14/1/98 Page 10 Issue H
6. Working principles
6.1 Analogue circuits
The incoming currents and voltages from the external transformers are converted to internal signals
in proportion to the current and voltage, via the internal input transducers and shunt resistors. The
noise signals caused by inductive and capacitive coupling are suppressed by an analogue RC filter
circuit. The analogue signals are fed to the A/D converter of the micro-processor and transformed to
digital signals through sample-hold circuits. The analogue signals are sampled at a rate of 16 times
the system frequency (40 - 70Hz), giving a sample time of 1.56 - 0.9mS for each measured value.
6.2 Digital circuits
The essential component of the MPR 3E5 relay is a powerful micro-controller. All of the operations,
from the analogue digital conversion to the relay trip decision, are carried out by the micro-controller
digitally. The relay program, located in EPROM, allows the CPU of the micro-controller to calculate
the reactance values in order to detect a possible fault.
For the calculation of the current values, an efficient digital filter, based on the Fourier Analysis
(DFFT - Discrete Fast Fourier Transformation), is applied to suppress high frequency harmonics and
DC components caused by fault induced transients or other system disturbances. The actual
calculated values are compared with the relay settings. When a value exceeds the starting value the
unit starts the corresponding time delay calculation. When the set time delay has elapsed, a trip
signal is given.
The relay setting values for all parameters are stored in EEPROM, so that the actual relay settings
cannot be lost, even in the event of auxiliary supply interruption. The micro-processor is supervised
through a built in "Watch-dog" timer. Should a failure occur the watch-dog timer resets the micro-
processor and gives an alarm signal via the self supervision output relay.
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General