Open Access
State-of-charge-based droop control for stand-alone AC supply systems with distributed energy storage
(Elsevier, 2015) Urtasun Erburu, Andoni; Sanchis Gúrpide, Pablo; Marroyo Palomo, Luis; Ingeniería Eléctrica y Electrónica; Ingeniaritza Elektrikoa eta Elektronikoa; Universidad Pública de Navarra / Nafarroako Unibertsitate Publikoa
The droop method is an advantageous technique for stand-alone AC supply systems, allowing for power sharing among various inverters with no need for communication cables. However, in stand-alone systems with multiple distributed energy storage units, the conventional droop methods are unable to control the storage unit state-of-charge (SOC) in order to change simultaneously. Existing techniques endeavor to solve this problem by changing the slope of the P – f curve however this solution compromises the power response performance. As an alternative, this paper proposes a new SOC-based droop control, whereby the P – f curve is shifted either upwards or downwards according to the battery SOC. The proposed technique makes it possible to select the time constant for the battery SOC convergence and, at the same time, to optimize the power response performance. The paper also shows how the SOC changes when the ratios between the battery capacity and the inverter rated power are different and how the proposed technique can limit the SOC imbalance. Simulation and experimental results corroborate the theoretical analysis.
Open Access
Analytical design methodology for Litz-wired high-frequency power transformers
(IEEE, 2015) Barrios Rípodas, Ernesto; Ursúa Rubio, Alfredo; Marroyo Palomo, Luis; Sanchis Gúrpide, Pablo; Ingeniería Eléctrica y Electrónica; Ingeniaritza Elektrikoa eta Elektronikoa; Universidad Pública de Navarra / Nafarroako Unibertsitate Publikoa
In the last quarter of a century, high-frequency (HF) transformer design has been one of the major concerns to power electronics designers in order to increase converter power densities and efficiencies. Conventional design methodologies are based on iterative processes and rules of thumb founded more on expertise than on theoretical developments. This paper presents an analytical design methodology for litz-wired HF power transformers that provides a deep insight into the transformer design problem making it a powerful tool for converter designers. The most suitable models for the calculation of core and winding losses and the transformer thermal resistance are first selected and then validated with a 5-kW 50-kHz commercial transformer for a photovoltaic application. Based on these models, the design methodology is finally proposed, reducing the design issue to directly solve a five-variable nonlinear optimization problem. The methodology is illustrated with a detailed design in terms of magnetic material, core geometry, and primary and secondary litz-wire sizing. The optimal design achieves a 46.5% power density increase and a higher efficiency of 99.70% when compared with the commercial one.
Open Access
Comparison of linear and and small-signal models for inverter-based microgrids
(IEEE, 2014) Urtasun Erburu, Andoni; Sanchis Gúrpide, Pablo; Marroyo Palomo, Luis; Ingeniería Eléctrica y Electrónica; Ingeniaritza Elektrikoa eta Elektronikoa; Universidad Pública de Navarra / Nafarroako Unibertsitate Publikoa
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.
Open Access
Control of a photovoltaic array interfacing current-mode-controlled boost converter based on virtual impedance emulation
(IEEE, 2019) Urtasun Erburu, Andoni; Samanes Pascual, Javier; Barrios Rípodas, Ernesto; Sanchis Gúrpide, Pablo; Marroyo Palomo, Luis; Ingeniería Eléctrica, Electrónica y de Comunicación; Ingeniaritza Elektrikoa, Elektronikoaren eta Telekomunikazio Ingeniaritzaren
Due to the nonlinear characteristics of a photovoltaic (PV) array, its regulation is highly dependent on the operating point. Focusing on a dc-dc boost converter, this paper first shows how the PV voltage and inductor current controls are affected by the PV array. It then proposes to emulate an impedance virtually connected to the PV array, making it possible to greatly improve the control robustness. Thanks to the proposed strategy, the crossover frequency variation for the whole operating range is reduced from 42 times for the traditional control to 3.5 times when emulating parallel resistance or to 1.4 times when emulating series and parallel resistances, all with simple implementation. Experimental results with a commercial PV inverter and a 4-kWp PV array validate the theoretical analysis and demonstrate the superior performance of the proposed control.
Open Access
Implementation and control of a residential electrothermal microgrid based on renewable energies, a hybrid storage system and demand side management
(MDPI, 2014) Pascual Miqueleiz, Julio María; Sanchis Gúrpide, Pablo; Marroyo Palomo, Luis; Ingeniería Eléctrica y Electrónica; Ingeniaritza Elektrikoa eta Elektronikoa; Gobierno de Navarra / Nafarroako Gobernua
This paper proposes an energy management strategy for a residential electrothermal microgrid, based on renewable energy sources. While grid connected, it makes use of a hybrid electrothermal storage system, formed by a battery and a hot water tank along with an electrical water heater as a controllable load, which make possible the energy management within the microgrid. The microgrid emulates the operation of a single family home with domestic hot water (DHW) consumption, a heating, ventilation and air conditioning (HVAC) system as well as the typical electric loads. An energy management strategy has been designed which optimizes the power exchanged with the grid profile in terms of peaks and fluctuations, in applications with high penetration levels of renewables. The proposed energy management strategy has been evaluated and validated experimentally in a full scale residential microgrid built in our Renewable Energy Laboratory, by means of continuous operation under real conditions. The results show that the combination of electric and thermal storage systems with controllable loads is a promising technology that could maximize the penetration level of renewable energies in the electric system.
