– Pedro Robredo, product line manager at Eaton Corporation (http://powerquality.eaton.com/Default.asp), says:

If you have dealt with UPSs for any length of time, you have no doubt heard plenty of hype about high-efficiency or “ecomode” UPSs in the past. Those terms generally referred to UPSs that switched between modes to improve efficiency. Some form of multi-mode capability has been available on UPS products for years.

But, those conventional multi-mode UPSs have their limitations. They can be ineffective against many types of power problems, slow to respond to transient power conditions and vulnerable to high surges, downstream shorts and other electrical system conditions.

So, can you entrust your mission-critical data centers to multi-mode power protection?

The answer is yes, if you choose wisely and ask the right questions of a UPS vendor, here are five examples of questions you should ask before selecting a high-efficiency, multi-mode UPS:

Does the UPS sacrifice protection to gain high efficiency?
Some high-efficiency UPSs force a trade-off between power savings and power quality.

Internal design—topology—profoundly affects efficiency and protection levels. Line-interactive UPSs are efficient, but they only offer limited voltage regulation, surge suppression, and battery backup. Premium, double-conversion UPSs do the most processing to deliver clean power, but at a cost to efficiency.

New multi-mode UPSs offer the best of multiple topologies in one UPS. As mentioned earlier, these UPSs flash between different modes to match the conditions of the moment:

  • Under normal conditions, the UPS is in a high-efficiency mode that includes surge suppression and voltage regulation.
  • When input power is poor, the UPS uses double-conversion technology to deliver cleanest output power for equipment.
  • When power quality is very bad or goes out altogether, the UPS draws on internal or external batteries or a standby generator as needed, just as a typical UPS does.

How does the UPS achieve its high efficiency?
Models vary in how they switch between operating modes.

Conventional multi-mode or eco-mode UPSs usually operate in one of two ways. They either:

  • Run in standby mode under normal conditions, powering the load from a utility input source that bypasses the internal circuitry of the UPS. Whenever major power disturbances occur, the UPS has to start up and charge those internal components, synchronize the electrical waveform, and then transfer to double-conversion mode. During short power disturbances, critical loads are left exposed to potentially damaging conditions.

  • Run in line-interactive mode, with an inverter or some sort of power stage running all the time, to provide some surge suppression and voltage regulation when in high-efficiency mode and enable faster switchovers to double-conversion mode.

The first type of UPS takes too long to respond to power conditions. Damaging surges or transient power problems could still reach your valuable electronics. The second type of UPS is faster to jump into action but consumes more energy. This type of UPS tends to be no better than 96–98 percent efficient.

New multi-mode technology resolves both issues. In this newer type of UPS, the inverter is continuously charged but not processing power. The inverter remains connected, running all controls and synchronized with the input power, so the UPS can transition to double-conversion mode without delay and without compromising efficiency. Inverter filtering components are connected to the load all the time, providing surge conditioning comparable to that present in double-conversion mode.

How efficient is the UPS when lightly loaded?
Average efficiency in the real world can be quite different from stated efficiency.

Manufacturers usually state UPS efficiency ratings at full load, but most of today’s UPSs are markedly less efficient under lighter loads, which is how they are likely to be used. Since so many IT systems use dual-bus architecture for redundancy, the typical data center loads each power bus (and each corresponding UPS) at less than 50 percent capacity, often as little as 20 to 40 percent.

As a result, it is important to know UPS efficiency across the entire load range, not just under theoretical ideal UPS operating conditions. While many UPSs drop off markedly in their efficiency under light loads, others can perform at 99 percent efficiency even when lightly loaded, as much as 15 percentage points better than a traditional UPS.

Just how quickly does the UPS detect and respond to power events?Electronic equipment can only tolerate brief, sub-second interruptions.

Look for a UPS that is very quick to detect and respond to power problems—ideally, within two to four milliseconds (1000ths of a second)—for two key reasons:

  • Minimizing inrush current. After even a brief disruption in power, IT equipment draws a surge of energy to recharge its capacitors. The longer the interruption, the greater the inrush current. Even if the disruption was only 10–15 milliseconds, the surge current could be 10 times higher than normal draw. If hundreds of servers were all drawing inrush current, the UPS could be overloaded or circuits could trip. For this reason, you want a UPS that switches between modes with the least possible interruption in power—ideally, 2 ms or less.

  • Preventing disruption to downstream static switches. If a data center has A and B side power systems for redundancy, it probably has static switches in the power infrastructure to extend this A/B redundancy to single-corded loads. If an upstream UPS takes too long to change state— either from high-efficiency mode to double-conversion mode or back again—these downstream static switches could mistakenly perceive a disruption in power and switch between A and B power sources. To prevent these unwanted and unnecessary transfers from occurring, the UPS must have a faster detection/transfer time than the static switch.

Look closely at a vendor’s claim that a UPS changes modes in a stated number of milliseconds. The stated figure sometimes does not include all the steps involved. Transition time is a function of two factors: (A) How fast the UPS can turn on its inverter, and (B) How fast it can turn off the static switch, which opens the door for power to flow through the UPS internal circuitry. Only when both activities have been accomplished is a UPS truly online in double-conversion mode.

What extras does the UPS offer for maximum protection?
How does the UPS handle storms, overloads and load faults?

New high-efficiency UPSs have been proven reliable under prolonged and repeated power problems far greater than the typical commercial site would experience. Even so, some data center managers and facilities managers will still feel more comfortable knowing the UPS is fully in double-conversion mode at times, such as during thunderstorms or rolling brownouts. Some multi-mode UPSs offer options for locking in double-conversion mode under user-specified conditions.
Also look for a UPS that knows the difference between an upstream and downstream fault. The UPS should respond differently depending on the origin of the power problem.
Closing thoughts
Until recently, there were significant trade-offs to increasing energy efficiency. To offer the highest efficiency, the UPS had to expose downstream equipment to potentially harmful surges and could create other problems due to slow reaction times. New advances in high-efficiency UPSs eliminate these sacrifices. You can have it all in a single UPS—99 percent efficiency and premium protection.

This is the kind of relief data center managers have been looking for as they face intense pressure to reduce energy consumption and meet environmental regulations—without compromising uptime.