Analytical controller synthesis techniques for Voltage Source Inverter (VSI) cascaded dual loop voltage regulators
Creators: name: Siraj, Haris
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400803
subject: Electrical energy generation (incl. renewables, excl. photovoltaics)
classificationCode: 400907
subject: Industrial electronics

description: Voltage source inverters (VSIs) have been widely used in the power industry, such as in Uninterruptable Power Supplies (UPS), Distributed Generation (DG) units, and Dynamic Voltage Restorers (DVRs). Usually, these VSIs are controlled in a stationary frame cascaded dual loop scenario using linear regulators in which the inner current loop provides proportional (P) action while the outer voltage control loop is a Proportional Resonant (PR) or Resonant (R), Integral Resonant (IR) or Proportional Resonant Integral (PRI) controller which can provide infinite gain at the target fundamental frequency. The research theme of this thesis is focused on developing the analytical synthesis framework for designing the controller gains of these VSIs for optimal performance in standalone and grid-connected scenarios. The thesis explores the operation of a VSI with a second-order inductive-capacitive (LC) AC side filter controlled in a dual loop scenario in the stationary frame of reference. Various existing control frameworks have been investigated, and their performance is considered in terms of analysis, dynamic response, steady-state performance, and ease of implementation. It has been found that while a number of these techniques satisfy the above criteria, further investigation is required in the development of a synthesis/design framework that could be easily implemented and relates performance parameters like stability margins and control loop bandwidths with the dynamics of current and voltage controllers as well as the filter plant model. Moreover, the discretization of these voltage control architectures has proved challenging since different discretization strategies can have a significant impact on the stability margins of the system. Furthermore, the implementation of difference equations for various discretized forms in fixed point scenarios poses a challenge as well since it is difficult to capture the characteristic resonant peak for these controllers owing to the high variance of coefficients in their respective transfer functions. Therefore, in this research work, the dual loop voltage control of an LC filter-based VSI has been analysed, and a comprehensive, mathematical framework to design the voltage regulation using proportional resonant and integral dominant controllers has been developed. At the same time, the dependency of the control framework has been established on the system’s operational parameters. Later, the digitization of integral 2 dominant controllers was investigated using various transformation techniques, i.e., zeroorder hold (ZOH), first-order hold (FOH), impulse invariant (IMP), pre-warped Tustin (PRE), and pole-zero matched (PZM) methods. This has been followed by the formulation of differential equations for hardware implementation in a DSP-based fixedpoint environment. The developed frameworks have been validated using simulation and hardware results. The thesis starts by analysing the operation of an LC filter-based VSI in a dual loop scenario operated in a stationary frame of reference. The inductor or capacitor current of the LC filter has been regulated using proportional gain (P) in the inner current loop, and the outer voltage loop is a PR, R, IR, or PRI controller, regulating the LC filter capacitor voltage. The model of the system has been developed in continuous time, i.e., s-domain, and a synthesis framework comprising a set of formulae has been developed by treating the open loop transfer function of the developed model. Later, the thesis investigates the implementation of integral dominant controllers in discrete time using the abovementioned transformation techniques, and their performance has been analysed using bode plots and pole-zero maps. And finally, the hardware implementation of integral dominant controllers has been discussed in a fixed-point scenario, and it has been reported that the state space resonant forms are far more efficient in capturing the resonant peak for integral dominant controllers in comparison to the direct form II (DFII) and direct form II transpose (DFIIt). The final part of the thesis deals explanation of the hardware setup and co-simulation framework, which has been used for the validation of the dual loop control framework.