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.

Cascaded H-bridge multilevel (CHB-ML) inverters are an attractive alternative for supplying power to ac grids as they have high reliability and offer an acceptable quality of voltage at their output terminals. In order to achieve efficient operation in these CHB-ML inverters, they must work at low switching frequencies. The finite-control set model predictive control (FCS-MPC) scheme is a very intuitive strategy for controlling this type of converter, but traditional FCS-MPC algorithms generally have a steady-state error when operating at low sampling frequencies and/or if there are parameters mismatch in the prediction model, regarding those of the real system. In this article, a grid-connected CHB-ML inverter that uses an improved FCS-MPC scheme is proposed. The proposed strategy eliminates the steady-state error in an MPC operating at low sampling frequencies and maintains correct operation when a change in the control reference occurs. Experimental results from a grid-connected CHB-ML inverter with three units (seven levels) demonstrate the feasibility of the proposal.

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.