Sriwidjojo - JASA Teknik Kelistrikan INDONESLA

When a generator loses excitation, it runs as an induction generator and draws reactive power from the system. Because of the additional reactive demand from the system, there may be a widespread voltage reduction resulting instability. Undercurrent relays in the field circuit can be employed, as the field current falls below a predetermined value, however this type o fprotection is affected by alternating current induced from outside circuit. A more reliable protection is a suitable impedance type relay with its characteristics in the negative reactance area, since the impedance of the generator change to capacitive when the generator loses its excitation. By selecting Mho relay with suitable characteristics, it will be possible to detect the loss of excitation condition. This paper will analyse the zone area protection of Mho relay when loss of field occurs with case study of Suralaya Steam Power Plant.