- Email: [email protected]
Multi-string-converter with reduced specific costs and enhanced functionality
Pergamon
PII:
Solar Energy Vol. 69(Suppl.), Nos. 1–6, pp. 217–227, 2000 2001 Elsevier Science Ltd S 0 0 3 8 – 0 9 2 X ( 0 1 ) 0 0 0 6 7 – 6 All rights reserved. Printed in Great Britain 0038-092X / 00 / $ - see front matter
www.elsevier.com / locate / solener
MULTI-STRING-CONVERTER WITH REDUCED SPECIFIC COSTS AND ENHANCED FUNCTIONALITY ¨ MIKE MEINHARDT † , * ,‡ , GUNTHER CRAMER* ,‡ , BRUNO BURGER** and PETER ZACHARIAS** *SMA Regelsysteme GmbH, Hannoversche Straße 1-5, D-34266 Niestetal, Germany ¨ Solare Energieversorgungstechnik e.V., Konigstor ¨ **Institut fur 59, D-34119 Kassel, Germany Received 20 June 2000; revised version accepted 21 May 2001 Communicated by KLAUS PREISER
Abstract—The development of a PV-converter based on the advanced Multi-String concept results in significantly reduced specific costs while still profiting from the well-known advantages of the string-converter technology developed by ISET and SMA in the mid-1990s. The paper deals with the basic considerations from a system’s point of view that resulted in the development of the Multi-String-converter. The features of the Multi-String-converter concerning operational behaviour and PV-system design are described. Due to enhanced capacity of the operational control unit the Multi-String-converter can be used additionally for active compensation of harmonics and reactive power in order to improve the power quality. An evaluation of Multi-String-converters in comparison with string-converters or conventional central-converter concepts is presented. The selected topology and the control strategy for the Multi-String-converter are introduced. 2001 Elsevier Science Ltd. All rights reserved.
can be achieved primarily by increasing the nominal power of the PV-converter. Fig. 1 shows the specific PV-converter costs as a function of the nominal power of the converter unit including state-of-the-art string- and central PV-converter as well as the first and second generation of the new multi-string-converter introduced in the present paper. Costs of the state-of-the-art converters are taken from a market survey published in Kreuzmann (2001).
1. INTRODUCTION
1.1. Prices of PV-converters must come down further The continuously decreasing prices for PVmodules result in the fact that the reduction of the specific PV-converter costs (» / W) is becoming more and more important. A wider usage of photovoltaic energy systems requires a further reduction of the specific costs while keeping the very high standards that PV-converters have reached nowadays regarding safety, efficiency, reliability, electromagnetic compatibility and functionality (e.g. grid monitoring functions). String oriented PV-converters developed by ISET (Germany) and SMA Regelsysteme GmbH (Germany) have been very successful in the past decade due to their economical and technological advantages (e.g. reduced DC-installation, local MPP-tracking). Nevertheless the reduction of the specific costs of state-of-the-art PV-converters is still possible.
1.3. A new PV-converter concept is needed
1.2. Cost reduction due to increased nominal power of converter unit
Due to the limited isolation of PV-modules and voltage ranges of electronic components of the converter it is not possible to extend the nominal power of single strings just by connecting more PV-modules in series. On the other hand the particular advantages of the string technology are lost by connecting strings in parallel in order to increase the nominal power. Since the extension of the nominal power of string-converters beyond 3 kW cannot be achieved either by extensive series or parallel connection of PV-modules a new converter concept is needed. In Cramer and Greizer (2000) future trends of PV-converters are presented including a new type of converter called ‘multi-string-converter’ offering a solution to the above problem. The present paper gives a detailed description
The experience has shown that a cost reduction †
Author to whom correspondence should be addressed. Tel.: 149-561-9522-0; fax: 149-561-9522-100; e-mail: [email protected] ‡ ISES member. 217
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Fig. 1. Costs of different PV-converter concepts (small string converter and central PV-converter costs taken from Kreuzmann (2001)).
of the features, the operational behaviour and approach for the technical realisation of the new multi-string-converter. 2. STATE OF THE ART OF PV-CONVERTERS AND PV-SYSTEM CONCEPTS
2.1. PV-system concepts used today In order to compare the new multi-string-converter to state-of-the-art PV-converters, PV-system concepts commonly used today are presented. The three main grid-connected PV-system concepts and associated converters are: • plant-oriented system concept (central converter) • module-oriented system concept (string-converter, multi-string-converter) • module-integrated PV-system concept (module-integrated converter) Fig. 2 summarises the main features of the different PV-system concepts comprising general system structure, safety, energy yield, costs, operational behaviour, system aspects (e.g. monitoring, maintainability . . . ).
2.2. Comparison of string-converter and central PV-converter technology In recent years the central PV-converter and string converter have emerged as the main competitors in the field of PV-system technology. As shown in Fig. 2 each converter stands for its own PV-system philosophy. Real PV-systems are compared very often in order to identify ‘the best’ PV-converter concept. However, the comparison of string-converter and central PV-converter based on the evaluation of real PV-systems always has to take into account the specific conditions of the corresponding PV-system (e.g. degree of dirt,
orientation and temperature of the PV-modules) (Schmela, 2000). Therefore a general judgement cannot be made based on the comparison of real PV-systems. Consequently Fig. 3 lists the general advantages and disadvantages of the different converters from a system’s point of view. 3. THE MULTI-STRING-CONVERTER
3.1. The modular concept of the multi-stringconverter As illustrated in the block diagram in Fig. 4 the multi-string-converter consists of several modularly extendable DC / DC-converters each connected to one common inverter via a common DC-link. A functional block performing local MPP-tracking and monitoring tasks is assigned to each of the DC / DC-converters. Furthermore the multi-string-converter includes a common operational control unit (OCU) performing the following tasks. • Start / stop-control • Realisation of safety functions (e.g. protection against islanding) • Control of line current • Supervisory control of the common inverter section • Communication with operator or owner of PVsystem All components are located in a single housing (IP65) that can be used in harsh environment directly in the PV-field adjacent to the PV-modules.
