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The power electronic transformer (PET) is an emerging technology that is quickly becoming a key component of the next-to-come power distribution networks (PDNs), due to its versatility on energy management, as well as, the improvement on the quality of the energy. PDNs are characterized by their unbalanced conditions, causing that PETs driven by conventional dq0 controls introduce current distortions on the primary winding of the transformer. Such distortion is evidenced in the 2 $\boldsymbol{\omega }$ oscillations of ${V_d}$ and ${V_q}$ acting as harmonic sources. In this sense, this article proposes a novel control approach for PETs. The key idea behind of this proposal consists of operating each phase independently, which is achieved through the enclosed rectification and the mitigation of the 2 $\boldsymbol{\omega }$ oscillations in a dual active bridge topology. The attained advantages by this control scheme are: a) balancing of the primary winding currents; b) unitary power factor; c) negligible harmonic distortion; and d) 2 $\boldsymbol{\omega }$ oscillation mitigation on the dc bus.

A refined dynamic lumped-parameter model of a REBCO field coil synchronous generator including details of the thermoelectromagnetic phenomenology of the superconductor has been developed in the classic dq0 frame of reference. The idea is to build a generic modelling framework that can be used in power systems engineering studies to infer trends on the behavior of HTS field coil machines in power grids. In the present work, the transient stability of a hybrid High Temperature Superconducting (HTS) generator is analyzed under the most severe operating condition applying a three-phase short circuit at its terminals. For the study, we chose the parameters of a wind generator at 1 MW. Two cases are considered: 1) no full transition of the HTS to the normal-resistive state under stable condition, 2) full transition of the HTS under unstable condition. The response of the HTS machine is compared against that of a conventional machine.

Due to several reasons, the three-phase voltages of power grids cannot be balanced. The performance of AC–DC converters using dq0-based controls can be severely affected by the presence of unbalanced input AC voltages. Different to these proposals, this study presents a new easy-to-implement control scheme based on a single PI loop algorithm for VSC-PFC rectifiers using a phasorial approach. This new scheme has various significant advantages: (i) fast counteracting of large unbalanced voltage and current conditions; (ii) power factor = 1 at any unbalanced sag operating conditions; (iii) negligible current harmonic distortion and (iv) low-ripple DC voltage. All these features are concurrently obtained. The proposed single-PI loop VSC-PFC rectifier control strategy is theoretically and experimentally validated. A revision of the main results and characteristics of various proposed techniques that are similar to the one proposed in this study is also carried out, qualitatively indicating the main advantages featured by the proposed control strategy for VSC-PFC rectifiers.

In this paper, a modified transformerless inverter topology for photovoltaic (PV) applications is presented. This proposal is capable to produce a nine-level AC output from a single low DC voltage source with an efficiency of 95.59%, making it ideal as a micro-inverter. This is achieved by an elevation phase that generates two isolated DC outputs from a single source, using a redesigned DC/DC parallel boost converter that elevates up to ten times the input with no extreme duty cycles and performing it with fewer components compared to similar capacities topologies. These two isolated voltages are employed to feed an asymmetrical cascade cells DC/AC inverter. The theoretical fundaments and both simulation and experimental results presented in this paper validate the proposed topology.

This paper presents a practical approach for the real-power economic dispatch of AC/DC power transmission systems formed by VSC-HVDC equipment. It uses the Lagrange multipliers method combined with a comprehensive representation of the AC/DC power system. The power losses of the AC systems, DC grid and power converters are considered in this novel formulation by using incremental transmission loss factors. For realistic solutions, the different control strategies of the power converters forming any VSC-based power grid are considered. Interestingly, this approach permits to conclude that the AC grids coupled to converters charged with DC voltage control define the nodal marginal prices in the DC network, and in turn, the energy prices in the VSC-connected passive grids are inherited from the DC network. Overall, this modelling approach yields great flexibility to model any arbitrary VSC-based HVDC power grid interconnecting various AC systems. The developed method has been validated using a two-terminal VSC-HVDC network interconnecting two otherwise independent AC grids. Its results are compared against those obtained by the sequential quadratic programming method available in the optimisation toolbox of Matlab©, with both solutions exhibiting differences inferior to 0.30% in the total generation costs, therefore concurring very well between each other. The practicality of this approach is also demonstrated by carrying out the real-power economic dispatch of an AC/DC transmission system comprising seven VSC units which give rise to a thirteen-node DC grid.

