Sensitivity Study of the Dynamics of Three-Phase Photovoltaic Inverters With an LCL Grid Filter

[EN] An accurate small-signal model of three-phase photovoltaic (PV) inverters with a high-order grid filter is derived in this paper. The proposed model takes into account the influence of both the inverter operating point and the PV panel characteristics on the inverter dynamic response. A sensitivity study of the control loops to variations of the dc voltage, PV panel transconductance, supplied power, and grid inductance is performed using the proposed small-signal model. Analytical and experimental results carried out on a 100-kW PV inverter are presented.Figueres Amorós, E.; Garcerá, G.; Sandia Paredes, J.; González Espín, FJ.; Calvo Rubio, J. (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.2008.2010175S70671756

PI-based controller for low-power distributed inverters to maximise reactive current injection while avoiding over voltage during voltage sags

This paper is a postprint of a paper submitted to and accepted for publication in IET Power Electronics and is subject to Institution of Engineering and Technology Copyright. The copy of record is available at the IET Digital Library.In the recently deregulated power system scenario, the growing number of distributed generation sources should be considered as an opportunity to improve stability and power quality along the grid. To make progress in this direction, this work proposes a reactive current injection control scheme for distributed inverters under voltage sags. During the sag, the inverter injects, at least, the minimum amount of reactive current required by the grid code. The flexible reactive power injection ensures that one phase current is maintained at its maximum rated value, providing maximum support to the most faulted phase voltage. In addition, active power curtailment occurs only to satisfy the grid code reactive current requirements. As well as, a voltage control loop is implemented to avoid overvoltage in non-faulty phases, which otherwise would probably occur due to the injection of reactive current into an inductive grid. The controller is proposed for low-power rating distributed inverters where conventional voltage support provided by large power plants is not available. The implementation of the controller provides a low computational burden because conventional PI-based control loops may apply. Selected experimental results are reported in order to validate the effectiveness of the proposed control scheme.Peer ReviewedPostprint (updated version

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

Advanced Solutions for Renewable Energy Integration into the Grid Addressing Intermittencies, Harmonics and Inertial Response

Numerous countries are trying to reach almost 100\% renewable penetration. Variable renewable energy (VRE), for instance wind and PV, will be the main provider of the future grid. The efforts to decrease the greenhouse gasses are promising on the current remarkable growth of grid connected photovoltaic (PV) capacity. This thesis provides an overview of the presented techniques, standards and grid interface of the PV systems in distribution and transmission level. This thesis reviews the most-adopted grid codes which required by system operators on large-scale grid connected Photovoltaic systems. The adopted topologies of the converters, the control methodologies for active - reactive power, maximum power point tracking (MPPT), as well as their arrangement in solar farms are studied. The unique L(LCL)2 filter is designed, developed and introduced in this thesis. This study will help researchers and industry users to establish their research based on connection requirements and compare between different existing technologies. Another, major aspect of the work is the development of Virtual Inertia Emulator (VIE) in the combination of hybrid energy storage system addressing major challenges with VRE implementations. Operation of a photovoltaic (PV) generating system under intermittent solar radiation is a challenging task. Furthermore, with high-penetration levels of photovoltaic energy sources being integrated into the current electric power grid, the performance of the conventional synchronous generators is being changed and grid inertial response is deteriorating. From an engineering standpoint, additional technical measures by the grid operators will be done to confirm the increasingly strict supply criteria in the new inverter dominated grid conditions. This dissertation proposes a combined virtual inertia emulator (VIE) and a hybrid battery-supercapacitor-based energy storage system . VIE provides a method which is based on power devices (like inverters), which makes a compatible weak grid for integration of renewable generators of electricity. This method makes the power inverters behave more similar to synchronous machines. Consequently, the synchronous machine properties, which have described the attributes of the grid up to now, will remain active, although after integration of renewable energies. Examples of some of these properties are grid and generator interactions in the function of a remote power dispatch, transients reactions, and the electrical outcomes of a rotating bulk mass. The hybrid energy storage system (HESS) is implemented to smooth the short-term power fluctuations and main reserve that allows renewable electricity generators such as PV to be considered very closely like regular rotating power generators. The objective of utilizing the HESS is to add/subtract power to/from the PV output in order to smooth out the high frequency fluctuations of the PV power, which may occur due to shadows of passing cloud on the PV panels. A control system designed and challenged by providing a solution to reduce short-term PV output variability, stabilizing the DC link voltage and avoiding short term shocks to the battery in terms of capacity and ramp rate capability. Not only could the suggested system overcome the slow response of battery system (including dynamics of battery, controller, and converter operation) by redirecting the power surges to the supercapacitor system, but also enhance the inertial response by emulating the kinetic inertia of synchronous generator

