Photovoltaic inverters

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Inverters

"When wealth is lost, nothing is lost; When health is lost, something is lost; When character is lost, all is lost."
(German proverb)

Mastervolt inverter Steca inverter SMA Multi-String inverter Solar Fabrik inverter

Photovoltaic inverters produced by Mastervolt, STECA, SMA Technologie AG and Solar-fabrik
(courtesy: Mastervolt, Steca, SMA, Solar-fabrik).

Inverters are used for DC voltage to AC voltage conversion. According to output voltage form they could be rectangle, trapezoid or sine shaped. The most expensive, yet at the same time the best quality inverters, output voltage in sine wave. Inverter input voltage depends on inverter power, for small power of some 100 W the voltage is 12 or 24 V, and 48 V or even more for higher powers. Large inverters could be connected in parallel when higher powers are required. For large systems 3-phase inverters are available in 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 we have many different types of inverters, such as central inverters for wide power range from 1 kW to up to 100 kW or even more, string inverters and module inverters. Central inverters are used in large applications. Many times they 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 are used in small photovoltaic systems. Such solutions are applicable to larger systems, however, in practice cheaper, and less reliable solution of central inverter or string inverters are used. Special design inverters are available for the purposes of 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.

Inverters - features and applications

Technical parameters

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 - V_DC
MPP voltage range - V_MPP
Maximum DC voltage - V_DCmax
Switch off voltage - V_DCoff
Rated AC voltage - V_AC

Rated DC current - I_DC
Maximum DC current - I_DCmax
Rated AC current - I_AC
Maximum AC current - I_ACmax

Rated DC power - P_DC
Maximum DC power - P_DCmax
Rated AC power - P_AC
Maximum AC power - P_ACmax
Power factor - cos φ
DC power Off - P_DCoff
DC power On - P_DCon
Standby DC power - P_DCStandBy
Night mode DC power - P_night

Noise level - dB_A
Temperature range - T
Total Harmonic Distortion - k

Inverter - electronics, courtesy SMA Technologie AG

Inverters are most sophisticated widely used electronic devices. SMA's inverter electronic control section is presented on the picture above
(courtesy: SMA Technologie AG).

Inverter efficiency

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

η = P_AC / P_DC

To make comparison of different inverters and/or inverters that are operating under different climatic conditions possible, "Euro η" efficiency was defined:

η_EURO = 0.03×η_5% + 0.06×η_10% + 0.13×η_20% + 0.1×η_30% 0.48×η_50% + 0.2×η_100%

Output wave forms for line-commutated, PWM and cascade inverter

Voltage and current in line-commutated (top) in PWM self commutated inverters (middle) and in cascade inverters (bottom)

Inverter 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 as shown in the picture.

More about inverter technologies

IEA-PVPS: Grid-Connected Photovoltaic Power Systems: Survey of Inverter and related Protection Equipment; Report IEA PVPS T5-05: 2002, December 2002 (800 kB).

Thyristor and MOSFET bridge as switching element. Thyristor bridges are used in line-commutated inverters. In self-commutated inverters MOSFETs, IGBTs or GTO thyristors are most common switching elements used.

Inverter - main parts

Main parts of an inverter are presented on the picture below - example of a 2100TL SMA's inverter without output transformer:

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 (RS232, RS485 for example). For more information about MPP operation please see charge regulators section.

SB 2100TL inverter - courtesy SMA Technologie AG

SB 2100TL inverter
(source/copyright: SMA Technologie AG)

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.

Multistring inverter - courtesy SMA Technologie AG

Multi string inverter - each string is connected to its own input
(source/copyright: SMA Technologie AG)

Inverters - interconnection and control

Central or distributed inverter sollutions

Centralized or distributed inverter solutions: central inverter, "master-slave" string inverters, "PSC" or "Teamkonzept" connection.

