Dr. Espinosa-Neira, Eduardo

The interest on weak and micro-grid systems has grown up substantially last time, specially tied up to distributed power generation systems (DPGSs), isolated systems as aircraft, or islanding power systems. These kinds of grids are usually under significant variation in their quantities, specifically in their voltage amplitude and/or frequency. On this line, many studies about synchronization methods have been developed, which may work under variations on the frequency value, unbalanced voltage, and even with harmonic distortion. However, power converters connected to this class of systems are poorly documented-specifically controlled rectifiers. In fact, most of the controlled grid connected converters (GCCs) are defined to work in a fixed frequency and balanced system. This paper deals with a GCC connected to a variable-frequency and unbalanced voltage supply system control through a predictive algorithm with a fixed resolution sampling strategy. Furthermore, the current references are imposed in order to help the weak-grid source subjected to unbalancing, taking more power from the phase with highest voltage amplitude and relaxing the other phases. This issue makes to calculate every phase current reference independently and accordingly the voltage amplitudes keep the dc-link voltage in a desired value. The results show the feasibility of the proposed algorithm, where the performance is highlighted by simulated and experimental waveforms.

This work proposes a multicell topology based on current-source cells in order to inherit the advantages of current-source topologies such as reduced load dv/dt voltage and natural bidirectional power flow and to adopt a similar behavior of the multicell topology based on a voltage source converter such as voltage controlled behavior where n C cells are connected in series to feed one load phase. In order to check the technical feasibility and performance of the proposed topology, a mathematical model is introduced and studied and key design guidelines of passive components are defined. The analysis shows the possibility of using components with a lower voltage rating than that of the classic multilevel current source topologies and allows the use of low switching frequencies in both rectifier and inverter stages while at the same time obtaining a high-quality waveform in both load voltage and converter input currents. A case of example is used to corroborate the theoretical analysis and the component design methodology, as well as the performance of the topology using a low-power prototype.

The use of power converters has considerably grown up, in part, because refined control strategies have been recently proposed, including nonlinear schemes such as predictive control. This approach is used in this work considering a variable grid frequency environment in order to get an appropriate response for a wide ac mains frequency range. Indeed, in order to achieve appropriate, both dynamic and static, responses for all operating frequencies, the number of samples per period is kept constant and independent of the ac mains frequency. This allows a fixed resolution of the sensed voltages and/or currents, which is preferable if high-performance digital control schemes are required. However, imposing a constant number of samples per period requires a variable sampling time in systems that feature variable ac mains frequency. On the other hand, predictive control has been developed and well documented just for a constant sampling frequency. This work presents how to extend the predictive control algorithms for variable sampling time allowing high-performance waveforms and wider ac mains frequency range. Simulated and experimental results show the feasibility of the proposed control strategy which corroborates the mathematical and model analysis.