Do I Need an Uninterruptable Power Supply (UPS)?

Ups uninterruptible power supply equipment refers to power supply equipment that will not be interrupted by a short pause, can always supply high-quality power, and effectively protect precision instruments. Full name Uninterruptable Power System. It also has the function of stabilizing voltage, similar to a voltage regulator.

ups uninterruptible power supply

From the basic application principle, UPS is a kind of power protection equipment that contains energy storage device, with inverter as the main component, and stabilized frequency and stable output. It is mainly composed of rectifier, battery, inverter and static switch. 1) Rectifier: A rectifier is a rectifying device. In short, it is a device that converts alternating current (AC) into direct current (DC). It has two main functions: first, it converts alternating current (AC) into direct current (DC), and supplies it to the load after filtering or to the inverter; second, it provides the charging voltage to the battery. Therefore, it also functions as a charger;
2) Storage battery: The storage battery is a device used by UPS to store electrical energy. It is composed of several batteries connected in series. Its capacity determines the time for which it maintains discharge (power supply). Its main functions are: 1 When the mains power is normal, it converts electrical energy into chemical energy and stores it inside the battery. 2 When the utility power fails, convert chemical energy into electrical energy and provide it to the inverter or load;
3) Inverter: Generally speaking, an inverter is a device that converts direct current (DC) into alternating current (AC). It consists of inverter bridge, control logic and filter circuit;
4) Static switch: Static switch is also called static switch. It is a non-contact switch. It is an AC switch composed of two thyristors (SCR) connected in antiparallel. control. Divided into two types of conversion and parallel models. The transfer switch is mainly used in two-channel power supply systems, and its role is to achieve automatic switching from one to the other; the parallel switch is mainly used for parallel inverters and city power or multiple inverters.
UPS is divided into three types according to the working principle of backup, online and online interactive.
Of these, the ones we most commonly use are
Uninterruptible power supplies are now widely used in: mining, aerospace, industrial, communications, national defense, hospitals, computer business terminals,
1. The full name of UPS is Uninterruptable Power System (or Uninterruptable Power Supply). In the event of a power outage or poor power quality due to an accident, UPS can provide high-quality and most economical power to ensure the integrity of computer data and the normal operation of precision instruments.
2. What are the electrical performance indicators of UPS and how are they classified?
The electrical performance indicators of UPS include basic electrical performance (such as input voltage range, voltage stabilization rate, conversion time, etc.), certification performance (such as safety certification, electromagnetic interference certification), and appearance dimensions. Depending on whether the output voltage waveform has a transition time when the utility power is off, the UPS can be classified into two types: backup (Off Line, with transition time) and online (On Line, without transition time). Because Line Interactive still has a conversion time, it is considered a variant of the backup type, but the charging time is shorter than the backup type. Another main difference between backup and online UPS is the voltage regulation rate. The online voltage regulation rate is generally within 2%, while the backup type is at least 5% or more. Therefore, if the user's load equipment is high-level communication equipment, medical equipment, microwave receiving equipment, it is better to choose online UPS.
3. What is the load (such as a computer) for the conventional electrical performance indicators of the UPS and the range of its usage.
Computers, like other general office equipment, are rectifier capacitor loads. The power factor of such loads is generally between 0.6 and 0.7, and the corresponding crest factor is only 2.5 to 2.8 times. The load factor of other general motors is only between 0.3 and 0.8. Therefore, as long as the UPS is designed to have a power factor of 0.7 or 0.8, and a crest factor of 3 or more can meet the requirements of a general load. Another requirement of high-end computers for UPS is to have low zero-ground voltage, super lightning protection measures, short-circuit protection and electrical isolation.
4. What are the indicators that reflect the ability of UPS to adapt to the grid?
The adaptability index of UPS to the power grid should include: input power factor; input voltage range; input harmonic factor; the magnitude of conducted electromagnetic field interference.
5, UPS input power factor is low, what are the adverse effects?
The UPS input power factor is too low. For the average user, the user must invest in thicker cables and air circuit breaker switches. In addition, the UPS input power factor is too low for the power company (because the power company needs to provide more power to meet the actual power consumption required by the load).
6. What are the indicators reflecting UPS output capability and reliability?
UPS output capacity is the output power factor of the UPS. Generally, the UPS is 0.7 (small capacity 1-10KVA UPS), while the new UPS is 0.8, which has a higher output power factor. The indicator of UPS reliability is MTBF (mean time between failures). Above 50,000 hours is better.
7. What are the "online" meanings of online UPS and what are the basic characteristics?
Its meanings include: zero transition time; low output voltage regulation rate; can filter the input power surge, clutter and other functions.
