10
TB XRW1-4 04.01 E
4
Working principle
4.1
Analog circuits
The input voltages are galvanically insulated by the in-
put transformers. The noise signals caused by inductive
and capacitive coupling are supressed 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
XRW1-4
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 digi-
tal filter based on the Fourier Transformation (DFFT -
Discrete Fast Fourier Transformation) is applied to sup-
press high frequency harmonics and d.c. components
caused by fault-induced transients or other system dis-
turbances. The microprocessor continuously compares
the measured values with the preset thresholds stored in
the parameter memory (EEPROM). If a fault occures 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 "watch-
dog" timer. In case of a failure the watchdog gives an
alarm signal via the output relay "self supervision".
4.3
Voltage supervision
The voltage element of
XRW1-4
has the application in
protection of generators, consumers and other electrical
equipment against over/and undervoltage.
The relay is equipped with a four step independent
three-phase overvoltage (U
1
> ... U
4
>) and undervoltage
(U
1
< ... U
4
<) 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 con-
tinuously compared with the preset thresholds.
For the overvoltage supervision the highest, for the un-
dervoltage supervision of the lowest voltage of the three
phases are decisive for energizing.
Basic for this elements is the RMS of the voltage fun-
damental.
