Stability of photovoltaic and wind turbine grid-connected inverters for a large set of grid impedance values

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Stability of Photovoltaic and Wind Turbine Grid-Connected Inverters for a Large Set of Grid Impedance Values
Marco Liserre, Member, IEEE, Remus Teodorescu, Senior Member, IEEE, and Frede Blaabjerg, Fellow, IEEE
Abstract—The aim of this paper is to analyze the stability problems of grid connected inverters used indistributed generation. Complex controllers (e.g., multiple rotating dq-frames or resonantbased) are often required to compensate low frequency grid voltage background distortion and an LCL-filter is usually adopted for the high frequency one. The possible wide range of grid impedance values (distributed generation is suited for remote areas with radial distribution plants) challenge the stability andthe effectiveness of the LCL-filter-based current controlled system. It has been found out and it will be demonstrated in this paper that the use of active damping helps to stabilise the system in respect to many different kinds of resonances. The use of active damping results in an easy plug-in feature of the generation system in a vast range of grid conditions and in a more flexible operation ofthe overall system able to manage sudden grid changes. In the paper, a vast measurement campaign made on a single-phase system and on a three-phase system used as scale prototypes for photovoltaic and wind turbines, respectively, validate the analysis. Index Terms—LCL-filter, single-phase system.

I. INTRODUCTION HE increase of energy demand and the recent black-out problems in the U.S. andEurope are boosting the market and research attention toward distributed power generation systems like wind turbines, photovoltaic systems, fuel cells, micro gas turbines, small hydro units, and biomass units. The modern development of WT systems started in the 1980’s with sites of a few tens of kilowatts to multi-megawatt range wind turbines today. E.g., Denmark has a high penetration ( 20%) of windenergy in major areas of the country and in 2004 16% of the whole electrical energy consumption was covered by wind energy. A higher penetration level will even be seen in the near future. As the power range of the wind turbines increases the key parameters like control of active and reactive power become more and more important [1], [2]. Grid codes for WT are different from country to country.Moreover some of the countries have no codes at all. Hence, the development of new turbines is very difficult and the most important wind turbine producers tend to have a conservative approach in order to comply with all the standards worldwide
Manuscript received December 10, 2004; revised July 15, 2005. This work was supported by the Danish Research Council under Contract 2058-03-0003. Recommendedby Associate Editor K. Ngo. M. Liserre is with the Dipartimento di Elettrotecnica ed Elettronica, Politecnico di Bari, Bari 70125, Italy (e-mail: R. Teodorescu and F. Blaabjerg are with the Institute of Energy Technology—Power Electronic Systems, Aalborg University, Aalborg 9220, Denmark (e-mail:; Digital Object Identifier 10.1109/TPEL.2005.861185T

and to all the most likely ongoing changes in the near future [3]. -filter for the grid side and of a current Hence, the use of an controller which is able to perform harmonic compensation of the grid background distortion is almost mandatory [2]. The modern development of PV systems has been quite different with respect to WT systems. Until the 1990s, the creation of photovoltaic farms(in analogy to the wind farms) has been considered the preferred solution to increase the penetration of PV. However, centralized inverters adopted in the wind parks have been proven to be unreliable due to the loss of generation during inverter outages and mismatch of the strings. Hence, the string inverters, specifically designed for a system configuration of one string of PV modules, has emerged...
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