Modelling and Control of Parallel-Connected Transformerless Inverters for Large Photovoltaic Farms

[EN] This paper presents a control structure for transformerless photovoltaic inverters connected in parallel to manage photovoltaic fields in the MW range. Large photovoltaic farms are usually divided into several photovoltaic fields, each one of them managed by a centralized high power inverter. The current tendency to build up centralized inverters in the MW range is the use of several transformerless inverters connected in parallel, a topology that provokes the appearance of significant zero-sequence circulating currents among inverters. To eliminate this inconvenience, this paper proposes a control structure that avoids the appearance of circulating currents by controlling the zero-sequence component of the inverters. A second contribution of the paper is the development of a model of n parallel-connected inverters. To validate the concept, the proposed control structure has been applied to a photovoltaic field of 2 MW managed by four 500 kW photovoltaic inverters connected in parallel.This work is supported by the Spanish Ministry of Economy and Competitiveness (MINECO), the European Regional Development Fund (ERDF) under Grant ENE2015-64087-C2-2-R and the Spanish Ministry of Education (FPU15/01274).Liberos-Mascarell, MA.; González-Medina, R.; Garcerá, G.; Figueres Amorós, E. (2017). Modelling and Control of Parallel-Connected Transformerless Inverters for Large Photovoltaic Farms. Energies. 10(8):1-25. https://doi.org/10.3390/en10081242S125108Pazheri, F. R., Othman, M. F., & Malik, N. H. (2014). A review on global renewable electricity scenario. Renewable and Sustainable Energy Reviews, 31, 835-845. doi:10.1016/j.rser.2013.12.020Subudhi, B., & Pradhan, R. (2013). A Comparative Study on Maximum Power Point Tracking Techniques for Photovoltaic Power Systems. IEEE Transactions on Sustainable Energy, 4(1), 89-98. doi:10.1109/tste.2012.2202294Borrega, M., Marroyo, L., Gonzalez, R., Balda, J., & Agorreta, J. L. (2013). Modeling and Control of a Master–Slave PV Inverter With N-Paralleled Inverters and Three-Phase Three-Limb Inductors. IEEE Transactions on Power Electronics, 28(6), 2842-2855. doi:10.1109/tpel.2012.2220859Araujo, S. V., Zacharias, P., & Mallwitz, R. (2010). Highly Efficient Single-Phase Transformerless Inverters for Grid-Connected Photovoltaic Systems. IEEE Transactions on Industrial Electronics, 57(9), 3118-3128. doi:10.1109/tie.2009.2037654PowerGate Plus 500 kWhttp://www.satcon.comAgorreta, J. L., Borrega, M., López, J., & Marroyo, L. (2011). Modeling and Control of $N$ -Paralleled Grid-Connected Inverters With LCL Filter Coupled Due to Grid Impedance in PV Plants. IEEE Transactions on Power Electronics, 26(3), 770-785. doi:10.1109/tpel.2010.2095429Power Electronicshttp://www.power-electronics.comPVS980—1818 to 2091 kVAhttp://new.abb.comInfineon, Central Inverter Solutionshttps://www.infineon.com/cms/en/applications/solar-energy-systems/central-inverter-solutions/Xiao, H., Xie, S., Chen, Y., & Huang, R. (2011). An Optimized Transformerless Photovoltaic Grid-Connected Inverter. IEEE Transactions on Industrial Electronics, 58(5), 1887-1895. doi:10.1109/tie.2010.2054056Mazumder, S. K. (2003). A novel discrete control strategy for independent stabilization of parallel three-phase boost converters by combining space-vector modulation with variable-structure control. IEEE Transactions on Power Electronics, 18(4), 1070-1083. doi:10.1109/tpel.2003.813770Ching-Tsai Pan, & Yi-Hung Liao. (2008). Modeling and Control of Circulating Currents for Parallel Three-Phase Boost Rectifiers With Different Load Sharing. IEEE Transactions on Industrial Electronics, 55(7), 2776-2785. doi:10.1109/tie.2008.925647Ogasawara, S., Takagaki, J., Akagi, H., & Nabae, A. (1992). A novel control scheme of a parallel current-controlled PWM inverter. IEEE Transactions on Industry Applications, 28(5), 1023-1030. doi:10.1109/28.158825Figueres, E., Garcera, G., Sandia, J., Gonzalez-Espin, F., & Rubio, J. C. (2009). Sensitivity Study of the Dynamics of Three-Phase Photovoltaic Inverters With an LCL Grid Filter. IEEE Transactions on Industrial Electronics, 56(3), 706-717. doi:10.1109/tie.2008.2010175Mohd, A., Ortjohann, E., Hamsic, N., Sinsukthavorn, W., Lingemann, M., Schmelter, A., & Morton, D. (2010). Control strategy and space vector modulation for three-leg four-wire voltage source inverters under unbalanced load conditions. IET Power Electronics, 3(3), 323. doi:10.1049/iet-pel.2008.0281Albatran, S., Fu, Y., Albanna, A., Schrader, R., & Mazzola, M. (2013). Hybrid 2D-3D Space Vector Modulation Voltage Control Algorithm for Three Phase Inverters. IEEE Transactions on Sustainable Energy, 4(3), 734-744. doi:10.1109/tste.2013.224568

