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Lessons on: UPS Topologies

This is the first in a series of articles aimed at helping organizations learn the basics of mission critical infrastructure. This time we’re focusing on UPS installations. Electrical systems are among the most critical in a data center. And arguably the most critical electrical system is the UPS. It is always on, provides continuous conditioned power to critical IT equipment, and needs to be treated with tender loving care.

Let’s look at UPS designs to gain a basic understanding of UPS functionality.


A UPS is designed to provide protection from very short duration power loss; say from a few thousandths of a second up to fifteen minutes or so. The UPS does this by inserting itself into the path of power between the source and the load, and depending on the quality of power, acts to augment or filter that power to provide the best possible source profile to the load. The source monitoring, augmenting and filtering (collectively called “conditioning”) are all accomplished in different ways by different UPS topologies.

UPS Topologies

There are five basic UPS topologies:

  • Offline Single Conversion
  • Line-Interactive
  • Online Double Conversion
  • Delta Conversion
  • Rotary

Offline UPS System

Offline single conversion systems are the simplest but provides the least amount of conditioning. Power flows directly from source to load while the module monitors power quality. When the source fails, a transfer relay opens the source and closes into the stored energy source – normally batteries – and continues to provide power to the load through a DC/AC inverter. The main benefit of offline systems is their low cost. The main drawback to this topology is its lack of conditioning, both while the normal source provides the power, and during backup operation of the inverter.

Line-interactive UPS system

Line-interactive systems are similar to offline in that the power flows directly to the load, but inserts an inductor (auto-transformer) into the path to provide some conditioning capability. The main benefit of line-interactive is its better conditioning capability with only a small premium to offline systems. The main drawback is its inability to completely isolate the load from source anomalies. Minor, short duration distrubances will flow through the inductor and affect the critical load adversely.

Online UPS System

Online double-conversion is one of two robust static UPS topologies. Unlike the previous topologies, online systems insert themselves completely into the power path and provide complete isolation of source and load. The input power an AC/DC rectifier which simultaneously charges the batteries and provides continuous DC power to a DC/AC inverter which delivers conditioned AC power to the load. The main benefit of the online system is its isolation of source and load. The drawback is it’s more expensive and less efficient than offline or line-interactive systems.

Delta Conversion UPS System

Delta conversion systems are somewhat of a hybrid that provide the benfits of both online and offline systems while minimizing the drawbacks. The delta conversion system provides both complete isolation and direct connectivity while utilizing an input inductor, or delta transformer, to balance power flows from source to load through the unit. When normal power is delivered, it is passed thorugh to the load. When bad power arrives at the input, the system automatically delivers stored energy power to the load, conditioned through the main inverter.

Rotary technology is not so much a different topology as it is a different technology all together. The first four topologies we discussed are called “static” UPS modules because they don’t employ rotating machinery to re-create the AC sine wave. Instead, static UPS topologies take AC sine wave current and rectify it, or “chop” it up so that it looks and behaves like direct current to charge the batteries and provide input to the inverter. The inverter re-creates the AC sine wave by “pulsing” DC bursts of current that vary by frequency and duration. This is accomplished by the use of high speed silicon controlled rectifier transistors, or SCRs that are gated “switches” controlled through complex integrated circuits.

Rotary systems utilize the same rectifier technology as static topologies on the front end to create DC current from AC, but use spinning motor-generators (MG) to re-create the sine wave on the output. This provides a number of advantages. First, the sine wave produced by the MG is a pure sine wave of very high quality. Second, the load is isolated from the source both electrically and mechanically. Third, the inertia of the rotating machinery provides short-term ride-through of sub-second anomalies that would normally cause a static UPS to go to battery. This last benefit alone saves a significant amount of wear and tear on batteries and the corresponding costs to replace them sooner than would otherwise be the case.