Test reports available on-line
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

On-Line available configuration tools
	Fronius Configurator - xls sheet for photovoltaic system configuration using Fronius inverters. For german xls sheet please see Fronius Configurator web page. Languages:
	KacoCalc PRO - PV-Modul Auslegungsprogramm für KACO Wechselrichter. Available as ZIP file (file size 2200 kB). Languages:
	GenAu - PV-GenAu excel table for exact calculation of solar array. Separate sheet for wire sizing also available (german language only). Available as excel file (file size 5414 kB). Languages: Sunny Design - Software for easy design of a PV plants. Available as exe file (file size 28724 kB). Languages:
	MaxDesign - MaxDesign is a software for PV plants - where SolarMax (Sputnik Engineering) inverters are used - configuration. Available as ZIP or self extract exe file (file size 18621 kB). Languages:
	Profiplan Wechselrichter-Auslegungsprogramm - xls sheet for configuring photovoltaic system using inverters produced by Solar Fabrik AG (german language). Languages:
	PV system design calculator - xls sheet for configuring photovoltaic system using Mastervolt inverters. Languages:
	Conergy sizer - the Conergy Sizer is a user-friendly system configuration program for installers and planners. This software is an efficient tool for finding the appropriate inverters for a planned photovoltaic system. Languages:
	Aurora Designer - System Configuration & Sizing Tool (639 KB). Languages:
	Auslegung Powerstocc WR - software for configuring photovoltaic systems using Powerstocc inverters (german language only). Languages:
	SITOP Solar Select - The program calculates all suitable configuration data capabilities for SITOP solar inverters with the photovoltaic modules of your choice. The module allows you to find out details of all currently available on the market. Calculations are based on real measured values of irradiation in minute resolution and is available for download at no cost. Languages:
	SP SIZER - xls sheet for configuring photovoltaic system using "Sun Profi" inverters produced by Sun Power Solartechnik GmbH (german language only). Languages:
	GT Series String Sizing Tool - on-line sizing tool for grid-connected photovoltaic systems with Xantrex GridTie invertres. Languages:

Inverters - research topics
	PV Labor der HTI Bern - 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. Languages: Webmaster's choice - one of the best inverter research related web sites. Detailed inverters test reports and monitoring data available on-line. Detailed plant descriptions and comprehensive literature section.
Inverters - electronics, application notes
	Texas Instruments - Solar Power Inverters DSC application notes for power inverters. Languages:
Inverters - companies
	SMA - SMA Technologie AG has been successfully producing industrial computers, photovoltaic (PV) inverters, and energy supply systems for 20 years. Languages: Webmaster's choice - interesting download area with many excellent documents available as pdf, xls or graphic files.
	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. Languages: Webmaster's choice - interesting site with photo gallery and sizing section.
	Solar Max - Inverters produced by Sputnik Engineering. Languages:
	Conergy - produces inverters, modules and mounting structures. Languages:
	Kaco Gerätetechnik GmbH - KACO Gerätetechnik is a powerful partner for system solutions in the field of current inverters, power electronics and photovoltaics. Languages:
	PV Powered - Headquartered in Bend, Oregon, PV Powered designs, manufactures and markets utility interactive photovoltaic inverters for the renewable energy marketplace. PV Powered's one-of-a-kind StarInverter product line offers PV solutions for the residential and commercial markets. Languages:
	Xantrex - Xantrex Technology develops, manufactures and markets advanced power electronic products. The Xantrex's products convert raw electrical power from any central, distributed, or backup power source into high-quality power required by electronic and electrical equipment. Languages: Webmaster's choice - comprehensive download section. Many interesting documents available as pdf files.
	OutBack Power Systems - OutBack Power Systems is an engineer-owned, customer focused, power systems equipment manufacturer. Languages: Webmaster's choice - comprehensive download section - images, wiring diagrams, technical drawings, manuals, prices etc...
	Mastervolt - Mastervolt is one of the leading manufactures of high quality electrical systems for marine, mobile, industrial and solar applications in Europe and beyond. Languages: Webmaster's choice - comprehensive download and interesting sizing section.
	Magnetek Inc. - Magnetek, Inc. produces power control products and systems for communications, data processing and storage, medical electronics, consumer products, industrial automation, transportation, energy management and other applications. Languages:
	Solarstocc AG - inverter series ranging from 1 to 6 kW, using the latest technology. Languages:
	Siemens - SITOP and SINVERT inverter series. Languages:
	Sun Power Solartechnik GmbH - Sun Power is manufacturing custom made PV-system equipment since 20 years, being applied in more than 50 countries worldwide. Applications like village supply in Tibet or water pumping stations in Africa require very reliable system components to allow a long lasting system operation. With this background standard power conditioning units have been designed for various kind of application. Languages:
	Panelektron - Company Panelektron located in Hungary produces sine wave inverters, solar charge controllers, switch mode power converters, mains battery chargers and other electronic equipment. Languages:
	Grupo Ingeteam - Ingeteam Group is made up of 26 companies, all of which are at the technological forefront in their respective areas. Languages:

