Alireza Tajfar

Sequence-Based Control of an Isolated DC/AC Matrix Converter

This paper describes the control of an isolated multiphase high-frequency-link (HFL) matrix inverter by controlling the time evolution of its switching states (or, switching sequences). It is referred to as the optimal-switching-sequence-based control (OSBC). Unlike several conventional control schemes, where, typically the control is based on averaged model of the inverter and the modulation scheme sets a predetermined switching sequence, OSBC synthesizes the fundamental switching sequence depending on the control needs on the fly. For instance, OSBC can seamlessly play with the inverter switching sequences if the input voltage of the inverter changes as evident in solar or wind-based energy systems. This necessitates that OSBC use a switching-sequence-based discontinuous (instead of averaged) modeling approach. Further, the stability of the closed-loop inverter is determined in OSBC using an advanced composite Lyapunov-function based approach, which also enables one to predetermine the reachability of the inverter dynamics for a given switching sequence. Thus, optimal control in OSBC is ensured under stability bound of the switching sequence. This provides a fundamental difference between OSBC and model predictive control for inverters. This also implies that in a multiobjective OSBC, one can shift the weights of the individual cost functions to yield better inverter performance without compromising the stability.

A Fault-Tolerant Switching Scheme for an Isolated DC/AC Matrix Converter

A fault-tolerant scheme is proposed for a multistage isolated three-phase dc/ac matrix inverter (MI-TMI). It comprises a front-end dc/ac converter followed by an ac/pulsating-dc converter and a pulsating-dc/ac converter. The fault-tolerant scheme (FTS) ensures a consistent operation under different fault conditions in any of the three stages of the TMI mentioned previously. A fault-diagnosis scheme is outlined which identifies the source of the converter fault and the impact of when the fault onsets on the inverter is explored. Finally, detailed results on the fault-tolerant switching scheme as well as experimental validation using a fabricated MI-TMI prototype are presented.

A fault-tolerant switching scheme for a photovoltaic high-frequency-link inverter

A modulation-based fault-tolerant scheme is outlined for an isolated photovoltaic high-frequency-link (HFL) inverter. It comprises a front end dc/pulsating-dc dc/ac converter followed by a unidirectional ac/dc converter followed by an ac/ac converter. The fault-tolerant scheme ensures operation under different types of fault conditions in the front end dc/pulsating-dc dc/ac converter without redundant circuits. Detailed results on the switching scheme as well as experimental validation using a fabricated prototype are presented.

Control of high-frequency-link inverter using optimal switching sequence

This paper presents an Optimized Sequence-Based-Control (OSBC) scheme for an isolated multi-stage high-frequency-link (HFL) converter, which directly controls the switching sequences of the power devices without the need for an intermediate pulse-width modulator as in conventional power converter control schemes. OSBC is realized by solving an optimization problem, determining an optimal switching sequence (comprising a union of feasible sequences) and corresponding time horizon for each switching state of the sequence. The proposed model-predictive switching control scheme provides faster transient response compared to the conventional PI control scheme in the ac/ac converter. Another interesting feature of the OSBC is multi-objective control capability through joint-optimization of multiple control goals. In fact, in order to meet various control objectives, a compromise should be made between them so as to achieve the optimized performance and this is even more crucial in capacitor-less topologies (since there is no buffer stage to prevent cross-coupling between various control actions). For the capacitor-less HFL inverter in this paper, joint-optimization of the following control goals are implemented using the OSBC scheme: 1- output voltage regulation, 2- flux balance control (FBC), and 3- loss mitigation. A case study with a sudden change in the input dc voltage characteristics is considered to verify the effectiveness of the joint-optimization of the above control goals in improvement of the HFL inverter operation. The results are verified using simulation for a 4.5 kW, 40 V/208 V three-phase inverter.