3.2. The multi-string-converter combines the advantages of string-converter and central PVconverter technology Fig. 3 shows that the multi-string-converter summarises the positive aspects of the central-PV-
Multi-String-converter with reduced specific costs and enhanced functionality
Fig. 2. Comparison of PV-system concepts used today (Meinhardt and Cramer, 2000).
Fig. 3. Multi-string technology summarises the advantages of central PV-converters and string-converters.
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Fig. 4. General structure of a multi-string-converter.
converter and string-converter concepts and omits the drawbacks. The development of a PV-converter based on the new multi-string-concept leads to a significant cost reduction of string-converters while still
using the advantages of the string-converter technology. The main features of the multi-string technology are: • optimum energy yield • optimum monitoring of strings • low specific costs of PV-converter • minimum costs of PV-system installation • nominal power of converter unit not limited • modular extensibility
3.3. Multi-string-converter for optimum energy yield Since each string of PV-modules has its dedicated DC / DC-converter including local MPPcontrol and a monitoring function an optimum operational behaviour can be achieved. Consequently failures of the PV-generator can be detected instantaneously and the defective part of the generator can be located very precisely. The function of the PV-plant is not effected by a breakdown of a single PV-module. An optimal energy yield can be obtained using multi-string-converters especially in PV-systems consisting of strings with different operational behaviour and conditions. The examples in Fig. 5 show that the multi-string-converter is absolutely suitable for connecting strings with different nominal values, size or type of solar cells as well as strings with different orientations (west, south, east) or different degrees of shadings, to a single PV-converter.
3.4. Low specific costs of PV-converter Fig. 5. Application of multi-string-converters for PV-systems with strings having strongly different characteristics. (a) Strings with different nominal power and / or voltage, size, type of solar cells. (b) Strings with different orientation (west, south, east). (c) Strings with different degrees of shading.
Although string-converters have reached a very low price level the reduction of the specific costs (» / W) is still possible by using synergy effects. As shown in Fig. 6a a PV-system applying several
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Fig. 6. (a) PV-system with several string-converters. (b) PV-system with one multi-string-converter (OCU, operation control unit).
(n) string-converters comprises some converter components implemented n times although they are just needed once. In order to reduce the converter costs these redundant functional blocks and components can be left out: in the particular example of a PV-system illustrated in Fig. 6b one multi-string-converter replaces n string-converters. Consequently n 2 1 of each of the items listed below can be omitted: • measuring and monitoring devices for line voltage and current • control and drivers for inverter section • operational control unit (OCU) including
• mains monitoring device with allocated switching device (MSD, in German: ENS) • residual current operated protective device • communication module Furthermore the multi-string-converter replaces several (n) small components with single enlarged ones like: • housing of converter • connectors and relays for line (dis)connection • EMC-filter, AC-filter, heat sink . . .
3.5. Minimum costs of PV-system installation One of the main features that led to the success
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of the string-converters was the massive reduction of costs associated with the PV-system installation (e.g. reduced DC-cabling and DC-distribution, omitted series diodes and DC-connection cases). This advantage is adopted by the multi-stringconverter due to the string-oriented structure of the concept. Therefore the overall costs of the PV-system are minimised using a multi-stringconverter.
3.6. Modular extensibility for realisation of PV systems of any size Due to the modular concept the multi-stringconverter can be applied to PV-systems of any size by equipping the converter with the desired number of DC / DC-converters. Depending on the nominal power of the PVsystem the inverter section of the multi-stringconverter is realised either using a single-phase or a three-phase topology. Examples for three-phase realisations are given in Fig. 7.
3.7. Modular extensibility for an enhanced functionality of the operational control unit ( OCU) In recent years the requirements on PV-converters have shifted from ‘just-converting-PV-energy’
to PV-converters with numerous additional functions (e.g. communication via Powerline, providing a history of events, statistical analysis of delivered energy). Since this trend is expected to continue, the multi-string-converter has the capability of modular functional extensibility. Because of the increased nominal power of the multi-string-converter the implementation of the additional functions in the OCU of the multistring-converter is economical compared to small string-converters. The additional costs caused by the enhanced functionality of the OCU result in a marginal increase in specific converter costs (» / W) only. In particular the enhanced functionality comprises: • power quality control • wireless communication These functions are described in some detail in Section 5.2.
3.8. Modular concept of a power conditioning unit for PV-systems presented in the literature ¨ In the literature (Roth and Schonholzer, 1996) the development of a prototype of a power conditioning unit for a 20 kW PV-system consisting of four DC / DC-converters connected to an off-the-shelf inverter for drive application is
Fig. 7. Examples for the realisation of three-phase PV-systems using a multi-string-converter. (a) Three single-phase units, (b) one three-phase unit.
