Eugenio Gubia

Transformerless Single-Phase Multilevel-Based Photovoltaic Inverter

The elimination of the output transformer from grid- connected photovoltaic (PV) systems not only reduces the cost, size, and weight of the conversion stage but also increases the system overall efficiency. However, if the transformer is removed, the galvanic isolation between the PV generator and the grid is lost. This may cause safety hazards in the event of ground faults. In addition, the circulation of leakage currents (common-mode currents) through the stray capacitance between the PV array and the ground would be enabled. Furthermore, when no transformer is used, the inverter could inject direct current (dc) to the grid, causing the saturation of the transformers along the distribution network. While safety requirements in transformerless systems can be met by means of external elements, leakage currents and the injection of dc into the grid must be guaranteed topologically or by the inverters control system. This paper proposes a new high-efficiency topology for transformerless systems, which does not generate common-mode currents and topologically guarantees that no dc is injected into the grid. The proposed topology has been verified in a 5-kW prototype with satisfactory results.

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.

Ride Through of Wind Turbines With Doubly Fed Induction Generator Under Symmetrical Voltage Dips

This paper deals with the grid fault ride-through capability of doubly fed induction generators. These machines are very sensitive to grid disturbances. To prevent the damages that voltage dips can cause on the converter, most machines are equipped nowadays with a crowbar that short circuits the rotor. However, during the crowbar activation, the rotor converter must be disconnected, hence the power generated with the turbine is no longer controlled. In doing so, the crowbar impedes the wind turbine from carrying out the voltage stabilization required by most new grid codes. This paper proposes a novel control strategy that notably reduces the crowbar activation time. As a result, the control of the turbine might shortly be resumed and the turbine can furthermore supply a reactive power fulfilling the newest grid regulations. Experimental results of a complete system are included, demonstrating the viability of the proposed control.

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.

High-Efficiency Transformerless Single-phase Photovoltaic Inverter

Grid-connected photovoltaic (PV) systems usually include a line transformer in their power conversion stage. This transformer guarantees galvanic isolation between the grid and the PV system, thus providing personal protection and avoiding leakage currents between the PV system and the ground. Furthermore, it also ensures that no continuous current is injected into the grid. However, because of its low frequency (50 Hz), the transformer is big, heavy and expensive. As an alternative to the grid-connected PV systems with line transformer, new transformerless inverter topologies are being studied. If no transformer is used, a galvanic connection between the grid and the PV array exists. In these conditions, leakage currents due to the capacitance between the photovoltaic array and earth could occur and increase the electromagnetic emissions. To avoid these leakage currents, it is necessary to use inverter topologies that avoid commonmode voltages. In this paper, a new transformerless singlephase PV inverter with six IGBTs and one diode is proposed. This topology generates no common-mode voltage and has a higher efficiency than the existing topologies. The topology has been validated by simulation.

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.

Frequency domain model of conducted EMI in electrical drives

A harmful aspect of adjustable speed drives is the presence of large high-frequency stray currents. The most important, from the view of electromagnetic compatibility, are common-mode currents at the output and input sides of the converter. Currents at the output flow through the installation ground while those at the input flow through the grid ground. These common-mode currents can cause disturbances in other units that are connected within the same power section or placed close to the drive. This paper proposes a simulation model for a complete drive system based on the frequency domain. The model accurately reproduces the behavior of common-mode currents at any point of the system and allows the user to understand the influence of each system element on the currents. Thus, the model is useful both for designing filter structures and placing them at the proper position inside the adjustable speed drive. The model is validated by means of experimental results on a 5 kVA adjustable speed drive.

Analysis of neutral-point voltage balancing problem in three-level neutral-point-clamped inverters with SVPWM modulation

In this paper, the evolution and the control options of the neutral point potential in NPC inverters with SVPWM modulation are analysed. In this analysis, the different components that can be found in the phase current are taken into account: fundamental, DC current and harmonics. Firstly, the current injected in the neutral point by the nonredundant medium vectors is evaluated for each phase current component. The only components that inject DC current in neutral point are demonstrated to be the even and nonmultiple of 3 current harmonics. This DC current tends to unbalance the neutral point potential. Finally, the compensation of this DC current by means of both the small redundant vectors and the fundamental component of the phase current is analysed. This study allows the maximum amplitude of the phase current harmonics to be determined without losing the neutral point potential control with the SVPWM modulation.

Control of three-phase stand-alone photovoltaic systems with unbalanced loads

Three-phase four-wire supply with a neutral point is often required in stand-alone photovoltaic systems. In general, the neutral point is achieved in three-phase systems by using three H-bridge inverters, with a high number of semiconductors, or from the dc-bus medium point, which is affected by loads consuming dc currents. In photovoltaic systems, many regulations demand the use of a transformer. Then, the configuration consisting of a three-leg inverter and a delta-wye transformer is very interesting. This system presents a natural coupling between the variables. In order to control it, this paper proposes a double-loop strategy with new inner current and outer voltage control loops that include feed-forward and feedback compensations. Simulation results show that the proposed control strategy achieves an accurate and robust behavior.

Power control between two DC buses for on-board systems network stability support

Aircraft electrical networks are undergoing big transformations, such as the implementation of DC grids. These grids can entail stability problems because of the constant power loads that they integrate. The system stability decreases when the length of the wires that connect the buses and the loads increases due to their inductive behaviour. As a solution to reduce this effect, distributed DC buses are placed near the loads. However, stability problems might be shifted to links between the DC buses. If these DC links are controlled, stability problems will disappear. In this paper, the control of a DC-DC converter to manage these links is described.