DOC. MIE91093 Rev. 1.34
Page 39 of 145
1.5
D
IRECTIONAL RELAY TESTING
1.5.1 Introduction
These relays are used to protect MV feeders against Earth
faults by detecting the residual voltage V
o
, the residual current
I
o
and the relative angle. Normally the current is lagging; for
neutral isolated lines,
I
o
-90°; for other earth systems
(resistance, Petersen Coil), the phase angle can be less: 40°,
60°. This is also the angle of maximum sensitivity of the relay.
Since relay inputs are one current and one voltage, we’ll use I
main and V aux to perform the test.
The parameters that we will test are:
The characteristic angle, sometimes called MTA= max
torque angle (electromagnetic relays), and the Angle
sector, that is half of the operating angle;
The pick up of Io;
The pick up of Vo.
When testing one parameter we have to set the other two at a
value above the pick up.
Test
MTA
: we keep Vo and Io above the respective pick
ups.
Test the Angle sector: we keep Vo and Io above the
respective pick ups.
Test the pick up of
Io
: we keep Vo above the pick up,
and the current angle at the measured Characteristic
Angle.
Test the pick up of
Vo
: we keep Io above the
measured pick up, and the current angle at the
measured Characteristic Angle.
In this example we’ll suppose we are testing a relay with the
characteristic displayed in the graph.