8. What is the frequency stability of the UPS output voltage? What are the differences between various types of UPS?
The stability of UPS output voltage frequency refers to the magnitude of changes in UPS output voltage and frequency under no load and full load. Especially when the maximum and minimum values of the input voltage range are changed, the output voltage frequency stability is still good. In response to this requirement, online UPS is far superior to backup and online interactive, while online interactive UPS is almost the same as backup.
9. What factors should users consider when configuring and selecting a UPS?
The user should consider understanding the application of various architecture UPS; considering the requirements for power quality; understanding the required UPS capacity, and considering the total capacity of the future expansion equipment; choosing a reputable brand and supplier; Focus on service quality.
10. What kind of UPS should be used in power applications where the quality of the power grid is not good and 100% of power cannot be cut off? Which functional indicators of UPS should be valued for choosing UPS?
It is best to use long-delay (8 hours) on-line UPS in areas with poor grid conditions, and backup UPS can be considered in areas with medium or good grid conditions. Whether the input voltage frequency range is wide, whether it has super lightning protection, and whether it has passed anti-electromagnetic interference certification are all functional indicators that need to be considered when selecting UPS.
11. In the case of small power capacity or local power supply, which functional indicators should be valued to choose UPS?
When using a small capacity or local power supply, first choose a small capacity UPS, and secondly, choose an on-line or backup UPS based on its requirements for power quality. The backup UPS has 500VA and 1000VA, and the on-line UPS has 1KVA to 10KVA for users to choose.
12. In the case of large power capacity or centralized power supply, what functional indicators should be valued to choose UPS?
In the case of large power capacity or centralized power supply, a large-capacity three-phase UPS should be selected. And consider whether there is output short-circuit protection; can be connected to 100% unbalanced load; has an isolation transformer; can be used for hot backup; multi-language graphical LCD display; can be remotely monitored; has super-strong monitoring Software that can automatically page and send E-mail automatically.
13. For the occasions requiring long-delay power supply, what functional indicators are important to choose UPS?
Long-delay power supply UPS needs to be configured with high-quality, sufficient energy batteries at full load, and whether the UPS itself has an extra large and strong charging current to make the additional battery fully charged in a short time. UPS must have output short-circuit protection; super overload capacity; full-time lightning protection.
14. What kind of UPS should be used in places with high requirements for intelligent management of power supply?
The intelligent UPS that can be monitored by the network should be selected. With the support of the monitoring software that the UPS can monitor on the local area network, wide area network, and the Internet, users can achieve the purpose of network monitoring of the UPS. The monitoring software should do can automatically page and send E-mail automatically; can automatically broadcast by voice; can safely shut down and restart the UPS; can operate across different operating platforms; can be scheduled to start; can be used as a power supply Status analysis record; can monitor UPS operation status. And the monitoring software needs to be certified by Microsoft.
15. What kind of investigations should users do for UPS manufacturers?
Whether the manufacturer has ISO9000 and ISO14000 certification; Whether it is a well-known brand and attaches importance to customer interests and product quality; Whether it has a local repair center or service unit; Whether it has passed international certification in terms of safety standards and anti-electromagnetic interference; UPS Whether it has high added value, such as whether it can be used for network monitoring or intelligent monitoring in the future
In the process of using an uninterruptible power supply system, people often think that the battery is maintenance-free without paying attention. However, some data show that the proportion of UPS mainframe failure or abnormal operation due to battery failure is about 1/3. It can be seen that strengthening the right
I. Overview
With the rapid development of the economy and the deepening of the understanding of the central bank on network construction, the construction and renovation of central computer rooms has been in full swing in recent years. However, the ensuing is an ever-increasing amount of electricity costs. The investment in the construction of the central computer room, including electrical, power, cooling and other system facilities accounted for more than half of the investment proportion. The high amount of power consumption made the entire data center operating cost Gaobu Center's computer room faces the embarrassing situation of affordable but not affordable.
Reducing the operating costs of central computer rooms and saving energy and consumption has become a concern of the relevant central bank departments. Energy conservation can start with the following aspects. The first is the energy saving of the computer room environment, including the cooling environment and the power supply environment; the second is to save energy from IT hardware equipment to reduce the energy consumption of IT equipment; the last is the energy saving of various integrated circuits within the IT equipment, such as the energy saving of the CPU. The UPS is in the most important part of the AC power supply link. Almost all IT equipment in the equipment room is powered by the UPS. It is imperative to improve energy efficiency during operation. The energy saving of UPS must be carried out in all aspects from schemes, batteries, and power distribution.
Flexible planning for capacity expansion on demand
Generally, the construction of the city-level central computer room is not a one-step process, and the needs for the next 5 to 10 years will be considered. However, the UPS is generally one-step, and two sets of high-power UPS are installed in parallel at a time. 10% to 20% of the capacity, before the planned load enters the equipment phase-out period. This not only causes a waste of investment, but also fails to make the UPS operate at a higher efficiency point, resulting in a waste of electrical energy. How to avoid this situation, from the perspective of UPS power supply system, should include the following aspects.
