Andoni Urtasun

Frequency-Based Energy-Management Strategy for Stand-Alone Systems With Distributed Battery Storage

Distributed generation is an attractive solution for stand-alone ac supply systems. In such systems, the installation of two or more energy-storage units is recommended for system redundancy and may also be required when there is a consumption increase following installation. However, energy management with multiple energy-storage units has been, but vaguely analyzed in the literature and the few studies made are based on communication cables with a central supervisor. This paper proposes an energy-management strategy for a multiple-battery system which makes it possible to avoid the use of communication cables, rendering the system more cost effective and reliable. The strategy modifies the conventional droop method so that the power becomes unbalanced, allowing for the regulation of one or more battery voltages or currents, as required. Furthermore, whenever the frequency is high, the PV inverters reduce their power in order to prevent the battery from overcharge or high charging currents. On the other hand, whenever the frequency is low, then either the noncritical loads are regulated or the system stops in order to prevent the battery from overdischarge or high discharging currents. Simulation and experimental validation are performed for a system with two-battery inverters, two-PV inverters, and a number of loads.

Small Wind Turbine Sensorless MPPT: Robustness Analysis and Lossless Approach

The configuration permanent-magnet synchronous generator with a diode bridge is frequently used in small wind energy conversion systems due to its reliability and low cost. In order to perform a sensorless maximum power point tracking, a suitable method consists of imposing the relationship between the dc current and the dc voltage in optimum operation. However, this strategy requires having knowledge of the system parameters, which are inaccurately known and can vary in real applications. Thus, the optimum curve is not precisely obtained, leading to power losses. This paper evaluates to what extent the power is reduced due to parameter errors. It is shown how the power can be drastically decreased due to some parameter variation, whereas it is not affected by others such as the resistance, which can then be neglected in order to simplify the model. Simulation results for an actual wind profile validate the theoretical analysis.

Limiting the power generated by a photovoltaic system

Although photovoltaic (PV) systems are generally based on Maximum Power Point Tracking (MPPT), many situations such as stand-alone systems or microgrids increasingly require the PV system to operate below maximum power. The problem here is that the power regulation becomes unstable when the MPP power is lower than the reference power as a result of a decrease in irradiance. This paper proposes a control strategy for a DC/DC boost converter that makes it possible to operate in both modes: at either maximum or limited power point tracking. Simulation results validate the control, showing high dynamics and confirming that the system stability is guaranteed for any situation.

Control of a Single-Switch Two-Input Buck Converter for MPPT of Two PV Strings

In this paper, the two-input buck converter is proposed as the dc/dc stage for photovoltaic (PV) cascaded converters. This converter is attractive for this application because it is cost effective and reliable and can achieve dual maximum power point tracking (MPPT) with only one power transistor. However, due to the simplified and integrated structure, the nonlinear characteristics of the converter and the two PV arrays complicate the control. By means of a small-signal modeling, the control theme of the two PV voltages is formulated, and the effect of the nonlinearities is presented. It is shown that, while fast voltage dynamics are achieved for the first input, the second-input voltage response depends on the second-stage converter control. Simulation and experimental results are reported to validate the theoretical analysis, showing the dual MPPT capability.

High-Frequency Power Transformers With Foil Windings: Maximum Interleaving and Optimal Design

Foil conductors and primary and secondary interleaving are normally used to minimize winding losses in high-frequency (HF) transformers used for high-current power applications. However, winding interleaving complicates the transformer assembly, since taps are required to connect the winding sections, and also complicates the transformer design, since it introduces a new tradeoff between minimizing losses and reducing the construction difficulty. This paper presents a novel interleaving technique, named maximum interleaving, that makes it possible to minimize the winding losses as well as the construction difficulty. An analytical design methodology is also proposed in order to obtain free-cooled transformers with a high efficiency, low volume, and, therefore, a high power density. For the purpose of evaluating the advantages of the proposed maximum interleaving technique, the methodology is applied to design a transformer positioned in the 5 kW-50 kHz intermediate HF resonant stage of a commercial PV inverter. The proposed design achieves a transformer power density of 28 W/cm3 with an efficiency of 99.8%. Finally, a prototype of the maximum-interleaved transformer is assembled and validated satisfactorily through experimental tests.

Comparison of linear and small-signal models for inverter-based microgrids

Frequency and voltage regulation in droop-based microgrids is generally modeled using small-signal analysis. In order to ensure accuracy, existing models do not decouple real and reactive power responses. However, the models become complicated and hide the real decoupled dynamics. This paper proposes a simple linear model which makes it possible to discern the different dynamic properties and to readily design the control parameters. The proposed model is validated by comparison with an accurate small-signal model and by simulation results. The effect of not considering the load is also evaluated.

Small Wind turbines sensorless MPPT: Robustness analysis and lossless approach

The configuration Permanent Magnet Synchronous Generator (PMSG) with diode bridge is frequently used in small Wind Energy Conversion Systems (WESC) thanks to its reliability and low cost. In order to perform a sensorless Maximum Power Point Tracking (MPPT), a suitable method consists of imposing the relationship between the dc current and the dc voltage in optimum operation. However, this strategy requires having knowledge of the system parameters, which are inaccurately known and can vary in real applications. Thus, optimum curve is not precisely obtained, leading to power losses. This paper evaluates to what extent the power is reduced due to parameter errors. It is shown how the power can be drastically decreased due to some parameters variation whereas it is not affected by others such as the resistance, which can then be neglected in order to simplify the model. Simulation results for an actual wind profile validate the theoretical analysis.

RMS voltage control with harmonic compensation for parallel-connected inverters feeding non-linear loads

Droop method is an advantageous technique for generating the grid in stand-alone AC supply systems with distributed generation. Regarding the AC voltage regulation, an interesting approach is to control the RMS voltage instead of its instantaneous value. However, this regulation has serious drawbacks when feeding non-linear loads since the voltage THD increases and current harmonics are not equally shared by the inverters. This paper proposes a harmonic compensation which reduces the voltage THD and makes that the current harmonics are equally distributed among the inverters. Simulation results demonstrate the improvements of the proposed control.

Frequency-based energy management of stand-alone systems: Design of the control parameters for high versatility

Frequency-based energy management strategies have been proposed for stand-alone systems with either centralized or distributed energy storage. This paper proposes a methodology for the design of the control parameters which ensures stability in both type of systems. As a result, the inverter manufacturer can implement one sole strategy, regardless of whether the expected stand-alone system is centralized or distributed. In addition, the battery inverter becomes more versatile since it can be used alone or in parallel with other battery inverters. Simulation results corroborate the theoretical analysis and demonstrate that energy management is carried out with no need of communication cables for a system with 1, 2 or 3 battery inverters with the same firmware.

Optimal DC gapped inductor design including high-frequency effects

DC inductor design has recently drawn the attention of magnetic designers due to the rise in the number of DC-DC power converter applications, such as electric vehicles and renewable distributed generation systems. The design methods found in the literature are highly iterative and make it difficult to assess the design trends and optimize the power converter as a whole. In this paper, the DC gapped inductor design problem is analyzed. A unified inductor model is first developed and a novel inductor design methodology is then proposed. The comprehensive inductor model makes it possible to easily introduce all the phenomena present in the inductor operation in the design process, including winding high-frequency effects and fringing flux in the gap. With this so-called analytical inductor design methodology, the inductor energy density can be maximized while achieving high efficiencies. Finally, an inductor for a fuel cell 2.4 kW-22 kHz boost converter is optimally designed reaching an energy density of 0.61 J/dm3 and an efficiency of 99.04%.