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As business assets are becoming increasingly digitized, businesses rely more heavily on computers and other essential electronic equipment. An untimely power outage can be a real headache—or worse—causing lost data, scrapped parts, patient discomfort and more. Uninterruptible Power Supplies (UPS) are often used to protect sensitive downstream equipment from power fluctuations and outages. At least that is the plan. In reality, depending on the UPS technology employed, the protection may not be what you think—leaving businesses with less protection than they bargained for.
There are multiple UPS technologies available, but this article will focus on two technologies: line-interactive and online-double conversion. While they both do the same types of things—for example, provide back-up power to shut down systems—how they achieve these can have significant impact on the business assets they are designed to protect.
What's my line?
Line interactive UPS technology (also called single conversion) is the less expensive of these two technologies, and that is one reason why it is also the most common. Think of this type of UPS as "batteries in a box." This technology monitors the incoming line voltage and supplements it with battery power when the voltage drops below a certain level. Line interactive designs may include a tap-changing transformer, which adds voltage regulation by adjusting transformer taps as the input voltage varies. The transfer time, which is inherent with the line-interactive methodology, is typically four to six milliseconds; a period that is noticeable to both human observers and, more importantly, downstream sensitive electronic equipment.
Online UPS technology (also called double conversion) takes the incoming utility alternating current (AC) and converts is to direct current (DC), which charges the batteries. The direct current is converted back to AC, hence "double conversion," which is supplied to the protected load as clean power at the desired voltage. Because the inverter is always connected to the output of the UPS, the double-conversion technology produces a clean sine wave and also reduces harmonics, which further protects downstream equipment. Dropping the incoming voltage does not cause activation of the transfer switch (to batteries) because the input AC is not the primary source—it is the backup power source. An input power failure, therefore, does not cause a transfer so there is no transfer time.
With this understanding. let's look at some common situations and how each technology handles them. In our examples, we'll assume that the incoming line voltage is nominally 120V, though the scenarios could be presented for any line voltage, including 220, 240, 480 and others.
Power outage
A power outage is perhaps the most common reason why companies install UPS systems—they want to be protected in case of an emergency.
So what happens when the incoming line voltage drops? With a line interactive UPS the batteries switch on, providing power to the protected load. The time provided by the batteries is dependent upon such factors as how many batteries are available (or how the UPS is sized relative to the protected load) and the condition of the batteries.
