Three years ago, we saw the great Northeast power outage shut down the power to tens of millions, and last year Hurricanes Katrina and her kin devastated sections of the Gulf Coast. In between those two, there was the Sumatra tsunami, and this spring New England was flooded and the Danube once again overflowed its banks.

No matter where one lives, one has to be prepared against the floods, fires, earthquakes, blizzards, hurricanes and human error that can interrupt the power coming into the data center. Then there are the local hazards, such as drivers hitting a utility poles or transformers blowing. Even when power isn't completely cut, summer afternoon brownouts, simple voltage swings or changes in frequency can wreak havoc on computers and critical equipment. These problems are compounded by the new rack-dense and blade server architectures. Although they allow companies to fit ever more computing power into the same square footage, they are also power-dense architectures, raising the demand not only for electricity to run the servers, but also to cool them.

Many companies have battery powered UPS systems for immediate relief from short term outages, but low capacity prevents a company from keeping its computing grid or electrically controlled machinery running continuously, despite brownouts. To ensure they stay up regardless of what happens on the grid, AOL, Intel and other high tech firms use a flywheel/generator setup.

When power is cut, kinetic energy in the flywheel provides ride-through power until a diesel or natural gas engine can fire up and reach operational speed. In addition to providing power during blackouts, brownouts and voltage sags or surges, the generator can filter out line noise, harmonic distortion, high voltage spikes, switching transients (rapid increases in voltage) and frequency variations that affect power quality.

At the heart of the system is a self-synchronizing, gear-type clutch, or "SSS Clutch" which enables the engine to automatically connect to the generator when the engine reaches exactly the same speed as the rotating generator, without interrupting the flow of power generation.

Photo Courtesy of Hitech Power Protection BV

LOOKING BEYOND UPTIME

In providing services to end users, IT managers work on two basic criteria - availability and quality of service. Failure is, of course, the worst level of service but, once that is handled, it is a matter of how well that service is performing.

The same applies to those electrons before they start traversing the LAN. The power must be available, and it must be at the right voltage, frequency and phase. While that should ideally be taken care of by the utility, a grid designed to power everything from factory equipment to fluorescent lights isn't tailored to meet the exacting requirements of datacenter equipment. This is where rotating UPSes come into play. To begin with, as their name implies, they meet the power availability requirements when needed, but they can also be used to meet the power quality demands.

A traditional method of providing short term power has been to use large lead-acid batteries to provide the necessary short term power from a few minutes to one hour. Batteries do, however, have certain drawbacks even for short term use, including the need for replacement, environmental concerns and power inefficiencies. They also have a large footprint, which makes them inappropriate for areas where space is a premium. More importantly, batteries have been known to fail when they are most needed. In many instances, however, batteries are not enough for business continuity purposes. For longer-term outages, these batteries need to be supplemented by a standby emergency generator. Batteries may be enough to ride through a very short glitch in power, let the equipment be properly shut down without damage or data loss until power from the utility is restored, or to enable the standby generator to be started and connected.

To meet the demanding needs of many customers with sensitive electronic equipment, power vendors have developed modern methods for providing emergency power as well as power conditioning. These involve diesel or natural gas engine gensets designed to provide continuous conditioning of utility power during normal operations as well as provide seamless transition to local power generation in the event of a power outage. Additional energy storage devices such as hydraulic motors and high pressure accumulator systems have been used and flywheels (either stand alone or connected to a rotating generator) have been designed to store kinetic energy that can be employed for event ride-through purposes and to help minimize the effects on voltage and frequency during an event. Flywheels now seem to be the preferred method of ride through.
There are three basic approaches to using a flywheel to provide emergency power:

• a combined motor/flywheel spinning independently of the UPS, engine-driven generator;
• hard-coupled to the generator; and
• flywheel mass incorporated within the generator itself.
  
  

In all cases, the flywheel power is drawn from the grid to keep the flywheel, or flywheel combined with generator, spinning during normal operations, and then provides short-term power to override short-term glitches until the generator can supply power to the grid.

