MM30-D
Doc. N° MO-0037-ING
Rev.
4
Date
24.02.2003
Copyright 2010 - Microener
Pag
13
of
36
2.3 - OPERATION OF THE DIRECTIONAL EARTH FAULT ELEMENT
It is assumed :
O>
=
Set minimum pick-up residual current : (3Io)
Uo>
=
Set minimum residual voltage (3Uo) to enable O> pick-up
o
=
Set characteristic angle (max. torque displacement of residual current 3Io from residual
voltage 3Uo).
3Io
=
Actual
earth fault relay’s input current
3Uo
=
Actual earth fault relay’s input voltage
o
=
Displacement of 3Io from 3Uo
Io
=
Component of 3Io in the direction
o
Io
= 3Io cos(
o-
o)
Fig 1
Fig.2
The relay measurement is:
3Io x cos(
o -
o) = Io
The relay trips when
Io
>[O>] (fig.2)
i.e, when the component of the input current in the measuring
direction of the relay exceeds the set trip level Is. [O>]
Operation is enabled only if the input zero-sequence voltage 3Uo is above the set level [Uo>].
The sensitivity of the relay is then proportional to cos(
o -
o), it is maximum when
o
=
o
and its operation field is limited within the range:
(
o - 90°) <
o < (
+ 90)
(fig.1)
The characteristic angle of the relay must be selected according to the kind of earthing of the installation
which has to be protected against earth fault; typical setting are:
UNEARTHED NEUTRAL
o
=
90°
NEUTRAL EARTHED VIA RESISTOR
o
=
0°
SOLIDLY EARTHED NEUTRAL
o
=
60°
