Behavior of three-phase inverters under grid faults explained from a geometric perspective

The aim of this work is to analyze the dynamic behavior of inverters powered by renewable energy sources (RES) operating under grid faults by means of a geometric point of view. To this purpose, a three-phase grid-connected inverter subject to voltage sags is analyzed. First, the mathematical equations that describe its electrical model under voltage sags are given in the complex form of the transformed Park components. Second, simulations results show the resemblance between the curve depicted by the transformed Park components of the injected currents in the complex plane and well-known geometric curves. Finally, the geometric-based approach is used to describe easily the dynamic behavior of grid-connected inverters under voltage sags. This new approach could be used to propose new control techniques to achieve fault ride-through (FRT) capability of three-phase inverters.Peer ReviewedObjectius de Desenvolupament Sostenible::11 - Ciutats i Comunitats SosteniblesObjectius de Desenvolupament Sostenible::7 - Energia Assequible i No ContaminantPostprint (published version

A parametric approach for the study of three-phase inverters subject to grid faults

The aim of this work is to describe the behavior of three-phase inverters connected to a faulty grid by means of a parametric approach which gives rise to limaçon of Pascal curves and 3D figures formed by the injected currents during the fault. The analytical study is given in the complex form of the transformed Park variables, and the simulation results are obtained by means of MATLAB. The results show that the parametric analysis can be an easy and useful tool to predict the behavior of three-phase inverters operating under voltage sags, with the aim of achieving fault ride-through (FRT) capability.Objectius de Desenvolupament Sostenible::11 - Ciutats i Comunitats SosteniblesObjectius de Desenvolupament Sostenible::7 - Energia Assequible i No ContaminantPostprint (published version

Initialization of DFIG wind turbines with a phasor-based approach

The objective of this paper is to propose a simple approach to solve the steady state of a wind turbine (WT) equipped with a doubly-fed induction generator (DFIG) which can be used to initialize dynamic studies of the machine. The idea is to model the rotor-side converter (RSC) as a constant current source connected to the rotor of the DFIG. The resulting equivalent circuit consists of a voltage source in series with a reactance, which makes it possible to obtain simple phasor expressions that can be used to obtain the Park components of the variables. The proposed method is compared with the traditional Newton-Raphson algorithm, showing that it is easier and faster to implement, as it makes use of the phasor expressions and it does not require an iterative process to obtain the final solution. Finally, the results of the proposed method are used to simulate a 2 MW DFIG-based WT under three-phase faults, considering three different WT operating points. In these simulations, the idea of constant rotor current is extrapolated to the entire event. The simulated results show that both current at torque peaks are reduced. The analytical study and the simulations have been carried out in MATLAB.Postprint (published version

Integration of cold ironing and renewable sources in the Barcelona smart port

Cold ironing, which is the procedure of supplying shoreside electrical power to a ship at berth when its engines are turned off, arises as the eco-friendly way to deliver power to ships while they are berthed in ports, thus avoiding the use of diesel engines onboard, which provokes the emission of large amounts of greenhouse gases into the atmosphere. The aim of this study is twofold. Firstly, a survey of research developments on cold ironing is carried out in order to show the state of the art on the problem of greenhouse gases emitted by ships while docked and how to tackle it. Current regulations and examples of current ports that make use of technologies for cold ironing purposes are also shown. Secondly, the study proposes the use of a cold ironing system in the port of Barcelona, where the power generation is entirely given by renewable energy systems (wind turbines and photovoltaic panels). The idea is to contribute to the wide spread of cold ironing within smart port microgrids to achieve the goal of zero emissions from berthed ships.Peer ReviewedPostprint (author's final draft

New smart sensor for voltage unbalance measurements in electrical power systems

This paper deals with voltage unbalances and how they can be quantified according to the standards. Firstly, a comparison between the different unbalance voltage factors is conducted in order to remark on their divergences. Secondly, according to the standard that better represents the phenomenon, i.e., EN 50160, a new methodology is proposed to quantify the voltage unbalance factor (VUF). In order to do so, it is recommended to measure the voltage unbalance in three-phase installations by means of a new smart sensor based on a single voltage sensor, which measures the direct-current (DC) voltage at the output of a three-phase diode bridge rectifier, while current methods make use of three voltage sensors (which can measure either phase-to-neutral voltages or phase-to-phase voltages). Furthermore, both simulation and experimental results have been carried out to validate the proposed methodology. Finally, a new voltage unbalance factor (and the corresponding methodology to obtain it from the measured DC voltage) is proposed.This research was funded by the Spanish Ministry of Science and Innovation, grant number PID2021-123633OB-C33.Peer ReviewedPostprint (published version

