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Diodes and fuses to make parts of the plant safe from electrical nature damages

Published : 07/13/2018 15:27:24
Categories : Photovoltaic Panels

KINDS OF DIODES

Standard diodes are the most basic ones in the in diodes class. They usually have a upper-middle voltage drop of the direct voltage and a low maximum of rated current. A common example of a diode is the 1N4148.

A rectifier diode is instead a serial diode with a higher rated current. This superior rated current is usually at expense of a higher direct voltage, such as the 1N4001.

Another common diode is the Schottky diode, commonly used in photovoltaic plants. The semiconductor composition of a Schottky diode is slightly different from a normal one, and this means a minor voltage drop which is normally between 0.15V and 0.45V. The advantage of this diode typology is that it it is particularly useful to limit potential loss.

Other typologies are the Zener diodes an the photodiodes.

DIODES APPLICATIONS

This makes Schottky diodes a perfect choice as protection and inversion diodes.

Concise guide on the use of blockage diodes and fuses in photovoltaic plants.

Diodes are among the principal components of a photovoltaic plant, their main function is to control the direction of the current flow. Current passing through a diode can usually move towards a single direction which is forward, and the current which tries to flow in the opposite direction gets blocked. They can be used only when there are more panels linked between themselves in series, such as in the case of a photovoltaic string.

If the tension at the extremities of a diode is negative, any kind of current will be able to flow, in this situation the diode is called OFF or inversely polarized. When, on the contrary, the tension is not negative, the diode presents itself like a short circuit and it will carry the current, for this reason it is defined ON or polarized. Every diode is composed by two polarized terminals: the positive extremity is called anode and the negative pole is named cathode.

The circuit symbol of a standard diode is a triangle that crashes into a line, similar to this example:

Diodes differ by bypass diodes and blockage diodes. Bypass diodes electrically isolate a module (a series of cells) in the event that it presents in its internal part some cells that work as inversely polarized diodes in series with other cells. This can happen due to shadows or obscuration. The immediate effect is to reset the energy produced by other cells, but it can also arrive the phenomena in which the cell dissipates generated energy from the entire series, with the risk of local overheating and of damaging the cell itself. Generally a bypass diode gets installed every 12-18 cells series.

The utilisation of blockage diodes (together with bypass ones which are already installed in photovoltaic modules) is recommended in order to avoid the circulation of current between strings of photovoltaic modules placed in parallel, when it exist between them a difference of tension caused by partial obscuration and shadows. Accumulation systems avoid night dissipation by the fact that modules work as load in illumination absence. As disadvantage, the presence of a waster in the fuse box in which they find themselves and an accurate welding link to the anode are necessary.

It is also necessary that diodes support a sufficient Ifav that comes from photovoltaic strings and a sufficient Vrrm of the plant tension.

An important characteristic is the maximum current. In fact, such as in every component, a diode dissipates a certain energy quantity before burning. Every diode must enumerate the maximum of the current, the inverse voltage and power dissipation. In the event that a diode is subjected to more tension or current than they are able to manage, they can get overheated or worse, merge.

Every characteristic of the diode must be detailed in the technical schedule or in the data-sheet.

A) DIODES UTILISATION

Connect one or more diodes in series for one or maximum two strings; the linking between more strings by the blockage point is not allowed because the inverse current coming from other strings can damage the module. This is a simple example of connection:

B) FUSES UTILISATION

The utilisation of gPV fuses consists in the optimal solution for photovoltaic modules and inverters protection from high levels of direct or indirect current in the internal part of the plant caused by sudden power surges (lightings) or by power outages (shadows, night working). This fuses have been realised according to specific norms which guarantee a high degree of robustness compared to the typical cycle of the current passage to their inside. This is caused by a particular photovoltaic application in which high levels of current caused by breakdowns can be more elevated than normal ones but only in a small way, requiring to the fuse a minimal current of rupture, lower than the majority of the others fuses.

Fuses perform like circuit switches because on their inside we can find a filament that remains intact if it does not get too heat. Fuses have different characteristics but the main one, to which you should pay attention to, is the maximum voltage in DC that it can support. However, it is forbidden to use a fuse with more than a string for photovoltaic panel, because the eventual inverse current coming from another series in parallel could damage the modules connected to the string. This is a simple example of the connection:

In the alongside image you will find the cartridge fuse sold in security kits. The actual fuse is inside the black cartridge that you can unscrew.

For a correct application inside the plant you will be required to cut the red cable that keeps the two extremities of the cartridge united and to connect the two obtained ends to the positive polarity cable that goes from battery to charge regulator.


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