Multilin MLJ Synchronism Check Relay Application
DESIGN CHARACTERISTICS
Measurement accuracy
The differential angle measurement of the MLJ is high precision and is limited solely by errors in available
voltage transformers.
The measurement of the angle is practically independent of the voltage.
In the MLJ the measurement is obtained via a numerical calculation done on digital voltage samples, thus
achieving high precision. This allows for a rating of 2º, which is clearly better than the possible rating using
other technologies.
Influence of harmonics
The pillar of the MLJ measurement calculation is the discrete Fourier transform, which is in essence a
harmonics filter. For this reason the voltage and line measurements are not affected by frequencies other
than the fundamental.
The rejection of harmonics is added to the independence of measurements, both magnitude and phase,
relative to frequency signal variations, which is very important in a synchronism checking relay which, by its
own nature, works in variable frequencies.
Given that in power systems, synchronization or synchronism checking is carried out in a steady state, that
is with voltage magnitudes near or equal to the rated value, close enable is not emitted for very low voltages.
Therefore, for voltage of less than 9 volts, the relay stops measuring phase and frequency, not giving
permission to close under such conditions.
The MLJ also offers additional insensitivity to frequency measurement concerning harmonics, since this is
done via a hardware circuit, a zero-cross detector, with an intrinsic harmonics filter. Furthermore, it has a
software filter which operates by double-period measurement, both between the rising and falling edges,
averaging them out and allowing for better performance of algorithm frequency (improving security and
response).