Current Differential Relay
GARD 8000 SYS RFL Electronics
June 20, 2014 13-54 973.334.3100
Figure 13-37. Three Terminal Application
13.5.1.2.1 Current measurements
The basic current measurement used for the current differential algorithm takes the currents measured
between zero crossings. All of the individual samples (normally sixteen of them) are added together.
This essentially calculates the integral of the current during that half-cycle (or “area under the curve”)
multiplied by a scale factor.
This is related directly to the charge being transferred over the line and in the case of sinusoidal
currents the RMS value of the current during the half-cycle.
13.5.1.2.2 Measurement Qualifiers
The purpose of the half-cycle current measurements is to provide a standardized and consistent basis
for relays at both ends of the line so that meaningful comparisons can be made. As such certain
qualifiers are added to the measurements to ensure the validity of the data.
13.5.1.2.3 Communicating Current Measurements to Remote Relay
Each valid half-cycle current measurement is saved locally for the current differential processing
routines. Phase A, B, and C transmit their current measurements only for the positive half-cycle and
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op L R R
restra L R R
I I I I
I I I I
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