Functions
2.8 Earth Fault Protection in Earthed Systems (optional)
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
228
Determination of direction for lines with series compensation
The direction determination/directional characteristic of the earth fault protection is based on the assumption
of a mainly inductive zero-sequence system impedance. In case of a series-compensated line, however, this
assumption does not apply anymore. According to the degree of compensation, the zero-sequence system im-
pedance is more or less influenced regarding its capacity. The situation is especially unfavorable if the capacitor
is located on the busbar side of the voltage transformers.
In case of faults on the protected line, the zero-sequence voltage consists of two components: the voltage drop
on the source impedance (mainly inductive) and the voltage drop over the series capacitor.
If the capacity of the series capacitor is known (and constant), the voltage drop on the series capacitor can be
determined according to the following formula:
U
CO
= -jX
CO
3·
I
0
Figure 2-111
Correction of series compensation for the direction determination with zero-sequence system
The voltage drop on the series capacitor U
C0
= 3·
I
0
·
XserCap
(address
3187
) is subtracted from the measured
zero-sequence voltage 3U
0meas
. The resulting voltage3U
0Dir
is then assigned to the directional characteristic of
the earth fault protection, as shown on Figure 2-111.
Determination of direction with negative phase-sequence system
It is advantageous to use negative sequence system values for the direction measurement if the zero-sequence
voltages that appear during earth faults are too small for an analysis of the zero-sequence values.
Otherwise, this function operates the same way as the direction determination with zero-sequence current and
zero-sequence voltage. Instead of 3
I
0
and 3 U
0
, the negative sequence signals 3
I
2
and 3 U
2
are simply used
for the measurement. These signals must also have a minimum magnitude of
3I2>
or
3U2>
.
It is also possible to determine the direction with a zero-sequence system or a negative sequence system. In
this case the device determines whether the zero-sequence voltage or the negative sequence voltage is larger.
The direction is determined by the larger of the two values. The direction is not determined during the single-
pole dead time.
For the application of a teleprotection scheme, the direction determination must be performed at all terminals
with the same setting.
