Open Access
Effect of the inner current loop on the voltage regulation for three-phase photovoltaic inverters
(IEEE, 2020) 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
In three-phase grid-connected PV inverters, regulating the input voltage 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 is currently oversized. This paper reveals the important effect of the inner current loop in the voltage stability and proposes to use a Proportional (P) controller instead of a PI controller. If tuned following the guidelines provided in this paper, the P controller makes it possible to design a stable voltage loop without increasing the input capacitance, thus reducing the converter cost.
Open Access
An energy management system design using fuzzy logic control: smoothing the grid power profile of a residential electro-thermal microgrid
(IEEE, 2021) Arcos Avilés, Diego; Pascual Miqueleiz, Julio María; Guinjoan Gispert, Francesc; Marroyo Palomo, Luis; García Gutiérrez, Gabriel; Gordillo, Rodolfo; Llanos, Jacqueline; Sanchis Gúrpide, Pablo; Motoasca, Emilia; Ingeniería Eléctrica, Electrónica y de Comunicación; Ingeniaritza Elektrikoa, Elektronikoaren eta Telekomunikazio Ingeniaritzaren
This work deals with the design of a Fuzzy Logic Control (FLC) based Energy Management System (EMS) for smoothing the grid power prole of a grid-connected electro-thermal microgrid. The case study aims to design an Energy Management System (EMS) to reduce the impact on the grid power when renewable energy sources are incorporated to pre-existing grid-connected household appliances. The scenario considers a residential microgrid comprising photovoltaic and wind generators, at-plate collectors, electric and thermal loads and electrical and thermal energy storage systems and assumes that neither renewable generation nor the electrical and thermal load demands are controllable. The EMS is built through two low-complexity FLC blocks of only 25 rules each. The first one is in charge of smoothing the power prfile exchanged with the grid, whereas the second FLC block drives the power of the Electrical Water Heater (EWH). The EMS uses the forecast of the electrical and thermal power balance between generation and consumption to predict the microgrid behavior, for each 15-minute interval, over the next 12 hours. Simulations results, using real one-year measured data show that the proposed EMS design achieves 11.4% reduction of the maximum power absorbed from the grid and an outstanding reduction of the grid power profile ramp-rates when compared with other state-of-the-art studies.
Open Access
Power angle-frequency droop control to enhance transient stability of grid-forming inverters under voltage dips
(IEEE, 2022) Erdocia Zabala, Ioseba; Urtasun Erburu, Andoni; Marroyo Palomo, Luis; Ingeniería Eléctrica, Electrónica y de Comunicación; Ingeniaritza Elektrikoa, Elektronikoaren eta Telekomunikazio Ingeniaritzaren
Due to the replacement of synchronous generators, grid operators are currently demanding to control grid-connected inverters in grid–forming mode to make them participate in the maintenance of the grid. To carry this out, the traditional droop controls based on the active and reactive powers are usually adopted, achieving a satisfactory performance in normal operation. Nevertheless, the power-frequency (P-ω) droop may become transiently unstable under voltage dips. This is because of the modification of the active power response caused by the inverter current limitation together with the voltage reduction. To enhance this, the power angle-frequency (δinv-ω) droop is proposed, consisting in employing an estimation of the inverter power angle as input to obtain the inverter frequency. The proposed method provides the inverter with the same performance as the P-ω droop in normal operation, while enhancing considerably the transient stability margins under current limitation. This is thanks to the higher variation of the inverter power angle with the phase difference between the inverter and the grid. Simulation results show the transient stability problems of the P-ω droop as well as the superior performance of the proposed δinv-ω droop control, which has also been verified by means of HIL results.
Open Access
Dynamic analysis of the conductance-frequency droop control during current limitation
(IEEE, 2024-08-30) Urtasun Erburu, Andoni; Erdocia Zabala, Ioseba; Marroyo Palomo, Luis; Ingeniería Eléctrica, Electrónica y de Comunicación; Ingeniaritza Elektrikoa, Elektronikoa eta Telekomunikazio Ingeniaritza; Universidad Pública de Navarra / Nafarroako Unibertsitate Publikoa, PJUPNA2024-11695
In inverter-based stand-alone microgrids, the P-f and O-V droop methods are frequently used to keep control of the microgrid voltage. However, in the presence of overloads or short-circuits, in which the inverter must perform a current- limiting strategy, the P-f droop becomes prone to transient instability. In order to remain stable under any possible overload or fault, the conductance-frequency $({G-f})$ droop is a promising alternative, however no analysis about its dynamic response has been carried out so far. This paper proposes a small-signal model of the system during current limitation, proving that the ${G-f}$ droop is also superior to the existing droop methods in terms of rapidity. Simulation results validate the theoretical analysis.
