector.
13.5.1.6.2 Time Overcurrent elements (51)
The relay allows for independent configuration of the phase and ground time overcurrent elements.
Each has four configuration settings: enable/disable, type of characteristic curve to use, pickup level,
and time dial setting.
The relay supports several operate characteristic curves. All curves have an instantaneous reset
characteristic—if the current drops below the pickup level the integrator is reset and the algorithm will
restart.
13.5.1.6.2.1 Functional Overview
The general form of the operate function is:








 B
M
A
tdtimeoperate
p
1
_
Where td is the time dial setting and M is the multiple of the pickup current.
13.5.1.6.2.2 Multiple of Pickup Current
The multiple of pickup current is defined as:
setting pickup
current measured
M
The algorithm is fully specified within the range 1.5 < M < 20. Additionally there are practical
limitations due to the current measurement itself—the measurement will start to clip if the peak current
exceeds the range of the equipment.
The calculations are based upon the half-cycle equivalent RMS value measurements. The individual
half-cycle contributions to the algorithm are integrated to account for time-varying currents. When the
integrated signal exceeds the operate time the output is asserted. As previously mentioned the
integration is reset if the measured current drops below the pickup current.