Joaquin Eloy-Garcia

Modeling, Analysis, and Design of Stationary-Reference-Frame Droop-Controlled Parallel Three-Phase Voltage Source Inverters

Power-electronics-based microgrids (MGs) consist of a number of voltage source inverters (VSIs) operating in parallel. In this paper, the modeling, control design, and stability analysis of parallel-connected three-phase VSIs are derived. The proposed voltage and current inner control loops and the mathematical models of the VSIs are based on the stationary reference frame. A hierarchical control scheme for the paralleled VSI system is developed comprising two levels. The primary control includes the droop method and the virtual impedance loops, in order to share active and reactive powers. The secondary control restores the frequency and amplitude deviations produced by the primary control. Also, a synchronization algorithm is presented in order to connect the MG to the grid. Experimental results are provided to validate the performance and robustness of the parallel VSI system control architecture.

A New Variable-Frequency Optimal Direct Power Control Algorithm

This paper presents a new formulation for direct power control (DPC) with several improvements over previous DPC formulations such as an exact discrete-time expression for the predicted power variations yielded by a given inverter switching state and the inclusion of the computing delay time in the model. Based on this formulation, a variable-frequency optimal DPC algorithm is presented that selects the inverter switching state that minimizes the power error. This algorithm shows an excellent precision in estimating power variations, therefore reducing power ripple and unwanted current harmonics, while retaining the fast dynamics inherent to variable-frequency DPC. This makes this algorithm suitable for interfacing distributed generation to microgrids, where a fast and accurate power control is desirable in order to perform voltage and frequency regulation. Additionally, studies are carried out regarding the average switching frequency and the influence of the model parameters on the algorithm robustness.

DSP-based Direct Power Control of a VSC with Voltage Angle Estimation

This paper presents a direct power control (DPC) method applied to a three-phase voltage source converter (VSC) connected to the grid. The influence of distorted angle information due to disturbances in the grid voltage, such as harmonics, unbalances, faults, etc., on the direct power control of the converter is taken into account through the use of an algorithm for voltage angle estimation. The control performance is compared in both cases, with and without angle estimation, as a mean of establishing an improving voltage ride-through capability of the converter in case of grid disturbances. The system has been simulated in MATLAB/Simulink and the simulation results have been validated with experimental results, obtained implementing the DPC and the angle estimation algorithm on a real-time DSP-based board (dSPACE ds1102), verifying the good performance of the proposed method

Extended direct power control of a three-level Neutral Point Clamped Voltage Source Inverter with unbalanced voltages

This paper presents the control of a three- level Neutral Point Clamped (NPC) Voltage Source Inverter under unbalanced grid voltages. The control method used is the Extended Direct Power Control (EDPC), which is a generic approach for Direct Power Control of multilevel inverters based on geometrical considerations. It defines, for each inverter vector, two geometrical loci of no power change (a circumference for active power and a straight line for reactive power). Each locus divides the plane into two regions. The relative position of grid voltage vector in these regions determines whether the application of a certain inverter voltage vector will increase or decrease power. Besides, dc link middle point voltage is controlled by taking into account the switching state of the inverter (those states which make the middle point voltage changing). Finally, a decision algorithm chooses the inverter vector to be applied among those which fulfil the three requirements. In case of imbalances in grid voltages, negative sequence components appear and system variables start oscillating at twice the fundamental frequency, affecting the performance of the inverter. In fact, if power is kept constant, output current becomes distorted, presenting mainly third and fifth harmonic components. In order to obtain sinusoidal balanced output currents and thus improve inverter performance under voltage imbalances, power reference is compensated with oscillating terms so that negative sequence current components disappear. The whole control system has been tested on a three-level NPC Voltage Source Inverter connected to the grid and results confirm the validity of the method.

Discrete-Time Sliding-Mode Control of a STATCOM Including Voltage and Current Limitations for Wind Farm Applications

This paper presents a discrete-time sliding-mode control of a VSC based STATCOM, intended for its application to wind farm connection to the grid. A sliding-mode controller has been used for currents while pole placement with a slower dynamic response has been used for DC bus voltage control, as DC voltage has been assumed to have slower dynamics than currents. Besides, the voltage and current limitations of the system has been studied and their influence analyzed by means of limitation circles

Discrete-Time Dead-Beat Control of a VSC Transmission Scheme Including Voltage and Current Limitations for Wind Farm Connection

This paper presents a discrete-time nonlinear control of a voltage source converter (VSC) transmission system for wind farm connection to electrical network. After introducing discrete-time modeling of a VSC transmission scheme, discrete-time currents (i.e. P and Q) and DC bus voltage controllers synthesis is presented. Both voltage and current limitations are then taken into account through the introduction of voltage and current limitation circles. Control performances are then illustrated by simulation results. More particularly, transient behavior in case of voltage and current limitations is discussed. Finally, some conclusions and perspectives are given

On the Motivation of Last-year Undergraduate Students: An Academic Experience of Teaching Electric Drives

This paper presents a statistical analysis of the behaviour of last-year undergraduate students facing a pilot experience of teaching electric drives in the University Carlos III of Madrid. The students were offered the possibility of participating as teachers-tutors for their colleagues and other fourth-year students, as a way of reviewing their already-known knowledge of electric machines and testing their capability for information transmission and group management. The administration of the University Carlos III supported this pilot experience as a part of the adaptation process to the Bologna Declaration (based on the Recommendation of the Council 98/561/EC of 24 September 1998), an EU treatment for quality assurance in higher education. The pilot experience took place between October 2005 and January 2006. Beyond technical aspects, the pilot experience revealed a key point: the lack of motivation of students in the absence of a reward and the low motivation level even with quite generous academic rewards. In the opinion of the authors, this problem appears to be inherent in a social structure that forgets basic education and is economically oriented: earning overcomes learning

Dynamic performance of a back-to-back converter under grid disturbances with a classical DC-bus voltage control loop v.s. a DC-bus voltage control loop with Ni-Cd and Ion-Li batteries

The aim of this paper is to present a comparative study between the dynamic response of two back-to-back converter topologies, using grid-connected Voltage Source Converters (VSCs). The first topology consists on a back-to-back converter where the DC-bus voltage is controlled by means of a classical internal DC voltage control loop. In the second topology, the DC-bus voltage is controlled by a bi-directional boost-buck DC/DC converter connected to a battery stack. In this paper both, Ni-Cd and Ion-Li battery technologies were studied. Since both topologies have a linear plant we can apply Field Oriented Control (FOC) to control both, real and reactive power exchanged with the electrical grids, as well as the DC-bus voltage. The back-to-back converter with internal DC-bus control voltage is a very well known topology which allows drive variable speed electrical machines, control electrical machines connected to the grid and decoupled reactive power flows of two different electrical grids. On the other hand, the back-to-back converter with a bi-directional boost-buck DC/DC converter connected to batteries is a more recent topology which allows decuple both, real and reactive power flows when connected to two different electrical grids. The comparison between both topologies and control methods is verified through time simulations in the discrete domain under several grid disturbances as real and reactive power steps and voltage dips. The final goal of this article is to be able to apply hot-swapping control to connect or disconnect batteries according to their State of Charge (SOC).