Woodward
Manual MRIK3-C EN
18
DOK-TD-MRIK3-C Rev. A
5. Working principle
5.1 Analogue
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 R-C 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.
5.2 Digital
circuits
The essential part of the MRIK3-C 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 digitally.
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 currents and ground current
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 calculated actual current values are compared with the relay settings. If a phase current ex-
ceeds the pickup value, an alarm is given and after the set trip delay has elapsed, the correspond-
ing trip relay is activated.
The relay setting values for all parameters are stored in a parameter memory (EEPROM - Electri-
cally 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 by 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 supervision".
5.3 Status
descriptions
Reaction to protection events is possible at any time unless blocking is expressly desired (refer to
6.12.1). In the inactive and blocked state auto reclosing is not possible.
For the explanation of the functional sequence the following six status transitions are defined.
5.3.1
„Inactive“
The relay MRIK3-C is in "inactive" status if one of the following conditions is fulfilled:
The circuit breaker is in position "OFF",
the unit is in "blocked" status,
the unit is not in "starting/cycle" status
5.3.2
„Reclaim time“ tR
The relay MRIK3-C is in "reclaim time" status (tR) when the reclaim time
has not yet expired or
not interrupted by other incidents.
