Application Analysis of UPS Uninterruptible Power Supply in Factory DCS (Distributed Control System)/ESD (Emergency Shutdown System)

Keywords: DCS ESD UPS Petroleum Petrochemical Natural Gas Metallurgical Power Plant Mining Pharmaceutical Semiconductor Safety Production Control

1 Overview

According to the IDC Internet Data Center, the average probability of power outages in large cities in China is 0.5 times per month, 2 times per month in medium-sized cities, and 4 times per month in small cities or towns. There are at least 9 problems with the power grid: power outages, lightning spikes, surges, frequency oscillations, voltage mutations, voltage fluctuations, frequency drift, voltage dips, and pulse interference. Ensuring normal power supply under any circumstances is an important foundation for the petrochemical industry.

At present, large-scale chemical enterprises use the Distributed Control System (DCS) system for chemical operations, monitoring, and control. DCS is an important component of process production monitoring, which determines the stability and safety of the entire production. If the DCS system malfunctions, it can cause process fluctuations, affect product quality, and even shut down the entire production line. Therefore, the DCS system is crucial to ensure the continuity of production, The ESD (Emergency Shutdown Device) system, which is extremely important in terms of safety and other aspects, is a timely response and protection system for potential hazards that may occur in petrochemical production equipment or the state that will continue to deteriorate if no measures are taken, so that the production equipment enters a predefined safe shutdown condition, thereby reducing the danger to an acceptable minimum to ensure that personnel, equipment The safety of production and equipment. Due to the importance and complexity of DCS and ESD systems, it is very difficult to handle power failures like backyard fires. Power supply is an important link in whether DCS/ESD can work normally. To avoid system failures caused by power supply failures as much as possible, various control manufacturers attach great importance to the distribution part of hardware, so ensuring the quality and safety of DCS/ESD power supply is particularly important. Therefore, in addition to the normal power supply of the industrial power grid, UPS (Uninterruptible Power System) power supply system is also required. The UPS system is composed of rectifier module charging module, battery, inverter module, automatic bypass switch, manual maintenance bypass switch, etc. UPS can provide high-quality, safe and reliable power supply to DCS/ESD to ensure the safety, stability, and reliability of chemical production.

2. Working principle and process of UPS power supply

2.1 Composition and Working Principle

UPS is mainly used to provide uninterrupted power supply to important loads such as distributed control system DCS, safety protection system ESD, control actuators, solenoid valves, electrical converters, etc. Maintain normal operation of the load and protect the load software and hardware from damage. Although different enterprises choose different models and system capacities for UPS power supply systems, their principles and main functions are basically the same. From the basic application principle, UPS is a power protection device that contains energy storage devices, with inverters as the main components, and stabilizes voltage and frequency output. The working principle of the UPS system is to first convert the poor quality factory power supply into a DC power supply through the rectifier device. One way is to charge the battery through the charging module, and the other way is to use the high-frequency pulse width modulation (SPWM) method to convert the DC power supply into a pure high-quality sine wave power supply in the inverter device to supply power to the load. When the AC working power supply disappears or components such as the rectifier device fail, the battery pack supplies power to the inverter device through the battery switch. When the inverter device malfunctions and the output voltage is abnormal or overloaded, it will be switched to the automatic bypass switch and the auxiliary power will directly supply power to the load.

2.2 UPS power supply working process

When the mains power is normal, there is a DC voltage in the DC main circuit, which is supplied to the DC-AC AC inverter and outputs a stable 220Vac AC voltage. At the same time, the mains power charges the current. When the mains power is under voltage or suddenly loses power, the battery pack feeds power to the DC circuit through an isolation diode switch. There is no switching time from grid power to battery power. When the battery energy is about to run out, the uninterruptible power supply emits an audible and visual alarm, and stops the inverter operation at the lower limit of battery discharge, with a long beep alarm. The uninterruptible power supply also has an overload protection function. When an overload occurs (150% load), it jumps to the bypass state and automatically returns when the load is normal. When a serious overload occurs (exceeding 200% of the rated load), the uninterruptible power supply immediately stops the inverter output and jumps to the bypass state. At this time, the front air switch may also trip. After eliminating the fault, as long as the switch is turned on and restarted, the operation will resume.

