Photovoltaic Inverters

Inverters are used for DC to AC voltage conversion. Output voltage form of an inverter can be rectangle, trapezoid or sine shaped. Grid connected inverters have sine wave output voltage with low distortion ratio. Inverter input voltage usually depends on inverter power, for small power of some 100  the voltage is 12 to 48 V. For grid connected invertres common input voltage range is from 200 to 400 V or even more. Grid connected inverters can be connected in parallel when higher powers are required. For large systems 3-phase inverters are available on the market. Inverters connecting a PV system and the public grid are purposefully designed, allowing energy transfers to and from the public grid. According to working principle many different types of inverters are distiguished, such as central inverters for wide power range to up to 100 kW or even more, string inverters and module inverters. Central inverters are used in large PV power plants. Some inverters can be connected according to the master-slave criteria, when the succeeding inverter switches on only when enough solar radiation is available or in case of main inverter malfunction. Inverters connected to module strings are used in wide power range applications allowing for more reliable operation. Module inverters sometimes also called micro inverters are used in small photovoltaic systems. Such solutions are applicable to larger systems, however, in practice cheaper solution of central inverter or string inverters are used. Special design inverters are available for the purposes of off-grid or hybrid systems. In most cases a powerful inverter includes charge regulator electronics, and not only the inverter. Modern inverters are the most sophisticated electronic devices implemented in photovoltaic systems. On top of high reliable electronics, which must be used, great care should also be taken on lightning protection. Inverters are based on microprocessor circuits, classic or RISC, and on power MOS or IGBT transistors.

INVERTER TECHNOLOGIES AND CONSTRUCTION

Technologies

There are various types of inverters used in photovoltaic systems. In line-commutated inverters thyristors as switching elements are used. Line-commutated inverters are not suitable for use in standalone systems because AC voltage is required to turn off thyristors. Second group are self-commutated inverters which can operate without AC grid voltage. In this inverters IGBT, MOSFET or GTO (Gate Turn Off) thyristors are used. According to the inverter operation, voltage and current control scheme are distinguished. Because of some advantages in grid-connected inverters in most cases current control scheme is applied. Advantages are higher power factor, better transient current suppression, short circuit current is limited to rated AC current. AC current of a line-commutated inverter is a simple rectangle form which must be filtered with low pass output filter. AC current of a self-commutated inverter is most common PWM signal and in cascade inverters sum of partial rectangle forms which together represent quite good reconstruction of sine current.

Inverter construction
(courtesy: SMA)

Main construction parts

Input stage of a grid-tied inverter is usually buck or similar converter. With appropriate MPP algorithm conversion in at maximum power can be attained. For more information about MPP algorithms and MPP trackers see literature section below. Main parts of an inverter are presented on the picture below: Input, MPP unit, DC/DC converter, switching bridge, output inductance, output DC current detection (protection function), ENS protection. Control functions includes grounding monitoring, optional display, thermal and overvoltage protection, communication ports (WiFi, Powerline, RS232 etc.).

Main parts of an inverter
(credit: pvresources)

TECHNICAL DATA

Technical data

The most important inverter parameters are rated DC and AC power, MPP Voltage range, maximum DC/AC current and voltage and rated DC/AC current and voltage. Other parameters are power in standby mode, power in sleeping (night) mode, power factor, distortion, noise level etc. The following parameters can usually be found in inverter data sheets:

Rated DC voltage	VDC	V
MPP voltage range	VMPP	V
Maximum DC voltage	VDCmax	V
Switch off voltage	VDCoff	V
Rated AC voltage	VAC	V
Maximum system voltage	Vmax	V
Rated DC current	IDC	A
Maximum DC current	IDCmax	A
Rated AC current	IAC	A
Maximum AC current	IACmax	A

TABLE 1: Inverter, electrical parameters - voltage and current

Rated DC power	PDC	W
Maximum DC power	PDCmax	W
Rated AC power	PAC	W
Maximum AC power	PACmax	W
DC power Off	PDCoff	W
DC power On	PDCon	W
Power factor	φ	-
Standby DC power	PDCStandby	W
Night mode DC power	Pnight	W

