【Daily Question 20090219】Functions and technical requirements of UPS power supplies in control systems.

UPS (Uninterruptible Power System), also known as an uninterruptible power supply, is a type of power supply that maintains a constant voltage and frequency; it includes energy storage devices and an inverter as its main components. It is primarily used to provide uninterrupted power supply to individual computers, computer network systems, or other power electronic devices. When the mains power supply is normal, the UPS stabilizes the mains voltage before supplying it to the load; at this time, the UPS functions as an AC power regulator. It also charges the battery built into the device ； When the mains power is interrupted (due to a power failure), the UPS immediately uses the energy stored in its batteries to supply 220V AC power to the load through inverter conversion, allowing the load to continue operating normally and protecting its hardware and software from damage. As a protective power supply device, the performance parameters of UPS are of great significance and should be key considerations when making a purchase. A wide input range for mains voltage indicates a high capacity to utilize mains power (reducing battery discharge). A small output voltage and frequency range indicate strong capability to adjust to mains power, resulting in stable output. The waveform distortion rate is used to measure the stability of the output voltage waveform, whereas voltage stability indicates the stability of the output voltage when the UPS is suddenly switched from no load to full load. UPS efficiency, power factor, conversion time, and other parameters are also important indicators of UPS performance; they determine the capacity to protect the load as well as the efficiency with which electrical power from the grid is utilized. The better the performance, the stronger the protection capability. Generally speaking, offline UPS provides the weakest protection for the load, online interactive UPS offers slightly better protection, while online UPS can handle almost all common power problems. Of course, the cost also increases as performance improves. Therefore, when purchasing a UPS, users should choose different types of UPS based on the power requirements of the load and the importance of that load.

1) Normal startup sequence: Since there is an inrush current when a load is started, and the power components inside a UPS all have a certain safe operating range, although we leave some margin when selecting these components, excessive inrush currents can still shorten their lifespan or even cause damage to them. Therefore, efforts should be made to minimize the damage caused by inrush current during use. Generally, when a UPS is operating in bypass mode, it has strong shock resistance. We can take advantage of this feature by following this procedure during startup: first supply power from the mains to the UPS so that it operates in bypass mode, then turn on the loads one by one, starting with those that generate higher surge currents and moving on to those that generate lower surge currents. After that, activate the UPS panel to put it into inverter operation mode. Never turn on all the loads at the same time when starting up, nor should it be started while under load. 2) Shutdown sequence The shutdown sequence is as follows: first, turn off the loads one by one, then turn off the UPS panel, allowing the UPS to operate in bypass mode while the charger continues to charge the battery pack. If UPS output is required, completely shut down the UPS, and then disconnect the incoming mains power

