10
TB XRI1-IE 11.01 E
4
Working principle
4.1
Analog circuits
The incoming currents from the main current transform-
ers on the protected object are converted to voltage
signals in proportion to the currents via the input trans-
formers and burden. The noise signals caused by in-
ductive 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 at 50 Hz (60 Hz) with a
sampling frequency of 800 Hz (960 Hz), namely, a
sampling rate of 1.25 ms (1.04 ms) for every measur-
ing quantity. (16 scans per periode).
4.2
Digital circuits
The essential part of the
XRI1-IE
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 currents and
ground current 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".
4.3
Earth fault protection
4.3.1
Generator stator earth fault
protection
With the generator neutral point earthed earthed as
shown in figure 4.4 the
MRI1
picks up only to phase
earth faults between the generator and the location of
the current transformers supplying the relay.
Earth faults beyond the current transformers, i.e. on the
consumer or line side, will not be detected.
Figure 4.1: Generator stator earth fault protetion
4.3.2
System earth fault protection
With the generator neutral point earthed as shown in
figure 4.5, the
MRI1
picks up only to earth faults in the
power system connected to the generator. It does not
pick up to earth faults on the generator terminals or in
generator stator.
Figure 4.2: System earth fault protection