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Grid-connected power inverters are indispensable in modern electrical systems, playing a pivotal role in enhancing the integration of renewable energies into power grids. Their significance, primarily when functioning as grid-forming inverters, extends to maintaining the grid’s inertia and strength—a distinct advancement over traditional grid-following operations. As grid-forming inverters, these devices emulate the characteristics of synchronous generators and can act as robust voltage sources, providing essential ancillary services. This behavior is particularly relevant when integrating energy storage systems on the converters’ direct current side. Among the various inverter topologies, the current source inverter (CSI) has emerged as a promising yet underexplored alternative for grid-forming applications. CSIs, when paired with their AC output filters, can effectively operate as voltage sources, utilizing control strategies that facilitate the integration of renewable energies into the electrical system. Their design inherently manages output current fluctuations, reducing the need for restrictive current limitations or additional protective measures. This paper examines the operational region of CSIs, obtained through detailed modeling, to explore their advantages, challenges, and potential for enhancing grid-connected systems. Analyzing the operating region from the converter model verifies the limits of where the converter can operate in a plane of active and reactive powers. For a small prototype model operating with 7 amperes in DC and 120 V in AC, it is possible to supply or absorb active power exceeding 1000 W and manage maximum reactive power values around 500 VAr, as determined by its operating region. Simulations also confirm that small changes in the control reference, as little as 5%, towards the region’s right limits cause significant oscillations in the dynamic control responses. This research aims to deepen our understanding of CSIs’ operational capabilities and highlight their unique benefits in advancing grid-connected systems and promoting the integration of renewable energy using this technology.

The use of advanced modulation and control schemes for power converters, such as a Feedback Quantizer and Predictive Control, is widely studied in the literature. This work focuses on improving the closed-loop modulation scheme called Feedback Quantizer, which is applied to a three-phase voltage source inverter. This scheme has the natural behavior of mitigating harmonics at low frequencies, which are detrimental to electrical equipment such as transformers. This modulation scheme also provides good tracking for the voltage reference at the fundamental frequency. On the other hand, the disadvantage of this scheme is that it has a variable switching frequency, creating a harmonic spectrum in frequency dispersion, and it also needs a small sampling time to obtain good results. The proposed scheme to improve the modulation scheme is based on a Discrete Space Vector with virtual vectors to obtain a better approximation of the optimal vectors for use in the algorithm. The proposal improves the conventional scheme at a high sampling time (200 μs), obtaining a THD less than 2% in the load current, decreases the noise created by the conventional scheme, and provides a fixed switching frequency. Experimental tests demonstrate the correct operation of the proposed scheme.

Three-phase active front end rectifiers are a widely used power topology in applications such as renewable energy interfaces and motor drives, among many others. Its control is usually performed by a cascade combination of linear controllers which must be properly tuned for correct operation, and which depend on the operating point and system parameters. These family of controllers place inherent limits on the system dynamic response. On the other hand, the predictive control approach has been proposed as an alternative technique for these converters due to its fast dynamic response, simple concept, flexibility, multiple objective capability, among other desirable properties. In the case of power converter applications, short prediction horizons are usually used due to computational limitations. However, when short horizons are considered, the non-minimum phase characteristic of the rectifier voltage response can introduce stability issues, as its direct inversion is not possible without jeopardizing it. Indeed, this may lead to voltage regulation loss, and in the worst case, overcurrents that could damage the converter. To overcome such problem, this paper proposes a new concept for a predictive horizon one voltage controller for grid-connected three-phase active front end rectifiers. The proposal is based on the minimum and non-minimum phase plant factorization concept and the use of different sampling periods, to prevent the direct inversion of the non-minimum phase voltage dynamic. Simulated and experimental results are included and show its correct operation, even with a horizon one predictive voltage controller, thus ensuring the fastest response of the system for a given sampling time.

This paper presents a new finite control set model predictive control strategy that, contrary to conventional approaches, achieves (i) zero steady-state error in the converter's AC current, and (ii) both fixed and lower harmonic spectrum, similar to that achieved by pulse width modulation based control schemes. These characteristics are attractive for medium and high voltage applications where high dv/dt is prohibitive and reduced switching losses are a must, or in applications that use passive filters and where a spread harmonic spectrum can cause resonances. The proposed strategy achieves dynamic results similar to those of conventional predictive control and a steady-state performance similar to that of a modulated control strategy. To do so, the strategy utilizes a modulated integral action to incorporate an input restriction into a conventional predictive control cost function. A grid-connected cascaded H-Bridge multilevel inverter is used to validate the strategy. Simulated and experimental results in both steady and transient states are presented to verify the proposed strategy's performance in the converter.

Hybrid Distribution Transformers (HDTs) offer a compelling alternative to traditional low-frequency transformers (LFTs), providing auxiliary services in addition to standard functionalities. By integrating LFTs with power converters, HDTs enhance the operational capabilities of the system. The specific configuration in which converters are connected to the transformer allows for the provision of multiple services. This can not only prevent network failures but also extend the lifespan of its components, an outcome that is highly desirable in a distribution grid. This article discusses an HDT developed to mitigate voltage fluctuations in the grid and to decrease the reactive power drawn from the secondary side of traditional LFTs. A finite-control-set model predictive control (FCS-MPC), in conjunction with linear controllers, is utilized for the effective management of the HDT converters. Two separate control loops are established to regulate voltage and reactive power on the secondary side of the transformer. Results from Hardware-in-the-Loop (HIL) testing affirm the proficiency of HDT in reducing grid voltage variations by 15% and in cutting reactive power consumption by up to 94%. The adopted control strategy and topology are demonstrated to be effective in stabilizing voltage and reactive power fluctuations while concurrently facilitating the charging of the converters’ DC link directly from the grid.

Cascaded H-bridge drives require using a significant-size capacitor on each cell to deal with the oscillatory power generated by the H-bridge inverter in the DC-link. This results in a bulky cell with reduced reliability due to the circulating second harmonic current through the DC-link capacitors. In this article, a control strategy based on a finite control set model predictive control and a proportional-resonant controller is proposed to compensate for the oscillatory power required by the H-bridge inverter through the cell’s input rectifier. With the proposed strategy, a DC-link second harmonic free operation is achieved, allowing for the possibility of reducing the capacitor size and, in consequence, the cell dimensions. The feasibility of the proposed control scheme is verified by experimental results in one cell of a cascade H-bridge inverter achieving an operation with a capacitance 141 times smaller than required by conventional control approaches for the same voltage ripple.