Investigation of the influence of automatic excitation control system on turbine generator oscillatory behavior - Part II

Transients following faults in the network and/or the tripping of generating unit auxiliaries can excite oscillatory torques on the turbine-generator-rotor-shaft system. The oscillations can be damped or amplified with time. Damped oscillations affect the power quality and if the oscillations grow with time they may even lead to generating unit outages. Well designed excitation control system can contribute significantly in quickly bringing the generating unit to new stable state following a disturbance thus preventing possible unit outage (depending on the extent of disturbance). However since the events following disturbance are very quick such corrective action by excitation systems is possible when it is kept on "Auto" and not on "Manual". The excitation control if kept on "Manual mode" due to reasons of maintenance or lack of operator confidence may be sufficient for steady state voltage control and reactive power control with operator intervention but will not be helpful in bringing the generating unit to new stable state following a disturbance as unit operator take time in initiating actions. Thus the personnel responsible for the operation and maintenance of generating unit need to have a deeper appreciation of the turbine generator oscillatory behaviour with the excitation control system in "Automatic Mode" as compared to that when the system is kept in "Manual Mode" of operation. Such a comparative analysis would be very beneficial for motivating the plant personnel to ensure "Automatic Mode" of operation thus making the generating unit more stable following disturbance and preventing possible outages. This paper (Part H) is a contribution in this direction in continuation to the results reported in our earlier paper [1]. In this work, comprehensive dynamic model of synchronous generator system with automatic excitation control has been developed in software Matlab/Simulink. The oscillatory behaviour of the turbine generator has been investigated in time domain following 10% step reduction in infinite bus voltage causing "pole slipping" if the machine operates with fixed excitation input. The torques exerted on turbine generator rotor shaft sections, causing fatigue, due to pole slipping have been investigated using the rotor shaft model developed in Matlab/Simulink. The positive influence of automatic excitation control system in quickly bringing the generating unit to new stable state following disturbance, thus, preventing the rotor shaft stress has been clearly established.