Brad Nacke, Liebert Corp.
Power quality means: consistent voltage, steady frequency and absence of interruptions. According to a Hubler/Mills report (April, 2000), the U.S. electric grid network delivers over 3 trillion kWh per year with roughly 99.9 percent reliability. Electric power problems cost U.S. industry $26 billion per year in lost data, material and productivity. With the rapid growth of the Internet and Web hosting facilities and an explosion in the use of sensitive electronic equipment and controls in the manufacturing sector, reliability demands will start at six 9s or 99.9999 percent reliability.
Distributed generation (DG), by definition, is the use of small-scale power generation technologies located near the load being served. The move toward on-site distributed power generation is accelerating due to impending deregulation and restructuring of the utility industry. Used judiciously, DG technologies can improve power quality, boost system reliability, reduce energy costs and help delay or defray substantial utility capital investment.
Industries across the board can benefit from distributed generation when used in conjunction with other types of power. 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. Other customer advantages include:
- Allowing customers to generate their own electricity with or without grid backup,
- Permitting customers to generate power while serving their thermal or cooling loads,
- Generating a portion of electricity on-site during peak price periods,
- Improving customer power quality and reliability; and
- Serving niche applications, such as green power or remote power.
The DG environment
DG power plants provide commodity electrical power and energy benefits and can be used in conventional peak shaving, intermediate or base load operating modes. The current DG portfolio includes varied technologies, although appropriate economies of scale have not yet been achieved to make some technologies feasible options. For the burgeoning DG market, gas engines are expected to meet a significant portion of demand due to their competitive initial cost, high shaft efficiency, proven reliability, suitability for start-up operations and well-established sales/service infrastructure. Motor-generator sets (genset) are the optimum power-conditioning devices for diesel and natural gas engines because they completely isolate the load from all of the problems on a utility line.
There are a variety of potential benefits that can be derived from DG:
- Deferral of new R&D capital investment;
- Reduction in T&D electrical line losses;
- Cost-effective source of new peaking power;
- Improved power quality and reliability;
- Reduction in energy and electric demand charges;
- Self-reliant source of emergency or standby power;
- High-reliability power for sensitive facilities when coupled with uninterruptible power supply (UPS) systems; and
- Low overall emission rates per kW.
One problem is inherent with a diesel genset: the ability to make an immediate bridge from the utility to back-up power. In the case of emergency lighting, a temporary blackout of two seconds or less may be acceptable. But in a manufacturing process, Internet hosting center or any environment with critical electronics, a temporary outage causes vital electronics to shut down and processes to fail. Diesel- or gas-powered generators must have time to warm up or “soft load.” During soft load the utility load is slowly shifted to the genset or the genset is started and brought up to full capacity before switching the load. Unfortunately, in the case of power outage or genset malfunction, such luxury cannot be afforded and the savings associated with DG turn to losses.
UPS bridges the gap
Regardless of the distributed power generation technology involved, an integral part of any diesel or natural-gas fueled DG system is a UPS. In switching power from the utility to DG, a UPS provides the bridge between the utility and generator while it ramps up to 100 percent, preventing costly downtime due to power outages and balancing the load until it reaches full capacity.
There are three major types of UPS topologies available, but only one specifically developed to bridge the gap between genset and utility. A double conversion UPS system eliminates a wide range of potential power problems such as spikes, surges, including voltage and frequency variations common with standby generator operations. This is because a true double conversion device uses a dual conversion bus and rectifier to deliver power to an attached device during an outage. The double conversion UPS continually recharges the battery, so that if power goes down, backup is immediately available.
Generator interaction problems for off-line and line-interactive UPSs are well documented. Off-line and line-interactive products require stable source frequency and phase shift. Stable source frequency is required since inverters must track the supply frequency to provide the voltage and current correction. Therefore, the output frequency of the system is the same as the input frequency, unless the UPS is operating on battery.
A classic operational problem is the starting of other loads on the generator causing the generator’s output frequency to vary, which then causes the off-line or line-interactive UPS to cycle on to battery operation. The problem is pronounced with natural-gas-powered gensets. This repetitive battery cycling can cause the battery to discharge completely, while 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 don’t exist for conventional double-conversion UPSs. Double- conversion UPSs 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. Further, the major double-conversion UPS manufacturers have developed input current distortion reduction techniques that greatly improve the compatibility of UPS with generators to allow closer load sizing.
Switching between the two power sources must be a break-before-make connection. A number of distributed redundancy power distribution configurations can be devised. However, that redundancy needs to be as close to the load as possible to achieve its goal -namely, keeping power available at the load equipment level. For DG applications, the double-conversion UPS topology is the most appropriate. The input voltage and frequency may fluctuate, but the double conversion UPS doesn’t care, since the rectifier is only making DC power to feed the DC bus. The output inverter usually contains an isolation transformer that can produce a separately derived neutral. This enables the UPS to be electrically isolated and provide common mode noise protection for the load. This design is proven and very well understood. It has been applied successfully to every imaginable application. Medium and large three-phase UPS products are double-conversion topology and have achieved million-hour critical bus MTBF (mean time between failures).
Companies are seeking to achieve non-stop 24/7/365 operating objectives where unscheduled shutdowns due to power interruptions are unacceptable. As both utilities and customers increasingly embrace the concept of DG, 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 system.
The growing demand for continuous systems availability that drives the DG environment today requires on-line maintenance/repair that will not impact industrial processing operations. Emergent DG technologies will continue to require unprecedented levels of availability and reliability for all industrial processes.
Nacke is the industrial market manager at Liebert Corp. (www.liebert.com), an independent subsidiary of Emerson headquartered in Columbus, Ohio.