( I ) Power supply scheme design
At present, there are two main types of UPS power supply schemes: decentralized power supply and centralized power supply. The characteristic of decentralized power supply is that one UPS powers one or more load devices. The advantage of decentralized power supply is that it distributes risks and does not cause large-scale power outages due to the abnormal power supply of a UPS. The disadvantages are that the UPS is distributed and inconvenient to manage, and wiring is not easy to plan. The other is the use of centralized power supply
Case, a set of high-power UPS power supply system directly supplies power to all loads in the computer room. The advantage of centralized power supply is that it is easy to plan, easy to manage, and easy to maintain. The disadvantage is that if the UPS system is abnormal, it may easily cause large-scale power outages. This disadvantage can be avoided by using various parallel structures. Therefore, the above two schemes each have advantages and disadvantages. At present, the central computer room generally adopts a centralized power supply scheme, which also concentrates the risk of power supply. When the total installed capacity of the UPS in the equipment room exceeds a certain limit, it is recommended that the equipment room be divided into several areas for power supply according to the planning of several phases.
( B ) UPS online parallel capacity expansion function
For the planning of UPS capacity in the computer room, a gradual expansion plan can be adopted according to the load capacity requirements in different periods, making the investment plan more economical, and also enabling the UPS to work at a better efficiency point. At present, the UPSs in the middle and high power sections have redundant parallel functions, which not only improves the reliability of the system, but also provides conditions for the expansion of the computer room. As long as sufficient air switches are reserved in the distribution box before and after the UPS during planning, and the corresponding space is planned in the computer room, the UPS parallel capacity expansion function can be realized. The key is the process of paralleling. When multiple brands of UPSs are paralleled, the UPS settings need to be modified. At this time, the UPS must be operated in the maintenance bypass state. The UPS is directly loaded by the mains. Large or even power outages will cause large-scale paralysis of the system. Therefore, the parallel expansion must have the online parallel function, that is, when the UPS parallel expansion, only the new UPS software needs to be modified to be consistent with the original UPS system, and the new UPS can be directly upgraded without closing the original UPS system The original system can be used. Before and after the expansion, the UPS works in the online mode to avoid the high-risk operation of switching to bypass power supply.
( Three ) the use of modular UPS to achieve incremental expansion
At present, modular UPS has begun to be applied in China. The characteristics of modular UPS include: scalability, short mean time to repair (MTTR), and economical realization of "N + X" redundant paralleling. Take Delta C Series ~ IJUPS as an example, each module is 20kVA, the maximum capacity of the entire system can be expanded to 160kVA, and the capacity can be gradually expanded according to the actual capacity requirements of the equipment room, as long as the power distribution capacity is planned in the initial phase of the equipment room. At the same time, it is more cost-effective to achieve "N + X" redundancy. Take the 60kVA capacity to achieve "N + I" redundancy as an example. The traditional solution must expand a 60kVA UPS, while using a modular UPS, only a 20kVA Modules can be saved, saving a lot of money.
3. Improve the UPS's own energy efficiency and optimize the load efficiency curve
At present, UPSs are all on-line double-conversion architectures, and rectifiers and inverters have power losses during their operation. Take a UPS with a capacity of 60kVA as an example. Each unit of electricity is calculated at 1.2 yuan. For each 1% increase in UPS efficiency, the electricity cost saved for a year is 5045.76 yuan. It can be seen that improving the working efficiency of UPS can save a lot of electricity bills for the data center, and it is also the most direct way to reduce the energy consumption of the entire computer room. Therefore, the procurement of UPS should try to purchase more efficient UPS.
Of course, high UPS efficiency is not only high at full load, but also must have a high efficiency curve, especially in the "1 + 1" parallel system, according to the system plan, the capacity of each UPS must not be greater than 50%. This time the efficiency is only below 90%, even if the full load efficiency reaches more than 95%, it is meaningless, so it is required that UPS must take measures to optimize the efficiency curve, so that UPS efficiency can reach higher efficiency at lower loads.
In addition to improving the efficiency of the UPS itself, some functions of the UPS can also be used. For example, like the ECO economic operation mode, the principle is to activate this function in a better city power environment to make the UPS directly powered by static bypass. At this time, the inverter is in standby state and works normally but does not output energy. When the mains power is abnormal, the UPS immediately switches to the inverter power supply state. The switching time is generally within 1 millisecond. Since the inverter is in standby state, its own loss is very small. At this time, the overall efficiency of the UPS can reach ~ 1J97% or more. Reduce losses by more than 3% compared to normal mode.