DCS systems are used for chemical operations, monitoring, and control in chemical enterprises; therefore, it is extremely important to provide these systems with a high-quality, safe, and reliable power supply. The plant uses a UPS to supply power to the DCS system. The UPS system consists of a rectifier module, a charging module, batteries, an inverter module, an automatic bypass switch, and a manual maintenance bypass switch. The UPS can provide high-quality, safe, and reliable power to the DCS, ensuring the safety, stability, and reliability of chemical production. Working principle of the UPS system: The factory power supply, which has poor quality, is first converted into a direct current supply using a rectifier. One path of this current is used to charge the batteries through a charging module, while the other path utilizes high-frequency pulse width modulation (SPWM) to transform the direct current back into a pure, high-quality sine wave power supply, which is then supplied to the load. When the AC mains power is lost or components such as the rectifier fail, the battery pack supplies power to the inverter through the battery switch. When the inverter fails and results in abnormal output voltage or overload, it switches to the automatic bypass switch, allowing the plant power to supply power directly to the load. For plants with a small number of DCS system input and output points, a configuration with two AC power supplies and a single UPS is used; whereas for those with a large number of such points, a configuration with two AC power supplies and two UPS units working in parallel is employed. 1. Wiring for single UPS with dual AC inputs: The wiring configuration for a single UPS powered by dual AC inputs is shown in Figure 1. Under normal conditions, AC input 1 supplies power to the UPS; when there is a fault with AC input 1, power is automatically switched to AC input 2 to supply power to the UPS, with circuit interlocking between AC inputs 1 and 2. In normal operation, the UPS operates following the sequence of \"AC input 1 – rectification – inversion – load\" ； When the rectifier module fails, it operates in the \"battery—inverter—load\" mode ； Once the battery is completely discharged, it automatically switches to having the automatic bypass switch supply alternating current directly to the load ； When the inverter module fails, power is supplied directly to the load using the operation sequence of \"AC Input 1 – Automatic Bypass Switch – Load\" ； When the UPS has a serious fault that requires repair or replacement, power from the grid is supplied to the load by using the operation mode of \"AC Input 1 – Manual Maintenance Bypass Switch\"; at this time the UPS is short-circuited, allowing it to be repaired or replaced without affecting the normal power supply to the load. Figure 1: Wiring scheme with dual AC inputs and a single UPS for power supply. 2: Wiring scheme with dual AC inputs and two UPS units connected in parallel. The wiring scheme with dual AC inputs and two UPS units connected in parallel is shown in Figure 2: This involves connecting the outputs of two UPS units with identical power levels directly in parallel; it is required that the output voltages, frequencies, and phases of these two UPS units be identical, and that the load currents they handle be roughly equal. When a dual-UPS parallel system is operating normally, each UPS carries 1/2 of the load current. If one UPS fails, the other takes on the entire load, but it must do so at reduced capacity. The mean time between failures of this parallel dual-UPS system is 7 to 8 times that of a single-UPS system, thereby **improving the reliability of the system. However, there is a certain degree of connection in the control between the two UPS units in a dual-UPS parallel system; it is essential to understand the control logic of the parallel operation board when operating it. Otherwise, improper operation may lead to misjudgments by the parallel operation board, resulting in a drop in the output voltage and causing adverse effects on the load. Figure 2: Dual AC input with dual UPS units in parallel for power supply. 3. Precautions for using UPS: (1) When selecting a UPS, it is necessary to leave some headroom; for example, for a load of 4KVA, the UPS should have a capacity of at least 5KVA. (2) The UPS should be avoided from being turned on and off frequently; it is best to keep it running for extended periods of time. (3) The newly purchased UPS should be charged and discharged, as this helps to extend the lifespan of its batteries. Constant voltage charging is generally used; the initial charging current should not exceed 0.5*C5 amps (where C5 can be calculated using the battery’s rated capacity). The voltage of each battery should be maintained between 2.30 and 2.35 V to prevent damage to the batteries. If the charging current remains constant for 3 hours, it indicates that the battery is fully charged; the typical charging time is 12 to 24 hours. (4) If the plant power supply remains stable, the UPS has no opportunity to operate; its batteries may be damaged due to prolonged floating charge conditions. It is necessary to charge and discharge the UPS regularly, as this not only activates the batteries but also checks whether the UPS is functioning properly. (5) The UPS should be checked regularly; the float charging voltage should be inspected once a month. If the float charging voltage is below 2.2V, equalization charging of the entire battery bank should be performed. (6) Regularly wipe the battery with a soft cloth to keep its surface clean. (7) Temperature control during UPS operation: The temperature range during UPS operation is maintained between 20°C and 25°C in order to extend the lifespan of the UPS batteries. In an environment without air conditioning, temperature control of the UPS is particularly important. (8) The UPS should be charged immediately after use to restore the battery to its normal condition. (9) Keep the distance between the external battery pack and the UPS as short as possible, and use wires with a large cross-sectional area to increase their conductivity and reduce power losses in the wiring. This is especially important when high currents are involved, as the losses in the wiring cannot be ignored. 4. Daily maintenance of UPS and handling of common faults 4.1 Daily maintenance (1) Check the UPS operation display screen daily to confirm whether the status indicator lights of the UPS power supply are showing a normal operating condition. (2) Listen with your ear to check if there are any abnormal noises from the UPS power supply; these mainly include the cooling fans of the power modules and the isolation output transformer, as well as any abnormal vibrating sounds from the transformers. (3) Check whether there is any blockage in the exhaust vent of the UPS power supply. (4) Check the floating charge voltage of the battery, as well as the output voltage and current of the UPS, on the UPS display screen every week, and keep records. Compare these values with previous data; if there are significant changes, the cause should be identified promptly. 4.2 General fault handling: When an alarm is triggered while the UPS is in operation, do not panic and operate any of its switches or buttons, to avoid exacerbating the problem or even causing a disruption in the UPS’s output. When an alarm occurs on the UPS power supply, one should first check the status indicator lights on the display panel as well as the messages shown on the LCD screen. UPS has a self-diagnosis function; in the event of a fault, it provides information such as the type of fault and the time it occurred through status indicator lights and an LCD display. After identifying the fault type indicated by the status lights and the LCD display, take appropriate action based on that specific fault type. During the operation of a UPS, the following types of faults often occur. (1) AC plant power failure, that is, a power outage or significant fluctuations in the AC input. When this alarm is triggered by the UPS, a battery discharge alarm is also generated; at this time, the AC input indicator light on the UPS panel goes out, while the battery discharge indicator light comes on. The first step in handling this issue is to determine whether there is a fault in the plant power supply; check whether the UPS input switch has tripped and whether the fuses have blown. If it is indeed a plant power supply failure, and the time required for restoration is unknown, unimportant loads should be disconnected to ensure uninterrupted power supply to critical loads. (2) Inverter failure: When this alarm occurs, an alarm indicating that the automatic bypass switch is under load is also presented. First, check whether an inverter overload alarm appears on the panel as well; if so, view the output current on the LCD display. If it exceeds the rated current, turn off any unnecessary loads until the output current is below the rated value. Then press the reset button to eliminate the alarm, restart the inverter, and resume power output. If there is no overload alarm signal, it indicates that the inverter did not shut down as a result of inrush current, but rather due to a fault in the IGBT devices or the drive control system; the manufacturer should be notified promptly for repairs. (3) When the bypass fails and this alarm signal appears, an inverter out-of-sync alarm signal should generally appear at the same time. This indicates that the voltage and frequency of the input plant power exceed the set ranges, but still meet the requirements of the rectifier; in this case, the inverter oscillates on its own and stops tracking the bypass. At this point, large-capacity loads cannot be started again to avoid stressing the inverter.

