Disturbance observer based control: applications to AC power electronics and induction motor drives with input constraints
Creators: name: MCNABB, Luke
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400803
subject: Electrical energy generation (incl. renewables, excl. photovoltaics)

description: Composite disturbance estimation (or observer) based control structures have numerous benefits for a range of different control strategies. Their separability and readily bound stable dynamic structures make for easy design and implementation with secondary anti-windup properties. At the same time, they can also be used to mitigate various demerits associated with discontinuous compensation schemes, such as unmatched disturbance rejection and minimising actuation chatter, which has numerous on-effects. There are some analytical and design limitations still present regarding the various composite structures. Moreover, there remain numerous novel application examples and issues that need addressing when using these techniques for stationary frame power electronics applications and induction motor drives systems.
 
This thesis presents the following contributions. First, it is identified that disturbance estimation based networks utilised for compensation possess the same polynomial architectures as signal conditioning anti-windup and their equivalents, noting that the former exhibits these traits intrinsically, leading to a drastic reduction in design/implementation overhead. It is also demonstrated by example that the intrinsic partitioning of the controller by using a disturbance estimator in the common anti-windup framework tends to be better than traditional partitions used in a two-step anti-windup development.  Also, because high-performance AC current regulators are prone to heavy saturation, the benefits of using disturbance estimative based compensation are demonstrated, particularly in terms of implementation overhead due to the potential for high order disturbances (e.g. multi-resonant). An application-specific development regarding the continuity between the phase leg controllers and the saturated control flux imparted on multi-phase loads using vector centred pulse-width-modulation is also investigated. State-variant constraints result to ensure decoupled phase leg control is retained.
 
Next, an appropriate strategy is identified to integrate disturbance compensation for finite-control-set optimisation schemes via the constraint set rather than the cost function to maintain a simple and elegant problem structure. Appropriate bounding of the disturbance observer is also addressed in light of the persistent enforcement of control constraints in almost all instances. This problem has been addressed in context for the three-phase induction machine current control system in the AC stationary frame. One particular issue of note, which is addressed, is that the orthogonal current vectors are controlled via the same actuator, resulting in coupled constraints, similar to that experienced with vector centred pulse-width-modulation.
 
Finally, a novel design strategy for composite sliding mode control with disturbance estimation-based compensation is investigated for application to a cascade induction machine drive system. Stability and performance analysis of the cascaded closed-loop system is approached by solving a differential inequality for the Lyapunov function directly. The resulting solution enables the time-variant properties of the composite control law to be considered and closely bounds the exact Lyapunov function solution.  In doing so, a design process can be established with the actual closed-loop performance in mind and the benefits of utilising composite control analytically realised. This contrasts with methods currently described in the literature, which are highly conservative for this class of controller. The application of a composite sliding mode controller in the stationary frame for current control has not been observed before in the literature.