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The role of UPS technology in distributed generation. (Distributed Power Update).

All industries pay a high price for downtime and strive to maintain facilities at peak operating efficiency, regardless of outside conditions. This continues to be the "Grail quest" for industrial plant managers. Recent advancements in technology have led to more automated manufacturing processes which, in turn, have created an increased dependence on electrical distribution systems. Now more than ever, quality and reliability of electrical power dramatically affect facility uptime.

It's not surprising, therefore, that facility managers are paying close attention to the emergence of distributed generation (DG) as something that may impact productivity and profitability. Distributed generation--whether for primary power or peak shaving--can solve some but not all facility power quality problems. Some problems are still best solved with a UPS. Although on-site generating capacity gives you assurance that electrical power will be available on your premises, it does not provide the conditioned power that a UPS can deliver for today's sensitive electronics.

This quality electrical power consists of four elements: consistent voltage, steady frequency, continuity (absence of interruptions) and cleanliness (absence of power quality events such as high-frequency noise, harmonic distortion and voltage transients). A well matched UPS and gen-set combination can provide computer grade power, but if the UPS and gen-set do not work as a system, it can create more problems than it solves.

A DG plant is normally used for either standby power or to take all or part of the facility off the utility during peak periods. In addition to providing conditioned power, the UPS serves as an essential bridge, to support the facility load while the generating plant ramps up to 100 percent. The UPS will also maintain continuous power to the load while the upstream automatic transfer switch performs its break-before-make transfer.

In the process of switching from utility, to UPS, to engine generator, several things must be considered when specifying the gen-set and the UPS to ensure that they work as a system. As a practical matter, most facilities have operations that are not critical and do not need to be supported by the UPS. Noncritical loads on the generator include motor loads (A/C units) including starting inrush and steady state running currents, lights, phone, and security systems. These mixed loads upon the generator need to be factored in and result in the gen-set being rated for a higher power output than the UPS equipment.

For situations where the facility load equals the UPS/gen-set output, the generator should still be rated for a higher kW output than the UPS. This is due to:

* An undersized generator will experience voltage or frequency excursions in response to large load steps.

* The rating of the generator must allow for UPS battery recharge current in addition to normal operating load current, since the battery will normally be discharged when the UPS transfers to generator.

* If a generator is undersized to the UPS, the leading power factor presented by the UPS input filter at light loads (or no load) may cause the alternator to become over excited. This could force the generator output voltage to increase to the point of an over-voltage shutdown.

* The UPS acts as a filter to isolate the generator from nonlinear computer loads. If the UPS goes to bypass, the generator must be rated to support these loads.

Generator oversizing will depend on two factors: the type of UPS rectifier and the type of load. A "dedicated" UPS load is the rare situation where 100 percent of the facility is supported by the UPS. More common is the "mixed-load" application, where some portion of the facility load is supported directly by the generator, without UPS support. The accompanying table shows some minimum sizing guidelines.

In the dedicated UPS case this is the minimum rating of the generator. However if the generator is connected to the bypass input of the UPS, the rating of the generator may have to be increased. The reason for this is that if the UPS transfers to bypass and the load on the output of the UPS is a nonlinear load. The distorted load current will be seen by the generator and this distorted current may cause the generator voltage waveform to become highly distorted. This highly distorted voltage will appear on the output of the UPS and may cause problems with some of the loads present on the output of the UPS.

In the mixed load case the number N is the minimum rating. The calculation should read: Generator size in kW = (UPS input kW x N) + additional generator load.

In order to get the voltage distortion of the generator down, the generator manufacturer may install an oversized alternator in the gen-set. This effectively reduces the per unit output impedance of the generator, but it also increases the fault current that is available from the generator.

Typically, double-conversion UPS products can be used with a l.25x to 1.5x generator size without operational problems. Further, the major double-conversion UPS manufacturers have developed input current distortion reduction techniques that greatly improve the compatibility of UPS systems with generators to allow even closer load sizing. In particular, 12-pulse rectifiers greatly reduce UPS input harmonics and they consequently require less input filtration and present less of a leading power factor to the generator.

It is mandatory to have the gen-set supplier verify your calculations with a computer sizing program based on actual UPS input parameters. Gen-sets must be equipped with a permanent magnet exciter, a three-phase sensing AVR and isochronous electronic governor to work properly with a UPS. Also, based on actual load and UPS input parameters, a gen-set supplier should verify UPS input load with or without battery charging whether the UPS uses 6- or 12-pulse rectifiers.

There are three major types of UPS topologies available: off-line, line- interactive and double conversion. All three types can provide some measure of outage protection, but only double-conversion UPS products can satisfy the unique needs of a facility with most or all of its power generated on-site.

