Bonfiglioli’s products for the PV sector include indoor and outdoor rated inverters, from 3 MW turnkey solutions down to 30 kW compact devices. Thanks to a consequent application of a modular system design, Bonfiglioli inverters can be matched precisely to individual plants’ requirements and guarantee world class productivity and availability. All products are designed at Bonfiglioli’s inverter competence centre in Germany to deliver superior quality and lasting efficiency.

Some datasheets of Bonfiglioli's inverters and related products are available here:

RPS 450 Compact Outdoor
RPS Compact
RPS Station
RPS TL Modular Outdoor
RPS TL
SCB String Connection Box
RPS Endurance

RPS Log 1000
RPS Portal
RPS Sensor Box


Photovoltaic Inverters

"Anyone who conducts an argument by appealing to authority is not using his intelligence; he is just using his memory." (Leonardo da Vinci)

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 Inverter construction, courtesy SMA

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.).

Inverter construction, courtesy SMA

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:

PDC - DC array power, PAC - 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.

ONLINE CONFIGURATION TOOLS

INVERTER PRODUCERS

String and central inverters

INVERTER RELATED WEB SITES

Research and design

SOURCES AND ADDITIONAL INFORMATION

Books

book

Zacharias, P. (Editor): Use of Electronic-Based Power Conversion for Distributed and Renewable Energy Sources, ISET Kassel, 2008.

book

Häberlin, H.: Photovoltaik, Strom aus Sonnenlicht für Verbundnetz und Inselanlagen; VDE Verlag, 2007, ISBN 978-3-8007-3003-2.

book

Teodorescu, R. et al: Grid Converters for Photovoltaic and Wind Power Systems; Willey 2011, ISBN 9780470057513.

book

Castañer, L., Silvestre, S.: Modelling Photovoltaic Systems Using PSpice®, John Wiley&Sons, 2002, ISBN 0-470-845279.

Papers

zapiski

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.

zapiski

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.

zapiski

Bower, W. et al: Performance Test Protocol for Evaluating Inverters Used in Grid-Connected Photovoltaic Systems, Institute for Sustainable Technology. 2004.

zapiski

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.

zapiski

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.

zapiski

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

advanced

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.

advanced

Mariethoz, S., Rufer, A.: Resolution and efficiency improvements for three-phase cascade multilevel inverters; 35th Annual IEEE Power Electronics Specialists Conference, Aachen, 2004.

advanced

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.

advanced

Tolbert, L.M. et al.: Multilevel PWM Methods at Low Modulation Indices; IEEE Transactions on Power Electronics, Vol. 15, No. 4, July 2000.

Reports

report

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

report

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

report

International Guideline for the Certification of Photovoltaic System Components and Grid-Connected systems; Report IEA-PVPS T5-06: 2002, February 2002

report

Probability of Islanding in Utility Networks due to Grid-Connected Photovoltaic Power Systems; Report IEA-PVPS T5-07: 2002, September 2002

report

Risk Analysis of Islanding of Photovoltaic Power Systems within Low Voltage Distribution Networks; Report IEA PVPS T5-08: 2002.

report

Evaluation of Islanding Detection Methods for Photovoltaic Utility Interactive Power Systems; Report IEA PVPS T5-09: 2002, March 2002.

report

Impacts of Power Penetration from Photovoltaic Power Systems in Distribution Networks; Report IEA-PVPS T5-10: 2002, February 2002.

report

Grid-Connected Photovoltaic Power Systems: Power Value and Capacity Value of PV Systems; Report IEA-PVPS T5-11: 2002, February 2002.

report

Utility Aspects of Grid-Connected Photovoltaic Systems; Report IEA-PVPS T5-01:1998, December 1998.

Other

report

Bower, W. et al: Performance Test Protocol for Evaluating Inverters Used in Grid-Connected Photovoltaic Systems, Institute for Sustainable Technology. 2004.

report

Cramer, G.: PV-System-Technik Entwicklungsstand und Trends in der dezentralen Elektrifizierung; SMA Solar Technology AG.

report

Cramer, G., Kleinkauf, W., Ibrahim, M.: Modulare Systemtechnik für die Netzeinspeisung mit Photovoltaikanlagen; SMA Solar Technology AG, Institut für Elektrische Energietechnik, Universität Kassel.

report

Cramer, G.: Dezentrale Netzeinspeisung mit String-Wechselrichtern für das 1 MWp-PV-Dach der Fortbildungsakademie in Herne; SMA Solar Technology AG.

Last modified: 3/7/2013 10:34:29 PM