Multi-String-converter with reduced specific costs and enhanced functionality
presented. In contrast to the new multi-string converter the power conditioning system de¨ scribed in Roth and Schonholzer (1996) uses five separate housings (one for each DC / DC-converter and one for the inverter) and does not use stringtechnology but parallel connection of solar modules. The maximum efficiency is just 91% whereas the multi-string-converter reaches more than 95%.
4. FIRST GENERATION OF MULTI-STRINGCONVERTERS: SUNNY BOY 5000 MULTISTRING
4.1. Structure of the ‘ Sunny Boy 5000 MultiString’ The first generation of the multi-string-converter called Sunny Boy 5000 Multi-String will be launched in spring 2002. As shown in Fig. 8 the Sunny Boy 5000 Multi-String comprises three MPP-controlled DC / DC-converters connected to a common single-phase inverter section. The maximum AC-power is 5 kW. The OCU of the multi-string-converter includes all state-of-the-art functions common for stringconverters: SCI CDC
supervisory control of inverter section co-ordination and supervision of the DC / DC-converters. For this purpose the OCU communicates with the controller of the DC / DC-converters
ACC COM
MSD
RCD
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AC-current control for sinusoidal line current communication with operator of PV-system via: LCD, Powerline, RS485 or RS232 mains monitoring device with allocated switching devices (MSD, in German: ENS according to VDE 0126) residual current operated protective device sensitive to all kinds of currents.
4.2. Technical data of the Sunny Boy 5000 Multi-String The Sunny Boy 5000 Multi-String has been designed for the requirements of the PV-market. Due to the wide range of input voltage of 150 V (minimum MPP-voltage) to 750 V (maximum open circuit voltage) the Sunny Boy 5000 MultiString can be connected to almost every kind of PV-module available. Table 1 summarises preliminary technical data of the first generation of SMA’s multi-string-converters.
Table 1. Preliminary technical data of the Sunny Boy 5000 Multi-String Maximum AC-power Nominal AC-power Maximum efficiency Input voltage range Maximum power per string Weight Stand-by power consumption Ambient temperature range Housing (stainless steel)
Fig. 8. Structure of the first-generation multi-string converter: Sunny Boy 5000 Multi-String.
5 kW 4.6 kW 96% 150–750 V 2200 Wp ,30 kg <1 W 225– 1608C IP 65
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4.3. Topology of the ‘ Sunny Boy 5000 MultiString’ An extensive investigation in order to identify the most suitable topology for the Sunny Boy 5000 Multi-String has been carried out. The topology shown in Fig. 9 has proven the best solution according to the criteria: costs, efficiency, size, weight, system aspects. Like the string-converter Sunny Boy 2000 the Sunny Boy 5000 Multi-String is based on a transformerless concept. This offers a very high efficiency especially in partial load range. The DC / DC-converters use a boost topology in order to provide the wide input voltage range. The energy storage capacitors (C pos and C neg ) are located in the DC-link. Since the multi-stringconverter operates with a constant DC-link voltage the capacitors are running constantly close to their rated voltage. Furthermore no additional reactive power caused by the 100 Hz-pulsation of the AC-power has to be transferred by the DC / DC-converters. The inverter section of the Sunny Boy 5000 Multi-String is made up by a half-bridge inverter. This inverter topology has been identified in Calais et al. (1999) as the most compact topology (lowest number of switches and diodes) suitable for transformerless PV-systems. The main advantages of this topology are the nearly negligible AC-component of the PV-module-to-earth-voltage VPV– PE and the simplicity of the topology (just two switches and diodes needed). Due to the low
Fig. 10. Simplified structure of the control strategy for multistring-converters.
AC-component of VPV– PE the effort for EMCfiltering on the DC-side of the converter is much smaller compared to other inverter topologies and the size of the heavy and costly filter inductors can be reduced to a minimum. This leads to a specific weight (kg / W) of the Sunny Boy 5000 Multi-String comparable to other PV-converters in the same power range (Welter, 1998).
4.4. Control strategy of the Sunny Boy 5000 Multi-String The simplified structure of the multi-stringconverter control strategy is shown in Fig. 10. For reasons of simplicity only one out of three DC / DC-converters is depicted. This control strategy can generally be applied to multi-string-converters comprising any number of DC / DC-converters. The control is divided into two parts: control of inverter section (AC-control) and control of the
Fig. 9. Topology of the power electronics of the Sunny Boy 5000 Multi-String.
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DC / DC-converters (DC-control). The main control tasks are assigned as follows: • AC-control: control of line current (IAC ) • DC-control: MPP-control for each of the DC / DC-converters The controllers operate independently from each other although they are coupled via the controlled system. The mutual influence of AC- and DCcontrol loops as well as the line voltage (VAC ) and PV-current can be regarded as disturbance. VDC and IPV 1 are disturbances for the DC-control loop ] and IDC 1 , IDC 2 , IDC 3 and VAC are disturbances for ] ] ] the AC-control loop.
shown as an example for a second-generation multi-string-converter. In addition to the functional modules provided by first generation multistring (SCI, MSD, RCD and CDC: abbreviations explained in Section 3.7) a module for ‘wireless communication’ and a module for ‘power quality control’ have been added to the operational control unit.
