Introduction to UPS knowledge

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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 regulates the voltage of the mains power and supplies 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 an outage), 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 software and hardware 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, while 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 others are also important parameters that characterize UPS performance; they determine the capacity to protect the load as well as the efficiency in utilizing mains power. The better the performance, the stronger the protection capability. Generally speaking, offline UPS provides the weakest protection for the load, online interactive types offer slightly better protection, while online UPS can handle almost all common power problems. Of course, the cost also increases as performance improves. Therefore, when choosing a UPS, users should select different types of UPS based on the power requirements of the load and the importance of that load. 1) Normal startup sequence: Since ordinary loads generate inrush currents at the moment of startup, and the power components inside a UPS have specific safe operating ranges, 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, turn the UPS completely off first, and then disconnect the incoming mains power. 2) In chemical plants, DCS systems are used for chemical operations, monitoring, and control; therefore, it is very important to provide these DCS 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 working power supply 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. Classification: For DCS systems with a small number of input and output points, a configuration with two AC inputs and one UPS is used; for DCS systems with a large number of input and output points, a configuration with two AC inputs 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 \"AC Input 1 – Manual Maintenance Bypass Switch\" mode; in this case, the UPS is bypassed, allowing it to be repaired or replaced without disrupting the normal power supply to the load. 2. Dual AC input with dual UPS in parallel for power supply: The dual AC input with dual UPS in parallel configuration is shown in Figure 2. This involves connecting the outputs of two UPS units with identical power levels in parallel; it is required that the output voltages, frequencies, and phases of these two UPS units be the same, and that the load currents they handle be roughly equal. When the 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 has to do so at a 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 incorrect judgments by the parallel operation board, resulting in a drop in the output voltage and adverse effects on the load. 3. Precautions for using a UPS: (1) When selecting a UPS, it is necessary to leave some margin; 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 float charging. The UPS should be charged and discharged 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 floating charge voltage should be inspected once a month. If the floating charge 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 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 unusual 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 troubleshooting: 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 utility 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 turns on. The first step in handling the 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 continuous power supply to important loads. (2) Inverter failure: When this alarm appears, an alarm indicating that the automatic bypass switch is under load is also generated. First, check whether an inverter overload alarm is displayed on the panel as well; if so, view the output current on the LCD screen. If it exceeds the rated current, turn off any unnecessary loads until the output current is below the rated value. After that, 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. 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. It explains separately what functions the UPS for control system equipment such as DCS, ESD, and F&G should possess, as well as the technical requirements. UPS should meet the functional and technical requirements: 1. It must 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 branded 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. The use of a UPS as a power supply for control devices such as DCS/ESD is under consideration. Such a UPS should possess the following functions: (1) It should be able to provide a sine wave power supply with low distortion and high stability, whether in mains mode or battery mode, thereby ensuring the best power supply for the DCS. 　　 (2) When the mains power goes out or comes back, 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, thus 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. To illustrate the technical specifications of UPS: 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 units, they normally supply power to the load via AC → rectification → inversion. Once the mains power is interrupted, the UPS switches to supplying power to the load through batteries → inversion. 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 impacting 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, backup UPS units are efficient in operation, produce low noise, and are relatively inexpensive. Parameter Selection: AC Uninterruptible Power Supply (UPS):
1. Technical Requirements:
1.1 Input Parameters
Input Voltage: Three-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, single-phase output is preferred.
1.5 The selection of backup batteries should comply with the following requirements:
Backup Power Supply Time (i.e., uninterrupted power supply time): 15–30 minutes
Charging Performance: 80% of the rated capacity can be achieved in 2 hours of charging. Sealed, maintenance-free lead-acid batteries are preferred; cadmium-nickel 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 mechanism.
1.3 It should have a maintenance bypass function. The maintenance bypass should be connected after the voltage transformation and stabilization mechanism, or it can have its own voltage stabilizing transformer. The maintenance bypass should be capable of synchronizing 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).

DC Uninterruptible Power Supply (UPS):
1. Technical Requirements:
1.1 Input Parameters
Input Voltage: Three-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 (RMS value): Less than 40mV
Allowable Power Interruption Time: ≤3ms
Last edited by lizhiwenfirs on 2009-3-12 09:55