Open Access
Analysis of the active inertia power provided by grid-forming strategies during a RoCoF
(IEEE, 2024-08-30) Urtasun Salinas, Ibai; Urtasun Erburu, Andoni; Bautista Portillo, Guillermo Antonio; Marroyo Palomo, Luis; Ingeniería Eléctrica, Electrónica y de Comunicación; Ingeniaritza Elektrikoa, Elektronikoa eta Telekomunikazio Ingeniaritza; Institute of Smart Cities - ISC; Universidad Pública de Navarra / Nafarroako Unibertsitate Publikoa, PJUPNA2024-11695
Power electronic-based generators are becoming increasingly prevalent in the electrical grid, necessitating their support in disturbances previously handled only by synchronous generators. One of the tests proposed by regulations is to evaluate the response of grid-forming inverters to a Rate of Change of Frequency (RoCoF). However, there is no detailed analysis of the effect of control parameters on the active inertia power. This article presents the temporal response equation of an inverter subject to a RoCoF and introduces the concept of equivalent inertia showing that it also depends on the damping factor. Thanks to this analysis and the flexibility of inverters, the parameter design of existing grid-forming strategies is proposed to achieve the desired active inertia power and system damping ratio. Theoretical analysis and control strategies have been validated by simulation.
Open Access
Integration of wind and PV power in the spanish electric system using existing hydroelectric power
(IEEE, 2024-08-30) Garwa, Asheem; Pascual Miqueleiz, Julio María; Marroyo Palomo, Luis; Ingeniería Eléctrica, Electrónica y de Comunicación; Ingeniaritza Elektrikoa, Elektronikoa eta Telekomunikazio Ingeniaritza; Institute of Smart Cities - ISC
This paper analyses the potential reduction of electric energy generated with fossil fuels with 40 GW of PV power and 40 GW of wind power in the existing Spanish electric system and its integration by making use of existing hydroelectric power and reserves with different levels of flexibility. We use 10-minute data for 2 years and we firstly assume that hydro power remains the same as originally, observing an already high reduction in fossil fuel use in the electric mix. Then we increasingly allow more flexibility in hydro power use, i.e., we allow to use hydro reserves with different time horizons, and we analyze the incremental reduction in fossil fuel generation as we increase this flexibility. We observe that we can obtain notable gains in terms of fossil fuel reduction when increasing flexibility just slightly showing how hydro power can integrate large amounts of variable renewable energy in Spain with only a small change in the reserves profile.
Open Access
Outdoor performance of a CdTe based PV generator during 5 years of operation
(IEEE, 2022) Guerra Menjívar, Moisés Roberto; Parra Laita, Íñigo de la; Marcos Álvarez, Javier; García Solano, Miguel; Marroyo Palomo, Luis; Ingeniería Eléctrica, Electrónica y de Comunicación; Institute of Smart Cities - ISC; Ingeniaritza Elektrikoa, Elektronikoaren eta Telekomunikazio Ingeniaritzaren
Together with the huge growth of the traditional crystalline silicon (Si-x) PV manufacturers, other thin-film solar cells have also emerged such as cadmium telluride (CdTe) manufacturers. They are characterized by the fact that they were created to reduce costs and by the scarcity of silicon, from which the rest of the modules are made. Despite they need more space to generate the same amount of energy as crystalline modules, their price is supposed to be much lower, and argue that they have a better performance at high temperatures. However, real comparisons between the outdoor performance of CdTe and Si-x modules have been scarcely addressed in the literature. This paper provides a comparison under real operating conditions of a CdTe photovoltaic generator versus a conventional silicon generator during 5 years of operation in a mid-latitude area, identifying the causes of the differences observed.