1. DCS (Distribute Control System) has extremely strict requirements regarding its power supply. It needs a power source of high quality and high stability. In the event of a sudden power outage in the grid, there must be a power supply available to provide electricity for a certain period of time, so as to enable protective actions on the data within the DCS system and to keep the field instruments and control valves in safe positions. Additionally, it is necessary to ensure that emergency measures can be taken to prevent failures from occurring. Due to the fact that the static AC uninterruptible power supply UPS is isolated from the urban power grid through a DC energy storage stage, which eliminates power supply disruptions and transient interference while providing continuous power supply, it is widely used in DCS control systems owing to its compact size, high efficiency, ease of installation and maintenance, as well as high reliability. A UPS (Uninterruptible Power Supply) is a device that provides continuous power supply in the form of static alternating current; it is a high-quality, highly stable independent power source composed of a thyristor rectifier, an inverter, a static AC switch, and battery packs. Due to slight variations in the power supply frequency and waveform distortion, these factors can severely disrupt the normal operation of the DCS; therefore, it is necessary to use a UPS system that is of high quality, highly stable, and has a short switching time. 　　 Online UPS – the optimal power supply solution. Whether it is in terms of the voltage regulation range, frequency range, filtering of input noise, or the zero transition time between mains mode and battery mode, the online UPS is the best DCS power supply system. 　　 Performance of online UPS: (1) Pure sine wave output. Whether in mains power mode or battery mode, it can generate a sine wave power supply with low distortion and high stability, providing the best power supply support for DCS. 　　 (2) Zero switching time. When the mains power fails or is restored, the UPS switches between mains mode and battery mode without any transition time, thereby ensuring high reliability in the operation of the load. 　　 (3) High degree of intelligence. Through the data communication interface, it is possible to connect one UPS to multiple computer systems. Equipped with power monitoring software, it allows for the monitoring of power supply and UPS operation status directly on the computer screen. It also offers functions such as alerts for abnormal power conditions, the ability to shut down the system, and saving user applications, all of which help to simplify network management and enhance the reliability of computer systems. (4) Bypass protection function. The bypass power supply function significantly enhances the emergency response capability of the UPS. Meanwhile, when users have special requirements regarding power supply, online UPS units can provide bypass power supply with overvoltage protection, thereby protecting the DCS from high-voltage hazards. 　　 (5) Dual-machine hot backup. For DCS systems with high power supply requirements, in addition to redundant power supply, redundant UPS is also necessary. Online UPS systems allow for the setup of primary and backup switching units according to user requirements: in the event of a failure of the main unit, it automatically switches to another backup UPS for operation; when the main unit needs to be maintained or inspected, the DCS can be switched to the backup UPS, ensuring that the DCS always has access to the best power supply. Furthermore, online UPS units are equipped with a manual bypass switch device, which allows users to switch the DCS system to bypass power supply during UPS maintenance, thereby ensuring the stability of the DCS power supply.

