Manual MRN3 GB
Woodward
TD_MRN3_09.08_GB_Rev.New
13
4. Working principle
4.1 Analog
circuits
The input voltages are galvanically insulated by the 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 with a sampling
frequency of 16 x f
N
, namely, a sampling rate of 1.25 ms for every measuring quantity, at 50 Hz.
4.2 Digital
circuits
The essential part of the MRN3 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 voltage in order to detect a
possible fault situation in the protected object.
For the calculation of the voltage value an efficient digital filter based on the Fourier Transformation
(DFFT - Discrete Fast Fourier Transformation) is applied to suppress high frequency harmonics
and d.c. components caused by fault-induced transients or other system disturbances.
The micro-processor continuously compares the measured values with the preset thresholds
stored in the parameter memory (EEPROM). If a fault occurs an alarm is given and after the set
tripping 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 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".
4.3 Voltage
supervision
The voltage element of MRN3 has the application in protection of generators, consumers and other
electrical equipment against over-/and under voltage.
The relay is equipped with a two step independent three-phase overvoltage (U>, U>>) and
under voltage (U<, U<<) function with completely separate time and voltage settings.
In delta connection the phase-to-phase voltages and in star connection the phase-to-neutral
voltages are continuously compared with the preset thresholds.
For the overvoltage supervision the highest, for the under voltage supervision of the lowest voltage
of the three phases are decisive for energizing.
