Composition of photovoltaic junction box

No matter how the junction box changes, the basic structure remains basically unchanged, including the box body, cover, connectors, terminal blocks, diodes, etc. Some junction box manufacturers have designed heat dissipation fins to enhance the temperature distribution inside the box, while others have designed other details, but the overall structure has not changed.

1. Box body

The box body is the main part of the junction box, with built-in wiring terminals and diodes, external connectors and box covers. It is the frame part of the junction box and can withstand most of the weather resistance requirements. The production material of the box body is generally PPO (polyphenylene oxide).

2. Box cover

The box cover plays a role in sealing the box body, preventing water, dust, and pollution. The sealing performance is mainly reflected in the built-in rubber sealing ring, which prevents air and moisture from entering the interior of the junction box. Some manufacturers set up a small hole in the center of the box cover, which is equipped with a dialysis membrane. The membrane is breathable and impermeable, and no water can penetrate three meters underwater, playing a very good role in heat dissipation and sealing.

The box body and cover are generally made of weather resistant materials by injection molding, with good elasticity, temperature impact resistance, and strong aging resistance.

3. Connector

The connector connects the wiring terminals to external electrical devices such as inverters, controllers, etc. The connector is made of PC (polycarbonate) material, but PC is prone to corrosion by various substances. The aging of junction boxes is mainly reflected in the susceptibility of connectors to corrosion, and the tendency of plastic nuts to crack under low-temperature impact. Therefore, the service life of the junction box is reflected in the lifespan of the connector.

4. Wiring terminals

The terminal blocks connect the outgoing lines and connectors of the components. There are 2, 3, 4, 5, 6 and other specifications. The width of the terminal itself is 2.5mm, 4mm, and 6mm according to the different outgoing lines. The terminal box of different manufacturers has different terminal spacing.

There are two types of contact between the wiring terminal and the outgoing line: one is a physical contact type such as compression or tightening, and the other is a welding method. The advantages and disadvantages of both methods have been discussed earlier.

5. Diode

The diodes inside the photovoltaic junction box are used as bypass diodes to prevent hot spot effects and protect components.

When the component is working normally, the bypass diode is in the cut-off state, and there is a reverse current, i.e. dark current, which is generally less than 0.2 microamps. Dark current will reduce the current generated by the component, although the amplitude is small.

From the most ideal perspective, every photovoltaic cell should be connected with a bypass diode, but due to the cost impact of the bypass diode, dark current loss, and the presence of voltage drop under operating conditions, this is very uneconomical. In addition, the positions of each cell in the photovoltaic module are relatively concentrated, and after connecting the corresponding diodes, sufficient heat dissipation conditions must be provided for these diodes.

Therefore, a generally reasonable method in practical application is to use a bypass diode to provide protection for multiple interconnected battery groups. This can reduce the production cost of photovoltaic modules, but it will also have an adverse impact on their performance. In fact, if the output power of a certain battery cell in a string decreases, then this string of battery cells, including those that are working properly, will be isolated from the entire photovoltaic module system due to the effect of bypass diodes. The result will be an excessive decrease in the output power of the entire photovoltaic module due to the failure of a single cell.

In addition to the above issues, the connection between the bypass diode and its adjacent bypass diode must be considered comprehensively. In fact, these connections are subject to some stress, which is a product of periodic changes in mechanical load and temperature. Therefore, during the long-term use of photovoltaic modules, the above connections may fail due to fatigue, causing anomalies in the photovoltaic modules.

In addition, the effect of masking one battery cell is different from that of masking half of two battery cells, so when masking is unavoidable, try to mask as many batteries as possible and minimize the shadow on each battery.

6. Hot spot effect

In the construction of solar modules, individual cells are connected in series, which is called series connection to achieve higher system voltage. Once one of the battery cells is obstructed (such as a tree branch or antenna, etc.), the affected battery no longer functions as a power source, but becomes an energy consumer. Other unobstructed batteries will continue to transmit current through them, causing high energy loss, and "hot spots" will appear, even causing battery damage.

To avoid this issue, bypass diodes are connected in parallel to one or several batteries connected in series. The bypass current bypasses the obstructed battery cells and is transmitted through diodes.

When the battery pack is working normally, the bypass diode reversely cuts off and has no effect on the circuit; If there is an abnormal working battery cell in the battery pack parallel to the bypass diode, the entire line current will be determined by the minimum current battery cell, and the current size will be determined by the shielding area of the battery cell. If the reverse bias voltage is higher than the minimum voltage of the battery cell, the bypass diode will conduct, and at this time, the abnormal working battery cell will be short circuited.

It can be seen that the hot spot refers to the component heating or local heating, and the battery cells at the hot spot are damaged, which reduces the power output of the component and even leads to component scrapping, seriously reducing the service life of the component and posing a hidden danger to the safety of power generation and other power plants. The accumulation of heat leads to poor or damaged components.

The formation of hot spots on battery components is mainly caused by external factors such as the obstruction of the components or local components. Common obstructions include leaves, dust, clouds, animal and animal feces, and snow; The internal factors include the internal resistance of the solar cell and the magnitude of the reverse current of the solar cell itself. This conclusion can be analyzed from the actual equivalent circuit of the battery cell. The load is connected 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

Then the working power of the series resistor is:

P=I2Rs, so the impact of Rs on the temperature of battery cells is positive. For battery cells, the smaller the internal resistance, the better. The internal resistance mainly refers to the internal resistance generated by the manufacturing process of the battery cell itself, as well as the internal resistance generated by the welding strip. Therefore, the welding process of the battery cell should be given sufficient attention, and the selection of the welding strip should also choose the one with low internal resistance; As for the reverse current factor, it still needs to be analyzed from the actual equivalent circuit, as the dark current varies for different battery cells. Component short circuit, obstructing a certain battery cell on the component that cannot function properly, which is an internal resistance relative to the component and consumes:

P=I2 R (R: equivalent internal resistance of the obstructed battery cell).

The heat generation current of the obstructed battery cell is I=ID+ISh (I: reverse current, ID: dark current, Ish: leakage current), so solar cell silicon wafers with larger reverse current are more likely to generate heat spots under the same external environment conditions.

The module array temperature T installed in the external environment is related to the sunshine intensity L, the system ambient temperature Ts, and the temperature Ti generated by the internal resistance. The component temperature can be expressed as:

T=T0+ α Ts+ β L+Ti

(T0 α、β The coefficient obtained by processing experimental data using the least squares method is related to factors such as the type of solar cell used, installation location, and support form

The harm of hot spots is enormous, and if the component array power plant is unmanned for maintenance, the hot spot effect is also very easy to occur. How to avoid or reduce the adverse effects of hot spots on components has become an important issue in component design.

7. Principles for selecting diodes

The selection of bypass diodes mainly follows the following principles:

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

2. The current capacity is twice the maximum reverse working current;

3. The junction temperature should be higher than the actual junction temperature;

4. Low thermal resistance;

5. Low pressure drop.