SUMMARYAn advanced method of analysis for grid-connected PV systems is developed in this research. To investigate the issues which may arise in the clustered PV systems, a "Demonstration Project on Clustered PV Systems" was initiated in December 2002 in Oota, Japan, involving the installation of more than 500 residential PV systems in the demonstrative research area, and the development of battery-integrated PV systems to avoid restrictions on output power due to the raising of the grid voltage. The annual performance of commercial PV systems without battery was analyzed and a performance ratio of 80% on average was found to have been achieved. Overvoltage of power distribution lines and snow are two major factors capable of causing very low performance ratio on a daily basis. The effects of batteries have also been analyzed, and the results indicate that there will be some reduction of energy losses due to the grid voltage, but the PCS efficiency will be at least 8% worse than that of commercial PV systems. It was also found that nonoptimized battery operation sometimes results in a fully charged situation at noontime and maximum reverse power flow may not be minimized in this situation.
Voltage of power distribution line will be increased due to reverse power flow from grid-connected PV systems. In the case of high density grid connection, amount of voltage increasing will be higher than the stand-alone grid connection system. To prevent the over voltage of power distribution line, PV system's output will be restricted if the voltage of power distribution line is close to the upper limit of the control range. Because of this interaction, amount of output loss will be larger in high density case. This research developed a quantitative analysis method for PV systems output and losses to clarify the behavior of grid connected PV systems. All the measured data are classified into the loss factors using 1 minute average of 1 second data instead of typical 1 hour average. Operation point on the I-V curve is estimated to quantify the loss due to the output restriction using module temperature, array output voltage, array output current and solar irradiance. As a result, loss due to output restriction is successfully quantified and behavior of output restriction is clarified.
In the case that a lot of grid-connected photovoltaic (PV) systems are installed in the residential area, voltage increasing of power distribution line due to the reverse power flow from the PV systems would be the problem. To prevent the over-voltage of the power distribution line, Japanese PV system's power conditioning subsystems (PCS) is monitoring its own output voltage, and if it is higher than the specification of the voltage, PCS will automatically regulate its output. Because of this function, PV system's output will be restricted even if PV array is receiving enough solar irradiance.To investigate the issues which may happen in the clustered PV systems such as over voltage, islanding and harmonic, "Demonstrative research on clustered PV systems" has been conducting from December, 2002. More than 500 PV systems will be installed and connected to the commercial power grid in the demonstrative research area in Oota, Japan. Battery integrated PV systems (see Fig. 1) are also developed to avoid the restriction of output due to the voltage raising of power distribution line.Advanced analysis method is developed to analyze both commercial PV systems and developed battery integrated PV systems. Performance ratio is used for the analysis of each system's performance. The following loss factors are considered and energy losses are quantified for each factor.• PCS off • PCS capacity shortage • Output regulation due to the grid voltage • Array temperature increasing • Fluctuation • PCS DC/AC inversion • Variation of modules, Dirt, degradation • Reflection due to the incident angle • DC circuit resistance • Shading • MPP mismatch • Battery As a result, annual performance ratio of commercial PV systems Fig. 1. Schematic view of battery-integrated PCS which don't have battery is around 80% in average (see Fig . 2) and energy loss due to the grid voltage is only 0.3%. However, more than 50% of the possible energy out is lost due to the grid voltage in daily performance analysis. Another major loss factor is snow coverage. Except these two factors, PV system's performances are very stable. Total efficiency of battery integrated PCS is around 8% less than that of commercial PCS's. Energy loss due to the grid voltage is almost 0% (see Fig. 3). Maximum reverse power flows from the systems are also compared to see the effect of battery, almost no differences are observed in 9 months average results.
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