Uninterruptible power supply technical features

At present, the digital revolution and network economy are sweeping the world. With the application of various information systems in various industries, the uninterruptible power supply (UPS) has developed rapidly. As an important factor directly related to the safe operation of computer software and hardware, the reliability of power quality should be the primary consideration for small and medium-sized enterprises, schools, etc. UPS (Uninterruptable Power System) means uninterruptible power supply. It is a constant voltage and constant frequency uninterruptible power supply with energy storage device and inverter as the main component. It is mainly used to provide uninterruptible power supply to servers, computer network systems or other power electronic equipment. Uninterruptible power supply has quickly changed from a peripheral device of the computer, an unused role, to a key device of the Internet and the defender of e-commerce. As the cornerstone of the information society, UPS has begun its new historical mission. With the advent of the international Internet era, higher and higher requirements are put forward for the quality of power supply. Whether it is the equipment of the entire network or the data transmission path to provide end-to-end comprehensive protection, it requires the configuration of high-quality uninterruptible power supply.

1. Working Principle of UPS

UPS is a power protection device that contains energy storage devices and uses inverters as main components to stabilize voltage and frequency output. When the mains power is input normally, UPS stabilizes the mains power and supplies it to the load. At the same time, it charges the battery inside the machine and stores energy in the battery. When the mains power is interrupted (power outage due to various reasons) or input failure occurs, UPS will convert the energy of the battery inside the machine into 220V AC power to continue to supply the load, so that the load can maintain normal operation and protect the load software and hardware from damage.

UPS started to develop in the 1990s. In the early 1990s, UPS was required to provide uninterrupted power to ensure that user data would not be lost. In the mid-1990s, intelligent UPS equipped with RS232 interface and various power monitoring software on the computer monitoring platform focused on the integrity of user data. In the late 1990s, UPS focused on ensuring the system has "high stability" and "high availability". Different eras require different classifications and developments of UPS. The classification and development of UPS is a need for the rapid development of today's information society.

2. Basic classification and characteristics of UPS

There are different types of UPS on the market. According to the working mode of UPS, they can be divided into three categories: backup type, double conversion online type, and online interactive type.

1. Backup UPS power supply

It is the original form of static UPS, which is widely used, mature in technology, generally only used in a small power range, with simple circuit and low price. This UPS has basically no improvement on the adverse effects of voltage frequency instability, waveform distortion and interference from the power grid.

Its working performance characteristics: 1) High utility rate of mains electricity, up to 96%. 2) Strong output capacity, no strict restrictions on load current crest factor, surge factor, output power factor, overload, etc. 3) The output conversion switch is limited by the switching current capacity and action time. 4) The input power factor and input current harmonics depend on the nature of the load.

2. Online interactive UPS power supply

Also called a 3-port UPS power supply, it uses an industrial frequency transformer. From the perspective of energy transfer, its transformer has three energy flow ports; port 1 is connected to the mains input, port 2 is connected to the battery through a bidirectional converter, and port 3 is output. When the mains is powered, the AC power flows into the transformer through port 1, and the appropriate transformer tap is selected under the control of the voltage stabilizing circuit to pull it in. At the same time, under the action of the bidirectional converter at port 2, the output voltage on port 3 is jointly adjusted with the help of the energy conversion of the battery, so as to achieve a better voltage stabilization effect. When the mains power is off, the battery supplies power to the transformer through the bidirectional converter through port 2 to maintain the AC output on port 3. In the process of transformer tap switching of the online UPS power supply, the bidirectional converter works as an inverter and is powered by the battery, so the output voltage can be uninterrupted.

