Next Generation UPS Technology For The New Millennium.

From servers to mainframes, responsible IT managers know that clean, uninterrupted power is an essential ingredient to system reliability. However, today's IT manager faces a number of new challenges that affect the best way to achieve maximum reliability in a given situation. The move towards re-centralizing critical equipment after years of distributed network environments and new international requirements for electromagnetic compatibility require different solutions for enterprise power protection than those that have been successful in the past. Failure to address these changing needs can be very costly in initial equipment costs and ongoing utility expenditures.

Enterprise Power Needs

As the servers that have proliferated over the past decade start to look more like the mainframe computers they replaced, their power requirements have changed along with their processing capabilities. Not only do they require more power, but they also require different power.

As typical applications start to have power requirements above 10KVA, it is not uncommon that the electrical contractor or facilities manager will recommend using the three-phase power provided by the utility for better load balancing within the site. In some cases, higher power electronic equipment will actually require three-phase input. Uninterruptible Power Supplies (UPS) that provide three-phase power have a number of different operating characteristics compared to the line-interactive, single-phase UPSs that have proven ideally suited for networking applications.

The Traditional Solution

Traditionally, the UPS of choice for three-phase enterprise applications has used Double-Conversion technology (Fig 1). In a Double Conversion UPS, a rectifier receives AC input from the utility and creates DC power to charge the UPS batteries. At the same time, the rectifier provides DC power to an inverter that supplies connected equipment with continuous, regulated AC power. When the utility input fails, the batteries provide DC power to the inverter until the utility power is available again. All of these transitions occur with no interruption in the flow of power--assuring that the load receives continuous, regulated AC voltage of high quality. Such characteristics are inherent in online UPS systems.

Unfortunately, the double-conversion UPS has increasingly become a problem for utilities and power consumers who are not protected by an UPS. While these systems have been reliable workhorses over the years, their low power factor and the extensive, high-power switching that takes place during their normal operation generates harmonic currents from the UPS back upstream to the power line. These currents, known as harmonic distortion disturb the voltage quality on the power line causing a variety of disturbances within an affected facility. In extreme cases, these disturbances can result in severe equipment damage and even fires. Observable symptoms of excessive harmonic distortion can include: blinking lights; capacitor failure; circuit breakers tripping without obvious cause; malfunction of computers and other electronic equipment; blown fuses for no apparent reason; motor failures; overheating of metal enclosures; power interference on voice communications; transformer failures; and diesel generator instabi lity.

In addition, the rectification and inversion of all power that passes through the double-conversion process has inherently high losses. Since a normal UPS is operational 24 hours a day, the energy wasted can be quite significant at these power levels. For example, with a conservatively estimated eight percent increase in efficiency at 2OKVA, a user can expect to save approximately $1,500 per year with an electricity cost often cents per kilowatt-hour.

Furthermore, the system's low input power factor results in particularly inefficient use of the utility. Power factor, a frequently overlooked specification, has numerous ramifications throughout the power distribution network. In addition to the adverse affects of harmonic distortion mentioned above, low power factor also results in the need to oversize all aspects of the power installation--from the UPS itself all the way back to the utility transmission level. Any optional diesel generators would likewise have to be oversized to accommodate the low power factor load.

The New World Power

The cumulative cost of oversizing the UPS, generator, wiring, and local switch gear, coupled with the cost of wasted energy due to system inefficiency, can add up significantly. In addition, new international regulatory requirements designed to reduce the harmful effects of harmonic distortion have changed the nature of power requirements for computers and other sensitive electronics.

Historically, electrical equipment operated with ideal voltage and current waveforms. However, in the last two decades, the exponential increase in the use of solid-state electronics has come at a price. This technology provides superior product quality with increased productivity using smaller, faster, and lighter components at a lower cost than ever before. At the same time, it requires clean electric power and is especially sensitive to power distortion.

Meanwhile, these same modern electronic devices (non-linear capacitive loads) employ internal switching power supplies that draw current differently than non-electronic apparatus such as heaters, lights, and motor-driven equipment (inductive loads). Until recently, the switching power supplies within these devices had a low power factor and generated harmonic distortion back into the power distribution system (not unlike the traditional UPSs used to power them). In fact, the largest single contributor of reflective harmonic currents in commercial buildings has been the personal computer with its low power factor. As a result, the traditional double-conversion on-line UPS was well matched with its traditional computer load.

However, the tremendous proliferation of electronic devices has had such a significant adverse impact on utility transmission and distribution systems that new international regulations are being implemented to reduce harmonic distortion caused by these devices. The Electric Power Research Institute (EPRI) has estimated that, left unchecked, the total non-linear (computer-type) loads will reach 50 to 70 percent of total building power usage by the year 2000, up from about 15 to 20 percent in 1992. Compliance to these new electromagnetic compatibility standards requires that new equipment meet certain electromagnetic interference specifications. To achieve these specifications, most manufacturers are replacing traditional internal switching power supplies with new Power Factor Corrected (PFC) switching power supplies in computers and other electronic devices. Use of PFC technology eliminates the harmonic distortion caused by these devices and gives them a considerably less disruptive electrical profile. While that's great for the computers, the double-conversion UPSs used to protect them now become the biggest contributor of harmonic distortion in the power network.

