Woodward
Manual MRI3-ITER GB
14
DOK-TD-MRI3 ITERE Rev.A
4. Working principle
4.1 Analog
circuits
The incoming currents from the main current transformers on the protected object are converted to
voltage signals in proportion to the currents via the input transformers and burden. The noise sig-
nals caused by inductive and capacitive coupling are suppressed by an analogue RC filter circuit.
The analogue voltage signals are fed to the A/D-converter of the microprocessor and transformed
to digital signals through Sample and Hold circuits. The analogue signals are sampled at 50 Hz (60
Hz) with a sampling frequency of 800 Hz (960 Hz), namely, a sampling rate of 1.25 ms (1.04 ms)
for every measuring quantity.
4.2 Digital
circuits
The essential part of the MRI3-ITE relay is a powerful microcontroller. All of the operations, from
the analogue digital conversion to the relay trip decision, are carried out by the microcontroller digi-
tally. The relay program is located in an EPROM (Electrically-Programmable-Read-Only-Memory).
With this program the CPU of the microcontroller calculates the three phase in order to detect a
possible fault situation in the protected object.
For the calculation of the current value an efficient digital filter based on the Fourier Transformation
(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 calcu-
lated actual current values are compared with the relay settings.
In case, the time for which a current was above the preset pickup value, exceeds the trip delay or
the thermal capacity is reached, an alarm signal will be given. Dependent on their adjustment the
output relays will also be activated.
The relay setting values for all parameters are stored in a parameter memory (EEPROM –
Electrically Erasable Programmable Read-only Memory), so that the actual relay settings cannot be
lost, even if the power supply is interrupted. The micro-processor is supervised by a built-in
"watchdog" timer. In case of a failure the watchdog timer re-sets the microprocessor and gives an
alarm signal, via the output relay "self supervision".
Figure 4.1: Block diagram
