EARTH FAULT RELAY MCAG AREVA / ALSTOM

The MCAG relay, used with a stabilising resistor, is designed for applications where sensitive settings with stability on heavy through faults are required, and is recommended for balanced and restricted earth fault, bus-zone and certain forms of differential protection for generators, auto-transformers, reactors and motors. The relay operates as a high impedance unit protection scheme. The relay is an attracted armature unit of simple and robust construction. The operating coil of this unit is connected in series with a small choke and capacitor, forming a series resonant circuit. These components are energised from an auto-transformer which is tapped to provide seven current settings. Due to the simple electromechanical construction, the detection element, and the output contacts are one and the same device. Operation is therefore fast, and highly reliable. The relay circuit, tuned to the supply frequency, rejects the harmonics produced by current transformer saturation. The total impedance of the relay and series stabilising resistor is usually low enough to prevent the current transformers developing voltages over 2kV during maximum internal faults, but in some applications a non-linear resistor is required to limit this voltage. Types MCAG 14 and MCAG 34 relays are single and triple pole, respectively.