It is preferable to have the generator and flywheel spinning together. In this case, the engine for the emergency UPS generator brings the generator and the flywheel up to the speed necessary to synchronize with the electrical utility grid. After that, the generator draws power from the grid to keep it and the flywheel spinning. During periods when emergency power is not required, the generator reconditions the electricity going into the data center, bringing it to the exact voltage and frequency required and eliminating any noise or distortion it may have picked up out on the grid. In addition, the generator can act as a synchronous condenser and provide what is called "reactive power."

Photos courtesy of SatCon Power Systems

ROTARY POWER QUALITY

"Actual" power is the electrical energy which is used to drive equipment. Reactive power is a component of electrical transmission, which is out of phase with the actual power and which is needed to provide voltage support. If there is not enough reactive power, the voltage drops and transmission lines overheat. This can lead to brownouts or black outs.

For example, the investigation into the August 2003 outage which cut power to 50 million people in the Northeast U.S. determined that it was at least partially attributable to a lack of reactive power. "Proper reactive power management would have helped to prevent the initial system events and therefore would have delayed or possibly might even have prevented the resulting blackout," Pat Wood, III, chairman of the U.S. Federal Electricity Reliability Council (FERC), said.

The problem with ensuring an adequate supply of reactive power is that it must be supplied near where the power is consumed, not near where it is generated. Power plants are frequently located hundreds of miles away from the cities they serve, which means another source of reactive power must be created locally. By having the emergency generators running connected to the grid, they not only provide a rapid source of emergency power for the data center, but also provide voltage support for the grid, which lessens the chance that there will be a power outage in the first place.

MAKING THE CONNECTION

Setting up such a flywheel UPS requires the ability to quickly and seamlessly disconnect the engine from the generator. Once the engine has brought the generator and/or flywheel up to the speed necessary to connect to the grid, the engine is shut down and disconnected from the generator. Then, when there is an electrical outage, the flywheel or mass of the generator keeps the generator turning and powering the data center in conjunction with the kinetic energy source until the engine can be brought back up to speed and reconnected to the generator, generally in five to seven seconds.

Most rotary UPS installations do this by means of an SSS, or Synchro Self Shifting, clutch. This is a freewheel gear-type of clutch. When the engine accelerates to the generator speed, the clutch connects and the engine drives the generator. Then when the generator is up to speed and running off power from the grid, the engine is shut down and the clutch overruns. The moment the engine decelerates below the speed of the generator, the clutch automatically disengages, leaving the generator running at full speed and connected to the grid. This process allows the generator to be driven by grid power, the flywheel or the engine, seamlessly shifting between power sources as is appropriate.

These types of rotary UPS systems are ideal for large data centers or other power intensive installations which require uninterrupted operation. More than 450 flywheel/generator-type UPS systems totaling more than 7700 MW have been supplied by a number of different manufacturers worldwide, are in use at data centers and manufacturing facilities, and have proven themselves to be a reliable and cost-effective source of emergency power. They take up a fraction of floor space of lead-acid batteries, are safer, have a lower failure rate and require less maintenance resulting in a lower lifetime cost. ENS

Morgan Hendry, is president of SSS Clutch Co., which manufactures the SSS Clutches used in most flywheel/generator rotary UPS systems.

SSS CLUTCH VENDORS
To find out if it is the best operations for your company, contact your electrical equipment supplier or one of the vendors listed below.

Caterpillar Inc.
Peoria, Ill.
www.cat.com

Hitec Power Protection Inc.
Stafford, Texas
www.hitecups.com

KS Techniques S.A.
Grace-Hollogne, Belgium
www.kstechniques.com

Piller Inc.
New York, N.Y.
www.piller.com

Russelectric Inc.
Hingham, Mass.
www.russelectric.com

SatCon Technology Corp.
Boston, Mass.
www.satcon.com