Luis Marroyo

Transformerless Inverter for Single-Phase Photovoltaic Systems

When no transformer is used in a grid-connected photovoltaic (PV) system, a galvanic connection between the grid and the PV array exists. In these conditions, dangerous leakage currents (common-mode currents) can appear through the stray capacitance between the PV array and the ground. In order to avoid these leakage currents, different inverter topologies that generate no varying common-mode voltages, such as the half-bridge and the bipolar pulsewidth modulation (PWM) full-bridge topologies, have been proposed. The need of a high-input voltage represents an important drawback of the half-bridge. The bipolar PWM full bridge requires a lower input voltage but exhibits a low efficiency. This letter proposes a new high-efficiency topology that generates no varying common-mode voltage and requires the same low-input voltage as the bipolar PWM full bridge. The proposed topology has been verified in a 5-kW prototype with satisfactory results

Dynamic Behavior of the Doubly Fed Induction Generator During Three-Phase Voltage Dips

The use of doubly fed induction generators (DFIGs) in large wind turbines has become quite common over the last few years. These machines provide variable speed and are driven with a power converter which is sized for a small percentage of the turbine-rated power. A drawback of the DFIG is that it is very sensitive to grid disturbances, especially to voltage dips. However, the operation of the machine in these situations has only been studied in the literature by means of simulations. This paper develops a theoretical analysis of the dynamic behavior of the induction machine during three-phase voltage dips. The proposed analysis contributes to understanding the causes of the problem and represents a very useful tool to improve the existing solutions and propose new alternatives. Experimental results are in good agreement with those obtained theoretically and validate the proposed analysis.

Individual Voltage Balancing Strategy for PWM Cascaded H-Bridge Converter-Based STATCOM

This paper presents a new control method for cascaded connected H-bridge converter-based static compensators. These converters have classically been commutated at fundamental line frequencies, but the evolution of power semiconductors has allowed the increase of switching frequencies and power ratings of these devices, permitting the use of pulsewidth modulation techniques. This paper mainly focuses on dc-bus voltage balancing problems and proposes a new control technique (individual voltage balancing strategy), which solves these balancing problems, maintaining the delivered reactive power equally distributed among all the H-bridges of the converter.

Wind Turbines Based on Doubly Fed Induction Generator Under Asymmetrical Voltage Dips

Many large wind farms employ doubly fed induction generators (DFIGs). The main drawback of these machines is their large sensitivity to grid disturbances, especially to voltage dips. As the penetration of wind energy in the network increases, it becomes more relevant to understand the behavior of these machines under voltage dips. This paper analyzes the operation of the DFIG under asymmetrical voltage dips and explains why such kinds of dips are more harmful than are symmetrical dips. The influence of the dip type and instant of occurrence is also studied.

Boost DC-AC inverter: a new control strategy

Boost dc-ac inverter naturally generates in a single stage an ac voltage whose peak value can be lower or greater than the dc input voltage. The main drawback of this structure deals with its control. Boost inverter consists of Boost dc-dc converters that have to be controlled in a variable-operation point condition. The sliding mode control has been proposed as an option. However, it does not directly control the inductance averaged-current. This paper proposes a control strategy for the Boost inverter in which each Boost is controlled by means of a double-loop regulation scheme that consists of a new inductor current control inner loop and an also new output voltage control outer loop. These loops include compensations in order to cope with the Boost variable operation point condition and to achieve a high robustness to both input voltage and output current disturbances. As shown by simulation and prototype experimental results, the proposed control strategy achieves a very high reliable performance, even in difficult transient situations such as nonlinear loads, abrupt load changes, short circuits, etc., which sliding mode control cannot cope with.

Power Adaptor Device for Domestic DC Microgrids Based on Commercial MPPT Inverters

This paper presents a power adaptor device, referred to as smart panel device, allowing the connection of additional energy sources and storage elements to a domestic photovoltaic (PV) grid-connected system. The adaptor output port is designed to behave as a power source/sink, thus enabling its hot-swap parallel connection to renewable power sources without modifying their maximum power point (MPP). Moreover, the adaptor device features a power characteristic with a single controllable MPP and allows the control of the injected power within the operating range of the dc-ac grid-connected inverter. The work presents the design principles of such device by describing the operation of a sliding-mode controlled quadratic-boost converter. The proper operation of the device is experimentally verified for several scenarios in a small PV-based microgrid system including a fuel-cell stack, a 1-kW three-phase wind turbine, a battery charger-discharger, and commercial grid-connected PV inverters.

Control of Doubly Fed Induction Generator under symmetrical voltage dips

Doubly Fed Induction Generator (DFIG) is largely used in wind generators. It provides variable speed characteristics in a cost-effective way. The main drawback of this machine is its sensitivity to grid disturbances. Voltage dips cause overvoltages to appear in the rotor that can surpass the converter limits. Additionally, variations of the grid voltage cause the stator flux to oscillate at the grid frequency. This paper proposes a novel control strategy to overcome these problems; furthermore, it reduces the rotor voltage, improving the control of the rotor current and it accelerates the dumping of the flux oscillations.

Electronic controlled device for the analysis and design of photovoltaic systems

The characterization and design of photovoltaic systems is a difficult issue due to the variable operation atmospheric conditions. With this aim, simulators and measurement equipments have been proposed. However, most of them do not deal with real atmospheric conditions. This letter proposes an electronic device that first measures the real evolution of the I-V characteristic curves of photovoltaic modules and generators, and then physically emulates in real time these curves to test photovoltaic inverters. The device consists of a dc-dc converter, a microcontroller and a data storage unit. The two operation modes (emulation and measurement) are digitally driven by the microcontroller. The converter current is controlled by means of a variable-hysteresis control loop, whose reference is provided by the microcontroller. In addition, a digital voltage control loop is designed to find out the complete characteristic curves of the photovoltaic generators. A 15-kW prototype is designed and built that can measure three times per second the characteristic curves of up to seven generators and then emulate their electrical behavior to test photovoltaic inverters. With the proposed device, the optimal configuration and performance of photovoltaic modules and generators, as well as the operation of photovoltaic inverters can be thoroughly analyzed under real atmospheric conditions.

A Novel PWM Modulation Strategy for DC Voltage Balancing in Cascaded H-Bridge Multilevel Converters

This paper presents a novel Voltage Balancing PWM modulation strategy for Cascaded H-Bridge multilevel converters. These converters have been classically commu-tated at fundamental line-frequencies, but the evolution of Power Semiconductors has allowed to increase switching frequencies and the power ratings of these devices, permitting the use of PWM modulation techniques. This paper presents a new PWM modulation strategy that solves the DC Bus voltage balancing problems that appear in the cascaded converter working in reactive power compensation (STATCOM) applications.

Design, analysis and comparison of multilevel topologies for DSTATCOM applications

The aim of this paper is to present a design method of two different DSTATCOM multilevel topologies which are suitable to be used connected to the distribution grid. These devices have been classically commutated at fundamental line-frequencies, but the evolution of power semiconductors has allowed to increase switching frequencies and the power ratings of these devices, permitting the use of PWM modulation techniques. This paper mainly focuses the design issues of the NPC and cascaded H-bridge multilevel topologies for DSTATCOM applications, presenting a method for the dimensioning of the different elements that compose the device, paying special attention to the DC bus capacitor sizing and output filter inductance sizing