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Aug 24 2019

The PV Junction Box :The Key Part Of A Solar Panel


Catalog

I

The Working Principle Of The Junction Box

II

PV Junction Box's Main Role

III

PV Junction Box's Nominal Power

IV

Diode

Bypass Diode

Hot Spot Effect

Principle Of Diode Selection

V

PV Junction Box Selection


The Working Principle Of The PV Junction Box

 

image from powerfromsunlight.com

Solar PV(photovoltaic) junction box is a connector between solar cell array composed of solar cell modules and solar charge control device. It is a cross-field comprehensive design integrating electrical design, mechanical design and material science. The junction box of solar cell module plays an important role in the composition of solar module, which mainly connects the power generated by solar cell with external lines. The PV junction box is glued together with the backboard of the components by silica gel, and the outgoing wires in the components are connected with the internal wires in the junction box, and the internal wires are connected with the external cables to make the components and external cables conduct. The diode in the junction box ensures that the components can work normally when they are blocked from light.


PV Junction Box's Main Role

PV junction boxes connect and protect solar PV modules, connect the power generated by solar cells to external lines, and conduct the current generated by PV modules.


PV Junction Box's Nominal Power

The power of the solar PV junction box is tested under standard conditions: 25 ℃, AM1.5, 1000W/M2. It's usually WP, but it can also be W. The power measured under this standard is called nominal power.


Diode

The key to the junction box is the choice of diode, which varies according to the type of cell in the module. The diode inside the PV junction box should be used as a bypass diode to guard the module against hot spots.

Bypass Diode

Junction-Box.jpg 

image from powerfromsunlight.com

When the component is working normally, the bypass diode is in the cut-off state, and there is a reverse current, namely dark current, which is generally less than 0.2μA. The dark current reduces the current generated by the component, albeit by a small margin.

Ideally, each cell should be connected to a bypass diode, but it is not economical because of the cost of the bypass diode,the dark current loss and the voltage drop in the operating state. In addition, the positions of the PV modules' cells are relatively concentrated, and the corresponding diodes must can be connected under sufficient heat dissipation conditions.

Therefore, it is generally reasonable to use a bypass diode to protect multiple interconnected battery groups. This can reduce the production cost of PV modules, but also adversely affect their performance. In fact, if the output power of a cell in a string of cells drops,then this string of cells, including those that work properly, would be isolated from the PV module system by the bypass diode. The result is that the output power of the entire PV module falls too much due to the failure of a single cell.

In addition to the above problems, the connection between the bypass diode and its adjacent bypass diode must be considered. Actually, these connections are affected by stresses that result from periodic changes in mechanical load and temperature. Therefore, in the long-term use of photovoltaic modules, the above connections may fail due to fatigue, resulting in abnormal photovoltaic modules.

Masking one battery is different from masking half of each battery, so when masking is unavoidable, try to cover as many batteries as possible, with as few shadows per battery as possible.

Hot Spot Effect

In solar modules, individual cells are connected in series, so-called series, to achieve higher system voltages. Once one of the cells is blocked (e.g., a tree branch or an antenna, etc.), the affected battery ceases to work as a power source and becomes an energy consumer. Other uncovered batteries continue to pass current through them, causing high energy losses, "hot spots" and even battery damage.

To avoid this problem, bypass diodes are connected in parallel to one or more cells in series.So the bypass current passes through the diode, bypassing the shielded cell.

When the cell is working normally, the bypass diode will reverse cut off, which will have no effect on the circuit. If the bypass diode parallel battery groups had an abnormal working battery slices, the whole circuit current will be decided by the minimum current cell, the current size is determined by cell covered area, when the reverse bias is higher than the minimum cell voltage, the bypass diode conduction. At this point, the abnormal work cell is short-circuited.

The formation of hot spots of battery components is mainly caused by external factors, such as the components or local components being shielded by shielding objects. Common shielding objects include: leaves, dust, clouds, animals and animal feces, snow, etc. Internal factors include solar cell internal resistance and solar cell reverse current. This conclusion can be obtained from the actual equivalent circuit of the cell. The load is in series with the internal resistance of the solar cell and the current flowing through the load is obtained from the equivalent circuit diagram: I=Iph-ID-ISh, and the working power of the series resistance is P=I2Rs. Therefore, for the battery, the smaller the internal resistance, the better.

The internal resistance mainly refers to the internal resistance generated by the manufacturing process of the battery itself, and the internal resistance generated by the welding belt. Therefore, the welding process of the battery should be paid enough attention to. The choice of the welding belt should also choose the small internal resistance. As for the inverse current factor, it is necessary to analyze from the actual equivalent circuit. For different cells, the dark current is different.

The internal resistance mainly refers to the internal resistance generated by the manufacturing process of the battery itself, and the internal resistance generated by the welding belt. Therefore, the welding process of the battery should be paid enough attention to. The choice of the welding belt should also choose the small internal resistance. As for the inverse current factor, it is necessary to analyze from the actual equivalent circuit. For different cells, the dark current is different.

The component array temperature T installed in the external environment is related to sunshine intensity L, environmental temperature Ts of the system, and temperature Ti generated by internal resistance. The component temperature can be expressed as T=T+αTs+βL+Ti

damaged box 

image from photovoltaikbuero.de

The harm of hot spot is huge, and the hot spot effect is very easy to occur in the case that no one is maintaining the power station of the module array. How to avoid or reduce the adverse effect of hot spot on the module becomes an important problem in the module design.

Principle Of Diode Selection

The selection of bypass diode mainly follows the following principles:

1. The withstand voltage capacity is twice the maximum reverse working voltage.

2. The current capacity is twice of the maximum reverse operating current.

3. Junction temperature should be higher than the actual junction temperature.

4, small thermal resistance.

5. Small pressure drop.


PV Box Selection

To choose a PV junction box, the current size of the component is the main information , one is the working maximum current , one is the short circuit current. Of course, the maximum current that the components can output when short-circuit current, according to the short-circuit current calculation of the rated current of the junction box should be a relatively large safety factor, according to the maximum working current calculation of the junction box is a smaller safety factor.

Scientific way to selecte PV junction box should be according to the rule of cell voltage and cell current change with light intensity.You have to know what the peak light is in your area, then check the maximum current possible against the curve of the cell current with the intensity of the light, and then choose the rated current of the PV junction box.


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