TABLE 2: Inverter, electrical parameters - power

Noise level	GdB	dBA
Operating temperature range	Toper	°C
Total harmonic distortion	k (THD)	-

TABLE 3: Non-electrical parameters of inverters

Efficiency

Inverter efficiency is a ratio of AC power and DC power:

P_DC - DC array power, P_AC - output AC power

Other efficiency definitions include convertion efficiency, MMPT efficiency, dynamic efficiency and weighted (euro) efficiency. To make comparison of different inverters and/or inverters that are operating under different climatic conditions possible, weighted efficiency was defined. Following equation is valid for Europe:

For southern USA with higher irradiance values weighted efficiency with corrected factors is as follows:

PROTECTION FUNCTIONS

Islanding and line disconnect

Islanding operation can be detected or monitored by passive or active islanding detection method. Passive method includes detecting rate of change of frequency, voltage phase jump and three-phase voltage drop monitoring. With active islanding operation detection method frequency shift, active frequency drift - AFD, ENS (impedance measurement), and reactive power fluctuation are detected and monitored. ENS disconnect unit is a standalone unit which disconnects inverter from the grid.

ENS = Selbsttätig wirkende Freischaltstelle mit zwei voneinander unabhängigen Einrichtungen zur Netzüberwachung mit zugeordneten allpoligen Schaltern in Reihe.

Other description for such kind of protection is: MSD = Mains Monitoring Units with Allocated All-pole Switching Devices.

INVERTER TEST REPORTS AVAILABLE ONLINE

Several comprehensive test reports are available on-line. For detailed test reports about some Fronius, SMA, Sputnik and Sunways inverters please visit web site of PV Labor der HTI Bern, Switzerland. Reports are available as pdf files (about 70 pages each) in german language.

Fronius: IG 30, IG 40

SMA: SunnyMiniCentral 6000, SunnyMiniCentral 8000TL SunnyBoy 3800,

Solar-Fabrik: Convert 6T

Sunways: NT 4000

Sputnik Engineering: SolarMax 2000E, SolarMax 3000E, SolarMax 6000E, SolarMax 6000C, SolarMax 25C

ONLINE CONFIGURATION TOOLS

SIEMENS SINVERT Select is a dimensioning program for determining the best possible configuration for a PV system with Siemens SINVERT inverters.
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SMA - with Sunny Design, the design of solar systems is easier than ever: simply enter all required details and within a few minutes you will receive the optimal system configuration.
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Fronius - Fronius Configurator for the perfect sizing of your pv system. In the planning stage of a pv system, the Configurator supports you in the best possible way to get the perfect sizing.
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KACO - KACOCALC Pro is free, continuously updated PC software for dimensioning photovoltaic installations.
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Sputnik - MAX Design is windows-based Solarmax inverter design software.
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Solar fabrik - convertplan software for Solar fabrik inverters design.
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Mastervolt - SysCalc, fast and effective PV system design, can be run locally on any PC with Microsoft Excel installed. No internet connection needed. Clear graphical overview of module voltage/current and temperature..
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INVERTER PRODUCERS