Technical requirements for UPS uninterruptible power supply modules: 1. It should have a wide voltage input range, which reduces the need to use batteries and thus extends their lifespan. 2. Equipped with a built-in CPU microprocessor for full-featured intelligent control, including operation and management, battery management, automatic power on/off, real-time parameter measurement and display, as well as intelligent input detection and error correction. 3. Each module is a complete independent unit, featuring a complete circuit section, a control section, and a bypass system. 4. High reliability and low maintenance requirements. Built-in manual maintenance bypass switch. Its MTBF (Mean Time Between Failures) is ≥200,000 hours, and its MTTR (Mean Time To Repair) is ≤15 minutes.

UPS must meet the functional and technical requirements: 1. It should be able to provide power at full load for over 30 minutes, and at reduced load for over 4 hours (it should enable emergency shutdown in case of a power outage, and ensure that critical control systems remain powered during maintenance interruptions)； 2. Use high-quality brand UPS units (the cost of losing one UPS unit due to an accident is enough to buy many more UPS units) ； 3. If possible, use dual power supply; in the event of a failure in the UPS, it can switch to mains power to ensure continuous operation ； If the economy is strong, a power supply system with multiple UPS units for redundancy can be adopted ； 4. Under consideration~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

As a UPS for control devices such as DCS/ESD, it should possess the following functions: (1) It must be able to supply a sine wave power source with low distortion and high stability, whether in mains mode or battery mode, thereby providing the best power supply support for the DCS. 　　 (2) When the mains power fails or is restored, the UPS switches between mains mode and battery mode without any transition time, thereby ensuring high reliability in the operation of the load. 　　 (3) It should have a bypass power supply function; moreover, when the user has special requirements regarding the power supply, the UPS can provide bypass power supply with overvoltage protection, thereby protecting the DCS from high-voltage hazards. 　　 (5) For DCS systems with high power supply requirements, in addition to redundant power supply, redundant UPS is also necessary. Ensure the stability of DCS power supply. As an example of the technical specifications of UPS: Product models UH31-0100L, UH31-0150L, UH31-0200L. Input: Rated voltage is three-phase four-wire input with a rated value of 380 Vac. Voltage range: Phase voltage ranges from 176–276 Vac; at full load, it operates within 120–176 Vac, with performance decreasing linearly, and it can operate at half load at 120 Vac. Operation with one phase missing: If one phase is missing while the other two are normal, 50% of the rated capacity can be output ； If two phases are missing and the other phase is normal, 25% of the rated capacity can be output. Frequency: 50Hz to 5Hz; Power factor: 0.95. Output rated power: 10kVA/7kW, 15kVA/10.5kW, 20kVA/14kW. Voltage: 220Vac ±3%. Frequency: For mains-powered operation, when the bypass frequency is within the tracking range (50Hz ±10%, with 6%, 4%, 2%, and 1% as optional options; the default value is ±6%), the output follows the bypass frequency ； When the bypass frequency exceeds the limit, the output frequency is 50Hz±0.2% ； Battery-powered: Output frequency is 50Hz ± 0.2%; frequency tracking rate ≤ 1Hz/s; power factor is 0.7; peak factor is 3:1; voltage distortion is ≤3% (for linear loads) and ≤4% (for nonlinear loads). Dynamic response – under load changes from 0% to 100% or from 100% to 0%, the change in output voltage is less than 5% of the rated voltage. (Resistive load) Dynamic response recovery time: ≤20 mS. Overload capacity: 105–125% of the rated load for 10 minutes; 125–150% of the rated load for 1 minute. Bypass operating voltage: 120–253 Vac. Efficiency at mains power: 89%, 90%, 90%. Battery type: Sealed, maintenance-free lead-acid battery. Number of battery cells: 40 (in two groups). Rated voltage: 240 Vdc. Charging current: 4 A, can be increased to 8 A by paralleling multiple batteries (when the battery capacity exceeds 100 AH, an external charger is recommended). Switching time: Mains power – zero switching; Inverter – zero switching; Bypass – zero switching. Noise level (at a distance of 1 meter)

