OSCILLATION CONTROL IN POWER SYSTEMS

Habib Wajid

doi:10.7273/000007102

Forced oscillations have lately become a serious concern in power systems. Unlike natural oscillations, these are caused due to periodic excitation, that can be modelled as an external injection into the power system. Forced oscillations tend to linger in the system unless the under lying source is remedied or removed from the system. Unresolved forced oscillations can lead to equipment fatigue and damage, reduced system capacity, stability issues, unnecessary relay tripping, islanding and blackouts in extreme cases. Needless to say, these are undesirable and must be dealt with. In this dissertation, a two-step solution is proposed to address the problem of forced oscillations of unknown origin in power systems. In step 1, we inject a carefully designed control signal into the system to counter the effect of forced oscillations. This attenuates the amount of oscillation content in the power system and relieves the system of stress, buying system operator time to take permanent action against the problem. In step 2, we locate the element responsible for injecting the forced oscillations into the system using measurements and either fix it or disconnect it from the system. As part of this dissertation, we have developed a novel open-loop controller for forced oscillation mitigation in power systems. The controller has been tested on validated model of Western Interconnection and has been found useful to attenuate the forced oscillations from power systems. In another contribution of this dissertation, we have developed a novel, decentralized, efficient forced oscillation source location method which works by elimination. The method uses input-output properties of synchronous generator to find out if the subject machine is the source of forced oscillation or not. The method has been tested on the Mini-WECC system and real data from Peak Reliability and has been found useful for the purpose. Several methods have been proposed in literature for forced oscillation source location, but none of those claim a universal applicability. Some methods have still managed to grab attention from industry due to their simplicity and intuitiveness, namely mode-shape method and dissipation energy method. Recently, a variation of energy method, called cross-power spectral density method has also caught some attention. In this dissertation we have analyzed, in detail, through rigorous heuristic testing, the validity of these source location methods against different test systems, type of oscillations, source locations, and damping conditions. Mini-WECC system and validated model of WECC have been used for this study and it has been found that none of these methods perform good under all conditions. The framework proposed for these studies uses frequency response for analysis, and is much faster than its time domain counterpart for a study of similar size.

OSCILLATION CONTROL IN POWER SYSTEMS

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