4.5. Safety concept for transformerless multistring-converters
5.2.1. Power quality control. The use of PVconverters for improvement of power quality is becoming more and more important because of the deregulation of the European electricity market. The deregulation can result in a significant deterioration of the power quality. The increasing number of consumers with non-linear characteristics connected to the grid emphasises the necessity for countermeasures that improve the power quality. Since disturbances are generated all over the utility grid the compensation of harmonics and reactive power also has to be realised in a decentralised manner in order to have an optimum effect. Due to the decentralised character of the generation of photovoltaic energy, grid connected PV-converters are very much suited to be used as active filters and reactive power compensators (Burger and Zacharias, 2000). By adding a ‘power quality control’ module the multi-string-converter can additionally be used to improve quality of electrical power supply by means of reactive power compensation and active filtering for compensation of harmonics.
The safety concept of the Sunny Boy 5000 Multi-String provides a very high safety standard for the operation of a grid connected PV-system. It is based on SMA’s long-term experience with the award-winning transformerless string-converters Sunny Boy 1500 /2000 (Stiftung Warentest, 1999). Personnel safety is achieved by applying the following measures (according to EN 50178). • Class II PV-modules and cabling. • Class I housing of converter (connected to protective earth, PE). • Basic insulation plus supplementary insulation of electronic components inside the converter. • Insulation supervision (for detection of earth faults in stand-by mode of operation). • Residual current operated protective device sensitive to all kinds of current (RCD) on the AC-side (according to DIN VDE 0126) (for detection of earth faults in grid connected mode of operation). • Mains monitoring device with allocated switching devices (MSD, in German: ENS according to VDE 0126). 5. SECOND GENERATION OF MULTI-STRING CONVERTERS
5.1. Customer defined multi-string-converters Following the Sunny Boy 5000 Multi-String a second generation of multi-string-converters will be launched. Additional modules in the form of a ‘construction set’ will be offered in order to enable the customer to configure ‘his’ multi-stringconverter tailored to his specific requirements. In Fig. 11 a PV-converter with four separately MPP-controlled, galvanically isolated DC / DCconverters and a three-phase grid connection is
5.2. Additional modules projected for the second-generation multi-string-converters This section describes the additional modules for the ‘multi-string construction set’ depicted in Fig. 12.
5.2.2. Wireless communication. The number of electrical appliances with wireless communication capabilities is continuously increasing. Schramm (1999) gives an example for a typical household appliance equipped with a GSM communication module. In PV-systems wireless communication modules based on DECT-standard (digital enhanced cordless telecommunication) or GSM-standard (global system for mobile telecommunication) are versatile tools for remote monitoring purposes. The operator or owner of the PV-system has unlimited access to the information provided by the multi-string-converter and will be notified on demand or immediately in the unlikely event of a PV-system failure. Consequently numerous
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Fig. 11. Customer-defined multi-string-converter of the second generation (PV-strings and utility grid not shown).
maintenance tasks can be carried out from the operator’s desktop and costly trips to the PVsystems can be avoided. In case of a DECT-communication module the multi-string-converter can be coupled easily to the domestic telephone system as a digital extension.
5.2.3. Three-phase inverter section. Depending on the nominal power of the PV-system the multistring-converter is equipped with a single-phase or a three-phase inverter section. The use of a three-phase inverter section would be recommended for PV-systems above 10 kW.
Fig. 12. Additional modules for the ‘multi-string-construction-set’.
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Fig. 13. Innovative ideas of multi-string-converter development.
5.2.4. Galvanic isolation. In order to complete the range of additional modules two different concepts for the realisation of PV-systems with galvanic isolation are introduced: • 50 Hz transformer for large-scale systems • DC / DC-converter with galvanic isolation realised with HF-transformer
6. CONCLUSIONS
The introduction of the new multi-string technology can be regarded as a further milestone in the development of PV-system technology. The development of a PV-converter based on the new multi-string-concept leads to a significant cost reduction of string-converters while still using the advantages of the string-converter technology. Beside the reduced specific costs the main features of the multi-string-converter are best operational performance resulting in an optimum energy yield, modularity and extensibility. First units for field tests of the new ‘Sunny Boy 5000 Multi-String’ representing the first generation of PV-converters in multi-string technology will be available in spring 2002. Finally, Fig. 13 summarises the innovative ideas that resulted in the development of the multi-string-converter.
Acknowledgements—This work was partly funded by the ¨ Wirtschaft und Technologie (BMWi)’ ‘Bundesministerium fur
of the Federal Republic of Germany under contract number 0329864. The authors are responsible for the content of the paper.