Mexico's national electric grid comprises ten regional systems, with one isolated in the south of the Baja California Peninsula. In addition, providing electricity access remains a challenge in the country. Therefore, distributed renewable energy solutions are relevant to satisfy the electricity demand and promote grid development. Distributed renewable energy access (DREA) based on a wind energy conversion system integrated into low-voltage direct current (LVDC) networks is presented. The DREA is analysed and simulated under wind speed conditions measured and recorded for one year at Baja California Sur. A representative variability is applied to five type-4 wind turbines based on wind speed turbulence intensity analysis. The generated power is transferred through a bidirectional dual active bridge converter, controlled by a novel small-signal model using a single closed-loop proportional-integral control, owing to its modularity, power density, and ability to transfer power from LVDC to high-voltage direct current. The effectiveness and robustness of the proposed DREA are assessed via a complete mathematical model, corroborated by the simulation results in MATLAB-Simulink®, and validated by experimental results using the real-time simulator Opal-RT Technologies® and laboratory prototyping. The results indicate a DC-link voltage compensation and integration of a power capacity of 150 kW in the LVDC networks with an efficiency of 94%.

Phasor measurement units (PMUs) play an important role in many applications of power systems. In order to ensure a reliable performance, the phasor estimation algorithm has to satisfy a set of requirements stated in the IEEE Standard C37.118.1, which establishes the test conditions and requirements for steady-state and dynamic conditions. There, two classes of performance, P and M, can be found. In general, P-class is intended for applications that require fast response and M-class is used when greater precision is necessary. In this paper, a novel algorithm based on Hilbert transform for phasor estimation in compliance with the IEEE standard C37.118.1 for P-class is proposed. Advantages of the proposal are a fast response and a low computational burden due to the HT implementation as a low-order filter of one cycle. Further, two low-complex strategies of compensation are proposed. The proposal is validated using all the test conditions specified in the IEEE Standard C37.118.1. Besides that, real voltage and current signals of an electrical system are analyzed. The obtained results demonstrate that the new proposal can meet all the requirements for P-class performance.

This paper presents the design, modeling, implementation, and experimental results of an advanced current-sensorless control scheme for voltage source converter–power factor correction rectifiers. This proposal is aimed to reduce the complexity of the control while maintaining desirable features for ac/dc conversion, such as unity power factor, low total harmonic distortion in input current, continuous regulation of the dc voltage even facing ac voltage sags and dynamic load changes. Unlike other proposals, this scheme has a single feedback proportional-integral loop, uses few mathematical operations, and the dq 0 reference frame is not included in the control scheme. These characteristics simplify the implementation of the proposed control thus improving its efficiency. The theoretical analysis and simulation validate the technical feasibility of the proposed control, and finally, various cases of study and experimental results obtained with a laboratory scale-down prototype are presented to confirm the viability and performance of the control.

An efficient method for the real-time contingency analysis of meshed high-voltage direct current (HVDC) power grids fed by voltage source converter (VSC) stations is introduced here. A linearised AC/DC grid model is initially determined considering the control strategies of the various VSC units. This lays the foundations for the determination of linear sensitivity factors with which the contingency analysis is carried out to evaluate the real-time N−1 criterion in AC/DC grids, as demanded by system control centres. Distribution and power-injection factors are subsequently derived for efficiently assessing the impact of AC/DC transmission line outages and load/generator disconnections on the HVDC grid. Conversion factors are also derived to estimate the impact of the loss of VSC stations on the AC/DC network, this being another inherent contribution of this work. The efficiency and validity of this timely approach, which finds practical applicability to the real-time operation of HVDC power grids, is confirmed using a meshed DC network fed by three VSC stations. The disconnection of AC and DC transmission lines, generators, and VSC stations are dynamically simulated using Simulink and their post-disturbance steady-state conditions are compared against those computed by the introduced method where it is confirmed that both solutions concur very well with each other.

