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Currently, the digital revolution and the internet economy are sweeping the world. With the application of various information systems in different industries, it has further driven the rapid development of uninterruptible power supplies (UPS). As an important factor directly related to the secure operation of computer hardware and software, the reliability of power supply quality should be a top priority for small and medium-sized enterprises, schools, and other organizations. UPS (Uninterruptible Power Supply) refers to an power supply that provides continuous electricity. It is a type of power supply with energy storage devices, and its main component is an inverter; it ensures a constant voltage and frequency, and is used primarily to supply uninterrupted power to servers, computer network systems, or other electrical electronic devices. The uninterruptible power supply has rapidly transformed from a peripheral device of computers, playing a role that was not critical, into a key device on the Internet and a protector for e-commerce. As the cornerstone of the information society, UPS has embarked on its new historical mission. With the advent of the Internet era, there are increasingly higher demands for the quality of power supply. To provide end-to-end protection for both the devices in the entire network and the data transmission pathways, high-quality uninterruptible power supplies are necessary. I. Working principle of UPS: A UPS is a power protection device that includes an energy storage unit, with an inverter as its main component, and it provides stable voltage and frequency output. When normal mains power is supplied, the UPS stabilizes the voltage of the mains power before supplying it to the load. At the same time, it charges the battery built into the unit, storing energy in it. When there is a power outage (due to various reasons) or an issue with the input power, the UPS converts the energy stored in the battery into 220V AC power to continue supplying power to the connected devices, ensuring their normal operation and protecting their hardware and software from damage. UPS systems began to be used on a large scale mainly starting in the 1990s. In the early 1990s, the key requirement for UPS systems was to provide a power supply without interruptions, in order to prevent data loss for users ； In the mid-1990s, intelligent UPS units were introduced, equipped with RS232 interfaces and various power monitoring software on computer-based monitoring platforms, with the primary goal of protecting the integrity of users’ data ； By the late 1990s, UPS focused its protection efforts on ensuring that systems had \"high stability\" and \"high availability\". The needs of different eras have led to various classifications and developments of UPS systems; the classification and evolution of UPS systems are necessary to meet the rapid development of today’s information society. II. Basic Classification and Characteristics of UPSs There are various types of UPSs available on the market. Based on their operating mode, they can be divided into three categories: standby type, double-conversion online type, and online interactive type. 1. Standby UPS: It is the earliest form of static UPS. It is widely used, features mature technology, is generally employed in low-power applications, has a simple circuit design, and is inexpensive. This type of UPS provides virtually no improvement in terms of issues such as unstable voltage frequency, distorted waveforms, and interference coming from the power grid. Its operational characteristics are as follows: 1) High utilization rate of mains power, up to 96%. 2) It has a high output capacity, with no strict restrictions on factors such as load current peak factor, surge factor, output power factor, and overload. 3) The output conversion switch is limited by the switching current capacity and response time. 4) The input power factor and input current harmonics depend on the nature of the load. 2. The online interactive UPS power supply, also known as a 3-port UPS power supply, uses a power-frequency transformer. From the perspective of energy transfer, its transformer has 3 ports for energy flow ； Port 1 is connected to the mains power supply; port 2 is linked to the battery via a bidirectional converter. Port 3 serves as the output terminal. When powered by mains electricity, alternating current flows into the transformer through port 1, and under the control of the voltage stabilization circuit, the appropriate transformer tap is selected. Meanwhile, thanks to the bidirectional converter at port 2, the energy from the battery is utilized to jointly regulate the output voltage at port 3, thereby achieving a good voltage stabilization effect. When the mains power is lost, the battery supplies power to the transformer via the bidirectional converter through port 2, maintaining the AC output at port 3. During the transformer tap switching process of online interactive UPS systems, the bidirectional converter operates in inverter mode and is powered by batteries, thereby ensuring uninterrupted output voltage. Its operational performance features are: 1) High utilization rate of mains power, up to 98%. 2) It has a high output capacity, with no strict restrictions on factors such as load current peak factor, surge factor, output power factor, and overload. 3) The input power factor and input current harmonics depend on the nature of the load. 4) The converter is connected directly to the output terminal and is in hot standby mode. It suppresses interference caused by output voltage spikes. 