Functions
2.2 Phase Comparison Protection and Ground Differential Protection
SIPROTEC, 7SD80, Manual
E50417-G1140-C474-A1, Release date 09.2011
59
2.2.3
Ground Current Differential Protection in Grounded Systems
The ground current differential protection of the 7SD80 operates as a stabilized (restrained) differential protec-
tion in grounded systems. The two 7SD80 devices exchange the phasors of the ground currents and the asso-
ciated restraining quantities over their protection interfaces. The restraining currents and the current phasors
are summed up in each device and compared to a pickup characteristic. In the event of an internal short-circuit,
the associated circuit breaker is tripped.
2.2.3.1
Description
Basic Principle / Influencing Variables
In healthy operation, both ends of a line carry the same current. This current flows into one side of the consid-
ered zone and leaves it again on the other side. A difference in current is a clear indication of a fault within this
line section.
If the actual current transformation ratios are the same, the secondary windings of the current transformers
CT1
and
CT2
at the line ends can be connected to form a closed electric circuit with a secondary current
I
; a mea-
suring element
M
which is connected to the electrical balance point remains at zero current in healthy opera-
tion.
When a fault occurs in the zone limited by the transformers, a current i
1
+ i
2
which is proportional to the fault
currents
I
1
+
I
2
flowing in from both sides is fed to the measuring element. As a result, the simple circuit ensures
reliable tripping of the protection if the fault current flowing into the protected zone during a fault is high enough
for the measuring element
M
to respond.
Figure 2-10
Basic principle of the differential protection for a line with two ends
This principle only applies to the primary system as long as quadrature-axis components of current are negli-
gible. Quadrature-axis components of current can be caused by line capacitances or excitation currents of
transformers and parallel reactors.
The secondary currents which are applied to the devices via the current transformers, are subject to measuring
errors caused by the response characteristic of the current transformers and the input circuits of the devices.
Transmission errors such as signal jitters can also cause deviations of the measured quantities. As a result of
all these influences, the total sum of all currents processed in the devices in healthy operation is not exactly
zero. The ground current differential protection is stabilized against these influences.
Additional measuring errors which may arise in the device itself by hardware tolerances, calculation tolerances,
deviations in time or due to the „quality“ of the measured quantities such as harmonics and deviations in fre-
quency, are also estimated by the device and increase the local self-restraining quantity automatically. Here,
the permissible variations in the protection data transmission and processing periods are also considered.
For transient inrush currents the devices have a separate inrush current restraint feature.