The typical faults of solar panels have essentially mechanical and electrical nature. The solar cells are subject to a short-circuit failure or interruption, making the panel unusable for the production of electricity. In case of short circuit of the photovoltaic cell, the output voltage of the solar panel drops to about 0.5 volts becoming unusable. If the panel is connected in series with the load it is possible that still allows its use. In case of interruption of the connections panel becomes completely out of order. The interruption in a photovoltaic cell can be caused by external ruptures caused by foreign objects fallen on the panel or even the obscuration caused by the accumulation of leaves.
One of the main problems that can occur after the installation of solar panels is to water seepage. The infiltration clearly can damage the solar cells and even cause a short circuit. To overcome the problem of infiltration is necessary to pay great attention to the mounting brackets of the panel. Normally solar panels are anchored above the tiles using the appropriate brackets that can be affixed to balance the roof protecting it from dangerous inclinations which may facilitate the penetration of rainwater.
Shadow effect on the panel
When a panel is uniformly illuminated photovoltaic cells each provide around 0.5 volts and a current dependent on the load, the characteristics of the cell and insolation. In the moment in which a single cell is in the shade, for example, for the fall of external bodies and leaves, the cell behaves as a resistance to the normal flow of electricity. The voltage produced by other photovoltaic cells is concentrated at the ends of the cell vision that is polarized inversely. When solar panels are connected in series this condition is hardly tolerated by the single photovoltaic cell, on which falls the entire nominal voltage of the system. Conversely, when the panels are connected in series obscured the cell must withstand the maximum voltage generated by the panels to which it belongs.
Panel protection from lightning
For photovoltaic panels, which are located in isolated places (eg in high mountains) and with a surface, generally very large, lightning is an important component of risk, to assess both the direct effects of lightning surges on the panel for generated on the system.
The risk analyzes conducted on photovoltaic systems take into account the following criteria: size, structure and exposure of the PV system, but also density and frequency of lightning strikes at the installation site.
The effects of lightning on photovoltaic generators can affect the entire system, because of the interconnection between the photovoltaic system and the electrical system of the building. Moreover, evaluation of the investment on the implementation of a PV system must be taken into consideration the possible risks of economic losses.
To reduce this risk, it is useful to place a wire guard making sure that the shadow does not affect the performance of solar panels. Furthermore, it is useful to leave mackerel corners of the roof on which the panels are installed to reduce the risk of lightning side. The cage must eventually be grounded using the support structure of the solar panels. Given the consequences and dangers of lightning and the lightning protection grounding of the solar panels must always be carried out by professional electricians.
What are inverters?
An inverter is an electronic apparatus capable of converting direct current into alternating current possibly different voltage, or an alternating current in another of a different frequency. This device converts the direct current generated by the solar cells into alternating current, and connects synchronously to the grid to power the main uses of your home.
Since the control unit is powered by photovoltaic modules, the apparatus is switched off completely during the night and then absolutely does not consume energy.
It is a particular type of inverter designed specifically to convert the electrical energy in the form of direct current produced by the photovoltaic module, into alternating current to be input directly into the grid. These machines extend the basic function of a generic inverter with extremely sophisticated and advanced functions, through the use of special control systems software and hardware that allow you to extract the maximum power from the solar panels available in all weather conditions. This function is called MPPT, an acronym of English origin which stands for Maximum Power Point Tracker.
The photovoltaic modules in fact, have a characteristic V / I such that there exists an optimal working point, known as the Maximum Power Point, where you can extract all the power available. This point of the characteristic varies continuously as a function of the level of solar radiation that strikes the surface of the cells. It is evident that an inverter able to remain "hooked" at this point, will always obtain the maximum power available in any condition. There are various techniques of realization of the MPPT function, which differ for dynamic performance (settling time) and accuracy. Although the precision MPPT is extremely important, the settling time it is, in certain cases, even more. While all manufacturers of inverters are able to obtain great accuracy sull'MPPT (typically between 99 to 99.6% of the maximum available), only a few manage to combine precision speed.
It is in fact on days with variable cloudiness occurring fluctuations in solar output large and sudden. It is very common to detect changes from 100W / m² to 1000-1200W / m² in less than 2 seconds. In these conditions, which are very frequent, an inverter with settling times less than 5 seconds can produce up to 15% -20% of energy in more than one slow. Some PV inverters are equipped with modular power stages, and some are even equipped with a MPPT for each power stage. In this way the producers leave system engineering the freedom to configure a master / slave or independent MPPT.
Another important characteristic of a photovoltaic inverter, is the network interface. This function, generally integrated in the machine, must meet the requirements set by the regulations of the various entities of the supply of electricity. In Italy, ENEL DK5940 issued the legislation, currently junta edition 2.2. This legislation provides a number of security measures to prevent the introduction of energy into the electricity grid if the parameters of this, are outside the limits of acceptability.
We can distinguish two types of inverters, in relation to the type of application:
Inverter for grid-connected systems
Inverter for isolated systems.
Inverter for grid-connected systems
They have the following characteristics:
technology sine wave constructed with reference to the mains voltage
leading performance and stability under normal irradiation conditions
utilities available for single-phase and three-phase loads on a wide range of power
network protections and interface integrated
display for data display production
additional modules to measure the irradiation, temperature, etc.
remote data transmission for the purpose of supervision
inability to island operation (it requires the presence of mains voltage)
Inverter for isolated systems
Here are the main types:
Inverter square wave
risk of generation of odd harmonics
no adjustment of the output voltage (varies with load and with the input voltage)
Modified sine wave inverter
less harmonics of the square
precise adjustment of the voltage
appropriate for powering many devices (TV, engines, hacksaws)
Sine wave inverter
technique similar to that of the inverter for connection to the network, but with simpler circuits,
unprotected and network synchronization
high yields, suitable for practically all types of users
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