The assembly structure of the conventional frame assembly is shown in Figure 1. From top to bottom, the order is glass, front EVA, battery matrix (battery string), rear EVA, and backplane. After vacuum lamination, aluminum frame and junction box are installed. part.
Taking a 60-cell module as an example, the internal battery matrix of the module is arranged in 6 columns and 10 rows, that is, each column has 10 series batteries, a total of 6 columns (also known as 6 series in the industry), and finally 6 series of batteries are connected in series. If each solar cell is equivalent to a semiconductor diode device, its series connection can be represented by an equivalent circuit diagram, as shown in Figure 2.
For component structure design, four aspects should be considered.
First, consider the structural arrangement of the cells inside the module, which is generally referred to as a stacked circuit design. The distance between each cell in the module, including the horizontal and vertical distances, generally needs to be more than 2mm, which is mainly to ensure that the reliability of the module will not be affected after the connecting ribbon is folded up and down on the surface of the battery. Through reasonable design, part of the light irradiated in the cell gap can be projected onto the cell surface again through the two reflections of the back glass, so that the output power of the module can be increased. Too large a cell gap will reduce the conversion efficiency of the module, and too small a gap is not conducive to the bending of the ribbon, and may lead to cracks in the cell. Usually, the cell spacing in a photovoltaic module is 2~5mm.
Second, consider the component’s minimum clearance and minimum creepage requirements. Clearance is the shortest distance in space between two conductive parts; Minimum Clearance or Through Air of a component refers to the distance from the charged body inside the component (such as solar cells and bus bars) to the edge of the glass; Creepage distance refers to the shortest distance between two conductive parts along the surface of solid insulating material, see Figure 3 and Figure 4.
IEC61730 and UL1703 standards have strict requirements on the minimum electrical clearance and creepage distance of components. Because the packaging material will absorb moisture, and the packaging process cannot guarantee complete sealing, this requirement is directly related to the insulation material group, the degree of micro-environmental pollution of the component application, etc. Generally, the minimum clearance and creepage distance for component design are selected based on the micro-environmental pollution level 2 and material group 1la, and then the minimum clearance and creepage distance requirements are determined according to different application levels and system voltages. Of course, if It is possible to appropriately reduce the distance requirements by reducing the environmental pollution level of the component application. The application level is divided into three levels: level 0, level 11, and level 11 according to the requirements of different application methods of photovoltaic modules for module safety (the application classification is derived from 1EC 61140)
Level 0: Components certified at this level can be used in systems that limit public access by fences or feature areas;
Level 11: Components certified by this level can be used in systems with voltages higher than 50V or power greater than 240w, and these systems are likely to be in contact with or approached by the public. This is the most commonly used application level of photovoltaic modules at present;
Level 11: Components certified by this level can only be used in systems with voltages less than 50V or power less than 240w. A system used to limit public access by fences or feature areas that are likely to be contacted or approached by the public.
Table 1 shows the corresponding relationship between the minimum clearance and creepage distance and the maximum system voltage of PV modules. The requirements in the UL1703 standard are slightly lower than those in this table.
|PV module maximum system voltage/V||Minimum electrical clearance/mm||Minimum electrical clearance/mm||Creepage distance/mm||Creepage distance/mm|
|Level II||Level 0 and Level III||Level II||Level 0 and Level III|
For 1500V system voltage, the UL1703 standard specifically requires that the distance from the metal frame grounding component to the edge needs to be doubled. If the edge sealing is performed with insulating materials that meet the relevant requirements, the requirements for the metal frame component can be the same. At present, the existing 1000V system components with application level of level 11 on the market are generally designed to have an electrical clearance (the distance from the inner charged body to the edge) of more than 15mm. Some shifts, but also in order to ensure reliability, taking into account the requirements of 1EC and UI, the standard.
For the minimum electrical clearance of the 1500V system, although it is specified as 19.4mm in the table, because this clearance distance greatly changes the size of the component and affects the efficiency and cost of the component, the same distance as the 1000V component is generally used, and then passed the IEC 61730-2 The impulse voltage test section specified in MST14 is used to prove whether the electrical clearance of the component meets the safety requirements.
For the minimum creepage distance, the application level of 1000V/1500V system components of class 11 requires a minimum creepage distance of 20mm/30mm. At this time, the size of the components will be very large. The pollution level of the components can be determined by doing the 1EC 61730-2 series B1 test. Reducing it to 1 reduces the creepage distance requirements to 6.4mm and 10.4mm, so that the components only need to meet the minimum electrical clearance to meet the creepage distance requirements.
Third, bypass diode selection and design are also required. The bypass diode plays the role of conducting and protecting the battery when the battery is blocked in the photovoltaic module. Generally, a bypass diode can protect up to 24 solar cells, and it is best to control it within 20.
Fourth, for the design of the output power of the component, it is generally necessary to know the Pmax, Impp, Vmpp three parameters of the designed component, or 3 parameters of the Pmax, ISC, VOC, FF, so that you can determine The size of the cells and the number of cells in series and parallel.
To sum up, according to the gap of the battery, the distance from the inner charged body to the glass edge and the electrical performance parameters of the module, the size of the module can be designed to select the appropriate glass size, EVA, backplane and frame size.