بررسی از دست دادن کیفیت توان در ماژول پی وی ناشی از ارتعاشات حاصل از باد در وین

Mechanical vibrations of a solar module mounting rack cause oscillations in the orientation of the module towards the sun. The resulting intensity oscillations of the incident light generate an a.c. current at the module’s terminals. We have investigated this effect in the laboratory by means of a vibration table and outdoors, where wind forces induce vibrations to the mounting rack. Although the collected results are specific and restricted to our experimental set up and the regional environmental situation we deduce that vibration induced current transients and oscillations of a solar module’s output most often will be the dominant origin of distortion in the low frequency regime. Highlights ► Wind causes mechanical vibrations of a solar module mounting rack. ► This generates an a.c. current at the module’s terminals. ► This effect is the major source for distortion in the low frequency reMechanical vibrations of a solar module mounting rack cause oscillations in the orientation of the module towards the sun. The resulting intensity oscillations of the incident light generate an a.c. current at the module’s terminals. We have investigated this effect in the laboratory by means of a vibration table and outdoors, where wind forces induce vibrations to the mounting rack. Although the collected results are specific and restricted to our experimental set up and the regional environmental situation we deduce that vibration induced current transients and oscillations of a solar module’s output most often will be the dominant origin of distortion in the low frequency regime. Highlights ► Wind causes mechanical vibrations of a solar module mounting rack. ► This generates an a.c. current at the module’s terminals. ► This effect is the major source for distortion in the low frequency regime. ► The magnitude of the distortion depends highly on the circuit arrangement.gime. ► The magnitude of the distortion depends highly on the circuit arrangement.

With the steady growth of photovoltaic systems and the ongoing trend towards large scale grid connected power stations, new challenges in assuring power quality arise (Bartlett et al., 2009 and Bollen and Häger, 2005). This has invoked numerous investigations about the proper interfacing for preserving power quality and avoiding electromagnetic interference (Bollen et al., 2008 and Chicco et al., 2009). However only little research has been done in the study of distortion, caused by the power generator itself, which is transmitted to the subsequent electric conditioning system (Piazza et al., 2004). As yet the research focus clearly lies on dealing with the symptoms of distortion, rather than looking at its causes. Especially the effect of vibrational disturbances has barely been researched at all. This makes it very hard to avoid any unwanted effects during the planning and design of PV stations beforehand. For the optimization of a PV station the effect of vibrational disturbances is a very important variable. This applies particularly when choosing a proper mounting construction, or when designing the circuitry. Additionally it has to be taken into account when designing maximum power point trackers, especially those working with the perturbation and observation (P&O) method. The goal of this research is to provide a rough assessment of vibrational effects and their impact on a PV station. For this indoor and outdoor measurements have been carried through. Indoor we worked with a well defined laboratory set up, whereas the outdoor measurements are clearly exemplary and locally. However some novel results could be found. These include the behaviour of the solar cell under different circuit arrangements, the dependency of vibrational disturbances in regard to the operating point and a rough quantitative estimation of the magnitude of the effect. Beside the intrinsic noise of the photovoltaic cells numerous external sources can introduce current transients, spikes and oscillations, as illustrated by Fig. 1. These noise sources cover a wide frequency range from below 1 Hz typical for cloud movements (Jewell and Unruh, 1990) up to 10 GHz which is caused by satellite communication channels. Although the externally introduced current noise at the terminals of a single module may not exceed several mA in amplitude, electrical interconnections of an assembly of modules summarises the noise amplitudes, which are conducted to the power conditioning unit. Current transients in the low to very low frequency regime are extremely hard to remove in the circuitry by a low pass filter without the reduction of the useful d.c. output. Because of the need to optimise the exposure of solar modules to the incident sun light they are placed on mounting constructions which easily are stimulated to vibrate. These vibrations cause changes of the module’s surface orientation with respect to the incident sun rays. As a consequence light intensity oscillations cause current oscillations at the module’s output. In space applications scientists are aware of this problem and efforts were undertaken to reduce the vibrations transmitted from the satellite to the solar array paddle (Matsuno et al., 1996). For terrestrial photovoltaic applications the resonance frequency of a mounting construction is in the low to very low frequency range. As previously reported noise amplitudes below 100 Hz sometimes became the major contribution to the observed noise spectra and were attributed to wind induced vibrations (Drapalik et al., 1996). Especially large photovoltaic power stations, with at least 1 MWP are often placed in sparse regions with high sunlight exposure. This leaves the power plant exposed to wind forces, without there being any natural obstacles, resulting in higher wind speeds near ground level (Wieringa, 1986). The effect of wind velocities has been included many times as an important variable in the performance of photovoltaic power plants (Moore et al., 2005 and Moore and Post, 2008). However the effect of this variable remains uncertain or is directly linked to the thermal condition of a solar module (Hiyama and Kitabayashi, 1997 and Jones and Underwood, 2001). While the wind cooling results in a slightly increased performance of the PV-Plant, the entailed vibrational disturbances are often ignored. Currently we investigate the effect of vibration induced current oscillations in detail in the laboratory and under outdoor conditions in Vienna. In this contribution we present some exemplary results from our measurements in order to illustrate the importance of mechanical vibrations as a major source of distortion. In the laboratory we examined the effect of changing light intensity distribution caused by the vibrations relative to the incident light beam over the area of the solar cells, with respect to different loads and circuit arrangements (i.e. parallel, serial and single). Thus we were able to derive some basic characteristics of the behaviour of ultra low frequency distortion in solar modules. To supplement and confirm our results, a simulation with the circuit simulation program Qucs1 has been undertaken.

Although the presented exemplary results in a limited time range were obtained on an individual small photovoltaic system we found that vibration induced current oscillations caused by wind forces were by far the dominant source of low frequency distortions below 100 Hz. Once several conditions are met at the same time the magnitude of current oscillations easily exceeded 1% of its d.c. value. The effect of changing light intensity distribution over the area of the solar cells, with respect to different loads and circuit arrangements is yet to be fully understood. Unfortunately a clean comparison between indoor results and outdoor results, was not possible with our set up. However as well in the laboratory experiments as in the simulation it became clear that the ratio of a.c. current to amplitude is significantly higher when the cells are connected in series than when they are parallel connected. The simulation as well as the differentiated I(V)DC curve suggest that the maximum of the distortion signal lies close to but below the maximum power point of the characteristic I(V) curve in relation to the voltage. This however could not yet be confirmed or falsified by the experimental measurements conducted. In the outdoor observations the magnitudes of oscillations depend on the mounting construction and will increase with increasing wind forces. Due to thermal management of photovoltaic collectors, construction design favours good ventilation conditions, which in return eases wind attacks. Thus vibration induced distortion appears to be unavoidable. Low incident angles of direct sun radiation can reduce the magnitude of the distortion. Therefore tracked concentrating systems potentially will experience less distortion than building integrated photovoltaic facades. Although in the first case the mounting system permits large displacements caused by external forces it is permanently facing the sun. Due to their large inclination angles facades are considerably misoriented towards the sun most of their operation time.