Choosing the right UPS topology for your application will depend on your power requirements (i.e. whether you need to back up an editing deck or a transmitter) and the level of reliability you require (you will not want to spare any margin when backing up equipment critical to the final game of the playoffs). The most common types of UPSs are offline, line interactive and online double conversion.
The most basic (but effective) UPS type is “offline” topology. Under normal conditions, when utility power is satisfactory, the UPS will be offline to let the stable power flow through the UPS to the critical load. If the power voltage drops due to a brownout or blackout condition, the UPS will immediately switch online and begin to produce power from its battery. The transfer time between utility power failure and stable UPS battery power production is typically less than four milliseconds — quick enough to avoid interrupting power to sensitive devices. Better-quality offline UPSs also provide a level of surge protection to prevent power spikes and surges. Typically, offline UPSs will have 10 to 20 minutes of battery backup time at their rated output, with many manufacturers offering additional battery modules for longer run times. An advantage of offline topology is price: a high-quality offline UPS will cost about $150 for a 500VA UPS (perfect for a small workstation) to around $300 for 1200VA UPS (suitable for backing up an editing suite).
If a higher degree of power quality is required (between 500VA and 5kVA), then a line interactive UPS topology is the most appropriate. Similar to offline topology, the line interactive UPS conditions the utility power fed through the UPS output by boosting the voltage if it sags below the nominal level, so as to keep the output voltage stable regardless of the input voltage. Again, line interactive UPSs are traditionally under 5kVA, but a few manufacturers employ that topology for larger UPS systems due to lower manufacturing costs.
For applications requiring the highest level of power quality and reliability, an online, double-conversion UPS topology is the recognized solution. Double-conversion topology is called “online” because the UPS is always online generating 100 percent of the output power from the UPS inverter. When utility power is available, the rectifier on the input of the UPS converts the AC power to DC. The DC power charges the battery while also powering the inverter, which then converts the DC power to pure AC power. If utility power fails and the rectifier stops feeding DC power to the inverter, the inverter is already connected to the battery bank and continues to produce uninterrupted power, without having to transfer modes. Because the inverter generates the output power, it has perfect control over the output power quality, typically maintaining a perfectly stable output voltage with low noise and low distortion. Also, because an online topology isolates the loads from the utility bus at all times, it is the perfect choice for noise-sensitive devices such as audio equipment. Usually online, double-conversion UPSs will cost more than other topologies, but for broadcasting applications, they are well worth the investment.
Operating efficiency is an important consideration with larger three-phase UPSs in the 10kVA to 500kVA range. A typical UPS will only convert 86 to 95 percent (on the high end) of the input power to output power, rejecting the other 5 to 14 percent as heat. Considering that an energy efficiency differential of as little as 2 to 3 percent (i.e. a 92 percent vs. a 94 percent efficient UPS) will result in energy cost savings equal to the cost of the UPS in as little as three to five years at utility rates of $0.10/kWh, UPS operating efficiency makes a tremendous difference to operating costs. Furthermore, the air-conditioning costs resulting from the extra heat rejection of the less efficient UPS can add up to 30 percent to the operating costs alone. In addition, be careful when comparing operating efficiencies, as specification sheets often only quote best-case scenarios. Always ask the manufacturer to provide a test certificate showing the UPS's operating efficiency at your desired load level. Most UPSs operate at well below 100 percent of the rated output and UPS efficiency typically declines at load levels below 50 percent. Demand a factory test certificate at load levels of 25 percent to 50 percent load (where most UPSs operate).
Some transmitters may require special attention when selecting a large three-phase UPS; particularly lOT-based transmitters that employ a crowbar protection circuit (a circuit that prevents damage to the vacuum tubes by immediately shorting the input power to remove current from the tube). During a crowbar trip, the output current can have an instantaneous spike of several thousand amps, followed by a brief surge of two or three times the normal current (double if the tube is cold). When a UPS system exceeds its nominal overload rating — usually around 150 percent — it will immediately switch the inverter off and seamlessly transfer to utility power via a static transfer switch inside the UPS. It will then be up to the utility to maintain the overload current. The problem occurs when the static transfer switch on the UPS cannot sustain an overload condition during a crowbar short circuit, thus risking damage. To ensure the UPS will handle your transmitter load, have the manufacturer demonstrate that the UPS is capable of handling a dead short, (phase to phase) on the output as a condition of purchase.
With the power protection afforded by uninterruptible power supplies, broadcast facilities can operate at all times without fear of costly power problems and downtime. The required level of reliability and power quality requirements will determine which UPS topology is right for you.
Alan Katz is product manager for MGE UPS Systems,www.megups.com.
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