The presence of harmonic currents in three-phase distribution networks and the stochastic behavior of unbalanced loads with different power factors are undoubtedly responsible for power quality problems. This paper proposes a novel solution named Cross Phases Hybrid Transformer (CPHT) to address those issues. The CPHT incorporates a conventional distribution transformer into an AC/AC power electronic stage referred to as cross phase module. This new hardware configuration allows to achieve continuous three-phase energy supply of distinct loads as well as high energy quality indexes for diverse distribution networks. The proposed solution has been numerically and experimentally validated by means of simulations carried out in Simulink and with a scaled-down laboratory prototype respectively. The validations consider distinct case studies to show the operative advantages of the CPHT with respect to other topologies reported in the literature.

This work introduces the Proportional-Values Modulation (PVM), an advanced technique for modulating DC–DC DAB converters. The relative advantages of this technique are: (i) Full elimination of reactive power throughout the bidirectional operating range of the DAB; (ii) reduction of the apparent power in the high-frequency transformer, HFT, while maintaining the harmonic distortion low, and (iii) the output capacitance is lower than the one obtained using Single-Phase-Shift Control (SPSC) and Dual-Phase-Shift Control (DPSC) methods, while the AC and DC current peaks are not incremented. This set of advantages increases the overall technical efficiency and power density flow of the converter. As additional characteristics of interest, the technique uses a single feedback PI loop and no complex control rules are implemented. Both features increase the computational efficiency of the PVM above other proposals.

In this article, a technique for the reduction of total harmonic distortion (THD) in distributed renewables energy access (DREA) composed of wind turbines is introduced and tested under the wind speed conditions presented in Tamaulipas, Mexico. The analysis and simulation are delimited by a study case based on wind speeds measured and recorded for one year at two highs in the municipality of Soto La Marina, Tamaulipas, Mexico. From this information, the most probable wind speed and the corresponding turbulence intensity is calculated and applied to a wind energy conversion system (WECS). The WECS is composed of an active front-end (AFE) converter topology using four voltage source converters (VSCs) connected in parallel with a different phase shift angle at the digital sinusoidal pulse width modulation (DSPWM) signals of each VSC. The WECS is formed by the connection of five type-4 wind turbines (WTs). The effectiveness and robustness of the DREA integration are reviewed in the light of a complete mathematical model and corroborated by the simulation results in Matlab-Simulink&reg, The results evidence a reduction of the THD in grid currents up to four times and which enables the delivery of a power capacity of 10 MVA in the Tamaulipas AC distribution grid that complies with grid code of harmonic distortion production.

High performance, highly efficient DC-DC converters play a key role in improving the penetration of renewable energy sources in the context of smart grids in applications such as solid-state transformers, built-in power drives in electric vehicles and interfacing photovoltaic and wind-power systems. Advanced medium-frequency transformers (MFTs) are fundamental to enhance DC-DC converters and determining its behavior, therefore MFT design procedures have become increasingly important in this context. This paper investigates which type of core material, between nanocrystalline and silicon steel, has the best properties for designing MFTs for distinct applications. Unlike to other proposals, in this work, two 1 kVA-120 V/240 V-1 kHz lab MFT prototypes, with a different type of core material, are developed for the purpose of comparing its physical characteristics, behavior, and performance under real-life conditions. A final section, the experimental results show that the nanocrystalline MFT has greater power density and efficiency. The results of this work introduce nanocrystalline MFTs as an option in a wider range of applications in niches in which other materials are currently used.