String and central inverters

SIEMENS PV is among the fastest growing power generation technologies. Siemens, as an EPC, provides tailor-made utility-scale PV power plants from a single source.
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SMA Solar Technology - SMA Solar Technology AG develops, produces and sells solar inverters and monitoring systems for photovoltaic applications.
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Conergy - Conergy is the only solar company worldwide that delivers synchronised solar systems: The system manufacturer produces all components needed for solar installation under one roof. Furthermore, all necessary services related to the solar plants come from one single source.
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Fronius - Fronius has been engaged in solar electronics since 1992, in particular in the development and production of photovoltaic inverters for both grid-connected and independent power supplies.
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SolarEdge - SolarEdge provides next generation power conversion electronics that effectively remove the known system constraints across the photovoltaic energy space. Smart DC ASIC technology and active electronics enable increased production of clean, grid-ready energy at a lower cost per watt than any other competitive offering.
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Sputnik Engineering - the products of the SolarMax brand are suitable for any field of application, from photovoltaic systems in single-family homes up to solar power plants in the megawatts range.
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Elettronica Santerno - Elettronica Santerno a worldwide leading company in designing and manufacturing drives, electric motors and frequency converters for industrial automation and solar inverters for renewable energies.
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Delta Energy Systems - Delta Energy Systems (Germany) develops, manufactures and markets worldwide, innovative customized and standard power supplies. Part of company's portfolio are also inverters for renewable energy systems.
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IDS - IDS AG conducts business in power engineering and automation technology on a global scale. Solar inverters are part of their comprehensive product portfolio.
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KACO new energy - KACO new energy is a powerful partner for system solutions in the field of current inverters, power electronics and photovoltaics.
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Advanced Energy - Advanced Energy provides industry-leading companies worldwide with power conversion products that enable innovation in thin-film manufacturing and renewable energy markets.
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Schneider Electric - The Renewable Energies Business of Schneider Electric specializes in delivering grid tie inverters, off grid inverter-chargers and backup power inverter-chargers.
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AnswerDrives - The AnswerDrives SolarGate inverter family has expressly been projected to convert the direct current electric energy made by photovoltaic panel, in alternating current, for the directly introduction in the electric net.
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OutBack Power - OutBack Power Technologies is a leading designer and manufacturer of reliable power electronics for renewable energy applications including solar, wind, micro-hydro, and battery based energy systems.

power-one - with its focus on Renewable Energy and “Green” power solutions, Power-One has recently established itself as the world’s second largest provider of solar inverters.

Centrosolar - With our Powerstocc Excellent inverter series, we provide you with the ideal combination of performance and convenience in inverter sizes of 1 – 10 kW.
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Centrosolar - With our Powerstocc Excellent inverter series, we provide you with the ideal combination of performance and convenience in inverter sizes of 1 – 10 kW.
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Mastervolt - Mastervolt is a pioneer in the field of solar energy. Worldwide there are more than 100,000 solar inverters installed which bear the Mastervolt label. In individual houses, in entire residential districts, at a school in South America and at a medical post on the edge of the desert.
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Ingeteam - With more than 30 years' experience in the power electronics sector, and a 20 year track record in renewable energy, Ingeteam has designed an extensive range of grid-connected and stand-alone PV inverters, marketed under the Ingecon® Sun brand and with rated capacities from 2.5 to 625 kW.
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INVERTER RELATED WEB SITES

Research and design

PV Laboratory of the BFH-TI - PV laboratory was founded in 1988 and focuses on PV system technology, mostly on grid-connected PV systems, but also on stand alone systems. The laboratory is active in education and in research and development.
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Texas Instruments - Solar Power Inverters application notes for power inverters.

SOURCES AND ADDITIONAL INFORMATION

Books

	Zacharias, P. (Editor): Use of Electronic-Based Power Conversion for Distributed and Renewable Energy Sources, ISET Kassel, 2008.
	Häberlin, H.: Photovoltaik, Strom aus Sonnenlicht für Verbundnetz und Inselanlagen; VDE Verlag, 2007, ISBN 978-3-8007-3003-2.
	Teodorescu, R. et al: Grid Converters for Photovoltaic and Wind Power Systems; Willey 2011, ISBN 9780470057513.
	Castañer, L., Silvestre, S.: Modelling Photovoltaic Systems Using PSpice®, John Wiley&Sons, 2002, ISBN 0-470-845279.

