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4 - 2
Siemens AG ⋅ January 1999
The transformers of the measurement input group MI
transform the currents I
1
(summation current of local
line end) and I
a
(pilot wire current) and match them to
the internal processing level of the device. As well as
the transformers providing complete galvanic and
low-capacitance isolation, filters are installed for the
suppression of interference. These filters are optimiĆ
zed for measured value processing in terms of bandĆ
width and processing speed. The modified analog siĆ
gnals are passed to the analog input AI.
The analog input AI contains input amplifiers sample
and hold elements for each input, multiplexers, anaĆ
log/digital converters, as well as memory for the
transfer of data to the microprocessor.
The micro processor system not only controls and
monitors the measured signals, but also processes
the actual protection functions. These, in particular,
include the following:
D Filtering and conditioning of measured signals,
D Continuous calculation of values relevant for proĆ
tection starting,
D Calculation of tripping and stabilization currents,
D Frequency analysis of the measured signal diffeĆ
rential, recognition of current transformer saturaĆ
tion and sequential faults,
D Checking of pilot wires (devices with pilot wire
monitoring),
D Interrogation of thresholds and timing sequences,
D Decision on trip commands,
D Storage and output of events and disturbance reĆ
cord data for fault analysis.
Binary inputs and outputs from and to the processor
are controlled by the input/output modules. This is
where the processor gets information from the staĆ
tion (e.g. remote resetting) or from other devices. The
main outputs are trip commands to the circuit-breaĆ
kers, indications for the remote indication of imporĆ
tant events and states, as well as optical displays
(LED) and an alphanumeric display panel on the front.
Communication with the device is via an integrated
keypad and alphanumeric LCD display panel which
enables the input of all data required for operating,
such as setting values, station data, etc. These paraĆ
meters can also be read out here, and after a system
disturbance, the relevant data for analysis can be read
off. A front serial operator interface also enables
communication via PC.
A suitably rated power supply feeds the described
function modules with the necessary +5 V. Short-
time interruptions of the supply voltage of up to
50 ms in the case of U
H
w 110 V=, which can occur
in the event of short-circuits in the station's DC supĆ
ply system, are bridged by a DC buffer.
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