There are two conditions for using the ECO mode: first, the static bypass must use two sets of highly reliable thyristors, and the combination of contactors and thyristors must not be used, because the contact point will ignite when the contactor is attracted. Generally, it will work after hundreds of times. It does not work normally, and the thyristor does not have this problem, and it can shorten the switching time. Second, it is recommended to use it in a better power environment, such as a first-level power supply unit.
Fourth, reduce input current harmonics and improve power factor
The fundamental reason for the generation of harmonics is that the power lines present a certain impedance, which is equivalent to a passive network composed of resistance, inductance and capacitance. Due to the non-sinusoidal current generated by the non-linear load, the current and voltage distortion in the circuit is called harmonics. Harm of harmonics include: causing additional losses and heat generation of electrical components (such as capacitors, transformers, motors, etc.); temperature rise of electrical components, low efficiency, accelerated insulation aging, and reduced service life; interference with normal equipment operation; increased reactive power The active capacity of power equipment is reduced (such as transformers, cables, and power distribution equipment); the power supply efficiency is low; resonance occurs, especially when diesel generators generate electricity; the open circuit trips, fuses blow, and equipment is damaged for no reason. The UPS is a non-linear load to the power grid and generates a lot of harmonics when it is working. Taking a UPS equipped with a 6-pulse rectifier as an example, the input power factor is generally around 0.75, and the harmonic is greater than 30%. The main methods to reduce UPS32 as harmonics are as follows.
( A ) 12 pulse rectifier
The principle is to add a phase-shifting transformer and a 6-pulse rectifier on the input side based on the original 6-pulse rectifier. After adopting this technical scheme, the harmonic can be reduced to about 10%. The advantage is relatively simple, the harmonic improvement is obvious; the disadvantage is that the improvement of the power factor is limited, and the price is slightly higher.
( Two ) passive filters
According to the principle of the LC filter circuit, the harmonics generated by the UPS are filtered and the power factor is compensated. The advantage is that the technology is simple and the cost is low; the disadvantage is that it can only compensate the harmonics of the point order, and at the same time is greatly affected by the load impedance, which cannot be applied to the full power section.
( Three ) active filter
The principle is to use a controllable power semiconductor device to inject a current with the same amplitude and opposite phase to the harmonic source current into the grid, so that the total harmonic current of the power supply is zero, and the purpose of real-time compensation of the harmonic current is achieved. The advantage is that it can compensate harmonics of multiple orders, and it is not affected by the load impedance; the disadvantage is that the purchase cost is higher.
( 4 ) Design of high-frequency IGBT rectification and PFC power factor correction circuit
The principle of the rectifier is to use high-frequency PWM to control the IGBT to turn on, divide the input voltage waveform, make the input current waveform as close to a sine wave as possible, and compensate the input voltage and current phase difference. The advantages are light volume, cheap price, and good effect; the disadvantage is that the technical structure is complex, it is not easy to maintain, and it is affected by power devices, and the current capacity is limited.
For the comparison of the above several technologies, performance and investment, you can choose the appropriate solution according to the actual needs.
V. Battery Management and Distribution Management Technology
UPS are equipped with batteries, and the user's investment in the battery pack often accounts for a large proportion of the entire UPS power supply system investment, or even exceeds the UPS's own investment, and the battery life is significantly lower than the UPS host. Because the main materials of the battery are heavy metals lead, sulfuric acid, and non-decomposable plastics, they will cause serious pollution to the environment. Therefore, reducing the number of batteries used and extending the battery's cycle life not only saves direct and indirect battery investments, but also reduces the environmental pollution of the entire equipment in the equipment room. Therefore, UPS can achieve battery energy saving through the following technologies.
( I ) Parallel battery sharing function
The principle of the shared battery pack is to isolate the fault through a special rectifier, synchronize the rectification of two or more UPSs in the parallel system, and the busbars share the current, so that each UPS busbar in the system is directly connected in parallel, and then the system backup time requirements After the battery is connected in parallel, it is connected to a parallel bus system to realize battery sharing and reduce battery investment. Taking "1 + 1" as an example, the traditional UPS solution has a system backup of one hour. When one of the UPSs fails, the battery of UPS2 cannot be used for UPS1, so UPS1 and UPS2 must each be equipped with a battery of -4 hours. Group to ensure that the system can be backed up for one hour after power failure. After adopting the shared battery pack solution, since UPS1 fails, the battery in the system can still provide energy for UPS2, so the entire system only needs to be equipped with a set of one-hour batteries. This not only saves direct battery investment, but also saves investment in space, load-bearing and air conditioning in the machine room, and reduces environmental pollution.
( Two ) intelligent battery management technology
There are many factors that affect battery life, including temperature, charge, discharge, and cycle times. If the above factors can be comprehensively processed, the battery life can be greatly extended, the battery replacement cycle can be extended, and the battery investment can be saved. UPS's smart battery management strategy includes: battery equal float charge management (equal float charge control), intelligent discharge termination voltage control for charging temperature and month, in addition, it should also have electric detection and battery leakage detection functions. In addition, a wide range of UPS can be selected to reduce the number of battery discharges. Through the above technologies, battery life can be greatly extended by 2-3 years.
