10
TB MRIK3 12.00 E
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 signals caused by inductive and
capacitive coupling are supressed by an analog R-C fil-
ter 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 fre-
quency of 800 Hz (960 Hz), namely, a sampling rate
of 1.25 ms (1.04 ms) for every measuring quantity. (16
scans per periode).
4.2
Digital circuits
The essential part of the
MRIK3
relay is a powerful mi-
crocontroller. All of the operations, from the analog digi-
tal conversion to the relay trip decision, are carried out
by the microcontroller digitally. The relay program is lo-
cated in an EPROM (Electrically-Programmable-Read-
Only-Memory). With this program the CPU of the mi-
crocontroller 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 - Dis-
crete 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 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 re-
lay 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 un-
less blocking is expressly desired (refer to 3.1.3). In the
inactive and blocked state auto reclosing is not possi-
ble.
For the explanation of the functional sequence the fol-
lowing six status transitions are defined.
4.3.1
"Inactive"
The relay 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 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 is in position "AR-ready" status when the fol-
lowing conditions are fulfilled:
•
The circuit breaker is in position "ON",
•
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.
