TB MRI(K)3-C 03.02 E
11
4
Working principle
4.1
Analog circuits
The incoming currents from the main current transform-
ers on the protected object are converted to voltage
signals in proportion to the currents via the input trans-
formers and burden. The noise signals caused by in-
ductive and capacitive coupling are surpressed 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 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 measur-
ing quantity.
4.2
Digital circuits
The essential part of the
MRI(K)3-C
relay is a powerful
microcontroller. All of the operations, from the analog
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 situa-
tion in the protected object.
For the calculation of the current value an efficient digi-
tal filter based on the Fourier Transformation (DFFT -
Discrete Fast Fourier Transformation) is applied to sup-
press high frequency harmonics and DC components
caused by fault-induced transients or other system dis-
turbances.
The calculated actual current values are compared
with the relay settings. If a phase current exceeds the
pickup value, an alarm is given and after the set trip
delay has elapsed, the corresponding trip relay is ac-
tivated.
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 by a built-in "watch-
dog" timer. In case of a failure the watchdog timer re-
sets the microprocessor and gives an alarm signal, via
the output relay "self supervision".
4.3
Status descriptions
Reaction to protection events is possible at any time
unless blocking is expressly desired (refer to 5.11.1).
In the inactive and blocked state auto reclosing is not
possible.
For the explanation of the functional sequence the fol-
lowing six status transitions are defined.
4.3.1 „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
4.3.2 „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.
4.3.3 „AR-ready“
The relay
MRIK3-C
is in position "AR-ready" status
when the following conditions are fulfilled:
•
The circuit breaker is in position "ON",LED CB
emit red light
•
the reclaim time has expired,
•
the unit is not in "blocked" status,
•
the unit is not in "starting cycle" status.
Only in "AR-ready" status a reaction of the AR-unit to
the protection incidents is possible!
4.3.4 „AR-starting“ (dead time)
In "AR-starting" status the start conditions for an auto-
matic reclosing by means of the protection commands
and the circuit breaker position are checked.
