ABB MDAR Instruction Manual page 18

I.L. 40-385.5
current and voltage are updated once a cycle. The
values from the previous cycle are operated on while
the present cycle of data is being collected.
The Fourier coefficients and sums, as well as the
sum of squares of the inputs are calculated and
accumulated to provide RMS and phasor values of
current and voltage for distance measurement and
metering display. The RMS value of the ground cur-
rent is also needed for inverse-time ground protec-
tion.
Several functions are performed only in Background
Mode. The servicing of the operator interface is one
of these functions; the display is updated, pushbut-
tons are acknowledged, and the LED's are con-
5-2
trolled. Metering of the analog inputs, updating the
target information for display purposes, and checking
the validity of the settings in the nonvolatile memory
are other functions performed only in Background
Mode.
Fault detection is in effect at all time in Background
Mode. Each sampled value of current and voltage is
compared to the sampled value from the previous
cycle. If the difference exceeds the established
threshold, a fault is suspected and Fault Mode is
entered. The zone-2 and zone-3 time delay distance
protection are performed in both Background Mode
and Fault Mode.