Variable Frequency Power Supplies, Issues and Requirements: Variable frequency power supplies have the same environmental requirements as other electrical equipment such as medium-voltage switchgear and transformers: below 1,000m altitude, relative humidity less than 95% or 90%, and no condensation. The environmental temperature should be below 40°C. When the altitude exceeds 1,000m or the environmental temperature is above 40°C, variable frequency power supplies must be used at reduced capacity. The environmental gases should be non-corrosive, dust-free, and non-flammable or explosive. The cooling methods for variable frequency power supplies are water cooling and forced air cooling; the standard protection level for the cabinet is IP20 or IP21, which can be increased to IP42 or IP54 upon special customer requirements.
Based on the aforementioned simple environmental requirements for variable frequency power supplies, both engineering design units and users typically treat them as standard medium-voltage switchgear or transformers for indoor installation. It is common practice to locate variable frequency power supplies in the same room as other electrical equipment. In better conditions, a separate room for the variable frequency power supply, akin to a transformer room, is designed. It is widely believed that manufacturers of variable frequency power supplies have already given full consideration to ventilation and cooling factors, so generally, only simple measures such as increasing the indoor clear height or installing axial fans on the exterior walls are taken during installation. Aviation ground power, 36V aviation power, 400HZ variable frequency power supply, 115/200V power, 27V DC power, 400HZ power, aviation power supply
From an electrical perspective, it is uncontroversial to adopt an installation layout similar to that of medium-voltage switchgear or transformers for variable-frequency power supplies. However, when considering the power losses of the three, there are significant differences between them. Product Variable-frequency Power Supply Oil-immersed Transformer Dry-type Transformer High-voltage Switchgear Note Loss (%) 25 22 3 Negligible Variable-frequency power supply losses refer to the entire speed range, all three are rate products, and the losses account for only a small part of their total power. The losses of variable-frequency power supplies at rated frequency and power are 2%, but there are almost no frequent operations at the rated point. Their losses in the conventional operating range of 70% to 85% speed are about 4%. Taking a 1000kVA variable-frequency power supply commonly used in the water supply industry as an example, the corresponding losses during conventional speed control operation reach 40kW, all of which are converted into heat. The heat dissipation of oil-immersed transformers is achieved by circulating the insulating oil to transfer heat to the cooling fins on the transformer casing, which is then carried away by the flowing air, making its heat dissipation method much better than that of air-cooled variable-frequency power supplies. Although dry-type transformers can also be forced-air cooled like variable-frequency power supplies, the thermal time constant of transformers is as long as several hours, while the thermal time constant of variable-frequency power supplies is only a few minutes due to the characteristics of rectifying and inverting semiconductor devices. The ability to withstand high temperatures of the two is incomparable.
It follows that the installation and layout of variable frequency power supplies cannot be treated as conventional medium-voltage electrical equipment; instead, the temperature of the indoor cooling air must be fully considered. Manufacturers of variable frequency power supplies typically install centrifugal fans inside the cabinet or axial fans on the cabinet roof to provide forced air cooling for the power devices in the rectifiers and inverters, and they have outlined the aforementioned environmental requirements. However, they have not provided suggestions on how to ensure these requirements. In the absence of an adequate air exchange system between indoor and outdoor spaces, the majority of the heat emitted by the variable frequency power supply remains indoors, causing the environmental temperature to rise continuously for the power supply. This completely alters the environmental conditions required for air cooling.
