Abstract : This paper addresses the stringent requirements of DCS (Distributed Control System) power supplies by analyzing the performance and operation of online static AC uninterruptible power supply (UPS) systems. It clarifies that selecting an online UPS is the optimal power guarantee for DCS systems, and illustrates how to select the capacity of the UPS and its matching batteries using graphs. Finally, it introduces the daily maintenance methods and precautions for UPS systems. Keywords: Power supply, selection, maintenance I. Introduction DCS (Distributed Control System) has extremely stringent power supply requirements. It not only needs a high-quality, highly stable power supply, but also requires a power supply to maintain power for a certain period when the power grid suddenly fails. This is necessary to protect the DCS system data, ensure the safety of field instruments and control valves, and ensure emergency process control to prevent failures. Because UPS systems are isolated from the urban power grid through a DC energy storage stage, eliminating power supply and transient interference, and providing continuous and uninterrupted power, coupled with their small size, high efficiency, convenient installation and maintenance, and high reliability, they are widely used in DCS control systems. II. UPS Selection UPS (Uninterruptible Power Supply) is a static AC uninterruptible power supply device. It is a high-quality, high-stability independent power supply, which is composed of a thyristor rectifier, inverter, AC static switch and battery pack. Since slight changes in power supply frequency and waveform distortion will seriously interfere with the normal operation of DCS, it is required to select a high-quality, high-stability UPS system with short switching time. 1. Online UPS — The best power guarantee Whether considering the voltage regulation output range, frequency range, input noise filtering and zero switching time between AC mode and battery mode, online UPS is the best DCS power supply system. Performance of online UPS: (1) Pure sine wave output. Whether in AC mode or battery mode, it can output a sine wave power supply with low distortion and high stability, providing the best power guarantee for DCS. (2) Zero switching time. When the mains power fails or is restored, the UPS switches between mains power mode and battery mode without any conversion time, effectively ensuring the high reliability of the load operation. (3) High intelligence. Through the data communication interface, one UPS can connect to multiple computer systems. It is equipped with power monitoring software, which can directly monitor the power supply and UPS operation status on the computer screen. It has functions such as supporting abnormal power information alarm, closing and saving user applications, etc., to simplify network management and improve the reliability of computer systems. (4) Bypass protection function. The bypass power supply function greatly improves the emergency handling capability of the UPS. At the same time, when the user has special requirements for the power supply, the online UPS can provide bypass power supply overvoltage protection, so that the DCS is free from high voltage danger. (5) Dual-machine hot backup. For DCS systems with high power supply requirements, in addition to redundant power supply, there must also be UPS redundancy. Online UPS systems can be configured with a primary/secondary backup switching device according to user requirements: when the main unit fails, it automatically switches to another backup UPS; when the main unit needs maintenance or inspection, the DCS can be switched to the backup UPS, ensuring that the DCS receives optimal power at all times. Furthermore, online UPS systems are equipped with a manual bypass switch, allowing users to switch the DCS system to bypass output power during UPS maintenance, ensuring the stability of the DCS power supply. 2. Operating Mode of Online UPS Normal Operating Mode of UPS: When the UPS is operating normally, the mains power supply is filtered to remove high-frequency harmonic noise (as shown in Figure 1). One path charges the battery pack through the charger, maintaining the battery at full potential; the other path is rectified into DC power by the rectifier, then converted into a pure sine wave power by the inverter, and finally delivered to the user equipment via a static switch and filter. Figure 1: Normal Operating Mode When the mains power fails, the battery quickly supplies power to the inverter, and the power continues to be delivered to the user equipment via the static switch and filter, preventing any power outage to the load. Figure 2 Working mode when mains power fails 3. Working mode when UPS is powered by bypass There are five situations when the UPS is powered by bypass: (1) Overload; (2) Inverter failure; (3) When starting up, the UPS starts running slowly within 20 seconds after the start button is pressed; (4) When shutting down, the shutdown button is pressed; (5) The UPS is running due to internal overheating. Figure 3 Working mode of UPS when powered by bypass III. Capacity selection 1. Selection of UPS capacity The capacity of the UPS should be selected according to the power consumption of the DCS system. Generally, the power consumption of the DCS is 60% to 70% of the rated power of the UPS. 2. Selection of battery capacity The battery capacity is selected according to the process requirements and the power supply time required by the DCS. The battery power supply time is related to the power used by the load. Figure 3 is a curve comparing the power supply time of various battery capacities with the power used by the load, which can be used as a reference for battery capacity selection. IV. Operation, Maintenance and Precautions of UPS (1) The UPS should be placed in an indoor environment free from harmful gases, violent vibrations, strong magnetic fields, and with an ambient temperature of -10℃ to +40℃, relative humidity of less than 85%, and good ventilation. (2) The power supply should be disconnected at least once a month to allow the UPS to work in inverter mode for 30 minutes. If the inverter is not used for a long time, the battery inside the UPS will age, deteriorate, or fail to stabilize the voltage. Once a power grid system failure occurs, it will inevitably cause damage to the DCS system. (3) The UPS should be cleaned and maintained regularly to prevent dust accumulation and ensure the machine's lifespan. (4) When cleaning, use a soft cloth to wipe it. Do not use abrasive as a cleaning agent. (5) Check all connecting wires regularly every day to prevent collisions, loosening, and moisture. (6) Keep the UPS air inlet and outlet unobstructed. Check the air inlet and outlet for blockages by foreign objects every month.