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What Are the Types of Uninterruptible Power Supply (UPS) System?

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Posted on May 28, 2020
October 22, 2020
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Uninterruptible power supply (UPS) is a crucial component in the data center power system for providing backup power when the primary power source fails. Not all UPS systems are the same. They vary greatly in topology, size, capacity, form factor, etc. This post attempts to help you better understand the UPS system by presenting the UPS types from some common perspectives and pointing out their characteristics as well as applications in an elaborate manner.

Topology: Static UPS vs Rotary UPS

Static UPS and rotary UPS are classifications that are widely accepted by the public.Though they perform the same function, the components within the two systems and their features vary greatly.

Rotary UPS

Rotary UPS technology has been the oldest working system in use for many years. It is called “rotary” because there are rotating components (such as motor generators) within the architecture to transfer power into the load. The concrete components in the rotary UPS system may vary from vendors to vendors. The following diagram is a simple version of rotary UPS topology.


Figure 1: Rotary UPS

Rotary UPS has a low redundancy, typically 15-30 seconds, thus it is often coupled with a diesel generator to support loads for extended periods of time. It is applicable for industrial applications because it is manufactured with high capacity, 125 kVA or higher. For applications featuring multiple short inrushes of power, rotary UPS plays a prominent role. Some rotary UPSes are also used for high-security applications such as military applications with the aim of preventing electrical eavesdropping. Since rotary UPS is a fixed investment and the requirement of auxiliary equipment, the cost may be higher than that of static UPS. As for efficiency, rotary UPS sustain higher fixed losses and is less effective than static one. For these reasons, the rotary UPS takes up the minority UPS market presence.

Static UPS

Static UPS relies solely on battery power as an emergency source. Today the UPSes in most applications are static UPSes and sometimes the single term UPS is used to refer to static UPS. Static UPS has a wider swath of applications than rotary UPS and runs more efficiently with a significant advantage below 50% load. According to the working principle, the common static UPSes are made up of double conversion (also called on-line), passive standby (also called off-line), and the line interactive UPSes. UPS with on-line double conversion technology provides the highest level of power protection but is more expensive than the other technologies, while off-line standby technology requires the lowest cost but supply minimal power protection. Line-interactive technology requires lower cost than online but is easy to break on transfer to battery mode. To get more information about these three UPS technologies, you can refer to Comparison of UPS Topologies: Line-interactive vs Online vs Offline.


Figure 2: Static UPS

Transformer Design: Transformer-Free UPS vs Transformer-Based UPS

Categorized from the perspective of the use of transformers, the UPS system can be classified into transformer-free UPS and transformer-based UPS.

Transformer-Based UPS

The key components in a transformer-based UPS system are shown in Figure 3 below.


Figure 3: Simple version of transformer-based UPS diagram

In the transformer-based UPS system, the power flows over the rectifier (Rect), inverter (Inv), and transformer to the output and supplies the critical load. An automatic bypass switch (static switch) links the input and output, allowing the UPS to be bypassed. Transformer-based UPS differs from the transformer-free counterpart in the design of a transformer before the rectifier. The aim of the transformer in this UPS design is to help step up the voltage of the UPS output to provide the critical load with the same voltage rating of the input. Transformer-based UPS is a popular choice on the application requiring galvanic isolation, or achieve large KW.

Transformer-Free UPS

Transformer-free UPS gets its name because it doesn’t include an isolation transformer as part of its design. Though transformer-free UPS operates in the same way as transformer-based one, the transformer-free UPS system is more simplex with fewer number of components included. In a typical transformer-free UPS, there are insulated-gate bipolar transistors (IGBTs) dealing with high voltages, thus the need for a step-up transmission transformer after the inverter is eliminated.


Figure 4: Simple version of transformer-free UPS diagram

Most transformer-free UPSes are now constructed using modular units. Compared with those of transformer-based UPS, the physical size of transformer-free UPS is smaller, the operation is easier and the cost is lower, but it is limited in its power strength. To achieve larger KW size or redundancy, multiple UPS modules must be paralleled together, which may cause the system failure due to the increase of devices. Overall, transformer-free UPSes are more advantageous in applications carrying low loads.

Phase Voltage: Single Phase vs Three Phase UPS

Judging by the input and output phase voltage, UPS can be categorized into single-phase and three-phase.

Single Phase UPS

Single phase UPS can be also referred to as 1 phase or 1-phase. It has a single input and output source to the power equipment using only one sine wave voltage. It contains only two wires to cover the current—one conductor and one neutral. Applications with low kVA requirements (typically up to 20kVA), such as home use, small business, etc. are where 1 phase UPSes are typically applied.

Three Phase UPS

In a three phase power system, there are three separate conductors, which provide continuous power to the load. It contains a minimum of four wires, usually three conductors and one neutral, supporting three phase output or one phase output. Nowadays most data centers, commercial, industrial, and medical applications choose three phase UPS as the perfect solution for their large power system with critical loads.

Single phase UPS and three phase have their differences mainly on the number of conductors, sine waves, and voltage. For more information about the comparison of single phase UPS vs three phase UPS, see this post: Single Phase vs Three Phase UPS: What’s The Difference and How to Choose?

Form Factor: Tower vs Rackmount vs Modular UPS

UPSes are designed for various scenarios, which gives birth to different form factors.

Tower UPS

Tower UPS is the traditional vertical UPS, which is a structure standing alone on the ground or on the desk/shelf designed for high availability environments, demanding enough space for the standalone unit. It is applicable to home users, small offices, or small data centers. The following figure shows an on-line double-conversion tower UPS.


Figure 5: 3-Phase Tower UPS

Rackmount UPS

Rackmount UPS is the UPS unit that can be installed and operated on a rack. It is designed for use in rackmount environments, from large data centers to network closets and other remote edge locations. Compared with the traditional tower UPS structure, rackmount UPS has its advantages of simplifying layout between racks, utilizing space, saving space, and integrated monitoring and management.

There is also tower & rackmount design UPS which can achieve tower-rackmount conversion. That means you can use the UPS as either a standalone UPS or an integrated one into the existing rack cabinet. The diagram below shows a single-phase rackmount & tower UPS.


Figure 6: Single-phase Rackmount & Tower UPS

Modular UPS

Modular UPS can be installed in the standard cabinet. Similar to rackmount UPS, modular UPS is superior to traditional UPS in the space-saving and easy management characteristics. Modular UPS is generally hot-pluggable, able to be configured according to the current needs or added more in the future, greatly shortening the time and making it easier for installation or repair. What’s more, modular UPS offers greater extendibility since such a modular configuration enables a number of smaller UPSes operating in parallel without limitation on the number of modules instead of as a large unit.

Choose the Right UPS Type Based on Specific Requirements

From this classification guide, you may have some understanding of the different types of UPS systems. Since each type has its unique characteristics as well as suitable application, careful consideration of choosing UPS types is recommended for gaining a stable and reliable power system.