March 2022

​View in Web browser​

Hi Reader,

Last time, we discussed the major components of a UPS system. If you missed it, you can find it here, UPS 101, The Basics. This month, we are going to move onto UPS topologies.

For our purposes, we will define topology as the configuration of the major components in a single UPS unit. What pieces are used to ensure there is uninterrupted power to the critical load?

I did a google search for “UPS Topologies”. The first page yielded Standby (also called Offline), Line-Interactive, and Online. The next defined Double-Conversion and Line-Interactive. Additional sites listed even more.

So which is correct? This is where it’s gets complicated.

IEEE 446-1995* defines only two UPS topologies, Double Conversion and Single Conversion. Using only the IEEE Standard will remove any bias and ensure we are all speaking the same language.

When I started my career, the gold standard topology was the double conversion UPS system. Many still consider it the best UPS system.

The double conversion UPS converts AC power to DC power using a rectifier. It then converts DC power back to AC power through an inverter. All power is converted twice, thus the name Double-Conversion.

In recent years, manufactures have introduced Eco Mode to their double conversion UPS Systems. When in this mode, the UPS allows power to flow through its static switch. The inverter is on, but what we call phase back. Meaning it is not producing any power, but is ready and can start in milliseconds.

Single Conversion UPS systems are where the most confusion occurs. However, IEEE only defines three types of Single Conversion topologies.

Line Interactive-Interact with the line using transformers and inductors to buck or boost the utility voltage to the critical load.

Tri-Port-It’s name comes from the fact that the input, inverter, and load all share the same transformer core. This allows for corrections to the input power without supporting the entire load.

Ferroresonant-Uses a special transformer called a ferroresonant that, although not efficient, provides for stable voltage and frequency output.

Recently I’ve learned a two new terms, Forward and Reverse Transfer. These are used primarily in industrial settings, and will fit into a category defined by IEEE.

In a Forward Transfer UPS, the load is powered from Utility Power until it senses a problem. The power is then diverted to the inverter. Forward Transfer UPS would primary be Line-Interactive.

In a Reverse Transfer UPS system, condition power powers the load unless there is a failure of the UPS System, then the power is switched by the static switch to the bypass line. Reverse Transfer UPS systems would be principally Double Conversion systems. I would argue Tri-Ports and Ferroresonant are also Reverse Transfer systems.

Knowing what topologies UPS system is protecting your critical load is crucial to knowing its limitations. There is no right or wrong UPS system, only the one that works for you.

For more in-depth reading on UPS topologies, check out an exert from my book “UPS Handbook, A Layman’s Guide to Uninterruptible Power Supply Systems”.

​UPS Topology.pdf​

Next month we are going to talk about bypasses used with UPS Systems and which one works best for your application.

If you have questions or want to learn more, please check out my book or email me.

Thanks again for reading.

Rob

​Rob@robdelauter.com ​

​

​

​

*Institute of Electrical and Electronics Engineers Standard 446-1995 “Recommended Practice for Emergency and Standby Power Systems for Industrial and Commercial Applications