5) The input switch has an off-time, which results in a conversion time for the UPS output as well, but it is much shorter than that of a backup system. 6) The converter has a charging function as well, and its charging capacity is very high. 7) By connecting an inductor in series between the input switch and the automatic voltage stabilizer, the inverter can supply power to the load immediately when the mains power is lost. This prevents the risk of a short circuit occurring as the inverter feeds back into the power grid when the input switch has not been turned off. 3. Double-conversion online UPS power supply: It belongs to the series power transmission method. When mains power is available, it performs the AC-to-DC conversion function, supplying energy to the DC-to-AC inverter while also charging the battery. These rectifiers are mostly thyristor rectifiers, but there are also new-generation rectifiers based on IGBT-PWM-DSP high-frequency conversion. During inversion, it performs the DC-to-AC conversion function, providing high-quality electrical power to the output. Whether powered by mains electricity or switched to battery power, the conversion time is zero. When the inverter is overloaded or fails, it stops delivering power; the static switch then switches automatically, allowing mains electricity to supply power directly to the load. A static switch is an intelligent, high-power contactless switch. Its operational characteristics are as follows: 1) Whether or not there is a mains power supply, the entire power required by the load is provided by the inverter, ensuring high-quality power output. 2) Since all the load power is supplied by the inverter, the output capacity of the UPS is not ideal, which imposes restrictions on the load, such as the peak factor of the load current, overload capacity, and output power factor. 3) Controlled rectifiers also suffer from a low input power factor, high reactive power losses, and significant input harmonic currents that have a negative impact on the power grid. Of course, by using IGBT-PWM-DSP rectification technology along with power factor correction techniques, the input power factor can be increased to nearly 1. 4. Double-inverter voltage compensation online UPS power supply. This technology was introduced in recent years; it involves applying the voltage compensation principle (delta conversion) from AC voltage stabilization techniques to the main circuit of the UPS, thereby creating a new circuit configuration for UPS systems. It falls under the category of series-parallel power transmission. Its operational characteristics are as follows: 1) The inverter (II) monitors the output terminal and, together with inverter (I), contributes to the regulation of the main circuit voltage, thereby enabling the supply of high-quality electrical power to the load. 2) When the mains power is lost, the output voltage remains unaffected, with no conversion time ； When the load current is distorted, it is adjusted and compensated by the inverter (II), thus operating in an online mode. 3) When mains power is available, inverters (I) and (II) only adjust and compensate for the difference between the input voltage and the output voltage; the inverters handle only 20% of the maximum output power, thus maximizing the remaining power. It has a high overload capacity. 4) The inverter (I) simultaneously performs power factor correction at the input side. The input power factor can reach 0.99, with the input harmonic current being <3%. 5) When mains power is available, since the maximum power that each of the two inverters can handle is only 1/5 of the output power, the overall efficiency of the system can reach 96%. 6) When mains power is available, the power output of the inverter (II) is only 1/5 of its rated value; therefore, the reliability of the power components necessarily **increases significantly**. 7) Energy savings are achieved due to input power factor compensation. III. Selection of UPS units A UPS unit should be able to last for at least 3 years. When choosing a UPS, users should determine the selection criteria based on their own requirements, and opt for the UPS that best meets their business needs, rather than choosing the cheapest or most expensive one. Before purchasing a UPS, users should take into consideration various factors such as the importance of the data processed by the connected devices, the power quality requirements of different electrical equipment, installation and space requirements, as well as budget constraints. Additionally, the appropriate weight and size of the UPS are also key considerations when making a purchase decision. Next, the user should determine the capacity required for the UPS and consider the total capacity needed for future equipment additions. It is also essential to choose reputable brands and manufacturers. Of course, when purchasing a UPS, one must also pay attention to various factors such as its output power, the duration for which power can be supplied, the output voltage waveform, instantaneous response characteristics, output frequency stability, waveform distortion factor, output voltage stability, safety performance, maintainability, and price. In short, users should select the appropriate UPS product based on their own business needs, taking into consideration three aspects: technical performance, service guarantees, and product price. In computing centers and network management centers where high requirements are placed on power supply quality, to ensure reliable power supply to the equipment, redundant power supply systems with \"fault tolerance\" features are often employed, including: 1) Host-secondary \"hot backup\" redundant power supply system: The basic setup involves connecting the AC bypass of the host UPS to the inverter output of the secondary UPS; in the event that the host UPS fails, the secondary UPS can take over the task of supplying power. Due to the lack of a \"scaling up\" function and the potential for 4-millisecond power interruptions, this redundant operation mode has a limited range of applications. 