The new DISTRIBUTED GENERATION

Power quality and reliability are primary considerations in the design and configuration of 24-hour infrastructures for mission-critical systems. For telecom, these systems require clean, dependable power to run. In addition, the technologies utilized for power availability and protection can play an important role in efficient operations that affect the bottom line. For these reasons, many have focused attention on distributed generation along with uninterruptible power supply (UPS) technologies as a way of improving power quality and reliability in a cost-effective strategy.

By definition, distributed generation involves the technology of using small-scale power generation technologies located in close proximity to the load being served. The move toward on-site distributed power generation is accelerating because of the impending deregulation and restructuring of the utility industry. In the appropriate configuration, distributed generation technologies can improve power quality, boost system reliability, reduce energy costs and help delay or defray substantial utility capital investment.

There are six major advantages to using distributed generation over existing technologies. These include:

• Generation of electricity with or without grid backup

• Generating enough power to serve thermal or cooling loads

• Generating a portion of electricity on-site to reduce the amount of electricity purchased during peak price periods

• Improving the level of power quality and reliability

• Serving niche applications such as remote sites

Changing the electric industry structure

Four years ago, California became one of the first states to approve a deregulated electricity market. Since that plan opened up wholesale electricity markets to competition, consumers and utilities have struggled with high prices and low supplies.

The problems associated with California's power crisis have been numerous, from rolling blackouts to extremely high electricity prices. This has led many organizations to implement or strengthen their capabilities to ride through a blackout with proactive measures to efficiently and effectively avoid downtime.

This viewpoint is echoed in Salomon Smith Barney's 2000 “Power Quality Field Trip” report. According to Jeffrey Sprague, an analyst at Salomon Smith Barney, “Power quality and availability are becoming increasingly large concerns given the strains on the existing power grid and the forces of deregulation.”

During the past several decades, power generation has been highly centralized in large facilities. Customers are served primarily by utility distribution companies that have connections to large generation facilities using high-voltage transmission lines and connections to customers through their lower-voltage distribution lines. This grid system is referred to as the transmission and distribution (T&D) system. In an electrical system with distributed generation, smaller, widely dispersed generating units supply electric power in addition to — or instead of — centralized facilities.

One aspect of distributed generation's efficiency is the lowering of energy costs through effective demand reduction on the utility or negotiation to sell excess capacity back to the utility that results from regular usage of on-site power generation. The selling of electricity back to the utility can be done on an ad hoc basis or as part of a call program. On an ad hoc basis, the excess electricity produced from distributed generation can be sold back to the utility whenever it is practical to do so. A call program means that a certain amount of electricity is sold back to the utility per agreement with the utility on a regular basis.

For mission-critical telecom systems, the call program may not be appropriate because reserves of electricity may be drained beyond what is needed to maintain operations in the event of an extended outage. In this situation, even distributed generation is limited by the available fuel. Having to rely on distributed generation as a source of power during an outage and being obligated to sell a portion of the generated power back to the utility may result in a shortage of electricity available to run critical operations. For organizations that host or have vital e-business operations, anything less than 24-hour uptime can be catastrophic, which suggests planning distributed generation more toward a demand reduction strategy.

Another aspect of distributed generation efficiency is its ability to be sited in a wide variety of locations close to the point of demand for electric power and — through multiple units — at a wide range of capacity levels. For example, distributed generation units can be located or moved quickly to alleviate transmission congestion or other circumstances that raise costs or threaten system integrity, reliability and efficiency. The ability of distributed generation to respond quickly to system problems could reduce the investment in T&D and generation required to meet projected demand growth.

Power quality

Electric power problems cost U.S. industries $26 billion per year in lost data, material and productivity. Most commonly recognized power quality events include frequency instability, sustained over voltages, high frequency noise, harmonic distortions, high voltage spikes and voltage sags and swells (Figure 1).

Microprocessor technology and Web links are penetrating deep into facilities of virtually every organization, and power quality starts at six 9s, or 99.9999%, reliability. Given this scenario, power interruptions or poor power quality of any kind must be eliminated, and in many situations orderly shutdowns are no longer options for mission-critical systems.

UPSs are available to protect against power problems and for a bridge between distributed generation power and utility power. While there are a few different UPS topologies to choose from, only a double-conversion UPS system eliminates the potential power problems such as spikes, surges and voltage and frequency variations common with standby generator operations. In this configuration, power is continuously supplied to the batteries via the rectifier while the inverter is providing a constant flow of power to the load. This allows for a seamless transition in the event of a power outage while supplying the load with a continuous source of superior-quality AC voltage.

A classic operational problem in a distributed generation application is the starting of loads on the generator causing the output frequency to vary, which then causes an offline or line-interactive UPS to cycle to battery operation. The problem is especially pronounced with natural gas-powered gensets. This repetitive battery cycling can cause the battery to discharge completely, significantly shortening the 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 and then transfers back to battery operation when generator output dips again.

For distributed generation applications, the double-conversion UPS topology is the most appropriate (Figure 2). With this technology, the UPS can accommodate large swings in supply frequency while continuing to provide regulated, stable output frequency without the use of the battery.

Incoming AC power is rectified to DC power to supply the internal DC bus of the UPS. The output inverter takes the DC power and produces regulated AC power to support the critical load. Batteries attached to the DC bus are float-charged during normal operation. When the input power is out of spec, the batteries provide power to support the inverter and critical load. During this switch in power from the utility to distributed generation, the UPS provides the control bridge between the utility and generator. The UPS system thus balances the load until the genset reaches full capacity.

Advantages of this configuration include:

• The critical load is completely isolated from the incoming AC input power

• The critical load is always being supplied clean power by the output inverter, which is always being supplied from the internal DC bus. When input power fails, there is no transitional sag in the output voltage because the inverter is already operating on DC input

• The input voltage and frequency may fluctuate, but the double-conversion UPS doesn't care because 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

• The double-conversion UPS is inherently dual-input, meaning it has separate inputs for the rectifier and bypass circuits. The customer may request a single-input model as a convenience for installation, but dual-input UPS products are incrementally more fault-tolerant

• A fault on the input line causes the UPS to go to battery power, but the UPS rectifier will not allow power from the DC bus to flow upstream

Redundancy is the basis of success for UPS power protection both in systems and within the common elements. N+1 redundancy provides substantial improvements in reliability and can be cost effective and easy to implement for a fault tolerant solution. Continuous availability demands dual bus architectures, with redundancy throughout, for the elimination of single points of failure and maintainability. For power protection, a dual bus configuration provides a mirrored system, which may be electronically tied together. In the event that one of the busses goes down, the duplicated system is available to carry the load for a seamless transition (Figure 3).

For carriers, the benefits of distributed generation — along with the right UPS technology — can be important in keeping mission-critical systems up and running. With the proper power protection strategy in place, distributed generation may have an important roll in power quality — as well as efficient operations — which can substantially impact the bottom line for many organizations in the short term and for many years down the road.

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James Hall is the telecom market manager for Liebert. His e-mail address is info@liebert.com.