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.

A discrete-time linear control strategy for a multilevel three-phase unified power quality conditioner (UPQC) based on single-phase power cells is presented. The multi-variable, nonlinear, and coupled features of these topologies make the control strategy design a difficult task. Controlling this kind of system with single-variable linear controllers-as proposed in this work-presents significant advantages compared with other approaches as simplicity in the design steps due to the large amount of tools developed for this kind of schemes. Particularly, a classic design method based on the root locus approach is used to choose the controllers parameters in order to achieve a given dynamical behavior. Compensation of reactive power and fundamental frequency disturbances is presented in this paper as part of a general control strategy for multilevel active power filters. The proposed control strategy is implemented on the TMS320C6713 DSP-based system for a low-power laboratory prototype, and thus the controllers design is carried out on the discrete-time and -frequency domain. Also, due to the inherent asymmetries among the power cells in a modular topology, a dedicated local control strategy is proposed to ensure a symmetrical distribution of the power among the power cells. This feature allows the semiconductor devices of each module to operate under the same voltage and current ratings. Simulated and experimental results showing stationary and transient conditions demonstrate the feasibility of the control scheme.

An asymmetric inverter features different dc-link voltages for each power cell and depending on the dc voltage ratios, there are binary, trinary, etc., asymmetric topologies. In particular, the trinary one can provide a waveform with 3n levels, where n is the number of cells connected in cascade. Hence, for n = 3, an output voltage with 27 levels can be obtained, ensuring a low total harmonic distortion (THD) and reduced dv/dt. However, when the load consumes active power, some power cells of the inverter can operate in regeneration mode, complicating the topology and control. To overcome this problem, a new modulation method for asymmetric inverters is presented. It features an ac output voltage of 13 levels for n = 3, and ensures unidirectional power flow in every power cell. This new modulation method also provides a voltage waveform with low dv/dt and reduced THD (5.7%). To confirm the approach, power flow and sensitivity analysis to changes in the dc voltage ratios are performed. Finally, simulation and experimental results are provided.

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.

In the last decades, finite control set-model predictive control (FCS-MPC) has been extensively investigated. This control strategy uses the system model to determine the optimal input that minimizes a predefined cost function. This procedure is performed by evaluating the admissible inputs of the converter model. In order to achieve a proper performance of the control strategy, a small sampling time must be used. In practice, the computational effort and processing time are critical factors that depend directly on the number of admissible states of the topology. With the objective of reducing the execution time needed by the algorithm, it is possible to take advantage of the high degree of parallelism that can be obtained from a field-programmable gate array (FPGA). This paper deals with the implementation of FCS-MPC in an FPGA XC3S3500E to control a power cell of a cascaded half bridge (CHB) converter. The design is divided into subparts, called modules, which can be projected to control multiple cells using only one chip. In addition, with the aim to restrict the use of hardware to the available resources, the document presents a series of considerations related to the algorithm and its implementation. This paper presents a detailed guide with emphasis on the design and outlining of each module in order to reduce the developing times for researchers in the area of power converters.

Hierarchical control is a widely used strategy that can increase resilience and improve the reliability of the electrical network based on microgrid global variables. The large amounts of data required during transitions prompt the use of more reliable and flexible communications to achieve the control objectives. Such communications can involve potential cyber vulnerabilities and latency restrictions, which cannot be always addressed in real-time. To accurately capture the system’s overall operation, this paper proposes a co-simulation framework driven by flexible communications and a resilient control algorithm to regulate the frequency and voltage deviations in a networked microgrid. Model-based predictive control has been implemented, to avoid slow transient response associated with linear hierarchical control. Software-Defined Networking (SDN) is responsible for increasing the communication intelligence during the power-sharing process. The effects of critical communications and overall system performance are reviewed and compared for different co-simulation scenarios. Graphical Network Simulator (GNS3) is used in combination with model-based predictive control and SDN, to provide latency below 100 ms, as defined in IEC 61850. Testing of the proposed system under different cyber attack scenarios demonstrate its excellent performance. The novel control architecture presented in the paper provides a reference framework for future cloud computing-based microgrids.