MICROENER MM30-D - Directional Earth Fault Element Operation; Earth Fault Measurement and Tripping; Characteristic Angle Settings

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°