Papers

	Häberlin, H., Schaerf, Ph.: New Procedure for Measuring Dynamic MPP-Tracking Efficiency at Grid-Connected PV Inverters, 24th European Photovoltaic Solar Energy Conference, Hamburg, Germany, Sept. 2009.
	Valentini, M., Raducu, A., Sera, D., Teodorescu, R.: PV inverter test setup for European efficiency, static and dynamic MPPT efficiency evaluation, Optimization of Electrical and Electronic Equipment, 2008. OPTIM 2008, May 2008.
	Bower, W. et al: Performance Test Protocol for Evaluating Inverters Used in Grid-Connected Photovoltaic Systems, Institute for Sustainable Technology. 2004.
	Esram, T., Chapman, P.L.: Comparison of Photovoltaic Array Maximum Power Point Tracking Techniques IEEE Transactions on Energy Conversion, Vol. 22, No. 2, June 2007.
	Salas, V. et al.: Review of the maximum power point tracking algorithms for stand-alone photovoltaic systems; Solar Energy Materials and Solar Cells, Elsevier, January 2006.
	Koutroulis, E. et al.: Development of a Microcontroller-Based, Photovoltaic Maximum Power Point Tracking Control System; IEEE Transactions on Power Electronics, Vol. 16, no. 1, January 2001, p.46-54.

New technologies and mathematical background

	Chiasson, J. et al.: Elimination of Harmonics in a Multilevel Converter using the Theory of Symmetric Polynomials and Resultants; IEEE Transactions on Control Systems Technology, Vol. 13, No. 2, March 2005.
	Mariethoz, S., Rufer, A.: Resolution and efficiency improvements for three-phase cascade multilevel inverters; 35th Annual IEEE Power Electronics Specialists Conference, Aachen, 2004.
	Tolbert, L.M. et al.: Charge Balance Control Schemes for Cascade Multilevel Converter in Hybrid Electric Vehicles; IEEE Transactions on Industrial Electronics, Vol. 49, No. 5, October 2002.
	Tolbert, L.M. et al.: Multilevel PWM Methods at Low Modulation Indices; IEEE Transactions on Power Electronics, Vol. 15, No. 4, July 2000.

Reports

	Grid-Connected Photovoltaic Power Systems: Survey of Inverter and related Protection Equipment; Report IEA PVPS T5-05: 2002, December 2002.
	Grid-Connected Photovoltaic Power Systems: Survey of Inverter and related Protection Equipment; Report IEA PVPS T5-05: 2002, December 2002.
	International Guideline for the Certification of Photovoltaic System Components and Grid-Connected systems; Report IEA-PVPS T5-06: 2002, February 2002
	Probability of Islanding in Utility Networks due to Grid-Connected Photovoltaic Power Systems; Report IEA-PVPS T5-07: 2002, September 2002
	Risk Analysis of Islanding of Photovoltaic Power Systems within Low Voltage Distribution Networks; Report IEA PVPS T5-08: 2002.
	Evaluation of Islanding Detection Methods for Photovoltaic Utility Interactive Power Systems; Report IEA PVPS T5-09: 2002, March 2002.
	Impacts of Power Penetration from Photovoltaic Power Systems in Distribution Networks; Report IEA-PVPS T5-10: 2002, February 2002.
	Grid-Connected Photovoltaic Power Systems: Power Value and Capacity Value of PV Systems; Report IEA-PVPS T5-11: 2002, February 2002.
	Utility Aspects of Grid-Connected Photovoltaic Systems; Report IEA-PVPS T5-01:1998, December 1998.

Other

	Bower, W. et al: Performance Test Protocol for Evaluating Inverters Used in Grid-Connected Photovoltaic Systems, Institute for Sustainable Technology. 2004.
	Cramer, G.: PV-System-Technik Entwicklungsstand und Trends in der dezentralen Elektrifizierung; SMA Solar Technology AG.
	Cramer, G., Kleinkauf, W., Ibrahim, M.: Modulare Systemtechnik für die Netzeinspeisung mit Photovoltaikanlagen; SMA Solar Technology AG, Institut für Elektrische Energietechnik, Universität Kassel.
	Cramer, G.: Dezentrale Netzeinspeisung mit String-Wechselrichtern für das 1 MWp-PV-Dach der Fortbildungsakademie in Herne; SMA Solar Technology AG.