Phase Comparison Protection
Functional Description
General
The phase comparison protection evaluates the phase currents at both ends of the protected object. The 2
7SD80 devices at the ends of the protected object communicate over their protection interfaces. The phases-
pecific 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.
[lo-pvl-erf-20101117, 1, en_US]
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
polarity of the currents at the two ends of the protected object, they recognize whether the fault is external or
internal. An internal fault applies if the polarity of the fault currents is identical on both sides; an external 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 Ιdyn
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 Ιstat
The static element Ιstat operates directly with the filtered fundamental value. If the amplitude of the funda-
mental 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.
Pickup Logic
The dynamic and the static element pick up independently of each other selectively for each phase.
To prevent tripping during an energization, a separate dynamic switch-on threshold 87L Idyn close> is
used.
The pickup is maintained over 2 measuring cycles. After the 2 measuring cycles have expired, the dynamic
sign comparison is blocked.
2.2.2
2.2.2.1
Functions
2.2 Phase Comparison Protection and Ground Differential Protection
SIPROTEC 4, 7SD80, Manual 49
E50417-G1100-C474-A2, Edition 02.2018
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