REFERENCES Burger B. and Zacharias P. (2000) Improvement of power quality — decentralised use of power converters (Dezen¨ verbestraler Einsatz von Stromrichtern — Stromqualitat sern). Erneuerbare Energien 4, 29–32, in German. Calais M. et al. (1999) Multilevel converters for single-phase grid connected photovoltaic systems: an overview. Solar Energy 66(5), 325–335. Cramer G. and Greizer F. (2000) From line-commutated ¨ converters to multi-string (Vom netzgefuhrten Stromrichter zu Multi-String). Erneuerbare Energien 2, 10–12, in German. Kreuzmann A. (2001) Market survey PV-converters, ¨ Marktubersicht PV-Wechselrichter. Photon, 46–55, in German; March. Meinhardt, M. and Cramer, G. (2000) Past, present and future of grid connected photovoltaic- and hybrid-power-systems. In Proceedings of IEEE Power Electronics Specialists Conference, July 16–20, Seattle, Washington, USA ¨ Roth S. and Schonholzer E. T. (1996) 20-kW-photovoltaic system NTB / NOK: design, inverters, measurements (20 kW-Photovoltaikanlage NTB / NOK: Auslegung, Wechselrichter, Messdaten). Bull. SEV/VSE 10, 23–29, in German. Schmela M. (2000) The beauty and the beast. Photon Int. 3, 10–12. Schramm M. (1999) The washing machine communicates via internet and mobile telephone (Bei Anruf Schleudern — Die ¨ Waschmaschine Margherita kommuniziert uber Internet und Handy). Frankfurter Allgemeine Zeitung 14, T12, in German. Stiftung Warentest (1999) Electrical Energy — Home Made (Strom hausgemacht), no. 9, pp. 66–71. Stiftung Warentest, Berlin, in German. Welter P. (1998) Market survey — PV converters. Photon 3, 10–12, in German.
PII:
Solar Energy Vol. 69(Suppl.), Nos. 1–6, pp. 217–227, 2000 2001 Elsevier Science Ltd S 0 0 3 8 – 0 9 2 X ( 0 1 ) 0 0 0 6 7 – 6 All rights reserved. Printed in Great Britain 0038-092X / 00 / $ - see front matter
www.elsevier.com / locate / solener
MULTI-STRING-CONVERTER WITH REDUCED SPECIFIC COSTS AND ENHANCED FUNCTIONALITY ¨ MIKE MEINHARDT † , * ,‡ , GUNTHER CRAMER* ,‡ , BRUNO BURGER** and PETER ZACHARIAS** *SMA Regelsysteme GmbH, Hannoversche Straße 1-5, D-34266 Niestetal, Germany ¨ Solare Energieversorgungstechnik e.V., Konigstor ¨ **Institut fur 59, D-34119 Kassel, Germany Received 20 June 2000; revised version accepted 21 May 2001 Communicated by KLAUS PREISER
Abstract—The development of a PV-converter based on the advanced Multi-String concept results in significantly reduced specific costs while still profiting from the well-known advantages of the string-converter technology developed by ISET and SMA in the mid-1990s. The paper deals with the basic considerations from a system’s point of view that resulted in the development of the Multi-String-converter. The features of the Multi-String-converter concerning operational behaviour and PV-system design are described. Due to enhanced capacity of the operational control unit the Multi-String-converter can be used additionally for active compensation of harmonics and reactive power in order to improve the power quality. An evaluation of Multi-String-converters in comparison with string-converters or conventional central-converter concepts is presented. The selected topology and the control strategy for the Multi-String-converter are introduced. 2001 Elsevier Science Ltd. All rights reserved.
can be achieved primarily by increasing the nominal power of the PV-converter. Fig. 1 shows the specific PV-converter costs as a function of the nominal power of the converter unit including state-of-the-art string- and central PV-converter as well as the first and second generation of the new multi-string-converter introduced in the present paper. Costs of the state-of-the-art converters are taken from a market survey published in Kreuzmann (2001).
1. INTRODUCTION
1.1. Prices of PV-converters must come down further The continuously decreasing prices for PVmodules result in the fact that the reduction of the specific PV-converter costs (» / W) is becoming more and more important. A wider usage of photovoltaic energy systems requires a further reduction of the specific costs while keeping the very high standards that PV-converters have reached nowadays regarding safety, efficiency, reliability, electromagnetic compatibility and functionality (e.g. grid monitoring functions). String oriented PV-converters developed by ISET (Germany) and SMA Regelsysteme GmbH (Germany) have been very successful in the past decade due to their economical and technological advantages (e.g. reduced DC-installation, local MPP-tracking). Nevertheless the reduction of the specific costs of state-of-the-art PV-converters is still possible.
1.3. A new PV-converter concept is needed
1.2. Cost reduction due to increased nominal power of converter unit
Due to the limited isolation of PV-modules and voltage ranges of electronic components of the converter it is not possible to extend the nominal power of single strings just by connecting more PV-modules in series. On the other hand the particular advantages of the string technology are lost by connecting strings in parallel in order to increase the nominal power. Since the extension of the nominal power of string-converters beyond 3 kW cannot be achieved either by extensive series or parallel connection of PV-modules a new converter concept is needed. In Cramer and Greizer (2000) future trends of PV-converters are presented including a new type of converter called ‘multi-string-converter’ offering a solution to the above problem. The present paper gives a detailed description
The experience has shown that a cost reduction †
Author to whom correspondence should be addressed. Tel.: 149-561-9522-0; fax: 149-561-9522-100; e-mail: [email protected] ‡ ISES member. 217
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Fig. 1. Costs of different PV-converter concepts (small string converter and central PV-converter costs taken from Kreuzmann (2001)).
of the features, the operational behaviour and approach for the technical realisation of the new multi-string-converter. 2. STATE OF THE ART OF PV-CONVERTERS AND PV-SYSTEM CONCEPTS
2.1. PV-system concepts used today In order to compare the new multi-string-converter to state-of-the-art PV-converters, PV-system concepts commonly used today are presented. The three main grid-connected PV-system concepts and associated converters are: • plant-oriented system concept (central converter) • module-oriented system concept (string-converter, multi-string-converter) • module-integrated PV-system concept (module-integrated converter) Fig. 2 summarises the main features of the different PV-system concepts comprising general system structure, safety, energy yield, costs, operational behaviour, system aspects (e.g. monitoring, maintainability . . . ).