Open Access
Adaptive voltage control of the DC/DC boost stage in PV converters with small input capacitor
(IEEE, 2013) Urtasun Erburu, Andoni; Sanchis Gúrpide, Pablo; Marroyo Palomo, Luis; Ingeniería Eléctrica y Electrónica; Ingeniaritza Elektrikoa eta Elektronikoa; Universidad Pública de Navarra / Nafarroako Unibertsitate Publikoa
In the case of photovoltaic (PV) systems, an adequate PV voltage regulation is fundamental in order to both maximize and limit the power. For this purpose, a large input capacitor has traditionally been used. However, when reducing that capacitor size, the nonlinearities of the PV array make the performance of the voltage regulation become highly dependent on the operating point. This paper analyzes the nonlinear characteristics of the PV generator and clearly states their effect on the control of the DC/DC boost stage of commercial converters by means of a linearization around the operating point. Then, it proposes an adaptive control, which enables the use of a small input capacitor preserving at the same time the performance of the original system with a large capacitor. Experimental results are carried out for a commercial converter with a 40 μF input capacitor, and a 4 kW PV array. The results corroborate the theoretical analysis; they evidence the problems of the traditional control, and they validate the proposed control with such a small capacitor.
Open Access
Modeling of small wind turbines based on PMSG with diode bridge for sensorless maximum power tracking
(Elsevier, 2013) Urtasun Erburu, Andoni; Sanchis Gúrpide, Pablo; San Martín Biurrun, Idoia; López Taberna, Jesús; Marroyo Palomo, Luis; Ingeniería Eléctrica y Electrónica; Ingeniaritza Elektrikoa eta Elektronikoa; Universidad Pública de Navarra / Nafarroako Unibertsitate Publikoa
The Permanent Magnet Synchronous Generator (PMSG) with diode bridge is frequently used in small Wind Energy Conversion Systems (WECS). This configuration is robust and cheap, and therefore suitable for small WECS. In order to achieve Maximum Power Point Tracking (MPPT) with no mechanical sensors, it is possible to impose the relationship between the DC voltage and the DC current on the optimum operating points. However, this relationship is difficult to calculate theoretically since the whole system is involved. In fact, as there is no model of the whole system in the literature, the optimum curve IL*(Vdc) is obtained with experimental tests or simulations. This paper develops an accurate model of the whole WECS, thereby making it possible to relate the electrical variables to the mechanical ones. With this model, it is possible to calculate the optimum curve IL*(Vdc) from commonly-known system parameters and to control the system from the DC side. Experimental results validate the theoretical analysis and show that maximum power is extracted for actual wind speed profiles.
Open Access
On the stability criteria for inverter current control loops with LCL output filters and varying grid impedance
(IEEE, 2017) Lumbreras Magallón, David; Barrios Rípodas, Ernesto; Ursúa Rubio, Alfredo; Marroyo Palomo, Luis; Sanchis Gúrpide, Pablo; Ingeniaritza Elektrikoa eta Elektronikoa; Institute of Smart Cities - ISC; Ingeniería Eléctrica y Electrónica
The use of LC and LCL filters and grid impedance variations are creating new challenges on the controller design for current control loops of photovoltaic and wind turbine inverters. In the design process, stability criteria such as Bode and revised Bode are commonly used. This paper analyses the limitations of Bode and revised Bode criteria to reliably determine stability and proposes a sufficient and necessary stability criterion, based on the Nyquist criterion, but that makes use of the Bode diagram. The proposed criterion, named generalized Bode criterion, is always reliable and helps the controller design. Relative stability in complex control loops is also studied and a relative stability analysis is proposed. Finally, the generalized Bode criterion and the proposed relative stability analysis are illustrated with a practical example in which a PI is designed in order to guarantee stability and achieve relative stability.
Open Access
DC capacitance reduction in three-phase photovoltaic inverters by using virtual impedance emulation
(IEEE, 2019) Urtasun Erburu, Andoni; Sanchis Gúrpide, Pablo; Marroyo Palomo, Luis; Ingeniaritza Elektrikoa, Elektronikoaren eta Telekomunikazio Ingeniaritzaren; Institute of Smart Cities - ISC; Ingeniería Eléctrica, Electrónica y de Comunicación
DC voltage regulation in grid-connected three-phase PV inverters is a fundamental requirement. In order to reduce the influence of the PV non-linear behavior and ensure stability in the whole operating range, the input capacitance in high-power inverters is currently oversized, thus increasing the converter cost. This paper proposes a control method which emulates a virtual impedance in parallel with the PV generator, making it possible to reduce the capacitance by a factor of 5. Simulation results confirm that the proposed control is stable and fast enough in the whole operating range with such a small capacitor.
Open Access
Fuzzy-based energy management of a residential electro-thermal microgrid based on power forecasting
(IEEE, 2018) Arcos Avilés, Diego; Gordillo, Rodolfo; Guinjoan Gispert, Francesc; Sanchis Gúrpide, Pablo; Pascual Miqueleiz, Julio María; Marietta, Martin P.; Marroyo Palomo, Luis; Ibarra, Alexander; Ingeniería Eléctrica, Electrónica y de Comunicación; Ingeniaritza Elektrikoa, Elektronikoaren eta Telekomunikazio Ingeniaritzaren
In this paper, an energy management strategy based on microgrid power forecasting is applied to a residential grid-connected electro-thermal microgrid with the aim of smoothing the power profile exchanged with the grid. The microgrid architecture under study considers electrical and thermal renewable generation, energy storage system (ESS), and loads. The proposed strategy manages the energy stored in the ESS to cover part of the energy required by the thermal generation system for supplying domestic hot water to the residence. The simulation results using real data and the comparison with previous strategy have demonstrated the effectiveness of the proposed strategy.