Dynamic modeling of DC-DC converters with peak current control in double-stage photovoltaic grid-connected inverters

In photovoltaic (PV) double-stage grid-connected inverters a high-frequency DC-DC isolation and voltage step-up stage is commonly used between the panel and the grid-connected inverter. This paper is focused on the modeling and control design of DC-DC converters with Peak Current mode Control (PCC) and an external control loop of the PV panel voltage, which works following a voltage reference provided by a maximum power point tracking (MPPT) algorithm. In the proposed overall control structure the output voltage of the DC-DC converter is regulated by the grid-connected inverter. Therefore, the inverter may be considered as a constant voltage load for the development of the small-signal model of the DC-DC converter, whereas the PV panel is considered as a negative resistance. The sensitivity of the control loops to variations of the power extracted from the PV panel and of its voltage is studied. The theoretical analysis is corroborated by frequency response measurements on a 230 W experimental inverter working from a single PV panel. The inverter is based on a Flyback DC-DC converter operating in discontinuous conduction mode (DCM) followed by a PWM full-bridge single-phase inverter. The time response of the whole system (DC-DC + inverter) is also shown to validate the concept. Copyright © 2011 John Wiley & Sons, Ltd. In photovoltaic (PV) double-stage gridconnected inverters a high-frequency DC-DC isolation and voltage step-up stage is commonly used between the panel and the grid-connected inverter. This paper is focused on the modeling and control design of DC-DC converters with Peak Current mode Control (PCC) and an external control loop of the PV panel voltage, which works following a voltage reference provided by a maximum power point tracking (MPPT) algorithm. The sensitivity of the control loops to variations of the power extracted from the PV panel and of its voltage is studied. Copyright © 2011 John Wiley & Sons, Ltd. Copyright © 2011 John Wiley & Sons, Ltd.This work was supported by the Spanish Ministry of Science and Innovation (MICINN) under grant ENE2009-13998-C02-02. The company AUSTRIAMICROSYSTEMS co-financed this project.Garcerá Sanfeliú, G.; González Medina, R.; Figueres Amorós, E.; Sandía Paredes, J. (2012). Dynamic modeling of DC-DC converters with peak current control in double-stage photovoltaic grid-connected inverters. International Journal of Circuit Theory and Applications. 40(8):793-813. https://doi.org/10.1002/cta.756S793813408Carrasco, J. M., Franquelo, L. G., Bialasiewicz, J. T., Galvan, E., PortilloGuisado, R. C., Prats, M. A. M., … Moreno-Alfonso, N. (2006). Power-Electronic Systems for the Grid Integration of Renewable Energy Sources: A Survey. IEEE Transactions on Industrial Electronics, 53(4), 1002-1016. doi:10.1109/tie.2006.878356Kjaer, S. B., Pedersen, J. K., & Blaabjerg, F. (2005). A Review of Single-Phase Grid-Connected Inverters for Photovoltaic Modules. IEEE Transactions on Industry Applications, 41(5), 1292-1306. doi:10.1109/tia.2005.853371Ridley, R. B. (1991). A new, continuous-time model for current-mode control (power convertors). IEEE Transactions on Power Electronics, 6(2), 271-280. doi:10.1109/63.76813Femia, N., Petrone, G., Spagnuolo, G., & Vitelli, M. (2005). Optimization of Perturb and Observe Maximum Power Point Tracking Method. IEEE Transactions on Power Electronics, 20(4), 963-973. doi:10.1109/tpel.2005.850975Hua, C., & Lin, J. (2004). A modified tracking algorithm for maximum power tracking of solar array. Energy Conversion and Management, 45(6), 911-925. doi:10.1016/s0196-8904(03)00193-6Tan, Y. T., Kirschen, D. S., & Jenkins, N. (2004). A Model of PV Generation Suitable for Stability Analysis. IEEE Transactions on Energy Conversion, 19(4), 748-755. doi:10.1109/tec.2004.827707Femia, N., Petrone, G., Spagnuolo, G., & Vitelli, M. (2009). A Technique for Improving P&O MPPT Performances of Double-Stage Grid-Connected Photovoltaic Systems. IEEE Transactions on Industrial Electronics, 56(11), 4473-4482. doi:10.1109/tie.2009.2029589Chiu, H.