GE 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).

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.

There is also a time for sustaining the permission signal, which always has a fixed value of some 130 ms.,

being the result of a prefixed delay of 100 ms. added to the dropout time of the measurement units (one

cycle = 20 ms. at 50 Hz.) and the deactivation of the output relay (10 ms.).

Minimum settings

Barring any special requirements, it is not recommended to use settings that near the minimum limits for the

relays, (2 V for the voltage difference, 2º for the angle difference), so as to avoid being too restrictive in close

enabling, given the real characteristics of accuracy in the installations, measurement transformers, etc.