Functions
182
7UM62 Manual
C53000-G1176-C149-3
For machines in unit connection, the pickup value has to be chosen such that
displacements during network earth faults which are transferred via the coupling
capacitances of the unit transformer to the stator circuit, do not lead to pickup. The
damping effect of the loading resistor must also be considered in this case.
Instructions for the dimensioning of the earth current transformer and the loading
resistor are contained in the pamphlet “Planning Machine Protection Systems”, Order
No. E50400-U0089-U412-A1-7600. The setting value is twice the displacement value
which is coupled in at full network displacement. The setting value is finally determined
during commissioning with primary values according to section 3.4.6.1.
Delays
The stator earth fault trip is delayed by the time set under address
.
When setting the delay time, the overload capability of the loading equipment must be
considered. All set times are additional delay times and do not include operating times
(measurement times, reset times) of the protection function itself.
Earth Current
Addresses
and
are only of importance for machines in busbar connection,
=
directional
or
non–dir. U0&I0
has been set. The
following considerations are not applicable for machines in unit connection.
The pickup value
is set such that for an earth fault in the protected zone,
the earth current safely exceeds the setting.
Since the residual earth current in a compensated network is very small, an earthing
transformer with an ohmic loading resistor is normally provided to increase the
residual wattmetric current in the event of an earth fault. This arrangement also makes
the protection independent of network conditions. Instructions for the dimensioning of
the earth current transformer and the loading resistor are contained in the pamphlet
“Planning Machine Protection Systems”, Order No. E50400-U0089-U412-A1-7600.
Since the magnitude of earth fault current in this case is determined mainly by the
loading resistor, a small angle is set for
, e.g.
15°
. If the network
capacitances in an isolated network are also to be considered, then a larger angle
(approx.
45°
) can be set which corresponds to the superimposition of the capacitance
network current onto the loading current.
The directional angle
indicates the phase displacement between
the neutral displacement voltage and the perpendicular to the directional characteristic
(Figure 2-85), i.e. it is equal to the inclination of the directional characteristic to the
reactive axis.
If, in an isolated network, the capacitances to earth of the network are mainly decisive
for the earth fault protection, it is also possible to work without earthing transformer. In
this case, the angle is set to approximately 90° (corresponding to sin
ϕ
switching).
Example busbar connection:
Loading resistor
10
Ω
10 A
continuous
50 A
for 20 s
6.3 kV
3
-----------------
500 V
3
---------------
⁄
Earthing transformer
27 kVA
(limb transformation)