( Three ) intelligent UPS distribution management technology
The principle is to realize multiple power-off protection functions for different levels of loads in the equipment room by detecting the UPS battery voltage or management time, reducing battery investment and increasing battery usage. There are two main types of intelligent UP management technology: software implementation and hard. Taking Delta UPS as an example, the software method is to install the Deltashutd0wnAgent shutdown agent program on the load server during UPS monitoring. When the mains power is abnormal and the battery voltage or timing conditions are met, the system program is automatically saved, and then the server is shut down.
The hardware method is that the UPS output is equipped with an intelligent power distribution screen. The UPS battery voltage or timing requirements are detected by the PLC. When the above conditions are met, the intelligent power distribution screen shuts down a certain output according to the setting. At present, this solution has been applied to the UPS power supply system of many domestic subways.
6. Concluding remarks
The energy saving of the central computer room must be grasped from the top down, or from the infrastructure to the core equipment. The UPS is the core of the entire AC power supply link. Doing a good job in energy saving can not only save a large amount of equipment investment and maintenance costs, but also Significantly reduced the operating costs in the later stages.
An overview of the circuit topology of the three-phase uninterruptible power supply system, the overall circuit structure, and various popular control strategies were made. The problems in the design and application of uninterruptible power supply and the new hotspots in current research were pointed out. Finally, Made predictions for the development of UPS . [1]
O Introduction
For a long time to come, the situation of China's mains power grid is insufficient, the voltage fluctuates greatly, and severe interference will still exist. And the rapid development of various industries and fields has put forward higher and higher requirements for power supply quality, especially important systems, important departments and important electrical equipment with high real-time requirements for power supply quality and the actual situation of China's power grid. The contradictions are becoming increasingly acute. Therefore, uninterruptible power supply (UPS), as a kind of green power supply with constant voltage and stable frequency purification, has become the focus of people's attention. In order to continuously improve the performance of UPS, researchers have done a lot of research on UPS systems, and proposed many circuit topologies and control strategies.
Circuit topology of UPS
The reliable operation of the UPS is inseparable from the coordinated work of the various modules. The following briefly analyzes the circuit topology of the main functional modules of the UPS.
1.1 Rectification and power factor correction circuit
The rectifier circuit forms a DC power supply device in the application. It is an interface circuit between the public power grid and power electronic devices. Its performance will affect the operation and power quality of the public power grid. High-performance UPS requires a high input power factor and minimizes the harmonic components of the input current. The traditional single-phase UPS mostly adopts the simulation method, and the three-phase UPS mostly uses the phase-controlled rectifier circuit and the voltage-type single-tube rectifier circuit.
1.1.1 Traditional three-phase phase-controlled rectifier circuit and voltage type single tube rectifier circuit
The phase-controlled rectifier circuit uses a semi-controlled power device as a switch, which has the following problems:
1) The presence of grid-side harmonic current will reduce the power factor of the grid side of the equipment and increase reactive power;
2) The phase-controlled rectification commutation method causes the voltage distortion of the power grid during the commutation period, which not only affects its own circuit performance, but also interferes with the power grid and adversely affects other equipment between the same grounding points;
3) The phase-controlled rectification link is a time-delay link, which cannot achieve rapid adjustment of the output voltage.
Voltage type single tube rectifier circuit is the abbreviation of three-phase uncontrolled rectifier bridge plus Boost circuit. Its disadvantages are: the large current peak not only hinders the increase of system power, but also increases the conduction loss and switching loss; in order to maintain the grid-side power As the factor increases, the boost circuit must have a certain boost ratio, which will cause the DC output voltage to be too high for the three-phase circuit.
1.1.2 Current-type three-phase bridge rectifier circuit
The current-type three-phase bridge rectifier circuit is shown in Figure 1. Its advantage is that the feedback control is simple. It is not necessary to add current feedback to the control circuit. Only the duty cycle of each switching tube can be adjusted to achieve sinusoidal input current. DC The voltage on the side is low. The disadvantage is that the sine of the input current is not very good, and a parallel capacitor must be added on the input side to achieve phase shifting. This kind of circuit is now becoming one of the hot spots of research. This circuit is suitable for occasions with high power rectifier circuits and low power factor requirements.