-J., Huang, H.-M., Yang, H.-T., & Cheng, S.-J. (2008). An improved single-stage Flyback PFC converter for high-luminance lighting LED lamps. International Journal of Circuit Theory and Applications, 36(2), 205-210. doi:10.1002/cta.404Chiu, H.-J., Yao, C.-J., & Lo, Y.-K. (2009). A DC/DC converter topology for renewable energy systems. International Journal of Circuit Theory and Applications, 37(3), 485-495. doi:10.1002/cta.475Martins DC Demonti R Photovoltaic Energy Processing for Utility Connected System 1292 1296 10.1109/IECON.2001.975968www.focus.ti.com/lit/ml/slup127/slup127.pdf2003 http://www.fairchildsemi.comEsram, T., & Chapman, P. L. (2007). Comparison of Photovoltaic Array Maximum Power Point Tracking Techniques. IEEE Transactions on Energy Conversion, 22(2), 439-449. doi:10.1109/tec.2006.874230Liserre, M., Blaabjerg, F., & Hansen, S. (2005). Design and Control of an LCL-Filter-Based Three-Phase Active Rectifier. IEEE Transactions on Industry Applications, 41(5), 1281-1291. doi:10.1109/tia.2005.853373Liserre, M., Teodorescu, R., & Blaabjerg, F. (2006). Stability of photovoltaic and wind turbine grid-connected inverters for a large set of grid impedance values. IEEE Transactions on Power Electronics, 21(1), 263-272. doi:10.1109/tpel.2005.861185Figueres, 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.2008.2010175Ciobotaru M Teodorescu R Blaabjerg F Control of single-stage single-phase PV inverter P.1 P.10 10.1109/EPE.2005.219501Zmood, D. N., & Holmes, D. G. (2003). Stationary frame current regulation of PWM inverters with zero steady-state error. IEEE Transactions on Power Electronics, 18(3), 814-822. doi:10.1109/tpel.2003.810852Castilla, M., Miret, J., Matas, J., Garcia de Vicuna, L., & Guerrero, J. M. (2009). Control Design Guidelines for Single-Phase Grid-Connected Photovoltaic Inverters With Damped Resonant Harmonic Compensators. IEEE Transactions on Industrial Electronics, 56(11), 4492-4501. doi:10.1109/tie.2009.2017820Timbus A Teodorescu R Blaabjerg F Liserre M Synchronization methods for three phase distributed power generation systems 2474 2481 10.1109/PESC.2005.1581980Vorperian, V. (1990). Simplified analysis of PWM converters using model of PWM switch. II. Discontinuous conduction mode. IEEE Transactions on Aerospace and Electronic Systems, 26(3), 497-505. doi:10.1109/7.106127Reatti A Balzani M PWM switch model of a buck-boost converter operated under discontinuous conduction mode 667 670 10.1109/MWSCAS.2005.1594189Reatti, A., & Kazimierczuk, M. K. (2003). Small-signal model of PWM converters for discontinuous conduction mode and its application for boost converter. IEEE Transactions on Circuits and Systems I: Fundamental Theory and Applications, 50(1), 65-73. doi:10.1109/tcsi.2002.805709Lin, B.-R., Huang, C.-L., & Li, M.-Y. (2009). Novel interleaved ZVS converter with ripple current cancellation. International Journal of Circuit Theory and Applications, 37(3), 413-431. doi:10.1002/cta.480MIDDLEBROOK, R. D. (1975). Measurement of loop gain in feedback systems†. International Journal of Electronics, 38(4), 485-512. doi:10.1080/0020721750892042

