Setting Notes - Siemens SIPROTEC 7SD80 Manual

Functions
2.2 Phase Comparison Protection and Ground Differential Protection

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.
62
SIPROTEC, 7SD80, Manual
E50417-G1140-C474-A1, Release date 09.2011