Toit Mouton

Solid-state transformer topology selection

This paper considers the design criteria of the solid-state transformer circuit, with special focus on the high voltage side. Three plausible multilevel converter configurations are identified and compared in terms of cost and voltage balance requirements.

Model predictive control with a fixed switching frequency for an AC-to-AC converter

This paper discusses the control of an AC-to-AC converter that forms part of an 11 kV electronic voltage regulator. A novel finite-set model predictive control technique is presented to allow fixed frequency switching. A fixed switching period is defined and divided into smaller evaluation steps. The switching behaviour is obtained by using the analogy of PWM with a triangular carrier wave. During the first half of the switching period, the system may only switch from the on-state to the off-state and during the second half, only from the off-state to the on-state. To account for the repetitive nature of the switching waveform, an average receding horizon prediction method is introduced. An anticipated prediction is made by calculating the average value for one switching period ahead, according to the position and state in the current switching period. The method presented good practical results and proved to be effective in maintaining a fixed switching frequency.

Finite-set model predictive control of a flying capacitor converter with heuristic voltage vector preselection

This paper presents a finite-set model predictive strategy for current control of a resistive-inductive-active load with a three-phase three-level flying capacitor converter. A heuristic method is used to reduce the calculation effort and to increase the prediction horizon. A disturbance observer determines the active part of the load. A hysteresis-based voltage balancing algorithm makes use of the redundancy of the 64 switching possibilities of the converter. The proposed method significantly reduces the calculation effort compared to classical finite-set model predictive control methods.

Natural balancing of the two-cell back-to-back multilevel converter with specific application to the solid-state transformer concept

This paper considers the voltage balancing of the two-level series stacked back-to-back converter topology. Firstly it is proven that the cell voltages will be balanced in steady state. Further insight into the balancing process is found through analysis of the rebalancing process following an external perturbation.

A generic approach to implementing finite-set model predictive control with a fixed switching frequency

This paper presents a generic approach to practically implementing a finite-set model predictive control strategy with a fixed switching frequency for high sampling rates. With conventional FS-MPC methods, the equations for describing the predictions of the controlled variables are derived specifically for each given topology. In this paper, a more generic approach is presented by using established state space control theory to develop a method for modelling and implementing an arbitrary topology. Numerical methods and the use of a symbolic computational toolbox is used for the calculation of off-line prediction equations and using a lookup table to reduce on-line computation time. An implementation scheme for an FPGA is discussed and experimental results are provided to confirm a successful design.

Increasing the performance of Finite-Set Model Predictive Control by oversampling

This paper presents an FPGA-based implementation of Finite-Set Model Predictive Control (FS-MPC) which makes use of oversampling in order to achieve a better control result: In general the maximum IGBT switching frequency is limited to half the sampling frequency for all FS-MPC methods. In simulations and practical implementations, however, it can be seen that the real switching frequency is far lower than its theoretical maximum. Especially for lower power applications where a good quality of the output variables is important, FS-MPC shows very high ripples compared to modulation-based approaches. The main reason for this is that conventional control systems have sampling cycle times which are normally in the range of 20 μs to 200 μs. By using FPGAs, the sampling frequency of the control system can be significantly increased, as the control is completely implemented in hardware. Experimental results for current control of a single-phase resistive-inductive load and for output voltage control of a single-phase Uninterruptible Power Supply (UPS) with a Neutral Point Clamped (NPC) inverter demonstrate the power of the proposed method. A voltage balancing algorithm for the DC link capacitors of the two inverter legs is included in the cost function as well as certain weighting factors to limit the IGBT switching frequency.

Variable switching point predictive torque control

In this paper an approach to include a variable switching time point into predictive torque control (PTC) is introduced. In PTC the switching frequency is limited by the sampling frequency; its theoretical maximum value is half the sampling frequency. In reality, however, the switching frequency is lower than this value, resulting in high current and torque ripples compared to modulator-based control methods. In order to overcome this an optimization problem is formulated and solved in real-time. The goal is to find the time point at which the switches of the inverter should change state in order to not only achieve the regulation of the torque and the flux magnitude to their references, but also the minimization of the torque ripple. Further advantages of the proposed method include the design flexibility and great performance during transients. Experimental results that verify the performance of the presented control strategy are included.

Predictive torque control of an induction machine fed by a flying capacitor converter

This paper presents a model predictive control strategy for an induction machine fed by a three-level flying capacitor converter. In order to reduce the calculation effort (64 different switching possibilities for one prediction step) in a first step the best one of the 19 voltage vectors is determined. After that the best switching state which delivers this voltage vector is determined regarding the flying capacitor voltage balancing and the switching effort, making use of a hysteresis-based algorithm. The proposed control algorithm is verified by several simulations which clearly verify that an effective control of speed, torque, flux and of the flying capacitor voltages is possible.

Model predictive control of a flying capacitor converter with output LC filter for UPS applications

This paper presents a model predictive control strategy for a three-level flying capacitor converter with output LC filter for UPS applications. In order to reduce the calculation effort (64 different switching possibilities for one prediction step) in a first step the best one of the 19 voltage vectors is determined. After that the best switching state which delivers this voltage vector is determined regarding the flying capacitor voltage balancing and the switching effort, making use of a hysteresis-based algorithm. A disturbance observer is used to estimate the output currents of the LC filter which are normally not measured. The proposed principle can be easily extended to flying capacitor converters with more than three voltage levels.

Model predictive control of cascaded H-bridge inverters based on a fast-optimization algorithm

In this work, a Finite Control Set Model Predictive Control (FCS-MPC) strategy for Cascaded H-bridge (CHB) inverters is proposed. The key novelty of our proposal comes from the way the cost function is designed. Generally, in standard FCS-MPC formulations for power converters, the cost function only considers the current tracking error. In this proposal, the proposed cost function also takes into account the control input tracking error. This allows one to obtain a reduced common-mode voltage during the steady-state while achieving a fast dynamic response during transients, similarly to the one provided by standard FCS-MPC. To account for calculation time, a fast-optimization algorithm based on sphere decoding is also considered. To verify the performance of the proposed predictive strategy, simulation results for a three phase five-level CHB inverter governed by the proposed FCS-MPC are presented.