Abstract : This paper introduces a method for retrofitting the chilled water pump and cooling water pump control system of a central air conditioning system in a high-rise building using programmable logic controllers (PLCs) and frequency converters (VDCs). This scheme achieves a technological upgrade of the central air conditioning system, ensures low-energy operation of the equipment, and improves the reliability of the electrical control system. Keywords : High-rise building; Central air conditioning control system; PLC; Frequency converter; Retrofit; Air conditioning energy saving 0 Introduction Central air conditioning is a major power consumer in high-rise buildings, accounting for approximately 60% of the total power consumption. Therefore, energy-saving retrofitting of central air conditioning systems is particularly important. Central air conditioning mainly consists of a chiller, cooling water circulation system, fan coil system, fan, and cooling tower, and is mainly used to achieve constant temperature control indoors. Its working principle is shown in Figure 1. Typically, the capacity of the chilled water pump is designed based on the highest temperature, full fill rate, and a 10% to 20% margin. Throughout the year, the pump system operates at a fixed maximum water flow rate. Due to seasonal, diurnal, and occupancy rate variations, the actual heat load of the air conditioning system is much lower than the design load for most of the time. Compared to the maximum design load (100% load rate) that determines the pump flow and pressure, nearly half of the year's operating time is below 50% load rate. The typical design temperature for chilled water is 5-7℃, but in reality, the temperature is only 2-4℃ for most of the year. This means the pump operates at full power, increasing pipeline energy loss and wasting the pump's transport energy, resulting in ineffective energy use. Variable frequency drive (VFD) technology can achieve automatic flow regulation and significant energy savings. Upgrading traditional electrical control systems with PLCs can greatly enhance operational reliability. 1. Implementation of Variable Frequency Drive 1.1 Energy-Saving Principle of VFDs A VFD is an AC power drive device with a variable output frequency. The main working principle of VFD speed regulation is to convert the three-phase AC power supplied to the motor stator into DC power through high-power rectification, and then invert the DC power into three-phase AC power with adjustable frequency and amplitude using sinusoidal pulse width modulation technology, which is then used as the power source for the motor. (Full text download of PLC and VFD application in energy-saving renovation of central air conditioning in high-rise buildings is also available.)