Woodward
Manual MRS1 GB
8
DOK-TD-MRS1 Rev.A
4. Working principle
4.1 Analog
circuits
The incoming currents from the main current transformers of the protected object are converted to
voltage signals in proportion to the currents via the burdened input transformers. The noise signals
caused by inductive and capacitive coupling are suppressed by an analog R-C filter circuit.
The analog voltage signals are fed to the A/D-converter of the microprocessor and transformed to
digital signals through Sample- and Hold- circuits. The analog signals are sampled at fn = 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 MRS1 relay is a powerful microcontroller. All of the operations, from the
analog digital convertion to the relay trip decision, are carried out by the microcontroller digitally.
The relay program is located in an EPROM (Electrically Programmable Read-Only-Memory).
The calculated actual negative sequence current values are compared with the relay settings. If a
negative sequence current exceeds the pickup value, an alarm is given and after the set trip delay
has elapsed, the corresponding trip relay is 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 microprocessor is supervised through a built-in "watchdog" timer. In case of a failure the
watchdog timer resets the microprocessor and gives an alarm signal via the output relay "self su-
pervision".