PV STRING PER-MODULE MAXIMUM POWER POINT ENABLING CONVERTERS G.R. Walker, J. Xue and P. Sernia School of Information Technology and Electrical Engineering The University of Queensland Abstract Many grid connected PV installations consist of a single series string of PV modules and a single DC-AC inverter. This efficiency of this topology can be enhanced with additional low power, low cost per panel converter modules. Most current flows directly in the series string which ensures high efficiency. However parallel Cúk or buck-boost DC-DC converters connected across each adjacent pair of modules now support any desired current difference between series connected PV modules. Each converter “shuffles” the desired difference in PV module currents between two modules and so on up the string. Spice simulations show that even with poor efficiency, these modules can make a significant improvement to the overall power which can be recovered from partially shaded PV strings. 1 SERIES PHOTOVOLTAIC MODULE CONNECTION Three specific examples of such DC energy sources that will have a role in distributed generation and sustainable energy systems are the photovoltaic (PV) panel, the fuel cell stack, and batteries of various chemistries. These DC energy sources are all series and parallel connections of a basic “cell”. These cells all operate at a low DC voltage ranging from less than a volt (PV cell) to three or four volts (Li-Ion cell). These low voltages do not interface well to existing higher power systems, so the cells are series connected to create a module with a higher terminal voltage. Focusing on PV systems, a typical “12 Volt” PV module or panel has 36 series connected solar cells with a maximum power point (MPP) of approximately 15V at normal operating temperatures (approx 50°C). These system voltages are appropriate for lower power systems, but beyond powers of a few hundred Watts, these panels themselves are placed in series strings – PV arrays – to maintain lower currents and higher efficiencies. The terms PV cell, PV panel and PV array will be used in this paper, to avoid confusion with the term “module” which may be used to refer to the power electronic converter associated with a panel. These long strings of cells bring with them many complications. A problem occurs when even a single cell in the array is shaded or obscured. The photocurrent generated in a shaded cell may drop to perhaps 20% of the other cells. The shaded cell will be reverse biased by the remaining cells in the string, but current will continue to flow through it causing large localised power dissipation. Bypass diodes, generally placed in parallel around each 18 cells (half a panel), limit the reverse bias voltage and hence the power dissipation in the shaded cell to that generated by the surrounding half panel. However, all the power from that sub string is lost while current flows in the bypass diode [1,5,6]. Module MPP currents may be permanently unbalanced for other reasons. PV modules in a string are never exactly identical. Because PV modules in a series string are constrained to all conduct the same current, the least efficient module sets this string current. The overall efficiency of the array is reduced to the efficiency of this module. For similar reasons PV panels in a string should be given the same orientation, and be of identical size. This is not always possible or desirable for ascetic or other architectural reasons. An example of a PV array with a curved surface [2] is shown in Fig.1. 2 PV MODULE CONVERTER CONNECTION 2.1 Current approaches for Grid Connection In grid-connected inverters for PV applications, a number of different approaches have been developed and used over the last 20 years. An excellent review of such systems available in Europe is given in [3]. Only the two most common approaches used in smaller residential scale installations (1-3kW) are compared here. The original approach was to create a single high voltage DC series string connected to a single DC-AC inverter: In a residential system of say 1.5kW (a typical size) all the PV panels on the rooftop can be connected electrically in series, to create a high voltage (360V) lowcurrent (4.5A) DC source. This source is connected to a single DC-AC inverter within the roof or house. The AC then runs to the residential switchboard. Note that this approach uses a single series string of modules, and so can only search for and operate at a global Maximum Power Point (MPP). Fig.1 This curved roof of a commercial building conservatory space in the Netherlands is formed by PV modules [2]. Such curved PV surfaces limit string connection options. The more recent Module Integrated Converter (MIC) approach is to mount individual DC-AC inverters per PV module, mounted at the module on the rooftop. A 240Vac connection from the switchboard runs to the rooftop, and loops from inverter to inverter, panel to panel. Each panel is now effectively placed in parallel, via its own dedicated inverter. Each panel can now operate at its own MPP independent of other panels. To be small, light and low cost, module-integrated converters generally use high frequency switch mode techniques. They require several conversion stages to efficiently convert the module’s low DC voltage to the 240Vac grid voltage – a boost stage probably including an isolation transformer, rectification to a high voltage DC bus, and an AC inversion stage. An example of a European 100W 24Vdc – 240Vac MIC before potting [4] is shown in figure 2. Fig.2 A 100W module integrated converter [4]. Compared to a single central converter, a per module collection of these converters will certainly be more expensive to purchase. This may be justified by simpler installation and protection and by the advantages of per module conversion – individual MPP tracking and module independence and thus higher reliability. 2.2 Alternatives for MPP tracking of PV arrays Several alternative configurations have been proposed which will allow per module MPP tracking, without resorting to the MIC approach of attaching a complete DC-AC grid connection converter to each PV module. Shimizu [5,6] gives two versions of a “Generation Control Circuit” (GCC) which may be placed in parallel with each PV module of a series string of modules to allow independent MPP tracking. The first is an
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