Functions
2.2 Phase Comparison Protection and Ground Differential Protection
SIPROTEC, 7SD80, Manual
E50417-G1140-C474-A1, Release date 09.2011
60
Evaluation of Measured Values
The ground current differential protection in grounded systems evaluates the sum of the ground current pha-
sors.
Each device calculates a ground current at each end of the protected object (fundamental component of the
ground current) and transmits it to the partner device. The received and the locally measured ground current
phasor is added to the ground differential current. The ground differential current value equals the fault current
that the differential protection system „sees“. In the ideal case, it equals the short-circuit current. In healthy op-
eration, the differential current value is low and for lines about similar to the capacitive charging current.
The restraining quantity counteracts the ground differential current. It is the total of the maximum measured
errors at the ends of the protected object and is calculated from the current measured quantities and power
system parameters that were set. Therefore, the highest possible error value of the current transformers within
the nominal range and/or the short-circuit current range is multiplied with the current flowing through each end
of the protected object. The total value, including the measured internal errors, is then transmitted to the other
end. This is the reason why the restraint current is a replica of the greatest possible measurement error of the
entire differential protection system.
The pickup characteristic of the differential protection is derived from the restraining characteristic I
diff
= I
rest
(45°
curve) which is cut off below the setting value 87N L: I-DIFF>. It complies with the equation
I
rest
= 87N L: I-DIFF> + Σ (current transformer errors and other measuring errors).
If the calculated differential current exceeds the pickup threshold and the greatest possible measurement error,
the fault must be internal (grayed area in the illustration).
Figure 2-11 Pickup characteristic of the ground differential protection
If it is desired that an internal fault should initiate a TRIP command and additionally a local current of a specific
quantity should exist, the value of this current can be set at address 1225 87N L: I>RELEAS. The default
setting for this parameter is zero so that this additional criterion does not become effective.
The differential current and the restraint current 3I0diff and 3I0restr are included in the fault record.
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