Proper UPS Essential for High Reliability DG Gensets

Issue 2 and Volume 106.

Steve Blankinship,
Associate Editor

By Brad Nacke, Industrial Market Manager, Liebert Corp.

The emergence of distributed generation – whether for primary power or peak shaving – can solve some, but not all, facility power quality problems. Economics is the driving force behind the surge toward distributed generation. Large users of electricity want to lower high costs during peak times; utilities want to decrease demand at the same time.

Increasing the output of on-site generators as little as 50 to 200 hours per year can earn customers annual incentives from the utility worth hundreds of thousands of dollars. Industries across the board can benefit from distributed generation when used in conjunction with other types of power.

But while on-site capacity assures power will be available, it does not provide the conditioned power that an uninterruptible power supply (UPS) can deliver for today’s sensitive electronics. A well matched UPS and genset combination can provide computer grade power. But if the UPS and genset don’t work as a system, it can create more problems than it solves.

A DG plant is normally used either for standby power or to take part or all of the facility off the utility during peak periods. In addition to providing conditioned power, the UPS is an essential bridge to support the facility load while the generating plant ramps 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.

Several things must be considered in choosing the genset and UPS in order to ensure they work as a system while switching from a utility to UPS. Most facilities have operations that are not critical and do not need to be supported by the UPS. Non-critical loads on the generator include motor loads such as air conditioning units, lights, phone, and security systems. Both critical and non-critical loads must be considered so that the genset is rated for a higher power output than the UPS equipment.

For situations where the facility load equals the UPS/genset output, the generator should still be rated for a higher kW output than the UPS. This will protect against large load steps and over voltage shutdowns, allow for UPS battery recharge and provide 100 percent of the power load if the UPS switches to bypass due to nonlinear loads created by computers.

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 the UPS supports 100 percent of the facility. More common is the “mixed-load” application, where some portion of the facility load is supported directly by the generator, without UPS support.

In order to get the voltage distortion of the generator down, the generator manufacturer may install an oversized alternator in the genset. 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 can be used with a 1.25x 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. Twelve-pulse rectifiers greatly reduce UPS input harmonics, consequently requiring less input filtration and presenting less of a leading power factor to the generator.

It is critical to have the genset supplier verify your calculations with a computer sizing program based on actual UPS input parameters. A genset 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 genset supplier should verify UPS input load with or without battery charging, whether the UPS uses 6 or 12-pulse rectifiers.

Double-conversion UPS products are the accepted standard for high reliability power, typically 3-phase systems of 50 kW or more. These products continuously convert incoming AC power into DC power; the DC power maintains the battery charge and is fed to the UPS inverter to be converted back to AC to support the load. If the input AC is interrupted, the UPS continues to convert DC into AC power without any glitch or transition. The only difference is that the batteries become the primary DC 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 AC 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.

Brad Nacke brings 17 years of industrial experience to Liebert Corporation and is an active member of IEEE. As Industrial Market Manager, he is responsible for environmental systems, power protection and support systems, and focuses on expanding industrial markets. Nacke received his BSIE from the University of Iowa, MBA from Xavier and did post-graduate work at Marquette.