Current type three-phase bridge rectifier circuit
1.1.3 Voltage-type three-phase bridge rectifier circuit
The voltage-type three-phase bridge rectifier circuit is shown in Figure 2. It is characterized by the use of high-frequency PWM rectification technology. The device is in a high-frequency switching state. Because the on and off states of the device can be controlled, the current waveform of the rectifier is controllable. of. The advantage of this circuit is that it can get the input current in the same phase as the input voltage, that is, the input power factor is 1, the harmonic content of the input current can be close to zero; the energy can flow in both directions, and the energy is normally from the AC side to the DC side. Flow, when the DC output voltage is higher than a given value, the energy flows from the DC side to the AC side, which has a higher conversion efficiency. The disadvantage is that it belongs to the Boost type rectifier circuit, and the DC voltage requirement is higher. This circuit is also a research hotspot.
Voltage type three-phase bridge rectifier circuit
1.2 Battery pack and charging and discharging circuit
The battery pack is an energy storage unit for UPS. When the mains power is normal, it absorbs the energy from the mains power and stores it in the form of chemical energy. Once the mains power is interrupted, it converts the stored chemical energy into electrical energy to power the inverter and maintain it. Load continuity. In small and medium power UPS systems, the voltage of the battery pack is usually relatively low, so charging and discharging circuits that can flow in both directions are usually used [4]. In order to improve efficiency in high-power systems, the simplified circuit usually connects the battery pack directly to the DC bus.
1.3 Inverter circuit
The inverter is the core of the UPS. It converts DC power into AC power with a constant voltage and frequency. In the following, the research hotspots of inverters are still analyzed with three-phase inverters as the object.
1.3.1 Three-phase half-bridge inverter circuit
In the three-phase inverter circuit, the three-phase half-bridge bridge circuit is most commonly used. This circuit is characterized by the use of fully controlled devices to form the inverter, which has the advantages of high power density, good performance, small size and light weight. This circuit facilitates the use of new control strategies to improve the quality of the inverter. However, it is difficult to achieve 100% independent load.
1.3.2 H- bridge inverter
For ultra-large-capacity inverters, due to the large increase in power level, new requirements are put on the structure of the inverter, and the H-bridge inverter is one of the options. The inverter output transformer adopts multi-winding connection method, the primary side of the output transformer adopts 3 independent windings, and the inverter output adopts 3 independent H-bridges. This control is convenient, but the cost is high.
1.3.3 Three-phase four-bridge arm conversion technology
Due to the inherent defects of the three-bridge inverter in the three-phase circuit, people began to seek for a new circuit structure, so a three-phase four-bridge inverter appeared, as shown in FIG. 3. The output of this circuit structure is a three-phase four-wire system. The three-phase voltage can be controlled independently and the control method is flexible. However, the algorithm of this topology is relatively complicated. The PWM vector rotates in three-dimensional space. Digital control methods must be used to achieve the spatial PWM waveform The generation of this circuit has become one of the research hotspots.
Three-phase four-bridge arm conversion technology
1.4 Three-phase UPS complete circuit
1.4.1 Traditional three-phase UPS circuit structure
Traditional three-phase UPS structure, the input uses thyristor rectification, the output uses inverter, the battery is directly connected to the DC bus, and the rectifier is also used as a charger. The output adopts transformer isolation, which can realize the complete isolation of the input and output, to ensure that the disturbance of the power grid will not cause interference to the load. When the mains power fails, the battery outputs stable AC power through the inverter; when the inverter fails, the output voltage is bypassed to ensure the reliability of the power supply. The main disadvantage of this structure is the relatively large size and weight.
1.4.2 High-frequency chain three-phase UPS
In order to reduce costs and reduce the size and weight of the UPS, a high-frequency chain three-phase UPS has appeared, as shown in Figure 4. This circuit eliminates the need for a large power frequency transformer, and the input uses high frequency rectification to obtain a higher input power factor and lower input harmonic current. The disadvantage is that there is no transformer isolation between the input and output, and the disturbance of the power grid may cause disturbance to the output of the UPS; the output three-phase voltage depends on the middle point of the battery and the capacitor to form a neutral line, so the positive and negative DC voltage amplitudes must be equal in the control, otherwise The output neutral will have a large DC component, which is not good for the load and the transformer in the load; the input uses a three-phase four-wire system, and the current flowing through the neutral may cause the neutral potential to shift and cause interference to the load; the input and output are not isolated The problem of circulating current in parallel is difficult to solve.
High-frequency chain three-phase UPS
1.4.3 New Online Interactive UPS
Because the above two types of UPS have to go through two full power conversions, the efficiency of the system is low. From the perspective of improving system efficiency, a series-parallel compensation large-capacity structure has emerged, which is a new type of online interaction. Structure, as shown in Figure 5. This topology also has no transformer isolation for input and output, so there are disadvantages of high-frequency chain UPS. The output frequency of this UPS must be consistent with the power grid, and the ability to suppress the disturbance of the power grid is not strong, so the power supply quality is worse than the traditional three-phase UPS. Its characteristic is that the energy from input to output is not converted by full power, and is also composed of two high-frequency converters, but converter 1 can only bear a maximum of 20% of power. From a cost perspective, this structure Lower cost. In the control method, converter 1 is a voltage compensator for compensating the distortion of the grid voltage; converter 2 is a current compensator for compensating the harmonic current of the load, and it is used as full power when the mains power is cut off. The voltage type inverter supplies power to the load.