Literature and more information

Koutroulis, E., Kalaitzakis, K., Voulgaris, N.C.: 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.
Salas, V., Olías, E., Barrado, A., Lázaro, A.: Review of the maximum power point tracking algorithms for stand-alone photovoltaic systems; Solar Energy Materials and Solar Cells, Elsevier, January 2006, article available on-line (597 kB).
Cramer, G.: PV-System-Technik Entwicklungsstand und Trends in der dezentralen Elektrifizierung; SMA Technologie AG (518 kB).
Cramer, G., Kleinkauf, W., Ibrahim, M.: Modulare Systemtechnik für die Netzeinspeisung mit Photovoltaikanlagen; SMA Technologie AG, Institut für Elektrische Energietechnik, Universität Kassel (148 kB).
Cramer, G.: Dezentrale Netzeinspeisung mit String-Wechselrichtern für das 1 MWp-PV-Dach der Fortbildungsakademie in Herne; SMA Technologie AG (249 kB).
IEA-PVPS: Grid-Connected Photovoltaic Power Systems: Survey of Inverter and related Protection Equipment; Report IEA PVPS T5-05: 2002, December 2002 (800 kB).
IEA-PVPS: International Guideline for the Certification of Photovoltaic System Components and Grid-Connected systems; Report IEA-PVPS T5-06: 2002, February 2002 (1272 kB).
IEA-PVPS: Probability of Islanding in Utility Networks due to Grid-Connected Photovoltaic Power Systems; Report IEA-PVPS T5-07: 2002, September 2002 (1422 kB).
IEA-PVPS: Risk Analysis of Islanding of Photovoltaic Power Systems within Low Voltage Distribution Networks; Report IEA PVPS T5-08: 2002, March 2002 (805 kB).
IEA-PVPS: Evaluation of Islanding Detection Methods for Photovoltaic Utility Interactive Power Systems; Report IEA PVPS T5-09: 2002, March 2002 (1413 kB).
IEA-PVPS: Impacts of Power Penetration from Photovoltaic Power Systems in Distribution Networks; Report IEA-PVPS T5-10: 2002, February 2002 (657 kB).
IEA-PVPS: Grid-Connected Photovoltaic Power Systems: Power Value and Capacity Value of PV Systems; Report IEA-PVPS T5-11: 2002, February 2002 (830 kB).
IEA-PVPS: Utility Aspects of Grid-Connected Photovoltaic Systems; Report IEA-PVPS T5-01:1998, December 1998 (1948 kB).

Mathematical background and new sollutions

Chiasson, J., Tolbert, L.M., McKenzie, K., Du, Z.: 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 (414 kB).
Mariethoz, S., Rufer, A.: Resolution and efficiency improvements for three-phase cascade multilevel inverters; 35th Annual IEEE Power Electronics Specialists Conference, Aachen, 2004 (410 kB).
Tolbert, L.M., Cheng, P.F., Chiasson, J., Cunnyngham, T.: Charge Balance Control Schemes for Cascade Multilevel Converter in Hybrid Electric Vehicles; IEEE Transactions on Industrial Electronics, Vol. 49, No. 5, October 2002 (291 kB).
Tolbert, L.M., Cheng, P.F., Habetler, T.G.: Multilevel PWM Methods at Low Modulation Indices; IEEE Transactions on Power Electronics, Vol. 15, No. 4, July 2000 (222 kB).