Open Access
Long-term degradation rate of crystalline silicon PV modules at commercial PV plants: an 82-MWp assessment over 10 years
(Wiley, 2021) Pascual Miqueleiz, Julio María; Martínez Moreno, Francisco; García Solano, Miguel; Marcos Álvarez, Javier; Marroyo Palomo, Luis; Lorenzo Pigueiras, Eduardo; Ingeniaritza Elektrikoa, Elektronikoaren eta Telekomunikazio Ingeniaritzaren; Institute of Smart Cities - ISC; Ingeniería Eléctrica, Electrónica y de Comunicación
Due to high competitiveness in the PV sector, despite the low degradation rate of crystalline silicon PV modules (below 0.5%/year), it is still important for utilities to know its actual value due to its impact on energy yield and hence, profitability, over the lifetime of a PV plant. However, uncertainties related to both the influence of downtime periods due to problems that may appear under normal operation of a commercial PV plant and to the measurement of degradation rates at PV plant level make this a challenging task. In order to obtain a significant value, in this paper, three measuring methods with different uncertainty sources are used for 82 MWp of PV modules on different locations in Spain and Portugal over 10 years. According to the different methods used and PV plants analyzed, excluding PV plants with problems, a range of degradation rates between 0.01 and 0.47%/year has been found. The overall average value observed is 0.27%/year. The findings of this work have also revealed the great importance of good operation and maintenance practices in order to keep overall low degradation rates.
Open Access
High-dynamics P-E and Q-f control of PV inverters for strong and weak grids
(IEEE, 2023-08-31) Urtasun Salinas, Ibai; Urtasun Erburu, Andoni; Marroyo Palomo, Luis; Ingeniería Eléctrica, Electrónica y de Comunicación; Ingeniaritza Elektrikoa, Elektronikoa eta Telekomunikazio Ingeniaritza; Institute of Smart Cities - ISC
The increase of power electronic-based generators is causing the replacement of synchronous generators, which poses new challenges to electrical grid stability. In particular, when grid-following inverters are connected to weak grids, stability problems related to the PLL used for synchronization arise. To address this issue, grid-forming controls are widely proposed. However, the conventional implementations, such as droop control or virtual synchronous generator, lead to slow power controls, which are not suitable for photovoltaic systems with no storage. Thus, to improve the control dynamics, this paper proposes a new P-E and Q-f control. This control uses the reactive power for grid synchronization, avoiding the use of a PLL, and is valid for both inductive and resistive lines. Furthermore, thanks to the controller design developed in the paper, the control remains rapid and stable for very weak grids. Simulation results validates the control design and shows that the proposed control is much faster than the droop control for all types of grids.
Open Access
Control method to coordinate inverters and batteries for power ramp-rate control in large PV plants: minimizing energy losses and battery charging stress
(Elsevier, 2023) González Moreno, Alejandro; Marcos Álvarez, Javier; Parra Laita, Íñigo de la; Marroyo Palomo, Luis; Ingeniería Eléctrica, Electrónica y de Comunicación; Institute of Smart Cities - ISC; Ingeniaritza Elektrikoa, Elektronikoaren eta Telekomunikazio Ingeniaritzaren; Universidad Pública de Navarra / Nafarroako Unibertsitate Publikoa
This work presents a novel control method for multi-megawatt photovoltaic (PV) plants that is able to regulate each plant inverter and the battery system to mitigate PV power fluctuations. The proposed control method makes it possible to implement different PV ramp-rate control strategies based on the use of batteries and the limitation of inverters during positive fluctuations, which have been conceptually proposed in the specialized bibliography, but have omitted how to perform the coordination between PV generators. The dynamic model and the tuning of the control parameters are presented and the method is used to correctly implement different inverter-limitation strategies using 5-second data from a real 45 MWp PV plant. Furthermore, a new control strategy is proposed. This strategy reduces curtailment losses to negligible values and takes into account and addresses the intrinsic asymmetry in the battery charging and discharging capability, an issue that has been overlooked in the specialized bibliography. The results show that the proposed control method can effectively control each of the multiple inverters in order to obtain the desired PV plant operation to regulate the battery charging power, even during highly fluctuating scenarios.