UPS power supplies can be divided into two main categories based on their output waveform: square wave output and sine wave output. Based on their operation mode, they can be divided into standby and online types. Among them, the standby UPS supplies power directly to the load from the mains electricity when the mains supply is normal. When the mains power supply is interrupted, the battery supplies power to the inverter, which in turn provides alternating current to the loads through the UPS inverter. In other words, the inverter of the UPS is always in a state of providing backup power to the load. In the case of online UPS systems, under normal conditions they supply power to the load via AC → rectification → inversion. Once the mains power is interrupted, the UPS switches to a battery → inversion mode to supply power to the load. Only when the battery is discharged to its final voltage does the control circuit send a signal to activate the automatic switch, thereby switching to power supply from another AC bypass source. Once the mains power is restored, the UPS switches back to having the inverter supply power to the load. Therefore, in an online UPS system, under normal conditions, it is always the UPS’inverter that supplies power to the load, which prevents any effects of power fluctuations and disturbances caused by the mains electricity grid from affecting the supply of power to the load. Obviously, its power supply quality is significantly better than that of a standby UPS. The online type can provide stable voltage and frequency power supply to the load. However, standby UPS units are favored for their high efficiency, low noise, and relatively low cost.

Alternating Current Uninterruptible Power Supply (UPS): 1. Technical requirements: 1.1 Input parameters: Input voltage – 3-phase 380V ±15% or single-phase 220V ±15%; Input frequency – 50±2.5HZ. 1.2 Output quality specifications: Voltage – 220V ±5%; Frequency – 50 ±0.5HZ; Waveform distortion rate – less than 5%. 1.3 Overload capacity – greater than or equal to 150% (within 5 seconds). 1.4 For power supplies of 20KVA or less, a single-phase output is generally suitable. 1.5 The selection of backup batteries should comply with the following rules: Backup power supply time (i.e., uninterrupted power supply time) – 15–30 minutes; Charging capability – 80% of the rated capacity can be achieved in 2 hours. Sealed, maintenance-free lead-acid batteries are preferred, but nickel-cadmium batteries can also be used. 2. Functions: 1.1 It should have fault alarm and protection functions. 1.2 It should have a voltage transformation and stabilization circuit. 1.3 It should have a maintenance bypass function; this bypass should be connected after the voltage transformation and stabilization circuit, or it can have its own voltage stabilizing transformer. The maintenance bypass should be able to synchronize with the internal main circuit. The switching time between the maintenance bypass and the internal main circuit should be less than or equal to the allowable power interruption time. The average time between failures (MTBF) of the UPS should be no less than 55,000 hours (no less than 150,000 hours when an automatic bypass is available). Direct Current Uninterruptible Power Supply (UPS): 1. Technical requirements: 1.1 Input parameters: Input voltage – 3-phase 380V ±15% or single-phase 220V ±15%; Input frequency – 50±2.5HZ. 1.2 Output quality specifications: Voltage – 224±0.3V; Ripple voltage – less than 0.2%; AC component (r.m.s.) – less than 40mV; Allowable power interruption time – ≤3ms; Voltage drop – less than 10%