Functions
2.2 Phase Comparison Protection and Ground Differential Protection
SIPROTEC, 7SD80, Manual
E50417-G1140-C474-A1, Release date 09.2011
62
2.2.3.2
Setting Notes
General
The operating mode of the ground differential protection depends on the neutral point treatment in the protected
zone. In grounded systems, address
207
SystemStarpoint
must be set to
Grounded
.
The ground differential protection can be switched
ON
or
OFF
at address
1221
87N L: Protect.
. This re-
quires the ground differential protection to be set to
Enabled
at address
112
87 DIFF.PROTEC.
. The setting
Alarm only
is only relevant for ground fault detection in resonant-grounded or isolated systems.
If a device is switched off or if the ground differential protection is disabled or blocked in a device, calculation
of measured values becomes impossible. The entire ground differential protection system of both ends is
blocked in this case.
Pickup Value Ground Current Differential Protection
The current sensitivity is set at address
1222
87N L: I-DIFF>
. It is determined by the entire current flowing
into a protected zone in case of a fault. This is the total fault current regardless of how it is distributed between
the ends of the protected object.
This pickup value must be set to a value that is higher than the total steady-state quadrature-axis component
of current of the protected object. For cables and long overhead lines, the charging current has to be consid-
ered in particular. It is calculated from the operational capacitance (see Section 2.2.2.2).
Considering the variations of voltage and frequency, the value set should be at least 2,5 to 3 times higher than
the calculated charging current. Moreover, the pickup value should not be smaller than 15 % of the primary
rated current of the largest transformer in the protection configuration.
If setting is performed from a personal computer using DIGSI, the parameters can be set either as primary or
as secondary quantities. If secondary quantities are set, all currents must be converted to the secondary side
of the current transformers.
Time Delays
In special application cases, it may be advantageous to delay the tripping of the differential protection using an
additional timer, e.g. in case of reverse interlocking. The time delay
87N L: T-DELAY
(address
1224
) is only
started upon detection of an internal fault. This parameter can only be set in DIGSI at
Display Additional Set-
tings
.
With the inrush current restraint activated, the time delay
87N L: T-DELAY
must be at least 20 ms for the
blocking by the inrush current restraint to be effective.
If it is desired that a TRIP command is generated in the event of an internal fault only if simultaneously the
current of the local line end has exceeded a specific quantity, then this current threshold can be set for enabling
the differential protection TRIP at address
1225
87N L: I>RELEAS
. This parameter can only be set in DIGSI
at
Display Additional Settings
.