Reduction of Current Harmonic Distortion in Three-Phase Grid-Connected Photovoltaic Inverters via Resonant Current Control

The resonant current control has been extensively employed to reduce the current harmonic distortion in a wide range of grid-connected distributed generation applications, including photovoltaic (PV) inverters, wind and water turbines, and fuel-cell inverters. However, the performance of these systems is deteriorated when the utility grid voltage experiences abnormal conditions such as voltage harmonics and imbalances. Several advanced control solutions have been recently introduced to cope with this problem but at the cost of a significant increase in the control computational load. This paper first analyzes the limitations of the standard resonant current control operating under abnormal grid conditions and then introduces a control scheme that improves the current harmonic distortion in such adverse conditions without increasing the computational load of the standard current control. This theoretical contribution is validated by means of selected experimental results from a three-phase PV inverterPostprint (published version

Mitigation of DC Current Injection in Transformerless Grid-Connected Inverters

PhD ThesisWith a large number of small-scale PV plants being connected to the utility grid, there is increasing interest in the use of transformerless systems for grid-connected inverter photovoltaic applications. Compared to transformer-coupled solutions, transformerless systems offer a typical efficiency increase of 1-2%, reduced system size and weight, and reductions in cost. However, the removal of the transformer has technical implications. In addition to the loss of galvanic isolation, DC current injection into the grid is a potential risk. Whilst desirable, the complete mitigation of DC current injection via conventional current control methods is known to be particularly challenging, and there are remaining implementation issues in previous studies. For this reason, this thesis aims to minimize DC current injection in grid-connected transformerless PV inverter systems. The first part of the thesis reviews the technical challenges and implementation issues in published DC measurement techniques and suppression methods. Given mathematical models, the performance of conventional current controllers in terms of DC and harmonics mitigation is analyzed and further confirmed in simulations and experiments under different operating conditions. As a result, the second part of the thesis introduces two DC suppression methods, a DC voltage mitigation approach and a DC link current sensing technique. The former method uses a combination of a passive attenuation circuit and a software filter stage to extract the DC voltage component, which allows for further digital control and DC component mitigation at the inverter output. It is proven to be a simple and highly effective solution, applicable for any grid-connected PV inverter systems. The DC link sensing study then investigates a control-based solution in which the dc injection is firstly accurately determined via extraction of the line frequency component from the DC link current and then mitigated with a closed loop. With an output current reconstruction process, this technique provides robust current control and effective DC suppression based on DC link current measurement, eliminating the need for the conventional output current sensor. Results from rated simulation models and a laboratory grid-connected inverter system are presented to demonstrate the accurate and robust performance of the proposed techniques. This thesis makes a positive contribution in the area of power quality control in grid-connected inverters, specifically mitigating the impact of DC injection into the grid which has influences on the network operating conditions and the design and manufacture of the PV power converter itsel

Variable structure control in natural frame for three-phase grid-connected inverters with LCL filter

This paper presents a variable structure control in natural frame for a three-phase voltage source inverter. The proposed control method is based on modifying the converter model in natural reference frame, preserving the low frequency state space variables dynamics. Using this model in a Kalman filter, the system state-space variables are estimated allowing to design three robust current sliding-mode controllers in natural frame. The main closed-loop features of the proposed method are: 1) robustness against grid inductance variations because the proposed model is independent of the grid inductance, 2) the power losses are reduced since physical damping resistors are avoided, and 3) the control bandwidth can be increased due to the combination of a variable hysteresis controller with a Kalman filter. To complete the control scheme, a theoretical stability analysis is developed. Finally, selected experimental results validate the proposed control strategy and permit illustrating all its appealing features.Postprint (author's final draft

Towards the Development of High-Fidelity Models for Large Scale Solar Energy Generating Systems

Small and large scale solar photovoltaic energy generating systems have been observed to take a leading place in power systems around the world which are aiming to move away from the use of fossil fuels. Technical and other challenges associated with such systems have become the focus areas of discussion and investigation in recent years. Among a range of technical challenges, power quality issues associated with the power electronic converters, especially the harmonics, are an important aspect in order to ensure that their stipulated limits are maintained. While harmonics caused by small-scale inverters, for example, those used in rooftop systems, are managed through their harmonic current emission compliance requirements, the harmonics caused by large scale inverters used in solar farms need to be managed at network levels which is essentially the responsibility of the network owners and operators. To be successful in this management process, the relevant generator connection requirements and system standards, relevant data provided by inverter manufacturers, pre-connection and post-connection studies and procedures require attention. With regard to limits associated with harmonic voltage levels at medium, high and extra high voltage (MV, HV and EHV) levels, well-established international standards exist, whereas the pre-connection study procedures which have existed for many years are now being challenged, noting the increase in the number and capacity of inverter based resources (IBRs). With regard to pre-connection harmonic compliance studies associated with power electronic based grid integrated resources or devices, the most well-known approach is the use of equivalent frequency domain models of the systems on either side of the point of connection or the grid interface. The grid is often represented by an equivalent harmonic impedance together with a corresponding background harmonic voltage. The power electronic based resources or the devices are represented by Thevenin or Norton models at the harmonic frequencies of interest, which are provided by their vendors where the approaches or the conditions under which these models are determined are not comprehensively known. It is however understood that the parameters of such equivalent circuits are mostly determined based on site tests and represent worst case harmonic performance, which do not necessarily correspond to rated power output. There is also the anecdotal understanding that such models are determined based on mathematical or simulation modelling. The most significant concern associated with such frequency domain models is their suitability for representation of the actual harmonic behaviour at a given point in time, thus posing the question of their fidelity which forms the backbone of the work presented in this thesis