The off-line and line-interactive topologies are called "single-conversion," because the UPS is only performing one type of conversion at a time. When utility power is available, they pass it through to the load, doing only a minimal amount of power conversion to charge the batteries. When input power fails, they convert d.c. battery power to a.c. power. Line-interactive products are a bit more sophisticated than offline systems, providing "buck/boost" circuits to raise or lower the voltage of the incoming a.c. power, to keep the output voltage within specifications. Single--conversion UPS products are primarily used as desktop devices, providing small amounts of 120 Va.c. to home computers and small networks.

By contrast, double-conversion UPS products are the accepted standard for higher power, three-phase systems of 50 kW or more. These products continuously convert incoming a.c. power into d.c. power; the d.c. power maintains the battery charge and is fed to the UPS inverter to be converted back to a.c. to support the load. If the input a.c. is interrupted, the UPS continues to convert d.c. into a.c. power without any glitch or transition. The only difference is that the batteries become the primary d.c. power source until power is restored to the input of the UPS rectifier.

The double-conversion UPS provides an extra measure of isolation between the incoming a.c. power and the load. This enables the output of the UPS to be precisely controlled and stable, even when the incoming power is experiencing frequency variations. Some on-site generators--especially those powered by natural gas--may have frequency excursions in response to sudden load changes. Double-conversion UPS products are unaffected by these excursions.

Another consideration is that double-conversion UPS products can be synchronized to each other, even when their inputs are not synchronized or when they're both free-running on battery power. This is especially important if a portion of the facility is mission-critical and uses dual-bus, distributed-redundant power.

In recent years, a few companies have introduced line-interactive three-phase UPS products with power factor correction. These have enhanced power-conditioning features compared to off-line and typical line-interactive products. Complexity approaches (and sometimes exceeds) that of double conversion products.

These are line-interactive UPS' in the classic sense, meaning that the series transformer and output inverter interact with the incoming utility voltage to alter the output voltage.

Some products have a small input inverter/charger (sometimes called a "delta" inverter) to modify the input voltage. The small inverter typically attaches to the d.c. bus, which it uses as a pipeline to exchange power with the output (main) inverter. The output inverter is used for both input power factor correction and outage protection.

Generator interaction problems for offline and line-interactive UPS are well documented. Offline and line-interactive products require stable source frequency and phase shift. Stable source frequency is required since the inverters must track the supply frequency to provide the voltage and current correction. Therefore, the output frequency and other fluctuations of the system are the same as the input frequency (unless the UPS is operating on battery), thus passing these potentially damaging variations directly to the load.

A classic problem with on-site generation is when transformer inrush current or heavy motor starting loads cause the generator's output frequency to vary, which then causes the offline or line-interactive UPS to cycle onto battery operation. The problem is especially pronounced with natural gas-powered gen-sets. This repetitive battery cycling can cause the battery to discharge completely, while also significantly shortening battery life. Another potential problem is the generator instability that occurs when the UPS load is transitioned to the generator. The UPS load transfer causes the generator voltage and frequency to sag, causing the UPS to go back to battery operation. Soon thereafter, the UPS senses stable generator output, transfers the load back to the generator, then transfers back to battery operation when generator output dips again.

These problems are not a factor for a conventional double-conversion UPS. Double-conversion UPS systems rectify the input supply and can accommodate large swings in supply frequency while continuing to provide regulated, stable output frequency, without the use of the battery. A double conversion UPS system eliminates the wide range of potential power problems such as spikes, surges, including voltage and frequency variations common with standby generator operations.

Today's companies are seeking to achieve nonstop 24/7/365 operating objectives where unscheduled shutdowns due to power interruptions are unacceptable. As both utilities and customers increasingly embrace the concept ofDG, gas turbine and reciprocating engine technologies appropriate for such applications can be better protected in the delivery of power quality and reliability with an integrated UPS systems.
                        Generator          Generator
                     oversizing for      oversizing for
UPS Rectifier Type  Dedicated UPS (N)  Mixed-Load UPS (N)

6-Pulse Rectifier         1.15                1.6
12-Pulse Rectifier        1.10                1.4

Minimum sizing guidelines for generator-UPS matching.

Brad Nacke is industrial market manager at Liebert Corp., Columbus, OH. Liebert is part of Emerson Network Power, which includes ASCO, Copeland and Kop-Flex.
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Title Annotation:uninterruptible power supplies
Author:Nacke, Brad
Publication:Diesel Progress North American Edition
Geographic Code:1USA
Date:Jan 1, 2002
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