Waterproof and air duct design for small-power outdoor photovoltaic grid-connected inverter

O Introduction

The design of outdoor photovoltaic grid-connected inverters must be both reliable and waterproof and able to discharge the heat generated by power devices out of the box. If it is completely sealed without a reasonable air duct, the waterproofing problem is solved but the thermal design requirements cannot be met; if only a simple air duct is designed, the heat dissipation problem is solved, but it poses a problem for the box sealing and waterproofing. In response to the seemingly contradictory problems, this article proposes a special structure with upper and lower double-layer independent sealing and 90-degree air duct. After long-term practical application, it has been completely successful.

l Independent sealing of upper and lower double-layer cavities

In view of the different protection level requirements of different devices, the requirements for mutual isolation and shielding of weak current control circuits and strong current main circuits, and the requirements for heat dissipation of power devices, the overall structure of the product is divided into upper and lower double-layer cavities to achieve mutual isolation, shielding and independent sealing; the connection between the two layers is sealed through waterproof terminals. The composition of the upper and lower double-layer sealed cavities is shown in Figure 1.

1.1 Composition of the closed cavity of the upper control circuit

The power main circuit board 10, the distribution board 11, and the control board 12 are installed on the upper layer of the middle partition; the waterproof terminal 13 is installed on the bottom of the box body 1; the self-clamping sealing ring 14 is installed around the upper opening of the box body 1; the upper cover plate 15 is fixed to the box body 1; in this way, the upper half of the box body 1 forms an upper sealed cavity, which is completely waterproof and dustproof and can reach the protection level of IP65.

1.2 Composition of the lower closed cavity

The lower layer of the middle partition is equipped with a radiator 2, air duct plates 3, 4, reactor 5, and transformer 6; a fan 7 is installed at the bottom of the box 1; shutters 8 are installed on both sides of the box 1; the lower cover 16 is fixed to the box 1; thus, the lower layer of the middle partition of the entire box 1 forms a relatively closed cavity; waterproof sealant is applied to the periphery of the middle partition and the box, and the periphery of the radiator and the middle partition, and the reactor and transformer are epoxy potted to effectively waterproof. The entire cavity can reach the protection level of IP54.

2 The composition of the 90-degree air duct

The structure of the 90-degree turn air duct is shown in Figure 2.

1) A 90° air duct is formed by the fan 7, the radiator 2, the air duct plate 3/4, and the shutter 8.

2) The path of cold air flow: cold air → fan → two transformers → two reactors → flow through the radiator tooth surface → through the air duct formed by the air duct plate, turning the hot air flow 90° and then discharging it from the left and right side shutters.

3) In order to realize a 90° air duct, the teeth of the radiator need to be specially processed. A portion of several teeth are milled off from the middle to form an air guide port, as shown in Figure 3.

4) In order to achieve smooth airflow, reduce wind resistance and increase the air outlet area, the connection between adjacent blades of the blinds needs to be punched into a notch, as shown in Figure 4.

3 Application Examples

The upper and lower double-layer independent seals and 90-degree air duct structure have been successfully applied to the structural design of an outdoor 6kVA photovoltaic grid-connected inverter power supply (see Figure 5). The overall performance of the product is reliable and fully meets the special requirements of outdoor operation.

4 Conclusion

For the design of small-power outdoor photovoltaic grid-connected inverter power products, it is necessary to fully implement waterproof design and take thermal design into consideration; the upper and lower double-layer independent sealed cavities and the 90-degree air duct structure proposed in this article reliably solve the design requirements in these two aspects. This technical solution can be used in other similar outdoor products.