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CU-Boulder ECEN 4517 - Multilevel Converters

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Martina Calais Vassilios G. Agelidis Centre for Renewable Energy Systems Technology Australia (CRESTA) Curtin University of Technology GPO Box U1987, Perth 6845, Western Australia pcalaism9cc.curtin.edu.au Abstract - Multilevel voltage source inverters offer several ad- vantages compared to their conventional counterparts. By syn- thesising the AC output terminal voltage from several levels of voltages, staircase waveforms can be produced, which ap- proach the sinusoidal waveform with low harmonic distortion, thus reducing filter requirements. The need of several sources on the DC side of the converter makes multilevel technology attractive for photovoltaic applications. This paper provides an overview on different multilevel topologies and investigates their suitability for single-phase grid connected photovoltaic systems. Several transformer- less photovoltaic systems incorporating multilevel converters are compared regarding issues such as component count and stress, system power rating and the influence of the photo- voltaic array earth capacitance. I. INTRODUCTION Grid connected photovoltaic (PV) systems, in particular low power, mostly single-phase PV “rooftop” systems and their contribution to clean power generation is recognised more and more worldwide. Grid connected PV rooftop systems are generally privately owned, single-phase sys- tems in a power range of up to 10 kW. The main aim of a private operator who owns such a system is to max- imise its energy yield. Issues such as long life time (20 years and longer), high (part-load-) efficiency and good environmental conditions (availability of solar radiation) are hence of importance to the private operator. Other important requirements for these PV systems (see Fig. 1) are the fulfillment of standards concerning power quality, electromagnetic compatability, acoustic noise limitations as well as safety and protection requirements. The most important issues, however, for PV grid con- nected systems to gain wide acceptance are reliability and low cost. Figures from 1995 show that the operating inavail- ability of inverters for low power PV systems due to defects is 6 to 7 days per year [l], which compares unfavourably with household appliances such as refrigerators or washing machines. And today’s costs for commercially available low power sine-wave inverters for PV applications range from 0.9 to 3 USSNp where as drive converters in the same power range are available for 0.25-0.5 US$/Wp [2]. First commercially available grid connected PV inverters were line commutated inverters, followed by self commu- tated, puls width modulation inverters including either line or high frequency transformers, often incorporating several VAgelidis 9 curtin. edu. au Fig. 1. Issues regarding grid connected PV systems for the low power range. stages of power conversion [3]. Newest trends in this field are string based units with a power rating around 1 kW [4], [5] and transformerless concepts [6], [7], [8]. For larger systems the overall efficiency can be increased through application of several, small, string inverters replacing a single unit which avoids losses through module mismatch and decreases the DC wiring effort. Transformerless concepts (in particular inverters with high input voltages) are advantageous regarding their high efficiencies. Their peak efficiencies of up to 97% are equivalent to efficiencies reached in drives applications [5]. Avoiding the transformer has the additional benefits of reducing cost, size, weight and complexity of the inverter. However, the removal of the transformer and hence its isolation capability has to be considered carefully. Multilevel converter technology is based on the synthesis of the AC voltage from several different voltage levels on the DC bus. As the number of voltage levels on the DC side increases, the synthesised output waveform adds more steps, producing a staircase wave which approaches the sinusoidal wave with minimum harmonic distortion [9]. Multilevel converters are particularly interesting for high power applications such as FACTS since the need of filters is reduced and the efficiency is high because all devices switch at fundamental frequency [ 101, [ 111. In low power applications where switching frequencies are not as restricted as in high power applications various control methods such as multicarrier pulse width modulation or multiple hysteresis band control methods can be used to further reduce harmonics in the stepped waveforms [12], [ 133. Multilevel converter topologies are especially suitable for PV applications since due to the modular structure of PV arrays different DC voltage levels can easily be provided. This paper provides an overview on various multilevel 0-7803-4756-0/98/$10.00 1998 IEEE 224topologies which have been suggested or are considered for (transformerless), single-phase grid connected systems. Each topology is briefly described, listing advantages and disadvantages regarding issues such as component count and stress, system power rating and the influence of the photovoltaic array earth capacitance. Due to quick voltage and current transitions most power electronic equipment emits disturbances which propagate either by conduction or radiation. In transformerless systems additionally leakage currents due to the photovoltaic array earth capacitance can occur and increase electromagnetic emissions (both conducted and radiated). Since the paper focuses on transformerless systems the issue of leakage currents in transformerless photovoltaic systems will be discussed first. 11. LEAKAGE CURRENTS IN TRANSFORMERLESS PV SYSTEMS Avoiding the transformer in PV inverter topologies results in a galvanic connection of the grid and the PV array. Due to the capacitance between the PV array and earth, potential differences imposed on the capacitance through switching actions of the inverter inject a capacitive earth current. The PV array earth capacitance is then part of a resonant circuit consisting of the PV array, DC and AC filter elements and the grid impedance. Due to necessary efficiency optimisa- tion of PV systems the damping of this resonant circuit can be very small so that the earth current can reach amplitudes well above permissible levels. Also, the resonant frequency is not fixed due to the


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CU-Boulder ECEN 4517 - Multilevel Converters

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