What is an Online UPS (dual-conversion) and its advantages
An online UPS takes incoming AC power, converts it to DC, then back to AC. Hence the Term "Dual Conversion" or "True Online". During this conversion the UPS can easily condtion the power to a perfect sine-wave.
The true flow of power is AC power comes in, is converted to DC (generally through a device called a IGBT) which is called the converter, charges the batteries, and is then converterted back to AC through what is called the inverter section (also through IGBT devices). The flow of power in a true on-line UPS is AC->DC->AC. So if you take away the AC (which is your incoming power), all you have left is your DC-AC - or your batteries supplying your output. In doing this there is absolutely no "switching" to batteries involved. The batteries simply take over. This is the only way to get a true uninterruptible power supply.
Also included in most online UPS is a isolation transformer (sometimes optional). The purpose of the isolation transformer is to bond your neutral and ground on the output of your UPS. What the heck does that mean? Well what that means, is that if you get a surge, or any kind of power anomaly, most UPS only protect your Hot legs. This leaves your neutral wide open for any kind of bad power to be passed through to your equipment. By "bonding" the neutral to ground on the output of the UPS, you eliminate the potential for any power glitches to get through by sending them to ground. An isolation transformer is a must. If you are purchasing an online UPS, you have to get the transformer option. In my opinion, it should not even be an option.
Also noted on an online UPS is the existence of filters. Generally there are filters on the input and output of the UPS. They use devices known as MOVs to protect you against surges and such. The MOV will blow, sort of like a fuse, if too much power tries to reach your equipment....such as a lighting strike.
If you are looking at a one-line schematic, you may see a strange line that starts near the input of the UPS, and goes up and around all of the inverter and converter sections, and ends up before the transformer. This is called a static bypass switch. You might be saying to yourself "but I though you said there was no "switch" in an on-line UPS?". Well that is true. There is no switching between input and batteries. The static bypass has a specific purpose. If the UPS faults in anyway, either internally, or with a short inside your equipment, the UPS will go to static bypass. Basically the bypass takes the flow of power inside the unit and reroutes it around its Inverter, Converter, and batteries. So you will be running on utility power (dirty power). However, most on-line UPS systems Still route their bypass through the transformer and filters so that you do have some protection against power anomalies. The static bypass also allows for extreme overload conditions. If your UPS is too overloaded, it will simply go into static bypass instead of dropping your equipment like standby or line-interactive units. This allows you to remove the overload, and never have to turn your equipment off.
It is highly recommended to find a UPS that uses IGBT (Insulated Gate Bipolar Transistors) in their inverter and converter section. The IGBT is basically a very fast switch device. It allows you to "chop" up the incoming sine wave, and re-create a perfect sine wave on the output,which is exactly what your equipment likes to see. The IGBTs also allow for very low THD (total harmonic distortion) on the input and output of your equipment. In large facilities, if you reflect a large amount of THD back to your utility provider, you can be punished. If you have a lot of equipment that puts of a lot of input THD, and you plugged it into the UPS, you would see that the input side (or reflected harmonics) back to your utility would be very low. Usually less than 5%.
About the Author:
Damion McHenry is a power consultant that has worked in the power industry for many years. He worked for a major manufacturer of uninterruptible power supplies in marketing/sales/technical support.
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