New online interactive UPS
1.4.4 Input and output isolated high-frequency chain UPS
Because the input and output of the traditional power frequency UPS has an isolation transformer, the output has good isolation characteristics, and the high frequency chain UPS has good input characteristics. Therefore, this type of high frequency chain UPS with input and output isolation appears as shown in the figure 6 shown. Due to the shortcomings of high-frequency rectification, an autotransformer must be connected to the input side to reduce the voltage, which increases the weight and cost of the whole machine. In addition, because the input uses a high-frequency converter, the efficiency of the whole machine is higher than that of high-frequency chain and traditional UPS. Are inefficient. However, because the input power factor is 1, there is no harmonic current, so the total power consumed is lower than that of a traditional three-phase UPS.
Input and output isolated high-frequency chain UPS
1.4.5 UPS with input and output in parallel
In this kind of circuit, the input end is formed by multiple rectifiers connected in parallel to power the DC bus. At the same time, the DC bus provides DC voltage to multiple inverters. The output terminals of multiple inverters are directly connected to supply power to the load. This method can increase the capacity of the UPS, increase the reliability of the system, reduce costs, and enhance maintainability. However, the more modules connected in parallel, the more difficult it is to solve the problem of current sharing between modules.
Uninterruptible power supply control technology
With the rapid development of various microcontrollers with rich control theory and functions, various discrete control methods have appeared. The number of control feedback loops can be divided into single-loop, double-loop, and multi-loop control. Increase the number of feedback loops as much as possible under the conditions allowed by the hardware, which can improve the control effect. From the control principle, it includes digital PID control, state feedback control, deadbeat control, repetitive control, sliding mode variable structure control, fuzzy control, neural network control, space vector control and other methods.
The digital PID control has good adaptability and strong robustness; the algorithm is simple and clear, and it is easy to implement with a single-chip microcomputer or DSP. However, there are two limitations: on the one hand, the sampling and quantization error of the system reduces the control accuracy of the algorithm; on the other hand, the sampling and calculation delays make the controlled system a system with pure time lag, causing the PID controller to be stable The reduced domains increase the design difficulty.
Predictive control can achieve small output current distortion and strong anti-noise capability. However, this algorithm requires knowing the precise load model and circuit parameters, so it is poor in robustness, and the delay caused by numerical calculation is also a practical application. problem. Hysteresis control has fast response speed and high stability, but the switching frequency of hysteresis control is not fixed, which reduces the circuit reliability, and the output voltage spectrum becomes worse, which is not good for system performance.
The basic idea of deadbeat control is to calculate the PWM pulse width of the next switching cycle based on the state equation of the inverter and the output feedback signal. Therefore, theoretically, the output voltage can be very close to the reference voltage in phase and amplitude The output voltage error caused by load changes or non-linear loads can be corrected in one switching cycle. However, deadbeat control is a control method based on an accurate mathematical model of the controlled object, which has poor robustness.
Swipe control is a kind of non-linear control. The characteristic of this control is the discontinuity of control. This control can be used for both linear and non-linear systems. This control method is very robust. The disadvantage is that it is difficult to obtain a satisfactory slip surface.
Repetitive control is a control method based on the internal model principle. The purpose of the inverter's repeated control is to eliminate the periodic distortion of the output voltage waveform caused by the rectifier bridge load. Repeated controller can eliminate the steady-state error caused by periodic interference. However, due to the control characteristics of repeated control delaying one power frequency cycle, the dynamic characteristics of UPS inverters with repeated control alone are extremely poor.
Fuzzy control belongs to the category of intelligent control. The design of the fuzzy controller does not require an accurate mathematical model of the controlled object, so it has strong robustness and adaptability. Fuzzy control is similar to traditional PD control, so this control has a fast response speed, but its static characteristics are not satisfactory. Neuron network control is a control method that simulates the intelligent activity of the human central nervous system. Neural networks have the advantages of nonlinear mapping capabilities, parallel computing capabilities, and strong robustness. They have been widely used in the field of control, especially in the field of nonlinear systems. Some results have been achieved in the design of neural network structures and learning algorithms. However, due to the limitation of the hardware system, neural network control has not been able to achieve online control of the inverter output voltage waveform. Most applications use offline learning to obtain optimized control rules, and then use the obtained rules to achieve online control.
