Table of Contents
Advertisement
[7sd80-diff-grundprinzip-20110530, 1, en_US]
Figure 2-10
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 toler-
ances, deviations in time or due to the "qualität" of the measured quantities such as harmonics and deviations
in frequency, 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 consid-
ered.
For transient inrush currents the devices have a separate inrush current restraint feature.
Evaluation of Measured Values
The ground current differential protection in grounded systems evaluates the sum of the ground current
phasors.
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 operation, 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 Ι
(45° curve) which is cut off below the setting value 87N L: I-DIFF>. It complies with the equation
= 87N L: I-DIFF> + Σ (current transformer errors and other measuring errors).
Ι
rest
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).
SIPROTEC 4, 7SD80, Manual
E50417-G1100-C474-A2, Edition 02.2018
Basic principle of the differential protection for a line with two ends
2.2 Phase Comparison Protection and Ground Differential Protection
Functions
= Ι
diff
rest
55
Advertisement
Table of Contents
