Multi-String-Converter with Reduced Specific Costs and Enhanced Functionality Mike Meinhardt, Günther Cramer SMA Regelsysteme GmbH, Hannoversche Straße 1-5, D-34266 Niestetal, Germany, Phone: +49 561 9522-0 Fax: +49 561 9522-100, Email:
[email protected] Bruno Burger, Peter Zacharias Institut für Solare Energieversorgungstechnik e.V., Königstor 59, D-34119 Kassel, Germany, Phone: +49 561 7492-0 Fax: +49 561 7492-100, Email:
[email protected] 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-nineties. 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 Stringconverters or conventional Central-converter concepts is presented. The selected topology and the control strategy for the Multi-String-Converter are introduced.
1 INTRODUCTION 1 €/W
Prices of PV-converters must come down further The continuously decreasing prices for PV-Modules result in the fact that the reduction of the specific PVconverter 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 standard 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 PVconverters is still possible. Cost reduction due to increased nominal power of converter unit The experience has shown that a cost reduction can be achieved primarily by increasing the nominal power of the PV-converter. Fig. 1 shows the projected development of specific PV-converter costs as function of the nominal power of the converter unit including stateof-the-art String-Converter as well as the first and second generation of the new Multi-String-converter introduced in the present paper.
Small String-Converters First Generation of Multi-String-Converters (Sunny Boy 5000 Multi-String)
AC
Specific costs of PVconverter 0.5 €/W
Second Generation of Multi-String-Converters (Sunny Team)
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1 kW
5 kW
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Nominal AC power of PV converter unit
100 kW cost-reduction-eurosun.ppt
Fig. 1: Cost reduction due to increased nominal power of PV converter unit A new PV-converter concept is needed Due to the limited voltage proof of PV-modules and 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 2.5 kW can not be achieved either by extensive series or parallel connection of PVmodules a new converter concept is needed. In [1] 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 of the features, the operational behaviour and approach for the technical realisation of the new Multi-String-converter.
2 THE MULTI-STRING-CONVERTER 2.1 The modular concept of the Multi-String converter As illustrated in the block diagram in Fig. 2 the MultiString-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 contents 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 PVfield adjacent to the PV-modules. PV-Modules
Multi-String-Converter
Utility Grid
String 1
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String 2
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String-orientated MPP-controlled DC/DC-converter
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Common Inverter Common Operational Control Unit Common DC-Link Modular Extendability
its own PV-system philosophy and both concepts are compared very often in order to identify “the best” concept. However, the comparison of String-converter and Central PV-converter based on the evaluation of real PV-systems has always to take into account the specific conditions of the corresponding PV-system (e.g. grade of dirt, orientation and temperature of the PV-modules) [2]. Therefore a general judgement can not be made based on comparison of real PV-systems. Consequently Fig. 3 lists the general advantages and disadvantages of both concepts from a system’s point of view. String-Converter
- rated power of a converter unit is limited to approx. 2 kW + high energy yield due to local MPP-tracking + cost reduction in PVsystem technology + optimum monitoring of PV system
Central PV-Converter - losses due to missmatch of PV-modules - costly DC-installation
+ rated power of converter unit is not limited + very low specific converter costs
Multi-String-Converter
string -central-eurosun .ppt
Fig. 3: Multi-String Technology summarises the advantages of Central PV-Converters and StringConverters Fig. 3 shows that the Multi-String-converter summarises the positive aspects of Central-PV-converter and Stringconverter concept 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 extendibility
Block-eurosun.ppt
Fig. 2: General structure of a Multi-String-converter 2.3 Multi-String-converter for optimum energy yield 2.2 The Multi-String-converter combines the advantages of String-converter and Central PVconverter technology In recent years Central PV-converters and Stringconverters have emerged as the main competitors in the field of PV-system technology. Each converter stands for
Since each string of PV-Modules has its dedicated DC/DC-converter including local MPP-control and monitoring function an optimum operational behaviour can be achieved. Consequently failures of the PVgenerator can be detected instantaneously and the defect 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 MultiString-Converters especially in PV-Systems consisting of strings with different operational behavior and conditions. The examples in Fig. 4 show that the Multi-StringConverter 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 grades of shadings, to a single PVconverter.
