Functions
2.2 Phase Comparison Protection and Ground Differential Protection
SIPROTEC, 7SD80, Manual
E50417-G1140-C474-A1, Release date 09.2011
66
If only the V0 voltage is connected, only parameter
1226
87N L: 3V0>
is effective. The threshold checks
87N
L:Vph-g min
and
87N L:Vph-g max
(parameter
1227
and
1228
) are not relevant.
You will find the logic diagram for the differential protection trip in Section 2.2.5.
2.2.4.2
Setting Notes
General
The operating mode of the ground differential protection depends on the neutral point treatment in the protected
zone. In resonant-grounded or isolated system, you have to set
Peterson-C.Gnd.
or
Isolated
at address
207
SystemStarpoint
.
The ground differential protection can be switched
ON
or
OFF
at address
1221
87N L: Protect.
. If set to
Alarm only
, an indication will be output when a fault is detected. Tripping is not initiated. This requires the
ground differential protection to be set to
Enabled
at address
112
87 DIFF.PROTEC.
.
If a device is switched off at any end of the protected object or if the protection interface communication is in-
terrupted, a calculation of measured values becomes impossible. The function then operates locally and only
issues directional indications and pickup indications but no pickup and tripping indications of the ground fault
differential protection.
Pickup Values
The pickup threshold of the displacement voltage is set in address
1226
87N L: 3V0>
.
At address
1229
87N L: IN(s)>
you can enter the minimum current for direction determination. The pickup
current is to be set as high as possible to avoid false pickup of the device provoked by unbalanced currents in
the system and by current transformers. Dependent on the grounding of the network starpoint, the magnitude
of the capacitive ground fault current (for isolated networks) or the wattmetric residual current (for compensated
networks) is decisive.
In
isolated
systems, a ground fault in a cable will allow the total capacitive ground fault currents of the entire
electrically connected system, with the exception of the faulted cable itself, to flow through the measuring point
as the latter flows directly to the fault location (i.e. not back via the measuring point). Enter about half of this
ground fault current as pickup value.
In
resonant-grounded
systems directional determination is made more difficult since a much larger reactive
current (capacitive or inductive) is superimposed on the critical wattmetric (active) current. Therefore, depend-
ing on the system configuration and the position of the arc-compensating coil, the total ground current supplied
to the device may vary considerably in its values with regard to magnitude and phase angle. The device, how-
ever, must evaluate only the active component of the ground fault current, the ground fault residual current,
that is
I
E
·cos
ϕ
. This requires extremely high accuracy, particularly with regard to phase angle measurement of
all the instrument transformers. Furthermore, the device must not be set to operate too sensitive. When apply-
ing this function in resonant-grounded systems, a reliable direction determination can only be achieved when
toroidal current transformers are connected. Here the following rule of thumb applies: set the value to half the
expected measured current, whereby only the residual wattmetric current is used. Residual wattmetric current
predominantly derives from losses of the Petersen coil.
For phase determination
87N L:Vph-g min
(address
1227
) is the criterion that applies to the ground-faulted
phase if simultaneously the other two phase voltages
87N L:Vph-g max
(address
1228
) have been exceed-
ed. Accordingly, the setting
87N L:Vph-g min
must be set smaller than the minimum phase-to-ground
voltage that occurs during operation. This setting, too, is uncritical. 40 V (default setting) should always be cor-
rect.
87N L:Vph-g max
must be greater than the maximum phase-to-ground voltage occurring during oper-
ation, but less than the minimum phase-to-phase voltage occurring during operation. For V
N
= 100 V, the value
must therefore be 75 V (default setting). The definite detection of the phase affected by the ground fault is a
further prerequisite for alarming a ground fault. When connecting the voltage V0 (Appendix A.3, Figure A-11),
the check of the phase voltages does not take place.
