Manual MRG3
SEG Electronics GmbH
DOK-TD-MRG3, Rev. D
21
4. Working Principle
4.1 Analog circuits
The input voltages are galvanically insulated by the input transformers. Thereafter, the influence of
noise signals caused by inductive and capacitive coupling is suppressed by an analog R-C filter
circuit.
The analog measured voltage signals are fed to the A/D-converter of the microprocessor and then
trans-formed to digital signals through Sample- and Hold- circuits. Any further processing is then
based on these digitalized signals. 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 and every 1.04
ms for every measuring quantity at 60 Hz.
4.2 Digital circuits
The essential part of the MRG3 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.
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 "watch-dog" 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 supervision element of the MRG3 is to generally protect 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 separately adjustable pick-up values and delay times.
In delta connection the phase-to-phase voltages and in star connection the phase-to-neutral volt-
ages 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.
