Abstract : In recent years, thermal power plant units have experienced large peak loads and frequent load changes during operation. During low load periods, the low-pressure heater condensate flow decreases accordingly, but the output is not adjusted accordingly, resulting in a "large horse pulling a small cart" phenomenon and significant energy waste. Furthermore, the relay pump motor uses a contactor-controlled I-frequency start-stop method, resulting in a large starting current and significant impact on the motor. It is also prone to water hammer effects, directly affecting the mechanical and electrical lifespan of the system. Implementing frequency conversion technology upgrades can make the relay pump system more economical, energy-efficient, and automated, which is of great significance. Keywords : Pump system, frequency conversion speed regulation, functional parameters 1. Overview Electric motors are a large and widely used electrical device in thermal power plants. Improving the energy efficiency of electric motors will have a significant impact on the economic benefits of the enterprise. Currently, electric motors generally suffer from problems such as outdated equipment, oversized design, and lack of speed control, resulting in the wasteful consumption of large amounts of electrical energy. The solution to these problems is to adopt new technologies, especially frequency conversion speed regulation, to upgrade old equipment, achieving the goal of energy saving, consumption reduction, and improved efficiency. The relay pump motors of Units 5 and 6 at Yong'an Thermal Power Plant originally used 380V full-voltage direct starting, which resulted in a large starting current, causing significant impact on the motors and easily leading to water hammer effects. This directly affected the mechanical and electrical lifespan of the system and reduced its safety and reliability. Moreover, during peak shaving and start-up/shutdown of the units, the output was not adjusted accordingly due to the fixed speed, resulting in a "large horse pulling a small cart" phenomenon and serious energy waste. Therefore, it is imperative to retrofit the relay pumps with variable frequency speed control to achieve energy saving, consumption reduction, and improved efficiency. [b]2 Variable Frequency Speed ​​Control System Retrofit Method 2.1 Scheme Selection[/b] Each of Units 5 and 6 has two 40kW motors, one main and one standby, working alternately. Based on the actual operating conditions of the motors, in the variable frequency speed control retrofit of the relay pumps, one frequency converter is used to control the two motors separately, with mutual interlocking function. That is, a one-control-one-interlock operation mode is adopted, where one frequency converter system controls one of the two motors for variable frequency operation. When the operating pump fails, it automatically switches to the standby pump for variable frequency operation. Meanwhile, the system also has a manual control system as a backup for the automatic control system. When the automatic control system malfunctions or needs maintenance, the system can be switched to manual control via a "manual/automatic" switch in the control room of units #5 and #6. The original system functions are retained, using a full-pressure starting motor at industrial frequency. The liquid level is controlled by adjusting the outlet valve opening, and the original low-pressure interlock function of the outlet header is retained to ensure the safe and reliable operation of the relay pump. [b][align=center]For details, please click: Energy-saving application of frequency conversion technology in relay pump motors[/align][/b]