6 TD_MRI1-IU_05.04_GB
4.2 Digital circuits
The essential part of the MRI1-IU relay is a powerful
microcontroller. All of the operations, from the ana-
logue 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 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 acti-
vated.
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".
selection of current transformers. It implies that, if an
electromechanical relay is replaced by MRI1, a high
accuracy limit factor is automatically obtained by using
the same current transformer.
4.3 Voltage controlled tripping
characteristic
The voltage controlled time overcurrent relay MRI1-IU
is the combination of a time overcurrent relay (basic
unit MRI1) and an additional undervoltage supervision
unit.
The undervoltage supervision unit has an influence on
the tripping delay of the overcurrent and short-circuit
steps by switching two setting points. In normal opera-
tion (at nominal voltage) the MRI1-IU operates like a
normal time overcurrent relay with preselected tripping
characteristic (IDMT, DMT) and adjusted pick up value
IS.
The following diagram explains the switching-over to
another IS value. ISN is the pickup value during nor-
mal operation and ISL at undervoltage (low voltage).
Switching-over at undervoltage (Example ISL = 0.5)
0.4 0.6 0.8 11 2 4 6 8 1010 20
I/I
N
1
1010
100100
t[s]
1.0
1.0
t
I>
=
I
SL
=0.5
I
SN
=1.0
In case of failure (short-circuit of the alternator) the al-
ternator voltage decreases. The MRI1-IU will recognise
this and then switch over without delay to a lower
pickup value IS. The value ISL can be adjusted. As a
result, shorter tripping periods of the overcurrent and
short-circuit step can be achieved. The adjusted trip-
ping characteristics (normal inverse, very inverse, ex-
tremely inverse or DMT is maintained). Setting ranges
see in 7.3.
4.4 Demand imposed on the main
current transformers
The current transformers have to be rated in such a
way, that a saturation should not occur within the fol-
lowing operating current ranges:
Independent time overcurrent function: K1 = 2
Inverse time overcurrent function: K1 = 20
High-set function: K1 = 1.2 - 1.5
K1 = Current factor related to set value
Moreover, the current transformers have to be rated
according to the maximum expected short circuit cur-
rent in the network or in the protected objects.
The low power consumption in the current circuit of
MRI1, namely <0,2 VA, has a positive effect on the
selection of current transformers. It implies that, if an
electromechanical relay is replaced by MRI1, a high
accuracy limit factor is automatically obtained by using
the same current transformer.