2.2. Comparison of string-converter and central PV-converter technology In recent years the central PV-converter and string converter have emerged as the main competitors in the field of PV-system technology. As shown in Fig. 2 each converter stands for its own PV-system philosophy. Real PV-systems are compared very often in order to identify ‘the best’ PV-converter concept. However, the comparison of string-converter and central PV-converter based on the evaluation of real PV-systems always has to take into account the specific conditions of the corresponding PV-system (e.g. degree of dirt,
orientation and temperature of the PV-modules) (Schmela, 2000). Therefore a general judgement cannot be made based on the comparison of real PV-systems. Consequently Fig. 3 lists the general advantages and disadvantages of the different converters from a system’s point of view. 3. THE MULTI-STRING-CONVERTER
3.1. The modular concept of the multi-stringconverter As illustrated in the block diagram in Fig. 4 the multi-string-converter consists of several modularly extendable DC / DC-converters each connected to one common inverter via a common DC-link. A functional block performing local MPP-tracking and monitoring tasks is assigned to each of the DC / DC-converters. Furthermore the multi-string-converter includes a common operational control unit (OCU) performing the following tasks. • Start / stop-control • Realisation of safety functions (e.g. protection against islanding) • Control of line current • Supervisory control of the common inverter section • Communication with operator or owner of PVsystem All components are located in a single housing (IP65) that can be used in harsh environment directly in the PV-field adjacent to the PV-modules.
3.2. The multi-string-converter combines the advantages of string-converter and central PVconverter technology Fig. 3 shows that the multi-string-converter summarises the positive aspects of the central-PV-
Multi-String-converter with reduced specific costs and enhanced functionality
Fig. 2. Comparison of PV-system concepts used today (Meinhardt and Cramer, 2000).
Fig. 3. Multi-string technology summarises the advantages of central PV-converters and string-converters.
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Fig. 4. General structure of a multi-string-converter.
converter and string-converter concepts and omits the drawbacks. The development of a PV-converter based on the new multi-string-concept leads to a significant cost reduction of string-converters while still
using the advantages of the string-converter technology. The main features of the multi-string technology are: • optimum energy yield • optimum monitoring of strings • low specific costs of PV-converter • minimum costs of PV-system installation • nominal power of converter unit not limited • modular extensibility
3.3. Multi-string-converter for optimum energy yield Since each string of PV-modules has its dedicated DC / DC-converter including local MPPcontrol and a monitoring function an optimum operational behaviour can be achieved. Consequently failures of the PV-generator can be detected instantaneously and the defective part of the generator can be located very precisely. The function of the PV-plant is not effected by a breakdown of a single PV-module. An optimal energy yield can be obtained using multi-string-converters especially in PV-systems consisting of strings with different operational behaviour and conditions. The examples in Fig. 5 show that the multi-string-converter is absolutely suitable for connecting strings with different nominal values, size or type of solar cells as well as strings with different orientations (west, south, east) or different degrees of shadings, to a single PV-converter.
3.4. Low specific costs of PV-converter Fig. 5. Application of multi-string-converters for PV-systems with strings having strongly different characteristics. (a) Strings with different nominal power and / or voltage, size, type of solar cells. (b) Strings with different orientation (west, south, east). (c) Strings with different degrees of shading.
Although string-converters have reached a very low price level the reduction of the specific costs (» / W) is still possible by using synergy effects. As shown in Fig. 6a a PV-system applying several
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Fig. 6. (a) PV-system with several string-converters. (b) PV-system with one multi-string-converter (OCU, operation control unit).
(n) string-converters comprises some converter components implemented n times although they are just needed once. In order to reduce the converter costs these redundant functional blocks and components can be left out: in the particular example of a PV-system illustrated in Fig. 6b one multi-string-converter replaces n string-converters. Consequently n 2 1 of each of the items listed below can be omitted: • measuring and monitoring devices for line voltage and current • control and drivers for inverter section • operational control unit (OCU) including
• mains monitoring device with allocated switching device (MSD, in German: ENS) • residual current operated protective device • communication module Furthermore the multi-string-converter replaces several (n) small components with single enlarged ones like: • housing of converter • connectors and relays for line (dis)connection • EMC-filter, AC-filter, heat sink . . .
3.5. Minimum costs of PV-system installation One of the main features that led to the success
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of the string-converters was the massive reduction of costs associated with the PV-system installation (e.g. reduced DC-cabling and DC-distribution, omitted series diodes and DC-connection cases). This advantage is adopted by the multi-stringconverter due to the string-oriented structure of the concept. Therefore the overall costs of the PV-system are minimised using a multi-stringconverter.
3.6. Modular extensibility for realisation of PV systems of any size Due to the modular concept the multi-stringconverter can be applied to PV-systems of any size by equipping the converter with the desired number of DC / DC-converters. Depending on the nominal power of the PVsystem the inverter section of the multi-stringconverter is realised either using a single-phase or a three-phase topology. Examples for three-phase realisations are given in Fig. 7.
