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, IGBT or SiC transistors.

Inverter Construction

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

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:

TABLE 1: Inverter, electrical parameters - voltage and current

TABLE 2: Inverter, electrical parameters - power

TABLE 3: Non-electrical parameters of inverters

Efficiency

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

[Equ 1]

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:

[Equ 2]

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

[Equ 3]

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 = 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.

Web Sites

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.

Sources and Additional Information - Books

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

Reports

	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.

Papers

	Bendib, B. et al. (2015), A survey of the most used MPPT methods: Conventional and advanced algorithms applied for photovoltaic systems, Renewable and Sustainable Energy Reviews, vol. 45, pp. 637-648.
	Esram, T., Chapman, P.L. (2007), Comparison of Photovoltaic Array Maximum Power Point Tracking Techniques, IEEE Transactions on Energy Conversion, vol. 22, no. 2, pp. 449-459.
	Häberlin, H., Schaerf, Ph. (2009), New Procedure for Measuring Dynamic MPP-Tracking Efficiency at Grid-Connected PV Inverters, 24th European Photovoltaic Solar Energy Conference, Hamburg, Germany.
	Valentini, M. et al. (2008), PV inverter test setup for European efficiency, static and dynamic MPPT efficiency evaluation, Optimization of Electrical and Electronic Equipment, OPTIM 2008, May 2008.
	Salas, V. et al. (2006), Review of the maximum power point tracking algorithms for stand-alone photovoltaic systems; Solar Energy Materials and Solar Cells, vol. 90, no. 11, pp. 1555-1578.