Seg XRI1-IR, Руководство пользователя

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TB XRI1-IR 02.00 E 9
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-IR 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 Directional feature
A built-in directional element in XRI1-IR is available for
application to doubly infeeded lines or to ring net-
works.
The measuring principle for determining the direction is
based on phase angle measurement and therefore
also on coincidence time measurement between cur-
rent and voltage. Since the necessary phase voltage
for determining the direction is frequently not available
in the event of a fault, whichever line-to-line voltage fol-
lows the faulty phase by 90° is used as the reference
voltage for the phase current. The characteristic angle
at which the greatest measuring sensitivity is achieved
can be set to precede the reference voltage in the
range from 15° to 83°.
Figure 4.1: Relay characteristic angle
The TRIP region of the directional element is deter-
mined by rotating the phasor on the maximum sensitiv-
ity angle for
±
90°, so that a reliable direction deci-
sion can be achieved in all faulty cases.
Figure 4.2: TRIP/NO-TRIP region for directional element in
XRI1-IR. In this case the advance direction is
defined as TRIP region and the reverse direction
as NO-TRIP region.