Woodward
Manual MRI3 GB
26
DOK-TD-MRI3 Rev.A
By means of an efficient digital filter harmonics and fault transients in the fault current are sup-
pressed. Thus, the uneven harmonics which, for instance, are caused an electric arc fault, do not
impair the protective function.
4.6 Determining earth short-circuit fault direction
(SR/YR-relay
type)
The SR-relay type is used in solidly-earthed or resistance-earthed systems for determining earth
short-circuit fault direction. The measuring principle for determining the direction is based on phase
angle measurement and therefore also on the coincidence-time measurement between earth cur-
rent and zero sequence voltage.
The zero sequence voltage U0 required for determining the earth short-circuit fault direction is gen-
erated internally in the secondary circuit of the voltage trans-formers.
With SR/ISR-relay types the zero sequence voltage U0 can be measured directly at the open delta
winding (e-n). Connection A3/A2.
4.6.1 Directly – earthed system
Most faults in a characteristic angle are predominantly inductive in character. The characteristic
angle between current and voltage at which the greatest measuring sensitivity is achieved has
therefore been selected to precede zero sequence voltage U
0
by 110°.
Figure 4.11: Characteristic angle in solidly earthed-systems (SOLI)