A Control Scheme to Suppress Circulating Currents in Parallel-Connected Three-Phase Inverters

[EN] The parallel operation of inverters has many benefits, such as modularity and redundancy. However, the parallel connection of inverters produces circulating currents that may result in malfunctions of the system. In this work, a control technique for the elimination of the low-frequency components of the circulating currents in grid-connected inverters is presented. The proposed control structure contains n - 1 zero-sequence control loops, with n being the number of inverters connected in parallel. Simulation and experimental results have been carried out on a prototype composed of two 5 kW inverters connected in parallel. The results have been obtained by considering the following mismatches between both inverters: inductance values of the grid filters, unbalance of the delivered power, and the use of different modulation techniques.This research was funded by the Spanish "Ministerio de Asuntos Economicos y Transformacion Digital" and the European Regional Development Fund (ERDF), under grants RTI2018100732-B-C21 and PID2021-122835OB-C22.Liberos, M.; González-Medina, R.; Patrao Herrero, I.; Garcerá, G.; Figueres Amorós, E. (2022). A Control Scheme to Suppress Circulating Currents in Parallel-Connected Three-Phase Inverters. Electronics. 11(22):1-23. https://doi.org/10.3390/electronics11223720123112

DC-Bus Signaling control laws for the operation of DC-microgrids with renewable power sources

[EN] DC-Bus Signaling (DBS) is a proven method to coordinate different microgrid agents, using the DC voltage of the microgrid as a communication signal. The droop control applied in a DC microgrid achieves accurate power-sharing among converters while leading to a certain voltage regulation error in the microgrid bus. The behaviour of the different agents in a microgrid is managed by using the voltage at each node of the microgrid as the DBS signal. The technique proposed in this paper uses an improved DBS technique to coordinate interlinking converters, photovoltaic generators, Energy Storage Systems and loads in a microgrid. The operation of the renewable power sources of a microgrid at the full generated power is desired due to economic and environmental reasons. The DBS technique proposed in this paper is adapted to integrate renewable power sources in the microgrid. The DBS-controlled Energy Storage Systems (ESS) will store the surplus energy if the generated power exceeds consumption. In the case of fully charged ESS, the renewable generators will limit their output power to those demanded by the loads. The proposed control laws have been tested in an experimental microgrid.This research was funded by the Spanish Ministerio de Asuntos Económicos y Transformación Digital and the European Regional Development Fund (ERDF) under Grants RTI2018-100732-B-C21 and PID2021-122835OB-C22.Patrao Herrero, I.; Torán, E.; González-Medina, R.; Liberos, M.; Figueres Amorós, E.; Garcerá, G. (2023). DC-Bus Signaling control laws for the operation of DC-microgrids with renewable power sources. IEEE Journal of Emerging and Selected Topics in Industrial Electronics. https://doi.org/10.1109/JESTIE.2023.334579

Small-Signal Model of the NPC + GCC Multilevel Transformerless Inverter in Single-Phase Photovoltaic Power Systems

[EN] Photovoltaic transformerless inverters are very efficient and economical options for solar-power generation. The absence of the isolation transformer improves the converters' efficiency, but high-frequency voltage to ground can appear in the photovoltaic string poles. The high capacitance to ground of the photovoltaic generator leads to undesirable high-leakage currents. Using half-bridge topologies dramatically reduces the leakage to ground, and using a multilevel half-bridge inverters improves the output quality compared with classical inverters. The neutral point clamped + generation control circuit (NPC + GCC) topology is a multilevel single-phase transformerless inverter capable of tracking the maximum power point of two photovoltaic sources at the same time. This paper presents the control structure and the dynamic modeling of the NPC + GCC inverter. The pulse-width modulated (PWM) switch model in continuous conduction mode (CCM) was used to obtain the small-signal model of the two switching converters that make up the inverter. The resulting dynamic model was used to quantify the stability margins of both converters' current and voltage loops.This research was funded by the Spanish "Ministerio de Asuntos Economicos y Transformacion Digital" and the European Regional Development Fund (ERDF), under grant PID2021-122835OB-C22.Patrao Herrero, I.; Liberos, M.; González-Medina, R.; Torán, E.; Figueres Amorós, E.; Garcerá, G. (2023). Small-Signal Model of the NPC + GCC Multilevel Transformerless Inverter in Single-Phase Photovoltaic Power Systems. Electronics. 12(17):1-19. https://doi.org/10.3390/electronics12173545119121

