Uninterruptible Power Supply (UPS) systems are a critical component of data center backup power. Without them, power fluctuations and outages can take down workloads, damage hardware, and result in expensive payouts on Service Level Agreements (SLAs).
In a nutshell, a UPS battery turns on after the system senses a loss of power. Their purpose is to maintain the infrastructure until consistent power returns, or if needed, until longer-term emergency power backup systems kick in. They also suppress power surges so they don’t damage equipment. UPS systems convert AC power to DC power using a rectifier and convert it from DC power to AC power with an inverter. These two converters and a bypass circuit pass power through the UPS and into the computing equipment.
Types of UPS Systems
An off-line UPS system powers the load directly and only switches to backup power in the case of an outage. These are very basic and not suitable for most data centers as there can be up to 25 milliseconds between the power interruption and battery turning on.
A line-interactive UPS keeps the inverter in-line and switches to battery when the power is lost. Line-interactive systems can sustain extended periods of low voltage or brownouts, using an auto-transformer to supply additional power from the battery.
In an on-line or Double-Conversion UPS, The rectifier charges the batteries and generates AC power at a steady rate. After a power outage, the unit switches to battery power for the inverter, which can continue to generate AC power for the IT load until it runs dry. When energy is converted between AC and DC, there is a slight loss of wasted energy. Highly energy efficient UPS systems can therefore deliver long-term ROI.
Online UPS systems are the most common for data centers as they were initially designed for 10 kW or more and intended for sensitive devices. They can handle a greater current when rectifying AC to DC power and charging the battery. The batteries are always connected to the inverter, allowing a middle ground between input power and IT equipment.
It can be hard to choose which UPS is right for your data center, as they differ depending on your power densities, efficiency goals, and management capabilities. Look out for a follow-up post describing how to select the best UPS for your data center environment.