Functions
2.2 Phase Comparison Protection and Ground Differential Protection
SIPROTEC, 7SD80, Manual
E50417-G1140-C474-A1, Release date 09.2011
53
2.2.2 Phase Comparison Protection
2.2.2.1 Description
General
The phase comparison protection evaluates the phase currents at both ends of the protected object. The two
7SD80 devices at the ends of the protected object communicate over their protection interfaces. The phase-
specific comparison and the resulting decision to trip the circuit breaker is made separately for each end.
The digitalized currents are first filtered to suppress DC components and higher harmonics.
Figure 2-4 Phase comparison protection, determination of the input variables
These filtered values are available to a sensitive dynamic element and a static element. By comparing the po-
larity of the currents at the two ends of the protected object, they recognize whether the fault is external or in-
ternal. An internal fault applies if the polarity of the fault currents is identical on both sides; an internal fault or
a load step occurs with different polarities.
If the comparison shows without any doubt that a fault is present, the trip command is sent. It is maintained
over a set minimum command duration.
The phase comparison protection may trip only at one end in case of single-end infeed. The non-feeding end
can also be switched off by means of a transfer trip signal.
Element Idyn
The dynamic filter algorithm generates the value idyn(t). It represents the current change of the filtered value
(fundamental component) over two cycles. If the current change exceeds the set threshold 87L Idyn>, the
phase comparison protection is started.
The polarity of the current change is transmitted to the device at the remote end of the protected object.
The dynamic element operates very sensitively in case of internal faults. In case of external faults, the method
is very stable even during different primary rated currents or different saturation of the current transformers at
the two ends.
Element Istat
The static element Istat operates directly with the filtered fundamental value. If the amplitude of the fundamen-
tal component exceeds the set threshold 87L Isteady>, the phase comparison protection is started.
The polarity of the current is transmitted to the device at the remote end of the protected object.
The static element is insensitive towards low fault currents.
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