Abstract : This paper introduces the application of a PLC-based variable frequency speed control system in the automation transformation of a water system. It includes system composition, main functions, lower-level PLC hardware and software design, variable frequency drive control design, upper-level monitoring design, and network communication function design between the variable frequency drive, PLC, and upper-level computer. Keywords : PLC; variable frequency drive; control system; network communication; configuration software 1 Introduction The external circulating water cooling system of a thermal power plant provides cooling for the internal circulating pure water of the power plant's generator sets, playing a crucial role in ensuring the normal and reliable operation of the generator sets. A thermal power plant's pump room currently has 8 circulating water pumps supplying external cooling water to the plant. The number and operation of the circulating water pumps are determined based on production needs and the pressure at the pump outlet. Before starting all pumps, the inlet pipes of the pumps must be evacuated; two vacuum pumps are used to perform this task. The composition of the circulating water system and its vacuum subsystem is shown in Figure 1. Before the automation upgrade, the commissioning of water pumps, including the starting and stopping of vacuum pumps and the opening and closing of corresponding vacuum valves, was all done manually on-site. In case of emergencies, such as a running water pump suddenly tripping, manual response was often not fast enough, failing to activate the interlocking backup water pump in time, thus affecting factory production. Moreover, before the upgrade, the water pump motors always ran at the power frequency, resulting in high energy consumption. Power frequency starting also puts a significant impact on the pump itself, accelerating the aging of various components in the pump system. [b][align=center]For more details, please click: Application of PLC and Frequency Converter in Circulating Water Systems of Thermal Power Plants[/align][/b]