A Control Stage for Parallel-Connected Interlinking Converters in Hybrid AC-DC Microgrids

[EN] Having two or more interlinking converters connected in parallel in hybrid microgrids has some benefits, like modularity, flexibility, and redundancy. However, the parallelization of the inverters leads to circulating currents that can cause system malfunctions. This work uses a method for suppressing low-frequency circulating currents in interlinking converters by controlling the zero-sequence component of the phase currents, showing that the control structure is valid for interlinking converters. The proposed control scheme has been applied to two parallel interlinking inverters of 5 kW and 2.5 kW, respectively. The interlinking inverters are connected to the grid, and they control the voltage in the DC bus of the hybrid microgrid. To validate the concept, simulation and experimental results are shown.This work was supported by the Spanish "Ministerio de Ciencia e Innovacion" and the European Regional Development Fund (ERDF) under Grant RTI2018-100732-B-C21 and Grant PID2021-122835OB-C22.Liberos, M.; González-Medina, R.; Patrao Herrero, I.; Torán, E.; Garcerá, G.; Figueres Amorós, E. (2023). A Control Stage for Parallel-Connected Interlinking Converters in Hybrid AC-DC Microgrids. IEEE Access. 11:61800-61812. https://doi.org/10.1109/ACCESS.2023.328656961800618121

Performance Evaluation of the B4 Topology for Implementing Grid-Connected Inverters in Microgrids

[EN] The B4 topology is an interesting alternative to the conventional B6 inverter due to its reduced number of parts and lower cost. Although it has been widely used in the past, especially in low-power motor drive applications, its application as a grid-connected inverter is an open area of research. In this regard, this paper analyses the feasibility of the B4 inverter topology for grid-connected applications. A versatile 7 kW inverter prototype, which may be configured as B4 and B6, was built, allowing for a comprehensive evaluation of the performance of both topologies. Through an analytical study and experimental tests, the performance of the B4 and B6 topologies was comparatively evaluated in terms of efficiency, total harmonic distortion of line currents, current unbalance, cost, and mean time between failures. The study was carried out in the context of microgrid systems, highlighting their role in the integration of renewable energy and distributed generation.This research was funded by the Spanish Ministerio de Ciencia e Innovación and the European Regional Development Fund (ERDF) under Grant PID2021-122835OB-C22.Torán, E.; Liberos, M.; Patrao Herrero, I.; González-Medina, R.; Garcerá, G.; Figueres Amorós, E. (2024). Performance Evaluation of the B4 Topology for Implementing Grid-Connected Inverters in Microgrids. Electronics. 13(9). https://doi.org/10.3390/electronics1309175513

A Control Scheme to Suppress Circulating Currents in Parallel-Connected Three-Phase Inverters

The parallel operation of inverters has many benefits, such as modularity and redundancy. However, the parallel connection of inverters produces circulating currents that may result in malfunctions of the system. In this work, a control technique for the elimination of the low-frequency components of the circulating currents in grid-connected inverters is presented. The proposed control structure contains n − 1 zero-sequence control loops, with n being the number of inverters connected in parallel. Simulation and experimental results have been carried out on a prototype composed of two 5 kW inverters connected in parallel. The results have been obtained by considering the following mismatches between both inverters: inductance values of the grid filters, unbalance of the delivered power, and the use of different modulation techniques

Experimental Study of the Parameter Mismatch Effects on the Low Frequency Circulating Currents of Parallel Three Phase Inverters

When converters are connected in parallel, a system with some benefits, including modularity and redundancy, is obtained. However, in these circumstances, circulating currents can appear that produce some adverse effects. In this work, a study of the low-frequency circulating currents that appear in three-phase inverters connected in parallel is performed. The study is focused on the effects produced by the parameter mismatch, namely inductance mismatches, power imbalance, and the use of different pulse with modulation (PWM) techniques. The nature of the circulating current produced by each of these factors were analyzed separately. Both simulation and experimental results are shown, which were obtained using a three-phase 10-kW prototype composed of two 5-kW inverters connected in parallel