Woodward
Manual XRI1-ER
12
TD_XRI1-ER_12.03, Rev. C
4. Working principle
4.1 Analog circuits
The incoming current from the main current transformer on the protected object is converted to voltage signals
in proportion to the currents via the input transformers and burden. The noise signals caused by inductive and
capacitive coupling are suppressed 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 measuring quantity. (16 scans per
period).
4.2 Digital circuits
The essential part of the XRI1-ER relay is a powerful microcontroller. All of the operations, from the analog digi-
tal conversion to the relay trip decision, are carried out by the microcontroller digitally. The relay program is lo-
cated in an EPROM (Electrically-Programmable-Read-Only-Memory). With this program the CPU of the micro-
controller calculates the three phase currents and ground current in order to detect a possible fault situation in
the protected object.
For the calculation of the current value an efficient digital 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 disturbances.
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 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 timer re-sets
the microprocessor and gives an alarm signal, via the output relay "self supervision".