3. Selection of UPS for DCS/ESD system

3.1 DCS/ESD power supply requirements

DCS/ESD has extremely strict requirements for its power supply. It not only requires high-quality and stable power supply, but also requires a power supply to maintain a certain period of power supply in case of sudden power outage in the power grid system, in order to protect the data of the DCS system and ensure that on-site instruments and control valves are in a safe position. At the same time, it also ensures that the process undergoes emergency processing to prevent faults from occurring. The UPS, as a control device for DCS/ESD, should have the following functions:

(1) Whether in mains mode or battery mode: a sine wave power supply that can output distortion and high stability is the best power guarantee for DCS.

(2) When the mains power is cut off or restored, there is no switching time between the UPS mains power mode and the battery mode, effectively ensuring high reliability of load operation.

(3) Equipped with bypass power supply function, and when users have special requirements for the power supply, UPS can provide bypass power overvoltage protection to prevent DCS from high voltage hazards.

(4) For DCS systems with high power supply requirements, in addition to redundant power supply, there should also be UPS redundancy. Ensure the stability of DCS power supply.

3.2 UPS comes in two forms

One type is backup type, the other type is online type, and there is also an online interactive type between the two. Online UPS has the following advantages:

(1) Pure sine wave output. Whether in mains mode or battery mode, it can output a low distortion and high stability sine wave power supply, providing the best power guarantee for DCS and ESD,

(2) Zero switching time. When the mains power is cut off or transformed. The switching between mains mode and battery mode of UPS is time-free, effectively ensuring high reliability of load operation.

(3) Intelligentization can achieve the function of connecting one UPS to multiple computer systems through data communication interfaces. Equipped with power monitoring software, it can directly monitor the operation status of the power supply and UPS on the computer screen. It has functions such as supporting abnormal power information alarm, closing, and saving user applications to simplify network management work and improve the reliability of computer systems.

(4) Bypass protection function. The bypass power supply function greatly improves the emergency response capability of UPS. At the same time, when users have special requirements for the power supply, online UPS can provide bypass power overvoltage protection, preventing DCS and ESD from high voltage hazards.

Due to slight changes in power supply frequency and time-varying waveforms, which will seriously interfere with the normal operation of DCS/ESD, it is required to choose a high-quality, high stability, and short switching time UPS system. The online UPS power supply system has many advantages such as zero time switching of power supply, so petrochemical enterprises choose online UPS as the DCS and ESD power supply system in the selection of UPS power supply.

3.3 Working principle of online UPS

When the power supply is normal, the online UPS is rectified by the power supply to provide DC voltage to the inverter for operation. The inverter provides AC power to the load. When the power supply is abnormal, the inverter is powered by the battery, and the inverter is always in working state to ensure uninterrupted output. Its characteristic is that it has an extremely wide input voltage range, no switching time, and high output voltage stability and accuracy, especially suitable for occasions with high power requirements.

3.4 In addition, the selection of UPS should also consider the use of UPS battery capacity at full load for more than 30 minutes, ensuring power supply for more than 4 hours under reduced load, emergency power outage can be accident shutdown, and important control systems can be guaranteed not to lose power during power outage maintenance; The capacity of UPS should be selected based on the power consumption of the DCS system. Generally, the power consumption of DCS should be 60% -70% of the rated power of UPS.

4. UPS operation maintenance and accident handling

4.1 Daily maintenance

(1) UPS should be placed indoors without harmful gases, severe vibrations, strong magnetic effects, ambient temperatures ranging from -10 ℃ to+40 ℃, relative humidity less than 85%, and good air intake;

(2) UPS should be regularly cleaned and maintained, not contaminated with dust, to ensure the lifespan of the machine: When cleaning, use a soft cloth to wipe, and do not use frosting as a cleaning agent;

(3) UPS should avoid frequent startup, preferably in a long-term startup state.

(4) If the power supply in the factory remains normal and the UPS battery pack has no chance of working, the battery may be damaged in a long-term floating charge state. Regular charging and discharging of the UPS should be carried out to not only activate the battery, but also verify whether the UPS is in normal condition.

(5) UPS should be charged immediately after use to restore the battery to its normal floating charge state.