Manual MRI3 GB
Woodward
DOK-TD-MRI3 Rev.A
19
4.3 Directional
feature
A built-in directional element in MRI3 is available for application to doubly infeeded lines or to ring
networks.
The measuring principle for determining the direction is based on phase angle measurement and
therefore also on coincidence time measurement between current and voltage. Since the neces-
sary phase voltage for determining the direction is frequently not available in the event of a fault,
whichever line-to-line voltage follows the faulty phase by 90° is used as the reference voltage for
the phase current. The characteristic angle at which the greatest measuring sensitivity is achieved
can be set to precede the reference voltage in the range from 15° to 83°.
Figure 4.2: Relay characteristic angle
The TRIP region of the directional element is determined by rotating the phasor on the maximum
sensitivity angle for ±90°, so that a reliable direction decision can be achieved in all faulty cases.
4.3.1 Reversal in direction during the activation phase
Reversal of the current direction during the activation phase can lead to hyper functions. This main-
ly applies to installations where parallel connected lines are monitored by current relays with direc-
tional feature. For this reason the directional determination for the phase current is shown in a time
window; this applies to all SR versions. In case of activation due to a fault, a timer is started and
measures the time in the determined direction for max. 1 s. This timer runs backwards at half
speed if, during the activation phase, a fault causes reversal of the direction. Only when the timer is
at zero again, the MRI3 recognizes the reversal in direction. The switch-over time is max. 2 s. The
activation delays tl> and tl>> are not affected by the delayed recognition of direction.
Figure 4.3: Recording scheme of the fault recorder with lead time
1.0
1.5
0.5
[s]
0.5
1.0
1.5
Trip delay at direction reversal
max. time
Recognition of direction reversal
Recognition of
backward direction
<Red LED> is alight
Recognition of
forward direction
<Green LED> is alight
Timer runs at
half speed
actual
direction reversal
t=1/2