By Morgan Hendry, SSS Clutch Co.
Three years ago, we saw the great Northeast blackout shut down the power to tens of millions. Last year Hurricane Katrina and her kin devastated sections of the Gulf Coast. In between we witnessed the Sumatra tsunami. And this spring saw flooding in New England and parts of Europe as the Danube once again overflowed its banks affecting area-wide power delivery. In addition to major power outages, power fluctuations with voltage spikes and sags occur frequently in local areas worldwide which may affect critical load applications.
As economic output shifts from iron wares to information, ensuring an adequate supply of clean and continuous power for corporate and internet data centers, telecommunications and financial services companies, hospitals, to name a few, becomes a higher priority. The same applies to manufacturing processes where even a momentary interruption can cause millions of dollars in damages and lost production. Many companies rely on separate redundant hookups from different sections of the grid as a protective measure against the floods, fires, earthquakes, blizzards, hurricanes and human error which can interrupt power flowing into their facility. Then there are the local hazards, such as drivers hitting a utility pole 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.
Many companies have battery powered uninterruptible power supply (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 extensively, despite brownouts. Any organization that needs to maintain power no matter what happens on the grid should consider rotary uninterruptible power systems (UPS) with ride-through capability. The added benefit of these rotary systems is that the generator can act as a filter on the company’s electric system to filter out utility disturbances. In other words, besides handling complete loss of utility power, these systems provide short-term protection against brownouts, short-term voltage sags and voltage surges, as well as filtering out the line noise, harmonic distortion, high voltage spikes, switching transients (rapid increases in voltage) and frequency variations which affect power quality; whereas repeated use of batteries for this function results in reduced life expectancy.
AOL, Intel and other high-tech firms use a rotary flywheel/generator setup to ensure they stay up regardless of what happens on the grid. During periods of normal supply, the generator remains synchronized with the grid, operating as a motor, and can perform a power conditioning function. When power is cut, the flywheel’s kinetic energy provides ride-through power until the diesel or natural gas engine reaches operational speed. 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.
This article looks at the power availability and quality issues affecting data center operators and manufacturing plants and how rotary UPS systems address these issues. Utilities should have a basic understanding of these issues so they can best advise their customers in steps to take to mitigate any damages from a power interruption and or voltage spikes or sags.
Looking Beyond Uptime
Information technology managers work on two basic criteria – availability and quality of service. Failure is, of course, the worst level of service. Once that is handled, however, it becomes a matter of how well that service is performing. The same applies to electrons before they start traversing the data network. Power must be available and it must be at the right voltage, frequency and phase. While ideally that should be taken care of by the utility, a grid designed to power everything from factory equipment to fluorescent lights in not tailored to meet the exacting requirements of data center equipment and manufacturing plant operators with sensitive equipment controls and motors. This is where rotating uninterruptible power systems come into play. To begin with, as their name implies, they meet the availability power requirements when needed. But they also can be used to meet power quality demands.
A traditional method of providing short-term power has been to use large lead-acid batteries. Batteries, 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 per megawatt, 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 short glitch in power, allowing the equipment to be properly shut down without damage or data loss until utility power is restored, or to enable standby generator to be started and connected. Power can thus be restored but either a disruption to power supply has occurred and or there has been a severe swing in voltage and frequency potentially damaging plant equipment.
To meet the needs of many customers with sensitive electronic equipment, power vendors have developed 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. Flywheels (either stand alone or connected to a rotating generator) have been designed to store kinetic energy that can be used for event ride-through 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: 1) a combined motor/flywheel spinning independently of the UPS, engine-driven generator; 2) hard-coupled to the generator; and 3) 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 operation, and then provide 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 facility, bringing it to the exact voltage and frequency required and eliminating any noise or distortion the it may have picked up on the grid. In addition, the generator can act as a synchronous condenser and provide what is called “reactive power.”
Rotary Power Quality
Peak power demand is measured in kVA and is the apparent power that a utility sees as it meters the power flowing into a plant. The peak power demand in kW is the actual power consumed. The utility is responsible for getting the energy to the plant and one of the major losses experienced is the transmission I²R losses. The utility is selling kW but is delivering kVA and the percentage difference between kW and kVA costs the utility, and in turn the customer, real dollars. To minimize the I²R losses, the kW and kVA should be nearly equal (unity power factor). When they are not, the utility has to provide more kW. The cost of low power factor is passed on to the plant owner in the form of an additional monthly charge. This additional cost on an annual basis can be significant.
The other issue is voltage drop. If the user has a poor power factor, transmission lines to the user may experience large voltage drops resulting in low voltage or voltage sags at the facility. Low voltage or voltage sag may cause computer equipment failure, motor failures (overheating, short life, and so on) critical drives not reaching maximum speed or load and so on.
The generator of a rotary UPS can provide reactive power for load voltage regulation and power factor compensation (volt ampere reactive – VAR), bi-directional load harmonic filtering, high overload and short circuit capabilities, among other things.
Making the Connection
Setting up such a rotary UPS requires the ability to quickly 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 freewheeling 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 using 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 either grid power, the flywheel or the engine, seamlessly shifting between power sources as appropriate.
These types of rotary UPS systems are ideal for large data centers or other power intensive installations that require uninterrupted operation. More than 450 flywheel/generator-type UPS systems totaling more than 7,700 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.
Morgan Hendry, is President of SSS Clutch Company which manufactures the SSS Clutches used in many flywheel/generator rotary UPS systems.