Browsing by Author "Dorfling, Martinus"
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- ItemGeneralized model predictive pulse pattern control based on small-signal modelling(Stellenbosch : Stellenbosch University, 2021-03) Dorfling, Martinus; Mouton, Toit; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.ENGLISH ABSTRACT: Optimized pulse patterns (OPPs) are a pulse-width modulation method in which the switching pattern is computed offline. Typically, the harmonic distortions for a given switching frequency are minimized. OPPs are particularly beneficial for industrial power electronic systems that operate at low switching frequencies (such as medium-voltage drive systems). However, designing a controller with a high dynamic performance for higher-order converter systems that are modulated by OPPs is a difficult and somewhat unexplored task. For first-order converter systems, a state-of-the-art industrial control technique known as model predictive pulse pattern control achieves a high dynamic performance. This thesis proposes a generalized model predictive pulse pattern controller that is applicable to (linear) higher-order converter systems. Using the notion of small-signal modelling, the dynamic equations of the state variables of the converter system are linearized around the optimal steadystate trajectory that results from the OPP. Key to the control method is to model the modifications to a pulse pattern with the strengths of impulses, resulting in the modifications to the converter states being linear in the impulse strengths. The proposed controller is formulated according to the model predictive control methodology. Thanks to the linear internal dynamic model, the underlying optimization problem can be formulated as a convex quadratic program. Simulation results demonstrate that the proposed controller achieves a very short response time during transients and superior harmonic performance during steady-state operation. Importantly, an implementation of the control algorithm on a low-cost field-programmable gate array demonstrates that the controller can execute in real-time within a short sampling interval of 25 µs; thus far, none of the (few) existing OPP-based controllers for higher-order converter systems have been proven to be practically implementable. Additionally, the control method is augmented with constraints on the state variables. Specifically, the state variables are given bounds that they should remain within. The method is verified through simulation. Furthermore, balancing of the neutral-point potential is integrated in the controller. Simulation results show that the balancing method performs well under dynamic operating conditions, including during zero power factor at the converter terminals, where traditional balancing methods tend to fail.