Inverters for Single-phase Grid Connected Photovoltaic Systems - An Overview Martina Calais’ Johanna Myrzik2 Ted Spoone? Vassilios G. Agelidis4 School of Engineering, Murdoch University, Murdoch WA 6150, Australia Technical University of Eindhoven, The Netherlands School of Electrical Engineering and Telecommunications, University of New South Wales, Australia * Inter-University Centre for Economic Renewable Power Delivery (CERPD), University of Glasgow, U.K. Abstract - An overview on recent developments and a summary of the state-of-the-art of inverter technology in Europe for single- phase grid-connected Photovoltaic (PV) systems for power levels up to 5 kW is provided in this paper. The information includes details not only on the topologies commercially available but also on the switching devices employed and the associated switching frequencies, efficiency, price trends and market share. Finally, the paper outlines issues associated with the development of relevant international industry standards affecting PV inverter technology. I. INTRODUCTION The continuing decrease of the cost of the PV’s, the ad- vancement of power electronic and semiconductor technology and favourable incentives in a number of industrial countries in general had a profound impact on the commercial accep- tance of grid connected PV systems in the recent years. A core technology associated with these systems remains the inverter, which has evolved to quite mature technology offering a num- ber of advantages to customers that were not possible many years ago. The technology has changed from line commutated inverters to switch mode ones mainly due to the availability of high frequency fully-controlled switching devices. Most inverters on the market in the mid 1990’s were self or line commutated central inverters, with DC power ratings above 1 kW, suitable for PV system configurations with sev- eral strings in parallel as shown in Fig. 1. During the 1000 Roofs Program, a subsidy program of the German Federal and State Governments (which was accompanied by an extensive measurement and analysis program [ 1,2]) the disadvantages of central inverters became apparent. These include complete loss of generation during inverter outages and losses due to the mis- match of strings. String inverters, which are designed for a Rated ~tcurrem w) Fig. 1. PV invertem available in 1994 and 2002 shown versus DC voltage and DC current ratings. system configuration of one string of PV modules (see Fig. 1) have since become more popular. Module integrated or module oriented PV inverters with rated power below 500 W can be classified as a third group of PV inverters beside central and string inverters (see Fig. 1). They have been available on the market since the mid 1990’s and their modularity allows for small, simple systems which can easily be expanded by paralleling more “AC-modules”. The concept allows mismatch and losses due to shading to be reduced even further than with string inverters. A recent development is the multi string PV inverter con- cept, where several DC to DC converters are connected to one central inverter. Unlike the string inverter concept, the multi string inverter requires only one central inverter for all supervi- sory and protection functions. [3,4,51 The topologies used in the different PV system concepts are described and discussed in Section I1 of this paper. Information on employed switching devices and switching frequencies, ef- ficiencies, price trends and market shares is included as well. Section 111 provides some background information on the de- velopments on standards affecting this technology, Finally con- clusions are summarised in Section IV. 11. INVERTER TOPOLOGIES An inverter has to fulfill three functions in order to feed energy from a PV array into the utility grid: 1. To shape the current into a sinusoidal waveform; 2. To invert the current into an AC current, and 3. if the PV array voltage is lower than the grid voltage, the PV array voltage has to be boosted with a further element. The way these three functions are sequenced within an inverter design determines the choice of semiconductors and passive components and consequently their losses, sizes and prices. This section discusses different inverter topologies available on the European market and gives an overview on their market shares, efficiencies and price developments over the last decade.[6,7, 8,9, 10, 11,3, 12, 131 A. Central inverters Based on drive system technology the first PV inverters at the end of the 1980’s were line commutated inverters (see Fig. 2(b)) with power ratings of several kilo watts. Although these topologies are robust, highly efficient and cheap, their major drawbacks are a power factor between 0.6 and 0.7 [14], 0-7803-7262-X/02/$10.00 Q 2002 LEB. 1995Inverting element + Current me shaping Inverting element + Voltage Current weve shaping adjustment Full Bridge Utility yv< 850 v Inverter Grid L I DC Filter (b) Thyristor Bridge Full Bridge Line Frequency Utilii %V< 400 v inverter Transformer Grid I 1 I Utility Grid r I 114 Fig. 2. Transformerless PV inverters (a) step down, (b) line commutated. which has to be compensated with special filters as well as high harmonic content in-the output current. Due to the rapid developments in the semiconductor device industry, thyristors have been increasingly replaced by BJT's, MOSFET's or IGBT's. Currently employed switching devices in PV inverters are shown in Fig. 4. Today central inverters are mostly self- commutated inverters in the power range above 2 kW. Their topologies without and with transformer are shown in Fig. 2(a) and 3(a). They are composed of a PWM full bridge, switching at high frequencies (> 16 kHz) which shapes and inverts the input current into an AC current. Most of the bridges use IGBT's or a combination of IGBT's and MOSFET's (see Fig. 4). This concept is a well known, robust, efficient and cheap technology which provides high reliability and low price per watt. Their efficiencies are lower than in line commutated concepts (see Fig. 9) due to the high switching frequencies of Fig. 3(b) shows a magnetic coupled inverter [15] available on the American market. The inverter consists of three conventional single-phase full bridges each with their mid- points connected to the primary winding of a transformer. The secondary windings of the transformers
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