A novel primary-side-assisted soft-switching and fault-tolerance of a high-frequency-link inverter for renewable-energy systems

A modulation-based soft-switching scheme is outlined for a high-frequency-link (HFL) inverter, which comprises a front-end HF isolated dc/ac converter followed by an ac/dc converter and an ac/ac converter. The proposed zero-voltage and zero-current switching (ZVZCS) scheme provides loss mitigation for all three legs of the ac/ac converter. Because the output of the ac/dc converter is pulsating-dc in nature, it retains the multi-phase encoded information generated by the dc/ac converters sinusoidal modulation. Although the inverter only needs a two-phase HF transformer to generate a three-phase output, a third phase is used to yield higher fault-tolerance. That is, in case of a fault in one of the phases of the HF transformer, two other phases can carry on nominal operation. The results are verified using simulation and experiments on a 2 kW, 40 V/208 V three-phase inverter.

An Optimal Sequence-Based-Controller (OSBC) for a grid-connected three-phase photovoltaic HFL inverter

This paper presents discontinuous modeling and design of an Optimal Sequence-Based-Controller (OSBC) for a grid-connected isolated high-frequency-link (HFL) converter. The OSBC directly controls the switching sequences of the power devices without the need for an intermediate pulse-width modulator as in conventional power converter control schemes. OSBC is realized by solving an optimization problem, determining an optimal switching sequence (comprising a union of feasible sequences) and corresponding time horizon for each switching state of the sequence. Maximum Power Point Tracking (MPPT) is realized using the Perturb & Observe algorithm as embedded in a unified OSBC controller optimizing the overall efficiency of the PV + HFL inverter. The effect of optimizing various control parameters with slow and fast dynamics at different scales is demonstrated. The results are verified using Matlab simulation for a 3 kW, 40 V/208 V three-phase inverter.

A transformer-flux-balance controller for a high-frequency-link inverter with applications for solid-state transformer, renewable/alternative energy sources, energy storage, and electric vehicles

This paper outlines a controller that balances the flux of the isolation transformer in a multi-stage high-frequency-link (HFL) bi-directional inverter. It comprises a primary-side high-frequency (HF) dc/ac converter (referred to as Bridge I) followed by cascaded ac/pulsating-dc and pulsating-dc/ac converters (referred to as Bridge II and Bridge III) on the secondary side of the transformer. The overall inverter is preceded by a capacitive filter and is followed by an output LC filter. The issue with the flux balance arises when energy is fed back from the ac output of the inverter to the dc energy source. The problem arises in the HFL inverter in this bi-directional mode since without an intermediate dc-link capacitor, the output of Bridge III, which is pulsating, has an additional problem with the variation in the maximum amplitude of each pulse due to an underlying beat frequency. Without addressing the flux imbalance, the HF transformer may saturate and have an impact on its size and thermal sustenance. The simulation results of the flux-balance controller shows satisfactory performance.

A fault-tolerant switching scheme for a high-power high-frequency-link inverter

A modulation-based fault-tolerant switching scheme (FTSS) is outlined for an isolated high-frequency-link (HFL) inverter. It comprises a front-end Dc/pulsating-Dc converter followed by an Ac/Ac converter. At first, the lost phase in the Dc/Ac Converter is identified based on the type of distorted output line voltages and then modulation references are changed accordingly so as to restore the output line voltages. The proposed FTSS ensures operation under different types of fault conditions in the front-end Dc/pulsating-Dc converter without redundant circuits. Detailed results on the switching scheme as well as experimental validation using a fabricated prototype are presented.

Harmonics analysis for a high-frequency-link (HFL) inverter

The goal of this paper is to do a harmonic analysis for the output voltages, output currents, input-source currents, and pulsating-dc link voltage of an Isolated-Multistage High-Frequency-Link (HFL) inverter. A comparison between the frequency-spectrum for these variables plotted using the derived equations.

Sequence-based Lyapunov stability of power-electronic converters

This paper1 outlines a methodology [1] to analyze the reaching condition of a class of switching system, which will be referred to as switching power converter (SPC), using a piecewise linear (PWL) model and multiple Lyapunov method. The reaching criteria for orbital existence are formulated as a linear matrix inequality (LMI), which is solved using a convex optimization solver. Further, the criterion is modified to distinguish the different modes of convergences of the reaching dynamics. A case study using a basic SPC is provided to illustrate the applicability of the proposed methodology.