UPS Topology, A Refresher?
Several years ago when UPS topology came up as a subject for discussion the first criteria that was established was whether the discussion would revolve around single-phase UPS or three-phase UPS. This distinction was required because certain UPS topologies usually applied to one category and not the other, not necessarily 100% of the time, but certainly in overwhelming majority. The best example (back then) would be UPS that utilize “stand by” (off-line) technology. These were usually simple UPS that switched to battery back-up when the utility had a black-out. The UPS would run until either the utility came back or the battery run time was used up. They are the primary type of UPS a consumer would see for sale at Wal-Mart® or Office Max® style stores.
Today that is not really the only case anymore. The vast majority of standby UPS are still made for the consumer market, 1 kVA and below in rating. The idea of a standby UPS though has now reached into even the largest of data centers. Driven by the need to improve energy usage (increased efficiency of UPS as an example, not just at full load but at fractions of their full load capability) some UPS manufacturers are offering a “standby’ capability on even their largest and most modern three-phase UPS.
This paper discusses the primary Static UPS topologies relevant in the three-phase marketplace today with some discussion on their different strengths and weaknesses.
There are 3 primary types of UPS’s, Stand-By (sometimes called Off-Line), Line-Interactive, and On-Line. To help understand what each does we have illustrated below (table 1) what type of basic power anomaly each UPS type protects your business critical load from. We have also included a newer technology being advertised lately, that of the Hybrid UPS.
Stand-By: This is the simplest of the 3; it is used in mostly small applications, under 1kVA, such as home computers. Standard operation for these products is for the utility to be supplied to the load directly. When there is a power loss the static-switch switches to the inverter and the batteries begin to supply power to the load. When power is restored the static-switch switches back to the utility power. The charger then turns on and changes the depleted batteries. This type of UPS has the great advantage of simplicity. Because of its simplicity is operates at 98-99% efficiency, it is also inexpensive to manufacture as it doesn’t need a robust charger or inverter as it is only made to operate in short bursts. Its weaknesses are that it allows all transients beside power loss directly to the load. This includes surges, sags, and frequency deviations. The frequency deviations are especially problematic if a generator is in use. When using a Stand-By UPS you want to make sure and have some type of TVSS (Transient Voltage Surge Suppression).
Line-Interactive: This type of UPS is used in critical applications from 250VA-25MVA. Their operation is much the same as Stand-By but with some critical differences. To correct some of the TVSS issues there is a transformer on the UPS input that is able to correct voltage problems. Unlike Stand-By, Line-Interactive only has an inverter that also operates as a charger, it is also always connected to the output of the UPS. In normal operation the utility is supplying power to the load and to the inverter which is acting as a charger is charging the batteries. When the power fails the inverter begins supplying power to the load with the power from the batteries. The static-switch disconnects the unavailable utility power. When the power returns, the static-switch switches the utility back on to power the load and the inverter begins to charge the batteries. Because of the transformer the efficiency is not as good as a Stand-By UPS, but it is still very good at 97-98% efficiency. A Line-Interactive UPS suffers from the same weakness of Stand-By UPS’s in that it can let some TVSS through to the load, although the risk is lessened through the use of a transformer and the coupled inverter to the output. For that reason it is also import to have some type of TVSS in concert with a Line-Interactive UPS.
On-Line: This type of UPS, also called Double Conversion, is used mostly in large applications such as call centers or data centers. The operation of this UPS is different for the previous two in that the raw utility is not powering the load, the UPS’s inverter is. When the utility power is operating nominally it is feeding the charger. The DC power from the charger is used to charge the batteries if needed, it is also fed into the inverter. The inverter, using power from the charger, powers the load. If the power should be lost the operation is the same except that the charger would be off and the power from the batteries would be used to power the inverter, feeding the load. This operation eliminates any possibility of TVSS affecting the load. The down fall is efficiency, which is lower than the first 2 at around 96%.
Hybrid: It is easy to see why On-Line UPS’s are used in all large critical operations. But as the green movement moves into the data center, UPS manufactures have had to get creative with ways to make there UPS’s more efficient. Most of them are advertising some type of ECO mode. Most of these operate like a hybrid of Stand-By and On-Line UPS’s. Because the power is very reliable in the US they can run there UPS’s in Stand-By mode 99% of the time. Should the power become unreliable it switches to On-Line mode until the power becomes reliable again. Using this method, ECO UPS’s are able to achieve over 99% efficiency. The weakness would be the same as Stand-By in that there is still a chance that some type of TVSS could make it to your load. You also have to have reliable power for the UPS to operate in ECO mode, not an option in some places, especially overseas.
As you can see from above your choice is going to very depending on your application and environment. One word of caution when stating your search for a new UPS, manufactures are limited to what their product range is, not what is best for your application, environment or budget. Jantech being a 3rd party is able to search multiple manufactures to find the best UPS for your application. Find out more about the UPS’s we offer at our New UPS, or give us a call today: 1-800-452-6832
Jantech Services – www.JantechUPS.com – 1800.452.6832 – web@JantechUPS.com
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