Accurate and fast characterization of voltage variations helps to evaluate their severity on equipment and activate protections. In this paper, a methodology for tracking and characterization of voltage variations, sample to sample, is presented. It consists of a Hilbert transform to estimate the voltage of the signal’s envelope, a fuzzy logic system to track down the type of voltage variation, and a rule-based method for the final identification and decision making according to IEEE Std 1159-2009. Unlike some techniques presented in the literature for tracking voltage variations such as the Kalman filter and adaptive linear network techniques, the proposed methodology requires neither a harmonic model nor an algorithm to adjust the model parameters, which in many cases increases the computational burden and time tracking. It is worth mentioning that the proposed classification stage does not need a training stage; therefore, its development is easier and its efficiency does not depend on a data training set. The performance of the proposed methodology is validated and tested using synthetic signals as well as real measurements of voltage variations. In addition, an implementation of our methodology into an field-programmable gate array based system is performed in an effort to offer a low-cost and portable system-on-a-chip solution for online and real-time monitoring of voltage variations.

This paper presents and details the H-Bridge PFC rectifier control scheme. This control is a step forward in sensorless current control schemes for performance improvements of single-phase bidirectional rectifiers. The H-Bridge PFC rectifier is designed based on a single PI controller loop fed by DC reference signals. Regarding dynamic operating conditions, the proposed control can maintain a regulated DC voltage under varying conditions at input (voltage sags) and output (load step changes) ports while maintaining an unitary power factor (PF) and constant THD current input at any of these conditions. Different to other investigations, this control proposal uses SPWM, does not require a variable transformation into the dq0 reference frame, and the complex compensators accompanying to the PI controller stage are no longer needed. Therefore, this proposal is aimed to reduce the complexity of the control scheme and to improve the overall dynamic response of the PFC rectifier. The experimental results obtained from the scaled-down prototype setup, which was built for research purposes, validate the control law, mathematical model and quantitative comparison between simulation and experiment, both considering the parasitic resistances of the inductor and semiconductor switches.

The electric arc phenomenon associated with high impedance faults (HIFs) in distribution systems is an exciting subject that directly impacts over the reliability of electrical utilities, because fault currents are very small and present non-linearity and asymmetrical waveforms. Therefore, HIFs may be an undetectable phenomenon by overcurrent protections like fuses, re-closers, and relays. A HIF entails a challenge for its detection and location. In this sense, more realistic high impedance models, as well as reliable signal processing techniques, are needed to extract all transient characteristics aiming to detect HIFs in distribution networks. In this work, an efficient Resistive-HIF model is proposed and implemented into the Alternative Transients Program of the Electromagnetic Transients Program (ATP/EMTP) software to represent the main characteristics of HIFs. The model is compared against well-prove models by using two multiresolution approaches based on the Hermite transform and the Wavelet transform considering different frequency bands.

This paper introduces a PV system model useful for steady-state power flow studies of practical electrical networks. This multi-array PV system model features a comprehensive representation of the three main stages taking part in solar energy conversion systems: (i) PV arrays for the solar-to-electrical energy conversion, (ii) the DC boost converter useful for establishing the MPPT strategy and for stepping up the output voltage of the PV arrays, and (iii) the DC-to-AC power conversion by the voltage source converter (VSC) used to link the PV system with the AC grid. The derived PV system model is flexible and modular as it permits to consider any desired number of PV arrays with different irradiance conditions each. For validation purposes, a 1.5-MW PV system coupled to a 3-bus AC network was simulated. Its steady-state power flow results were compared against those obtained by a highly-detailed switching-based PV model implemented in Simulink©. It is shown that the proposed model retains sufficient accuracy since the computed relative errors were inferior to 2% between both fundamentally different methods. The IEEE 57-bus test grid is also used to incorporate five PV plants, thus showing the practicality of the introduced modeling approach for distributed PV systems.