3.7. Modular extensibility for an enhanced functionality of the operational control unit ( OCU) In recent years the requirements on PV-converters have shifted from ‘just-converting-PV-energy’
to PV-converters with numerous additional functions (e.g. communication via Powerline, providing a history of events, statistical analysis of delivered energy). Since this trend is expected to continue, the multi-string-converter has the capability of modular functional extensibility. Because of the increased nominal power of the multi-string-converter the implementation of the additional functions in the OCU of the multistring-converter is economical compared to small string-converters. The additional costs caused by the enhanced functionality of the OCU result in a marginal increase in specific converter costs (» / W) only. In particular the enhanced functionality comprises: • power quality control • wireless communication These functions are described in some detail in Section 5.2.
3.8. Modular concept of a power conditioning unit for PV-systems presented in the literature ¨ In the literature (Roth and Schonholzer, 1996) the development of a prototype of a power conditioning unit for a 20 kW PV-system consisting of four DC / DC-converters connected to an off-the-shelf inverter for drive application is
Fig. 7. Examples for the realisation of three-phase PV-systems using a multi-string-converter. (a) Three single-phase units, (b) one three-phase unit.
Multi-String-converter with reduced specific costs and enhanced functionality
presented. In contrast to the new multi-string converter the power conditioning system de¨ scribed in Roth and Schonholzer (1996) uses five separate housings (one for each DC / DC-converter and one for the inverter) and does not use stringtechnology but parallel connection of solar modules. The maximum efficiency is just 91% whereas the multi-string-converter reaches more than 95%.
4. FIRST GENERATION OF MULTI-STRINGCONVERTERS: SUNNY BOY 5000 MULTISTRING
4.1. Structure of the ‘ Sunny Boy 5000 MultiString’ The first generation of the multi-string-converter called Sunny Boy 5000 Multi-String will be launched in spring 2002. As shown in Fig. 8 the Sunny Boy 5000 Multi-String comprises three MPP-controlled DC / DC-converters connected to a common single-phase inverter section. The maximum AC-power is 5 kW. The OCU of the multi-string-converter includes all state-of-the-art functions common for stringconverters: SCI CDC
supervisory control of inverter section co-ordination and supervision of the DC / DC-converters. For this purpose the OCU communicates with the controller of the DC / DC-converters
ACC COM
MSD
RCD
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AC-current control for sinusoidal line current communication with operator of PV-system via: LCD, Powerline, RS485 or RS232 mains monitoring device with allocated switching devices (MSD, in German: ENS according to VDE 0126) residual current operated protective device sensitive to all kinds of currents.
4.2. Technical data of the Sunny Boy 5000 Multi-String The Sunny Boy 5000 Multi-String has been designed for the requirements of the PV-market. Due to the wide range of input voltage of 150 V (minimum MPP-voltage) to 750 V (maximum open circuit voltage) the Sunny Boy 5000 MultiString can be connected to almost every kind of PV-module available. Table 1 summarises preliminary technical data of the first generation of SMA’s multi-string-converters.
Table 1. Preliminary technical data of the Sunny Boy 5000 Multi-String Maximum AC-power Nominal AC-power Maximum efficiency Input voltage range Maximum power per string Weight Stand-by power consumption Ambient temperature range Housing (stainless steel)
Fig. 8. Structure of the first-generation multi-string converter: Sunny Boy 5000 Multi-String.
5 kW 4.6 kW 96% 150–750 V 2200 Wp ,30 kg <1 W 225– 1608C IP 65
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4.3. Topology of the ‘ Sunny Boy 5000 MultiString’ An extensive investigation in order to identify the most suitable topology for the Sunny Boy 5000 Multi-String has been carried out. The topology shown in Fig. 9 has proven the best solution according to the criteria: costs, efficiency, size, weight, system aspects. Like the string-converter Sunny Boy 2000 the Sunny Boy 5000 Multi-String is based on a transformerless concept. This offers a very high efficiency especially in partial load range. The DC / DC-converters use a boost topology in order to provide the wide input voltage range. The energy storage capacitors (C pos and C neg ) are located in the DC-link. Since the multi-stringconverter operates with a constant DC-link voltage the capacitors are running constantly close to their rated voltage. Furthermore no additional reactive power caused by the 100 Hz-pulsation of the AC-power has to be transferred by the DC / DC-converters. The inverter section of the Sunny Boy 5000 Multi-String is made up by a half-bridge inverter. This inverter topology has been identified in Calais et al. (1999) as the most compact topology (lowest number of switches and diodes) suitable for transformerless PV-systems. The main advantages of this topology are the nearly negligible AC-component of the PV-module-to-earth-voltage VPV– PE and the simplicity of the topology (just two switches and diodes needed). Due to the low
Fig. 10. Simplified structure of the control strategy for multistring-converters.
AC-component of VPV– PE the effort for EMCfiltering on the DC-side of the converter is much smaller compared to other inverter topologies and the size of the heavy and costly filter inductors can be reduced to a minimum. This leads to a specific weight (kg / W) of the Sunny Boy 5000 Multi-String comparable to other PV-converters in the same power range (Welter, 1998).
4.4. Control strategy of the Sunny Boy 5000 Multi-String The simplified structure of the multi-stringconverter control strategy is shown in Fig. 10. For reasons of simplicity only one out of three DC / DC-converters is depicted. This control strategy can generally be applied to multi-string-converters comprising any number of DC / DC-converters. The control is divided into two parts: control of inverter section (AC-control) and control of the
Fig. 9. Topology of the power electronics of the Sunny Boy 5000 Multi-String.