2) Utilize a dual-unit redundant power supply system: This is achieved by connecting two UPS units with identical power outputs in parallel, ensuring that their outputs are at the same amplitude, phase, and frequency. During normal operation, each of the two UPS units handles 1/2 of the load current; in the event that one of the UPS units fails, the remaining UPS unit takes on the entire load. The mean time between failures (MTBF) of such a parallel system is 7–8 times that of a stand-alone UPS, thereby **improving the reliability of the system**. IV. UPS Maintenance As computers become increasingly widespread, UPS, the guardian of computers, has also seen wide use. UPS is a device that is simple to use but prone to damage. Proper use and maintenance will extend the lifespan of the UPS. 1) Try to avoid connecting inductive loads. This is because the starting current of inductive loads often exceeds 3–4 times the rated current, which causes an instantaneous overload in the UPS. It affects the lifespan of the UPS. Inductive loads include electric fans and refrigerators that are commonly used in summer. 2) It is not advisable to operate it at full load or with excessive light load. Do not use it according to its rated power, and do not connect other appliances to the empty ports thinking they should not remain unused. A long-term full-load condition will directly affect the functionality of the UPS. 3) Protect the battery; the battery is a very important component of a UPS. Currently, most medium-sized UPS units use maintenance-free sealed lead-acid batteries. Although it appears to require no maintenance, neglect can still lead to problems; moreover, this type of battery is quite expensive and needs to operate in an environment with a temperature range of 0–30 degrees. 4) Regular maintenance. Generally, the terminal voltage of the battery in a UPS should be measured every six months. If the voltage exceeds 1V, balanced constant-voltage current limiting (0.5A) charging should be used. If that doesn’t work, the battery has to be replaced. If there is no power outage in the area for an extended period, it is necessary to manually interrupt the power supply on a regular basis (usually every three months) to allow the UPS to discharge under load. 5) Be careful to avoid lightning strikes. It is essential to ensure proper shielding and grounding protection for the UPS. V. Development trends of UPS systems: As new technologies are continuously developed and gradually put into use, it is foreseeable that UPS systems will move in the direction of higher frequencies, increased intelligence, networking capabilities, as well as larger capacities with redundant single units. High frequencyization: Although traditional online technologies are already highly mature, their development prospects are limited due to various insurmountable problems inherent in them. The introduction of the concept of high frequency has brought many new ideas and possibilities for the development of UPS systems. With the advancement of high-frequency technology and related components, high-frequency online UPS systems with a capacity of 3 KVA or less have now become mature in terms of technology and product design. Their performance and reliability are superior to those of traditional UPS systems. High frequency plays an important role in reducing size and costs, as well as in providing better performance when dealing with nonlinear loads. Intelligentization: The use of microprocessors in UPS units was previously limited to large and medium-sized UPS systems; in recent years, it has gradually extended to smaller and micro UPS units. As a result, UPS systems have become more intelligent, with improvements in areas such as control, monitoring, and communication. UPS systems are increasingly managed by computers, which and their peripherals are able to handle some foreseeable problems autonomously, carry out automatic management and adjustments, and transmit relevant information over the network to the operating system or network administrators to facilitate remote management. Networking: The need to treat the UPS as a member of a networked home is becoming increasingly urgent, as it forms the foundation for the proper functioning of the network. It is required that the UPS have a larger capacity for storing energy, be able to serve multiple computers or other peripherals simultaneously, and facilitate dynamic allocation of load through some mechanism. High-capacity single-unit redundancy: As networks place increasing demands on the reliability of UPS systems, one way to ensure this reliability is through redundancy, in addition to relying on components that are highly reliable in themselves. Internal redundancy in single units of small-capacity UPSs has been implemented. Currently, large-capacity UPS systems still have to be achieved through parallel connection. But doing so would require too large an investment from the users. But there is no doubt that UPS systems monitored using Internet technology will become one of the mainstream trends in UPS technology in the future.