M-LIB3 Low Impedance Bus Differential Relay
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6.1 GENERAL CHARACTERISTIC
Currents from the CTs of the Feeders connected to each protected zone are conveyed to a wiring bus
(one for each phase) that drives the positive and negative halfwave of the current on two different wires.
The wiring bus also carries the signal from the CT’s saturation detectors.
One M-LID3 differential relay is connected to the wiring bus of its protected zone and from the wiring bus of each
phase it can measure :
q The bias current IR proportional to the arithmetic summation of all the incoming and outgoing currents of the
feeder connected to the protected busbar zone.
q The differential current Id proportional to the vector summation of all the currents of the zone.
The CTs saturation detectors shunt the measurement of Id during saturation due to faults external to the protected
zone.
The Id measuring circuits are transformer isolated whereas the IR measuring circuits are optoisolated.
6.2 POWER SUPPLY
The auxiliary power is supplied by a fully isolated and self protected, built-in, interchangeable module.
Two options are available:
24V(-20%) / 110V(+15%) a.c. 80V(-20%) / 220V(+15%) a.c.
a) -b) -
24V(-20%) / 125V(+20%) d.c. 90V(-20%) / 250V(+20%) d.c.
Before energizing the unit check that supply voltage is within the allowed limits.
6.3 MEASUREMENTS ACQUISITION AND OPERATION PRINCIPLE
For enhanced reliability the A/D converters and the signal measuring circuits are totally duplicated.
Furthermore all the signal-processing elements are duplicated.
It means that any step of operation takes place only if two elements produce the same output.
The basic detection elements are :
q The CTs Circuit Supervision element which compares the actual differential current Id with the CTs Circuit
Supervision Level ISV :
operation when Id> [ISV]
q The zone differential current element which compares the actual differential current Id with the set minimum
pick-up level IdS :
operation when Id> [IdS]
q The zone Biased Differential Current element which compares the actual differential current with the actual
trip level “IS” calculated in function of the zone through current IR and the bias coefficient R
operation when Id> ([IdS] + R • IR)