Abstract : To further improve the application level of variable frequency technology in power systems, this paper analyzes the principle of variable frequency technology for direct air-cooling blowers using a graphical method. It also introduces the existing problems and solutions during application and provides some suggestions. Keywords : Thermal power unit; direct air-cooling system; blower; frequency converter [b][align=center]Principle of Air- Cooling Blower Variable-Frequency and Trouble-Analysis LIU Xuan (Shanxi Zhangshan Electric Power Company Ltd, Changzhi 046021, China)[/align][/b] Key Words : generating unit; direct air-cooling system; blower; variable-frequency governor. Thermal power plants mostly use water-cooling technology. Faced with increasingly urgent water scarcity, newly built large-scale thermal power plants in China are using direct air-cooling technology to replace traditional wet cooling (water-cooling) technology. Direct air-cooling systems use air to directly cool the turbine exhaust. After passing through radiators located at the top of the air-cooling island, the turbine exhaust steam, under the cooling air action of axial flow fans below the radiators, experiences a pressure reduction and temperature drop, condensing into water and returning to the condensate tank. Uncondensed steam and air are drawn away from the top of the radiators by a vacuum pump, preventing the formation of dead zones inside the air-cooled condenser during operation, which would reduce heat exchange efficiency and cause freezing in winter. The core of air-cooling technology lies in controlling the turbine back pressure. Because thermal power generation is a very complex energy conversion process, the turbine back pressure is affected by many factors and varies complexly. Therefore, direct air-cooled units often use variable frequency technology to control the axial flow fan speed to regulate the turbine back pressure. 1. Analysis of the Inverter Principle of Air-Cooled Fans Inverters are classified into two types: AC-AC and AC-DC-AC. AC-AC inverters directly convert industrial frequency AC into AC with controllable frequency and voltage, also known as direct inverters. AC-DC-AC inverters, on the other hand, first convert industrial frequency AC to DC through a rectifier, and then convert the DC back to AC with controllable frequency and voltage, also known as indirect inverters. The air-cooled fan inverters used by Zhangshan Power Generation Co., Ltd. (hereinafter referred to as Zhangshan Company) are indirect inverters. Their principle is shown in Figure 1, consisting of a main circuit (including a rectifier, intermediate DC link, and inverter) and a control circuit, described below. [b][align=center]For more details, please click: Inverter Principle and Fault Analysis of Air-Cooled Fans[/align][/b]