GE MLJ1005B010H00C Synchronism Check Relay

GE MLJ1005B010H00C Synchronism Check Relay

The main applications of the MLJ are:

• Connecting a generator to the system.  

• Re-establishing the connection between two parts of the system.  

• Manual closing of circuit breakers  

• Automatic reclosing of a breaker after a relay trip.

The MLJ is a digital synchronism-checking relay that measures bus and line voltages. It tests:  

• Voltage difference  

• Frequency slip  

• The phase angle between both voltages

The relay functions in two modes:  

• Continuous mode: In this mode synchronism is checked continuously.   

• Manual mode: This is activated when voltage is applied through a manually activated input, thus  beginning synchronism control when voltage applied through another digital input for initial checking.

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.

Close enable

Close enable is defined by three conditions in the system:

• The difference in amplitude of the two voltage signals to be synchronized defines a circular sector shifted  from the axis of abscissa a distance equal to the magnitude of the minimum vector and having a  maximum anchor equal to the difference ΔV.  

• The angular difference, allowed either in positive or negative, between the two approaching vectors,  forms a cone on which the close enable takes place.   

• The frequency slip, Δf = ⏐f1 - f2⏐, which has to be less than that specified, configures a third and basic  condition of closure, optimum for carrying out closure procedures in ideal conditions, with similarity  between signals.

Synchronism

In general, synchronism check is intended primly for application where the two parts of a system to be joined  by the closure of a circuit breaker are interconnected at other points throughout the system. Usually,  performing synchronism check measurements is done with relatively long times in order to make sure the  voltages are synchronized. Nevertheless, this long timer, which can be on the order of 10 to 20 seconds, is  not appropriate when both ends of the line are to be reclosed at high speed. If the measurement time is  lower, then the synchronism check can be done faster, although this means that the reclosing could be done  under no-synchronism conditions, with greater frequency slip than for ideal condition.

Application of voltage

As the MLJ is equipped with instantaneous undervoltage monitoring in two inputs (line and bus), and with  undervoltage and overvoltage levels (live and dead), it can be used as a relay in a large number of  applications such as:  

• Undervoltage  

• Overvoltage  

• Voltage within the limits  

• Voltage outside of limits  

• Instantaneous or time delayed