Distributed cooperative control for stepper motors synchronization

This paper describes the design and simulation of a distributed cooperative control algorithm based on multi-agents to synchronize a group of stepper motors. Modeling of the two-phase hybrid stepper motor in closed loop is derived in d-q rotary reference frame, based on field-oriented control techniques to provide torque control. The simulation obtained by MATLAB-Simulink shows that the distributed cooperative control effectiveness depends on the network topology defined by the graph.Postprint (published version

Control strategies for DFIG wind turbines under grid fault conditions

The classical control techniques for regulating the active and reactive power delivery in doubly fed induction generators (DFIG), for wind power applications, are normally based on voltage oriented control (VOC) strategies. Among these algorithms, those that work in a synchronous reference frame, attached to the magnetic flux vector, became very popular. In spite of the good behaviour of such algorithms their performance depends highly on an accurate detection of the stator flux position, something that can be critical under unbalanced or distorted grid voltage conditions. This paper presents a new VOC strategy able to control the operation of a DFIG in the αβ reference frame, with no need of flux position estimation, something that conducts to a more simple and robust algorithm. In order to evaluate the advantages of this new control proposal, namely VOC-RRF, their performance will be compared with the response obtained with a classical VOC algorithm by means of PSCAD/EMTDC® simulation models.Peer ReviewedPostprint (published version

Symmetrical and unsymmetrical voltage sag effects on the three-phase synchronous machine stability

This paper focuses on the effects of voltage sags, both symmetrical and unsymmetrical, on the three-phase Synchronous Machine (SM). Voltage sags on SM cause speed variations, current and torque peaks and hence may cause tripping and equipment damage. The consequences of voltage sags on the machine behaviour depend on different factors such as sag’s magnitude (or depth), duration, initial point-on-wave and the parameters of the electrical machine. In this study, three SMs of different rated power have been considered in order to simulate the voltage sag effects for specific conditions and analyze the machine stability.Postprint (published version

Voltage sag influence on controlled three-phase grid-connected inverters according to the Spanish grid code

© 2020 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting /republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other worksTo ensure the safe operation of the grid, there are some requirements to be taken into consideration to connect power converters. During abnormal conditions – e.g. during voltage sags –, the control of the converters is a very important key to guarantee power quality and good behaviour of the distributed generation system. The aim of this study is to employ two possible control strategies for a grid-connected inverter according to the Spanish grid code, and to analyse the behaviour of the output voltages during both symmetrical and unsymmetrical voltage sags. The analytical development shows the sag influence on currents, voltages, active and reactive powers. These influences are explained through Ku transformation in the synchronous reference frame, thus giving a representation for electrical variables easiest to analyse. The results show how control strategies affect the converter behaviour and how they can support the main grid during faults through the control of active and reactive power injection. Sags with different durations and depths have been taken into account, which can provoke critical values for electrical magnitudes and can lead to the violation of the grid code. The proposed control strategies study has been validated by means of both simulations in MatlabTM–Simulink and experimental resultsPeer ReviewedPostprint (author's final draft

Testing of three-phase equipment under voltage sags

This paper provides insight into the testing of three-phase equipment exposed to voltage sags caused by faults. The voltage sag recovers at the fault-current zeros, leading to a ‘discrete’ voltage recovery, that is, the fault is cleared in different steps. In the literature, the most widespread classification divides ‘discrete’ sags into 14 types. The authors study shows that it is generally sufficient to consider only five sag types for three-phase equipment, here called ‘time-invariant (TI)’ equipment. As the remaining nine sag types cause identical equipment behaviour in Park or Ku variables, the number of laboratory tests (or of extensive simulations) on equipment under sags is reduced by a ratio of 14/5. The study is validated by simulation of a three-phase induction generator and a three-phase inverter, which are ‘TI’, and a threephase diode bridge rectifier, which is not ‘TI’. Both analytical study and simulation results are validated by testing a three-phase induction motor and a three-phase diode bridge rectifier.Postprint (author's final draft