1130 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 19, NO. 4, JULY 2004Cascaded DC–DC Converter Connection ofPhotovoltaic ModulesGeoffrey R. Walker, Member, IEEE, and Paul C. SerniaAbstract—New residential scale photovoltaic (PV) arrays arecommonly connected to the grid by a single dc–ac inverter con-nected to a series string of pv panels, or many small dc–ac inverterswhich connect one or two panels directly to the ac grid. This paperproposes an alternative topology of nonisolated per-panel dc–dcconverters connected in series to create a high voltage string con-nected to a simplified dc–ac inverter. This offers the advantagesof a “converter-per-panel” approach without the cost or efficiencypenalties of individual dc–ac grid connected inverters.Buck, boost, buck-boost, and Cúk converters are considered aspossible dc–dc converters that can be cascaded. Matlab simulationsare used to compare the efficiency of each topology as well as eval-uating the benefits of increasing cost and complexity. The buck andthen boost converters are shown to be the most efficient topologiesfor a given cost, with the buck best suited for long strings and theboost for short strings. While flexible in voltage ranges, buck-boost,and Cúk converters are always at an efficiency or alternatively costdisadvantage.Index Terms—Cascaded converter, grid connected photovoltaic(PV), Matlab efficiency simulation, maximum power pointtracking (MPPT), module integrated converters (MICs),multilevel converter, series connected converter.I. INTRODUCTIONWITH AN increasing worldwide interest in sustainableenergy production and use, there is renewed focus onthe power electronic converter interface for dc energy sources.Three specific examples of such dc energy sources that will havea role in distributed generation and sustainable energy systemsare the photovoltaic (PV) panel [1], the fuel cell stack [2], andbatteries of various chemistries [3].II. SERIES CONNECTED PV PANELSThese dc energy sources are all series and parallel con-nections of a basic “cell.” These cells all operate at a lowdc voltage, ranging from less than 1 V (PV cell) to 3 or 4 V(Li–Ion cell). These low voltages do not interface well toexisting higher power systems, so the cells are series connectedto create a battery, a fuel cell stack, or a PV module or panelwith a higher terminal voltage. (The term PV panel rather thanPV module will be used in this paper to avoid confusion withthe proposed attached power electronic modules.)For example “12-V” PV panels have 36 solar cells with amaximum power point (MPP) of approximately 16–17 V understandard test conditions. These system voltages are appropriateManuscript received February 13, 2003; revised September 12, 2003. Rec-ommended by Associate Editor Z. Chen.The authors are with the School of Information Technology and ElectricalEngineering, The University of Queensland, Brisbane 4072, Australia (e-mail:[email protected]; [email protected]).Digital Object Identifier 10.1109/TPEL.2004.830090for lower power systems, but beyond powers of a few hun-dred Watts (W), these panels themselves are placed in seriesstrings to maintain lower currents and higher efficiencies. Theselong strings of panels (and hence cells) bring with them manycomplications.PV panels in a string are never exactly identical. Because PVpanels in a series string are constrained to all conduct the samecurrent, the least efficient panel, and indeed cell, sets this stringcurrent. The overall efficiency of the array is reduced to the ef-ficiency of this cell. This also means that PV panels in a stringmust be given the same orientation and be of identical size.A more profound problem occurs when even a single cellin the array is shaded. The photocurrent generated in a shadedcell may drop to perhaps 20% of the other cells. The shadedcell will be reverse biased by the remaining cells in the seriesstring, but current will continue to flow through it causing largelocalized power dissipation. A diode around a group of 18 cells(half a 12-V panel) limits the reverse bias and hence the powerdissipation in the shaded cell. However, all the power from thatsub string is lost while current flows in the bypass diode [1].Placing a dc–dc converter on each half-panel or panel sub-string, and then connecting theseconverters in series stringsavoids many of these problems. This paper examines the ad-vantages, difficulties, and implementation issues of using a cas-caded converter connection for a series string of PV panels, ormore generally dc energy sources. A proposed residential gridconnected solar installation consisting of twelve 12-V 60-W PVpanels is used where a specific example is helpful in developingthe discussion.III. CONVERTER INTERFACE OF PV PANELSIn grid-connected inverters for PV applications, a number ofdifferent approaches have been developed and used over the last20 years. An excellent review of such systems available in Eu-rope is given in [4]. Only the two more common approachesused in smaller residential scale installations (1–3 kW) are com-pared here (see Fig. 1).A. Single DC String, Single DC–AC InverterIn a residential system of say 2 kW or less, all the PV panelson the rooftop can be connected electrically in series, to createa high voltage low current dc source. This source is connectedto a single dc–ac inverter within the roof or house. The ac thenruns to the residential switchboard.B. Individual DC–AC Inverters per Panel (Module IntegratedConverters)In this more recent approach, each PV panel has its own dc–acinverter, mounted at the panel on the rooftop. A 240-V ac con-nection from the switchboard runs to the rooftop, and loops from0885-8993/04$20.00 © 2004 IEEEWALKER AND SERNIA: CASCADED DC–DC CONVERTER CONNECTION 1131Fig. 1. Comparison of three grid connected PV inverter topologies discussedin the text—a single dc–ac inverter connected to a single dc PV string (top); amodule integrated dc–ac inverter for every PV panel (middle); or the proposedseries connected panel integrated dc–dc converters connected to a centraliseddc–ac inverter (bottom).inverter to inverter, panel to panel. Each panel is now effectivelyplaced in parallel, via its own dedicated inverter.To be small, light and low cost, module-integrated convertersgenerally use high frequency switch mode techniques. To effi-ciently convert the panel’s low dc voltage to the 240-V ac gridvoltage they invariably require a
View Full Document
Unlocking...