TB MRF3 04.01 E
9
4
Working principle
4.1
Analog circuits
The input voltage is galvanically insulated via the input
voltage transformer. The noise signals caused by the
influence of inductive and capacitive couplings are
then suppressed by RC-analog filter circuits. The ana-
log voltage signals are fed to the A/D-converter of the
microprocessor and then transformed into digital sig-
nals via sample- and hold-circuits. These digital values
are then used for further processing. The analog sig-
nals 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). The input voltage is
also passed through an analog filter for frequency
measurement and is then converted into square wave
signals via comparators. The frequency is determined
by measuring complete cycles.
4.2
Digital part
The essential element of the protection relay is a pow-
erful microcontroller. All functions - from the analog
digital conversion to the relay tripping decision are
carried out by the microcontroller digitally.
The relay program is located in an EPROM (Electri-
cally-Programmable-Read-Only-Memory). With this
program the microcontroller's CPU calculates the value
of the measured voltage of the fundamental frequency.
Harmonics are suppressed by an efficient digital filter
based on the Fourier transformation (DFFT = Discrete
Fast Fourier Transformation) When the measured volt-
age falls below the voltage threshold U
B
, all frequency
functions are blocked.
The frequency is established from the time difference of
two similar voltage zero passages. The microprocessor
compares continuously the frequency measured values
and df/dt measuring values with the preset pickup
values (setting value) stored in the parameter memory
(EPROM). If a fault occurs an alarm is given and after
the set tripping delay has elapsed, the corresponding
tripping relay is activated.
The relay setting values for all parameters are stored in
a parameter memory (EPROM - 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 failure the watch-
dog timer resets the microprocessor
and gives an
alarm signal via output relay "self supervision".
4.3
Principle of frequency supervision
Frequency relay
MRF3
protects electrical generators,
consumers or electrical operating equipment in general
against over- or underfrequency.
The relay has, independent from each other, four fre-
quency elements f
1
- f
4
with a free choice of parame-
ters, with separate adjustable pickup values and delay
times as well as two elements for supervision of fre-
quency gradient df/dt. With the aid of the frequency
gradient sign both frequency increase and frequency
decrease can be supervised.
The measuring principle of the frequency supervision is
based in general on the time measurement of complete
cycles, whereby a new measurement is started at each
voltage zero passage. The influence of harmonics on
the measuring result is thus minimized.
J
KJ
6
6
Figure 4.1: Determination of cycle duration by means of
zero passages.
In order to avoid false tripping during occurence of in-
terference voltages and phase shifts the relay works
with an adjustable measuring repetition (see chapter
5.2.2)
Frequency tripping is sometimes not desired by low
measured voltages which for instance occur during al-
ternator start-up.
All frequency supervision functions can be blocked
with the aid of an adjustable voltage threshold U
B
in
case the measured voltage value is below this value.
Summary of Contents for MRF3
Page 1: ...MRF3 Frequency Relay...
