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When selecting an uninterruptible power system (UPS), determining the optimal topology is one of the most important considerations. Because a standby UPS offers only the most basic level of protection, the majority of organizations choose to deploy either a line-interactive or a double-conversion online model. Before making this decision, it is important to understand the differences between the two topologies. Consider the following five key factors:

1. Your power environment. A line-interactive system conditions and regulates AC utility power, shielding connected equipment from five of the nine most common power problems: blackouts, sags, surges, and under- and over-voltage conditions. These models are best suited for applications where utility power is, for the most part, consistently clean. In facilities where the AC line voltage is unstable, distorted or fluctuates wildly, line-interactive UPSs will often resort to their batteries, which can reduce available runtime for an extended outage as well as require frequent battery replacement.

In applications where the power quality is less stable, an online UPS is generally the preferred solution, as it is the only type of UPS that completely isolates connected equipment from raw utility power by converting power from AC to DC and then back to AC. As such, the topology safeguards equipment from all nine common power problems; in addition to those protected by a line-interactive UPS, the online model also remedies electrical line noise, frequency variations, switching transients and harmonic distortion.

2. The equipment being protected. When determining the optimal UPS topology, it is essential to consider the equipment it will be tasked with protecting. How sensitive are the devices? Are they critical to your organization’s availability and uptime? Knowing these basic requirements will go a long way toward establishing the topology that will best serve the application. Keep in mind that a double-conversion online UPS is the only topology to offer zero transfer time to the battery, making it ideal for sensitive and mission-critical equipment. And because this type of unit safeguards equipment from all nine common power problems, it affords the highest level of protection.

3. Your capacity requirements. How much equipment are you expecting to protect with your UPS? Generally speaking, line-interactive models extend up to around 5000 VA. Above this capacity, the topology has historically been impractical due to its larger size and greater cost. Conversely, double-conversion online models are rarely considered below 750 VA because line-interactive is more practical for smaller loads. However, when it comes to selecting a UPS within the 750 to 5000 VA power range, the functional and economic advantages of each topology aren’t always so clear-cut. Choosing the best topology within this overlap range will depend on the specifics of the installation, as well as weighing all advantages and possible shortfalls.

4. Financial ramifications. When considering the monetary investment required by the two topologies, it is critical to take into account not only the upfront price tag, but also the costs of potential downtime. For mission-critical facilities, the cost of downtime should be paramount in deciding what level of protection is required. Although a line-interactive system may be less expensive, it will not provide the same level of protection as an online system — leaving equipment more vulnerable to damage and the organization more susceptible to downtime.

5. Total cost of ownership. Another financial factor to think through is the ongoing operating and service costs that each topology requires. For instance, a double-conversion online UPS will consume more energy over time than a line-interactive model, which operates with greater efficiency. In addition to higher electricity demands, online models also produce more heat, which translates to a need for extra cooling. While these may seem like nominal charges, they can add up if an organization deploys multiple UPSs across an enterprise, or even when considering the total lifetime energy consumption of one unit. And while line-interactive UPS systems protect connected devices during a complete power outage, they don’t safeguard sensitive equipment against all power anomalies, which can lead to degradation and premature equipment failure. Service requirements can also vary between the two topologies, with certain design aspects that theoretically increase or decrease operating life and reliability.

As you can see, there is no single answer or distinct deduction when deciding between line-interactive and online UPS topologies. If you’re still weighing the pros and cons and trying to determine the optimal solution for your environment, give Unified Power a call. We would be happy to help establish the ideal power protection solution for the unique requirements of your organization and applications.

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