Its working performance characteristics: 1) High utility rate of mains electricity, up to 98%. 2) Strong output capacity, no strict restrictions on load current crest factor, surge factor, output power factor, overload, etc. 3) Input power factor and input current harmonics depend on the nature of the load. 4) The converter is directly connected to the output end and is in hot backup state. It has a suppressive effect on output voltage spike interference. 5) The input switch has a disconnection time, which causes the UPS output to still have conversion time, but it is much smaller than the backup type. 6) The converter also has a charging function, and its charging capacity is very strong. 7) If an inductor is connected in series between the input switch and the automatic voltage regulator, when the mains power is off, the inverter can immediately supply power to the load, which can avoid the danger of short circuit caused by the inverter feedback to the grid when the input switch is not disconnected.

3. Double conversion online UPS power supply

It belongs to the series power transmission mode. When the mains is present, the AC->DC conversion function is realized, providing energy to the DC->AC inverter and charging the battery at the same time. The rectifier is mostly a thyristor rectifier, but there is also a new generation of IGBT-PWM-DSP high-frequency conversion rectifier. When inverting, the DC->AC conversion function is completed, providing high-quality electric energy to the output end. Whether it is powered by the mains or switched to the battery, the conversion time is zero. When the inverter is overloaded or fails, the inverter stops outputting, the static switch automatically switches, and the load is directly powered by the mains. The static switch is an intelligent high-power contactless switch.

Its working performance characteristics: 1) Regardless of whether there is a mains supply, all the load power is provided by the inverter to ensure high-quality power output. 2) Since all the load power is provided by the inverter, the output capacity of the UPS is not ideal, and restrictions are imposed on the load, such as load flow peak factor, overload capacity, output power factor, etc. 3) For the controlled rectifier, there are still low input power factor, large reactive power loss, and input harmonic current that has a great impact on the power grid. Of course, if the IGBT-PWM-DSP rectification technology is used, the success factor correction technology can be used to increase the input power factor to close to 1.

4. Dual inverter voltage compensation online UPS power supply

This technology was proposed in recent years. It mainly applies the voltage compensation principle (delta conversion) in AC voltage stabilization technology to the main circuit of UPS, producing a new UPS circuit structure type, which belongs to series-parallel power transmission.

Its working performance characteristics: 1) Inverter (II) monitors the output end and participates in the adjustment of the main circuit voltage with inverter (I), which can provide high-quality electric energy to the load. 2) When the mains power is off, the output voltage is not affected and there is no conversion time; when the load current is distorted, the inverter (II) adjusts and compensates, so it is an online working mode. 3) When the mains power is available, inverters (I) and (II) only adjust and compensate for the difference between the input voltage and the output voltage. The inverter only bears 20% of the maximum output power, so the power surplus is the largest. Strong overload capacity. 4) Inverter (I) also completes the power factor correction function of the input end. The input power factor can reach 0.99, and the input harmonic current is <3%. 5) When the mains power is available, since the maximum power borne by the two inverters is only 1/5 of the output power, the overall efficiency can reach 96%. 6) When the mains is on, the power intensity of the inverter (II) is only 1/5 of the rated value, so the reliability of the power device must be greatly improved. 7) Due to the input power factor compensation, it has energy-saving effect.

3. Selection of UPS power supply

A UPS can be used for at least 3 years. When choosing a UPS, users should determine the selection criteria according to their own requirements and choose the UPS that best suits their business needs, rather than the cheapest or most advanced. Before purchasing a UPS, users should consider the importance of the data processed by the load equipment, the requirements of various electrical equipment for power quality, installation and space requirements, and economic budget. In addition, whether the weight and size of the UPS are appropriate is also a key issue that we should pay attention to when purchasing.

Next, users should understand the required UPS capacity and consider the total capacity of future equipment expansion. It is also essential to choose a reputable brand and manufacturer. Of course, when purchasing a UPS, you should also pay attention to its output power, power supply time, output voltage waveform, transient response characteristics, output frequency stability, waveform distortion coefficient, output voltage stability, safety performance, maintainability and price. In short, users should choose appropriate UPS products according to their business needs and consider three aspects: technical performance, service guarantee and product price.

