GEK - 105560
5
2. APPLICATION
2.1 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).
Closing time delay
The minimum closing time, set at 100 ms, is actually 160 ms, as it is necessary to add the time
required by the measurement units and the operation of the output relays to the set 100 ms. It should
be kept in mind that during the first cycles, from the time of a closure, the voltage is stabilizing both in
the line as well as in the buses and it is no desirable to allow a closure under these conditions. What is
more, it is necessary to take into consideration the response of both the high voltage transformers and
the internal transformers of the relay.
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