a)
= = = =
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blocks and components can be left out: In the particular example of a PV-System illustrated in Fig. 5.b one MultiString-converter replaces n String-converters. Consequently n-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 relais for line (dis)connection • EMC-Filter, AC-Filter, heatsink … String 1
a)
West South East
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PV
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PV
PV
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Converter n 230V / 400 V 50 Hz / 60 Hz
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OCU Utility Grid
= = String 1 Opt- yield- eurosun. ppt
Fig. 4: Application of Multi-String-converters for PVSystems 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 grades of shading
b)
String 2
String n ...
PV
PV
PV
... . . . .
. . . .
. . . .
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OCU
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Multi-String converter
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= 1,3
230V / 400 V 50 Hz / 60 Hz
2.4 Low specific costs of PV-converter 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. 5.a a PV-system applying several (n) String-converters comprises some converter components implemented n times although they are just need once. In order to reduce the converter costs these redundant functional
Utility Grid S t r i n gmultistring .ppt
Fig. 5: a) PV-system with several String-Converters b) PV-system with one Multi-String-Converter (OCU: Operation Control Unit)
2.5 Minimum costs of PV-system installation One of the main features that lead to the success 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-String-converter due to the string-oriented structure of the concept. Therefore the overall costs of the PV-system are minimised using Multi-String-converter.
2.6 Modular extendibility for realisation of PV systems of any size
trend is expected to continue the Multi-String-converter has the capability for modular functional extendibility. Because of the increased nominal power of the MultiString-converter the implementation of the additional functions in the OCU of the Multi-String-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 4.2.
Due to the modular concept the Multi-String-converter can be applied to PV-systems of any size by equipping the converter with the desired number of DC/DCconverters. Depending on the nominal power of the PV-System the inverter section of the Multi-String-Converter is realised either using a single-phase or a three-phase topology. Examples for three-phase realisations are given in Fig. 6. a)
b) = =
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3 FIRST GENERATION OF MULTI-STRINGCONVERTER: Sunny Boy 5000 Multi-String 3.1 Structure of the “Sunny Boy 5000 Multi-String” The first generation of the Multi-String-Converter called Sunny Boy 5000 Multi-String will be brought to market in spring 2001. As shown in Fig. 7 the Sunny Boy 5000 Multi-String comprises three MPP-controlled DC/DCconverters connected to a common single-phase inverter section. The maximum AC-power is 5 kW.
N L1 L2 L3
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3-phase MultiString-Converter PVString
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1-phase MultiString-Converter
common singlephase inverter
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Three-phase.ppt
Fig. 6: Examples for the realisation of three-phase PVsystems using Multi-String-Converter a) three single-phase units b) one three-phase unit
2.7 Modular extendibility 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 PVconverters with numerous additional functions (e.g. communication via Powerline, providing a history of events, statistical analysis of delivered energy). Since this
DC/DC-converter MPPControl
PVString
DC-link
=
DC/DC-converter Sunny Boy 5000 Multi-String sb5000eurosun.vsd
Fig. 7: Structure of the first generation Multi-String converter: Sunny Boy 5000 Multi-String The OCU of the Multi-String-converter includes all stateof-the-art functions common for String-converters: SCI Supervisory Control of Inverter section CDC Co-ordination and supervision of the DC/DCConverters. For this purpose the OCU communicates with the controller of the DC/DCconverters ACC AC-current Control for sinusoidal line current
COM Communication with Operator of PV-system via: LCD-Display, Powerline, RS-485 or RS-232 MSD Mains monitoring device with allocated Switching Device (MSD, in German: ENS according to VDE 0126) RCD Residual Current operated protective Device sensitive to all kinds of currents (RCD).
3.2 Technical data of the Sunny Boy 5000 Multi-String The Sunny Boy 5000 Multi-String has been designed according to 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 Multi-String 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-converter. 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):
String 1
5 kW 4.6 kW 96 % 150 V ... 750 V 2200 Wp < 30 kg