Computing centers and network management centers that have high requirements for power supply quality often need to use the following redundant power supply systems with "fault-tolerant" functions to ensure that the power supply to the load is foolproof: 1) Host-slave "hot backup" redundant power supply system: The basic structure is to connect the AC bypass of the host UPS to the inverter power output of the slave UPS. In case of a failure of the host UPS, the slave UPS can be used to carry the load. This redundant working mode has limited application due to the lack of "capacity expansion" function and the possibility of a 4-millisecond power supply interruption. 2) Use a dual-machine redundant power supply system: Directly connect two UPSs with the same power output in parallel by placing them in the same amplitude, phase and frequency. During normal operation, each of the two UPSs bears 1/2 of the load current. In case of a failure of one of the UPSs, the remaining UPS will bear the entire load. The mean time between failures (MTBF) of this parallel system is 7-8 times that of a single UPS, which greatly improves the reliability of the system.

4. UPS Maintenance

With the increasing popularity of computers, UPS, the computer's guardian, has also been widely used. UPS is a simple-to-use but easily damaged device. Scientific use and maintenance will extend the life of UPS.

1) Try not to connect inductive loads. Because the starting current of inductive loads often exceeds the rated current by 3-4 times, which will cause instantaneous overload of UPS and affect the life of UPS. Inductive loads include electric fans and refrigerators commonly used in summer.

2) Do not fully load or lightly load the UPS. Do not use the UPS according to its rated power, and do not think that the empty ports should be left idle and connect other appliances. Long-term full load will directly affect the mission of the UPS.

3) Protect the battery. A very important part of UPS is the battery. At present, most medium and small UPS use sealed lead-acid batteries that do not require maintenance. Although it does not require maintenance on the surface, it will also go wrong if not taken care of properly. Moreover, this kind of battery is very expensive and requires working in a 0-30 environment.

4) Regular maintenance. Usually, the terminal voltage of the battery should be measured every six months. If the voltage exceeds 1V, it should be charged with a balanced constant voltage and current limit (0.5A). If it does not work, the only option is to replace the battery. If the local power supply does not stop for a long time, the power supply must be interrupted regularly (usually every three months) to discharge the UPS with load.

5) Pay attention to lightning protection. Be sure to ensure effective shielding and grounding protection of the UPS.

5. Development trend of UPS power supply

As new technologies are constantly being developed and gradually applied in practice, it can be predicted that UPS power supplies will develop in the direction of high frequency, intelligence, networking and large-capacity single-machine redundancy in the future.

High frequency: Although the traditional online technology is very mature, it has many problems that cannot be overcome, which limits its development prospects. The introduction of the high frequency concept has brought many new ideas and space for the development of UPS. With the development of high frequency technology and devices, the technology and products of high frequency online UPS of 3KVA and below have matured, and their functions and reliability should be higher than those of traditional UPS. High frequency plays an important role in reducing volume, reducing costs, and having a better response to nonlinear loads.

Intelligence: In the past, the application of microprocessors in UPS was only used in large and medium-sized UPS. In recent years, it has gradually developed towards small and micro UPS. The result is the intelligent development of UPS, including control, detection, and communication. UPS is gradually managed by computers, and computers and peripherals can "autonomously" deal with some foreseeable problems, can automatically manage and adjust, and can pass relevant information to the operating system or network administrator through the network, which is convenient for remote management.

Networking: The demand for UPS as a member of the network family is becoming more and more urgent, because it is the basis for the normal operation of the network. UPS is required to have a larger storage capacity, can serve multiple computers or other peripherals at the same time, and can achieve dynamic configuration between loads through some mechanism.

Large-capacity single-machine redundancy: As the network's requirements for UPS reliability are getting higher and higher, the way to solve the reliability problem is to use redundancy in addition to requiring high reliability of the components themselves. Single-machine redundancy for small-capacity UPS has already appeared. However, large-capacity UPS must currently be achieved through parallel connection. However, doing so will cause users to invest too much. However, there is no doubt that UPS systems monitored by Internet technology will become one of the mainstream UPS technologies in the future.