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DC / DC-converters (DC-control). The main control tasks are assigned as follows: • AC-control: control of line current (IAC ) • DC-control: MPP-control for each of the DC / DC-converters The controllers operate independently from each other although they are coupled via the controlled system. The mutual influence of AC- and DCcontrol loops as well as the line voltage (VAC ) and PV-current can be regarded as disturbance. VDC and IPV 1 are disturbances for the DC-control loop ] and IDC 1 , IDC 2 , IDC 3 and VAC are disturbances for ] ] ] the AC-control loop.
shown as an example for a second-generation multi-string-converter. In addition to the functional modules provided by first generation multistring (SCI, MSD, RCD and CDC: abbreviations explained in Section 3.7) a module for ‘wireless communication’ and a module for ‘power quality control’ have been added to the operational control unit.
4.5. Safety concept for transformerless multistring-converters
5.2.1. Power quality control. The use of PVconverters for improvement of power quality is becoming more and more important because of the deregulation of the European electricity market. The deregulation can result in a significant deterioration of the power quality. The increasing number of consumers with non-linear characteristics connected to the grid emphasises the necessity for countermeasures that improve the power quality. Since disturbances are generated all over the utility grid the compensation of harmonics and reactive power also has to be realised in a decentralised manner in order to have an optimum effect. Due to the decentralised character of the generation of photovoltaic energy, grid connected PV-converters are very much suited to be used as active filters and reactive power compensators (Burger and Zacharias, 2000). By adding a ‘power quality control’ module the multi-string-converter can additionally be used to improve quality of electrical power supply by means of reactive power compensation and active filtering for compensation of harmonics.
The safety concept of the Sunny Boy 5000 Multi-String provides a very high safety standard for the operation of a grid connected PV-system. It is based on SMA’s long-term experience with the award-winning transformerless string-converters Sunny Boy 1500 /2000 (Stiftung Warentest, 1999). Personnel safety is achieved by applying the following measures (according to EN 50178). • Class II PV-modules and cabling. • Class I housing of converter (connected to protective earth, PE). • Basic insulation plus supplementary insulation of electronic components inside the converter. • Insulation supervision (for detection of earth faults in stand-by mode of operation). • Residual current operated protective device sensitive to all kinds of current (RCD) on the AC-side (according to DIN VDE 0126) (for detection of earth faults in grid connected mode of operation). • Mains monitoring device with allocated switching devices (MSD, in German: ENS according to VDE 0126). 5. SECOND GENERATION OF MULTI-STRING CONVERTERS
5.1. Customer defined multi-string-converters Following the Sunny Boy 5000 Multi-String a second generation of multi-string-converters will be launched. Additional modules in the form of a ‘construction set’ will be offered in order to enable the customer to configure ‘his’ multi-stringconverter tailored to his specific requirements. In Fig. 11 a PV-converter with four separately MPP-controlled, galvanically isolated DC / DCconverters and a three-phase grid connection is
5.2. Additional modules projected for the second-generation multi-string-converters This section describes the additional modules for the ‘multi-string construction set’ depicted in Fig. 12.
5.2.2. Wireless communication. The number of electrical appliances with wireless communication capabilities is continuously increasing. Schramm (1999) gives an example for a typical household appliance equipped with a GSM communication module. In PV-systems wireless communication modules based on DECT-standard (digital enhanced cordless telecommunication) or GSM-standard (global system for mobile telecommunication) are versatile tools for remote monitoring purposes. The operator or owner of the PV-system has unlimited access to the information provided by the multi-string-converter and will be notified on demand or immediately in the unlikely event of a PV-system failure. Consequently numerous
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Fig. 11. Customer-defined multi-string-converter of the second generation (PV-strings and utility grid not shown).
maintenance tasks can be carried out from the operator’s desktop and costly trips to the PVsystems can be avoided. In case of a DECT-communication module the multi-string-converter can be coupled easily to the domestic telephone system as a digital extension.
5.2.3. Three-phase inverter section. Depending on the nominal power of the PV-system the multistring-converter is equipped with a single-phase or a three-phase inverter section. The use of a three-phase inverter section would be recommended for PV-systems above 10 kW.
Fig. 12. Additional modules for the ‘multi-string-construction-set’.
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Fig. 13. Innovative ideas of multi-string-converter development.
5.2.4. Galvanic isolation. In order to complete the range of additional modules two different concepts for the realisation of PV-systems with galvanic isolation are introduced: • 50 Hz transformer for large-scale systems • DC / DC-converter with galvanic isolation realised with HF-transformer
6. CONCLUSIONS
The introduction of the new multi-string technology can be regarded as a further milestone in the development of PV-system technology. The development of a PV-converter based on the new multi-string-concept leads to a significant cost reduction of string-converters while still using the advantages of the string-converter technology. Beside the reduced specific costs the main features of the multi-string-converter are best operational performance resulting in an optimum energy yield, modularity and extensibility. First units for field tests of the new ‘Sunny Boy 5000 Multi-String’ representing the first generation of PV-converters in multi-string technology will be available in spring 2002. Finally, Fig. 13 summarises the innovative ideas that resulted in the development of the multi-string-converter.
Acknowledgements—This work was partly funded by the ¨ Wirtschaft und Technologie (BMWi)’ ‘Bundesministerium fur
of the Federal Republic of Germany under contract number 0329864. The authors are responsible for the content of the paper.
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