Harmonic injection PWM technology, the utilization rate of DC bus voltage can basically reach loo%. This method is very effective for voltage open-loop control systems, but in closed-loop control systems, the initial phase of the harmonic injection must be consistent with the fundamental wave. In the instantaneous voltage control, the initial phase of the voltage fundamental wave cannot be accurately positioned and is difficult. application.
Space vector PWM has many advantages, such as small current distortion, high DC bus voltage utilization rate, and easy digital implementation. This control method also requires an accurate model of the circuit.
The various control schemes mentioned above have their advantages, but also have their disadvantages. At the same time, different control methods to form a composite control control scheme have been widely used in practice and achieved good results.
Problems in Uninterruptible Power Supply Design and Application
The American UPS manufacturer APC company summarizes and summarizes the five aspects of the UPS power supply system currently facing, which must be solved in the future:
1) Life cost cycle issues;
2) Adaptability and scalability of uninterruptible power supply systems;
3) Improve the availability of uninterruptible power supplies;
4) Manageability of uninterruptible power supply to the power supply system;
5) Serviceability issues.
Recent developments in uninterruptible power supplies
The development trend of uninterruptible power supply is the multi-machine parallel redundancy of UPS. The redundant parallel technology is used to improve the capacity and reliability of the UPS. The hardware devices with richer functions are used to achieve full digital control and various advanced complex control algorithms Can be used to continuously improve the performance of UPS, that is, to digital and high-frequency development; further intelligent and networked UPS, making the computer network an uninterrupted network.
4.1 UPS multi-machine parallel technology to achieve redundancy
UPS's parallel technology can bring the following benefits:
1) The capacity of the power system can be flexibly expanded;
2) Parallel redundant systems can be formed to improve the reliability of operation:
3) High system maintainability. When a single power supply fails, it can be easily replaced and repaired by hot plugging.
The use of parallel technology can form a redundant power supply system with fault tolerance. From the information available, there are mainly the following redundant configuration schemes:
1) Centralized parallel control;
Parallel redundant UPS
2) Master-slave parallel control;
3) Decentralized parallel control;
4) Ring-chain parallel control;
5) Wireless parallel control.
These several parallel methods, from the perspective of reliability, the centralized type is the worst and the wireless control is the best. It has also become a research hotspot.
4.2 Digitization and high frequency of UPS
The original UPS used analog control methods with many limitations. With the continuous improvement of the calculation speed of the digital processor, various advanced digital control methods are realized, which makes the design of the UPS have great flexibility, shortens the design cycle, and greatly improves the performance. The high frequency of the UPS effectively reduces the volume and weight of the device, eliminates the audio noise of the transformer and inductor, and improves the dynamic response of the output voltage. Digital control method has become a research hotspot in the field of AC power supply. An inevitable development trend is that various methods penetrate each other and combine with each other to form a composite control scheme. Digital compound control is a development direction of UPS control.
4.3 Intelligent and Networked UPS
In order to adapt to the development of computer networks, UPS has begun to configure RS232 interface, RS485 interface, USB interface, SNMP card and MODEM to become a part of computer network.
UPS Power SNMP Network Card
1) Real-time monitoring function It performs real-time high-speed sampling of each analog parameter of the UPS and the switching value indicating the working state to realize digital monitoring.
2) Self-diagnosis and self-protection function The UPS analyzes and compares the simulated parameter and working state data collected in real time and the data of key hardware devices in the system with normal values to determine whether the UPS has hidden trouble. If there is a fault, the alarm will be reported with a friendly graphical interface and text prompts on the display of the control panel according to the corresponding level of fault information. Dial the phone and other means to make an alarm and make corresponding protective actions.
3) Man-machine dialogue control mode Large UPS can provide users with a monitor LCD screen, which displays the workflow and parameter information in graphics and text. Visual menus can be provided for user operation. And guide the user to help troubleshoot according to the established way with help and continuous prompts, effectively preventing misoperation.
4) Remote control function In the networked era, UPS should not only provide protection to the hardware equipment directly powered by it, but also protect the entire network.
UPS remote monitoring topology
Run programs and data and data transmission channels are fully protected to make them an uninterrupted network. This means that the UPS should be configured with the corresponding power monitoring software and SNMP (Simple Network Management Protocol) manager to enable it to have remote management capabilities. Users can perform remote monitoring and network communication operations of data between the UPS and the network platform. UPS becomes an important part of the network system. In this way, the network administrator monitors multiple UPSs through the network management software, and the managed UPSs can be on the same LAN or different LANs, and can even be incorporated into the network management system to manage the UPSs through the Internet.
Due to the future widening and globalization of the network, it will inevitably bring complexity of the network, and various forms of network systems are connected together. As a part of the network system, UPS is required to be able to monitor on various network platforms, and with the ultra-high-speed development of the Internet, intranet and e-commerce, users' requirements for network availability will become higher and higher. The protection of critical equipment extends to the protection of the entire network path.

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