New UPS Technology For New Applications

To address this developing harmonics and power factor mismatch between critical computer loads and the UPSs traditionally used to protect them, the Delta-Conversion online UPS has been developed. Compared to a traditional Double-Conversion UPS, the new technology provides:

* Harmonic distortion reduction from near 30 percent down to less than three percent

* Energy waste reduction down from approximately ten percent to as little as three percent with total efficiency approaching 97 percent

* Increased power infrastructure utilization from about 80 percent up to nearly 99 percent

Unlike the Double-Conversion UPS, the Delta-Conversion Online UPS does not use a rectifier to charge the batteries. Instead, it uses two bi-directional converters (sometimes called four-quadrant inverters) that can pass power in either direction, eliminating the need for the rectifier (Fig 2). Both converters are connected to a common battery. The Converter 1 is rated at about 30 percent of the output power of the UPS. It is connected in series between the utility power supply and the load. Converter 2 is rated for the full UPS kW load.

Hew It Works

In normal operation, pure power from the utility is passed on to the load via the pure power path using an ac to ac conversion process via Converter 1. losses are greatly reduced because the power is not converted twice. The converters are in full control of the output power to the load, as well as charging of the battery. When a power failure occurs, the Delta-Conversion Online system instantaneously takes power from the batteries through the continuously operating second converter to the load, just as a Double-Conversion UPS would.

Converter 1, called the Delta Converter, compensates for any difference between the UPS output voltage and the voltage from the utility. It also controls the input power factor by assuring that the utility sees current that is sinusoidal and in phase with the utility voltage. In addition, the Delta Converter directs charging of the battery.

Converter 2, called the Main Converter, keeps the voltage to the load stable and precisely regulated. In the idealized form, all power is processed ac to ac through the Delta Converter to the load with nothing converted ac to dc or dc to ac, hence no losses. In practice, there are, of course, small losses resulting from the power circuitry, magnetic components, and fans. Like a double conversion system, if the load has a reactive or harmonic component, the Main Converter supports the reactive or harmonic current since it cannot be taken from the utility. While such reactive or harmonic currents will increase total losses slightly, efficiency remains exceptionally high.

During an undervoltage condition, the Delta Converter takes additional current (power) from the utility and passes it to the output of Main Converter, which passes the excess power, i.e., that power not consumed by the load, backwards through the DC link and, finally, forward through the Delta Converter. Although this is a true double-conversion process with related losses, only the delta power between input and output is converted and only while the undervoltage condition exists (Fig 3).

In the event of an over-voltage situation, the Delta Converter must "absorb" the excess utility voltage to assure proper output, again via a double-conversion process with similar loss considerations. The difference is that the direction of power flow through the delta conversion process has been reversed from the undervoltage condition. During battery charging, the Delta Converter takes extra power from the utility, passed backward through the Main Converter, and absorbed in the battery as charging current. The two converters have a "job-sharing" relationship. The Main Converter maintains synchronization to the utility and controls the output voltage in all operational modes. (During battery operation, its frequency is controlled by an internal frequency reference, as in all other on-line UPS.) The Delta Converter controls the input by "importing" more or less power from the utility as necessary for supporting the load and makes up for any differences in voltage levels and waveform between the utility and ou tput voltages.

How Delta-Conversion On-line Stacks Up

Even though the Delta Conversion On-line UPS offers next generation technology with many advantages, it still offers the same no-break transitions expected of a traditional on-line UPS. Yet unlike the traditional UPS, it maintains significantly higher efficiency through a wide range of operation, as well as assuring input factor greater than 0.99 percent through a similar range. The table shows a side-by-side comparison of the Delta Conversion On-line UPS compared with a typical legacy double-conversion model. While it would be possible to address many of the deficiencies of the legacy system with additional hardware for power factor correction and harmonic filtering, the cost would be considerable and total system efficiency would drop even lower.

Solutions Change With The Times

As every astute IT Manager knows, yesterday's brilliant solution can be tomorrow's white elephant. Fortunately for everyone, the useful life cycle of UPS is far longer than most of the other devices in the computer room. The double conversion UPS was the technology of choice for nearly two decades, but like the equipment it protected in the 80s, it has become an anachronistic solution from the past. Fortunately for users of the latest computer equipment and peripherals, the Delta-Conversion On-line UPS offers the highest available level of protection without the many drawbacks of its predecessor.

Jorgen P. Madsen is the product manager of the three phase high power products at American Power Conversion (West Kingston, RI).

                        Comparison of UPS Features
       Description         Double-Conversion UPS  Delta-Conversion UPS
  True On-Line Function             YES                   YES
True 2-way Filter Function          YES                   YES
   Input-side Harmonics    YES [greater than] 30%          NO
 Unity Input Power Factor    NO (0.67 to 0.8PF)           YES
 Energy Losses (Typical)    [greater than]8-12%     [less than]4%
Capability of Running SMPS          YES                   YES
  Load without Derating