Sergio Martinez

Low-Voltage Ride-Through Capability for Wind Generators Based on Dynamic Voltage Restorers

The increasing penetration of wind energy in power systems has forced operators worldwide to develop new grid codes to integrate this form of generation. One important aspect of the problem is the low-voltage ride-through capability of wind generators. This paper describes a solution for wind farms with squirrel-cage asynchronous generators based on the use of a dynamic voltage restorer. Along with a specifically developed control scheme, it provides the wind generator the ability to remain connected during a voltage disturbance and, at the same time, to fulfill the demanding reactive power requirements imposed by recent grid codes. The main experimental results of extensive laboratory tests on system performance are also presented.

Computer-Based Simulation and Scaled Laboratory Bench System for the Teaching and Training of Engineers on the Control of Doubly Fed Induction Wind Generators

Among the existing renewable sources, wind energy is reaching production rates that are becoming important on the worldwide energy scene. Since the control of these wind generators is a very technical discipline, practical teaching methodologies are of special relevance. Paradoxically, in the past, the training of engineers specializing in this area has lacked the practical component represented by field tests, due to the difficulty of access to this kind of installation. This paper presents a system designed for use both in teaching and training procedures for control strategies for wind generators with doubly fed induction generator (DFIG) technology. The system includes two phases or levels of use: the first being a simulation phase based on computer models, and the second, an advanced level which allows for the conducting of tests on a laboratory scaled workbench composed of a wind turbine emulator coupled to an electric generator. With this equipment, the effectiveness of the wind generator regulation systems can be analyzed from the point of view of the maximum power point tracking control strategy, as well as from that of the contribution produced by the wind generator to the control of the operation of the electric grid to which it is connected.

A Resistance Emulation Approach to Optimize the Wave Energy Harvesting for a Direct Drive Point Absorber

In general, a major challenge for the exploitation of renewable energies is to improve their efficiency. In electricity generation from the energy of ocean waves, not unlike other technologies, the converter must be optimized to make the energy harvesting economically feasible. This paper proposes a passive tuning control strategy of a point absorber in which the power captured is maximized by controlling the electromagnetic force of the generator with a resistance emulation approach. The proposed strategy consists of mapping the optimal values for regular waves and applying them to irregular waves. This strategy is tested in a wave energy converter in which the generator is connected to a boost rectifier converter whose controller is designed to emulate a resistance. The power electronics system implemented is validated by comparing its performance with the case in which the generator is directly connected to a resistive load. The simulation results show the effectiveness of the proposed strategy as the maximum captured power is concentrated around the optimal values previously calculated and with the same behavior for both excitations.

Emulation of an OWC Ocean Energy Plant With PMSG and Irregular Wave Model

Ocean energy is a promising resource for renewable electricity generation that presents many advantages, such as being more predictable than wind energy, but also some disadvantages such as large and slow amplitude variations in the generated power. This paper presents a hardware-in-the-loop prototype that allows the study of the electric power profile generated by a wave power plant based on the oscillating water column (OWC) principle. In particular, it facilitates the development of new solutions to improve the intermittent profile of the power fed into the grid or the test of the OWC behavior when facing a voltage dip. Also, to obtain a more realistic model behavior, statistical models of real waves have been implemented.

Fast-Frequency Response Provided by DFIG-Wind Turbines and its Impact on the Grid

This paper presents a methodology for the analysis of frequency dynamics in large-scale power systems with high level of wind energy penetration by means of a simplified model for DFIG-based wind turbines. In addition, a virtual inertia controller version of the optimized power point tracking (OPPT) method is implemented for this kind of wind turbines, where the maximum power point tracking curve is shifted to drive variations in the active power injection as a function of both the grid frequency deviation and its time derivative. The proposed methodology integrates the model in a primary frequency control scheme to analyze the interaction with the rest of the plants in the power system. It is also proven that, under real wind conditions, the proposed version of the OPPT method allows us to smooth the wind power injected into the grid, thereby reducing frequency fluctuations.

A Simplified Electro-Mechanical Model of a DFIG-based Wind Turbine for Primary Frequency Control Studies

In recent years, world-wide power systems are experiencing a steadily growth of wind power penetration. A common concern in the operation of such systems is related to the frequency stability. Modern variable speed wind turbines have a limited capacity in providing ancillary services, such as fast-frequency response and primary frequency regulation. Recent developments in wind plant controllers allow performing active power control from all its units to respond to system frequency deviations. It is thus important to study the effects of the presence of these plants in the system frequency response. Existing detailed models of wind turbines are not suitable to this goal because their complexity makes them impractical for system-wide studies. This paper presents a simplified model of a wind turbine with Doubly-Fed Induction Generator specifically conceived for such studies, along with its validation with a detailed model.

Comparison of current control strategies applied to a boost-rectifier connected to a direct drive wave energy converter

Direct drive wave energy converters are characterized by a direct conversion of the wave energy into electricity with no intermediate mechanical conversion system. For this reason, optimization methods for maximizing the absorbed power have to be designed for acting on the electrical generator. Besides, this type of system requires power electronics to be connected to the grid. This paper evaluates three different current control strategies applied to a boost-rectifier in the ac-dc stage as part of an optimization method. The system is implemented and simulated in MATLAB/Simulink with real random waves. In direct drive wave energy conversion, unlike the original application of these controllers, the input electrical power is highly variable, both in amplitude and frequency. The controllers are assessed under these conditions and their advantages and disadvantages are presented.

Energy conversion efficiency assessment of a direct drive wave energy converter with different current controllers

Linear permanent magnet generators are commonly used as the power take-off system in direct drive wave energy converters. The electrical power generated is characterized by a high variability in amplitude and frequency, so a power electronic interface is needed for grid connection. In addition to its coupling function, the electronic converter can be used as a tool to improve the efficiency of the wave energy conversion. An approach to this goal is to use the generator-side converter to emulate a resistance as seen from the generator. For this optimization method, this paper describes the performance of three different current controllers of the converter from the point of view of the overall conversion efficiency.

Study of voltage fluctuations caused in a distribution grid by the connection of a wave energy converter and corrective actions based on reactive power compensation

Abstract Renewable energies, in general, have considerably increased their penetration in electrical power systems. However, they are intermittent energy sources, and large amounts of this power could create problems for the proper operation of the electrical network. In this sense, this paper studies the effects of wave energy generation on the voltages in a distribution network, using the IEEE 34-bus test feeder. In addition, corrective actions based on reactive power compensation, such as reactive compensation from the grid-side converter of the wave energy converter and the use of a STATCOM, are presented. The results show that the use of the grid-side converter as a reactive power compensator improves the voltage quality in the network, although its capability to mitigate voltage fluctuations is limited. The use of a complementary external STATCOM can provide for extra compensation if needed.

A novel educational proposal: Devising an electric power system

The study of electric power systems within the field of Electrical Engineering is usually approached by computer simulation because any actual test is quite complex to be implemented, especially with renewable energies. Having the aim to improve student learning about this topic, a new subject called “Devising an Electric Power System” was organized following a CDIO (Conceive-Design-Implement-Operate) approach. The subject is programmed for one academic year and based entirely on laboratory work. The students are divided into three teams. Every team would have to work on a power system that includes a solar PV generator and a pumping controlled drive, both connected to a three-phase grid. The third and last part of the subject is focused on “electric utility” business strategy. In the final day of the course a competition between the three teams takes place.