Medium frequency transformers (MFTs) are a key component of DC&ndash, DC dual active bridge (DAB)-type converters. These technologies are becoming a quintessential part of renewable energy solutions, such as photovoltaic systems and wind energy power plants, as well as in modern power grid interfaces functioning as solid-state transformers in smart-grid environments. The weight and physical dimensions of an MFT are key data for the design of these devices. The size of an MFT is reduced by increasing its operating frequency. This reduction implicates higher power density through the transformer windings, as well as other design requirements distinct to those used for conventional 60/50 Hz transformers, therefore, new MFT design procedures are needed. This paper introduces a novel methodology for designing MFTs, using nanocrystalline cores, and tests it using an MFT&ndash, DAB lab prototype. Different to other MFT design procedures, this new design approach uses a modified version of the area-product technique, which consists of smartly modifying the core losses computation, and includes nanocrystalline cores. The core losses computation is supported by a full analysis of the dispersion inductance. For purposes of validation, a model MFT connected to a DAB converter is simulated in Matlab-Simulink (The MathWorks, v2014a, Mexico City, Mexico). In addition, a MFT&ndash, DAB lab prototype (1 kVA at 5 kHz) is implemented to experimentally probe further the validity of the methodology just proposed. These results demonstrate that the analytic calculations results match those obtained from simulations and lab experiments. In all cases, the efficiency of the MFT is greater than 99%.

This paper proposes an improved Multilevel Boost Converter, also known as Multiplier Boost Converter, in which the spike-type current among capacitors is eliminated through one resonant inductor, achieving some resonant-type current waveforms. Experimental results demonstrate the applicability of the proposition.

This paper introduces an advanced transformerless multilevel hybrid-conversion topology intended for the interconnection of renewable DC sources at small-scale. The most important contribution presented in this paper is the generation of two isolated DC sources from a single DC source without the use of any type of transformer. The DC sources feed a nine-level DC/AC hybrid cascade multilevel converter. This advanced topology is achieved by redesigning the conventional DC/DC Buck topology, attached to the multilevel converter, and embedding a suitable switching strategy along with a Field Programmable Gate Array (FPGA)-based control. The advantages of the proposed structure, when compared to other proposals in the literature, are higher efficiency, reduced number of power switches, and high power density derived of transformerless characteristic. As a way to highlight differences and advantages of this converter over other options recently available in the literature, this paper carries out a quantitative evaluation comparing the number of voltage levels and the number of elements involved in the structure of DC/AC multilevel converters. The mathematical model and control strategy of the converter are explained and analyzed by means of simulations. Finally experimental results, obtained from a laboratory-scale prototype, show the performance of the system and demonstrate its relative advantages.

Voltage control based on reactive power compensation is a fundamental aspect of the operation of ac electric power systems. This paper presents a novel shunt compensation scheme based on a virtual air gap variable reactor. The scheme is fully developed, from the adaptation of the virtual air gap principle to high-voltage applications and the determination of its expected performance, to the proposal of a digital cascade control using internal model and proportional-integral controllers. The suitability and flexibility of the device, and the voltage control and reactive power compensation scheme are verified by means of laboratory tests performed in a small-scale prototype. Measured results show that the proposed device and its control provide a robust load compensation scheme for ac systems.

In this paper, the active front-end (AFE) converter topology for the total harmonic distortion (THD) reduction in a wind energy system (WES) is used. A higher THD results in serious pulsations in the wind turbine (WT) output power and several power losses at the WES. The AFE converter topology improves the capability, efficiency, and reliability in the energy conversion devices; by modifying a conventional back-to-back converter, from using a single voltage source converter (VSC) to use pVSC connected in parallel, the AFE converter is generated. The THD reduction is achieved by applying a different phase shift angle at the carrier of digital sinusoidal pulse width modulation (DSPWM) switching signals of each VSC. To verify the functionality of the proposed methodology, the WES simulation in Matlab-Simulink® (Matlab r2015b, Mathworks, Natick, MA, USA) is analyzed, and the experimental laboratory tests using the concept of rapid control prototyping (RCP) and the real-time simulator Opal-RT Technologies (Montreal, QC, Canada) is achieved. The obtained results show a type-4 WT with a total output power of 6 MVA, generating a THD reduction up to 5.5 times of the total WES current output by Fourier series expansion.