Jesús López

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.

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.

Modeling and Control of

Designing adequate control laws for grid-connected inverters with LCL filters is complicated. The power quality standards and the system resonances burden the task. In order to deal with resonances, system damping has to be implemented. Active damping is preferred to passive damping so as to improve the efficiency of the conversion. In addition, paralleled grid-connected inverters in photovoltaic (PV) plants are coupled due to grid impedance. Generally, this coupling is not taken into account when designing the control laws. In consequence, depending on the number of paralleled grid-connected inverters and the grid impedance, the inverters installed in PV plants do not behave as expected. In this paper, the inverters of a PV plant are modeled as a multivariable system. The analysis carried out enables to obtain an equivalent inverter that describes the totality of inverters of a PV plant. The study is validated through simulation and field experiments. The coupling effect is described and the control law design of paralleled grid-connected inverters with LCL filters in PV plants is clarified.

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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.

-Paralleled Grid-Connected Inverters With LCL Filter Coupled Due to Grid Impedance in PV Plants

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.

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

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.

Ride Through of Wind Turbines With 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.

High-Efficiency Transformerless Single-phase Photovoltaic Inverter

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.

Control of Doubly Fed Induction Generator under symmetrical voltage dips

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.