8
TB MRM3 07.01 E
3.3
Analog part
The alternating currents injected by the CTs are con-
verted into galvanical isolated voltages via input
transmitters and burden in the analog part.
The effect of inductive and capacitive coupled interfer-
ences are suppressed by RC analog filters. The meas-
uring voltages are fed to the analog inputs (A/D trans-
former) of the micro-processor and then converted into
digital signals by means of sample and hold circuits.
These digitized values are then used for further proc-
essing. The measuring values are acquired at fn = 50
Hz (fn = 60 Hz) with a sampling frequency of 800 Hz
(960 Hz), and thus the instantaneous values of the
measured quantities are acquired every 1.25 ms
(1.04 ms).
3.4
Digital part
The protection relay is equipped with a powerful mi-
cro-controller, being the core element of the protection
unit. With this micro-controller all tasks are completely
digitally processed, from discretization of the measur-
ing quantities to protective tripping.
With the protection program, stored in the program
storage (EPROM), the micro-processor processes the
voltages applied to the analog inputs and from this
calculates the fundamental harmonics of the current.
Digital filtering (DFFT-Discrete Fast-Fourier-Transforma-
tion) for suppression of harmonics as well as suppres-
sion of DC components during the short-circuit is used
in the process.
The micro-processor compares the existing current with
the threshold value (setting value) stored in the parame-
ter storage (EEPROM) and up-dates the thermal image.
If a current exceeds the threshold value for longer than
the trip delay or if the thermal image exceeds its rated
value, a fault signal occurs. Dependent on their set-
tings, the output relays pick up as well. When setting
the parameters, all setting values are read-in by the
micro-processor and saved in the parameter storage.
The program flow is continuously monitored by the in-
corporated "Hardware-Watchdog". Processor failure is
signalled by the “Selfsupervision” output relay.
Figure 3.7: Block Diagram of Protective Functions
Содержание MRM3
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