Distributed generation (DG) is making a comeback. In the late 1970s, federal legislation sparked a flurry of interest in the topic. Today it is once again an industry buzzword. Standards development, equipment testing, and project demonstrations are flourishing. However, this renewed interest poses the following unresolved questions. What are the requirements for interconnection with electric distribution systems? How will significant amounts of DG actually impact real-world distribution systems? And how do we go about analyzing, designing, and operating DG systems that no one has seen?
A Brief History
In the past, powering electric loads depended on local generation. Before 1910, for example, the generating capacity at industrial sites exceeded that of all utility companies combined. As electric power caught on, generation technologies evolved quickly. Electricity production continued on a large scale into the 1960s, when industries reached the power limit that existed with the materials (and other technologies) of the day.
The distribution and transmission infrastructure that grew to support the electric power industry also played an early role in the history of DG. By interconnecting transmission systems, utilities found it possible to share energy and increase system reliability without necessarily building more generating capacity.
Demand for electric energy in the United States slowed dramatically in the early 1980s, as did construction of large central-station generating facilities. In addition, the Public Utilities Regulatory Policies Act of 1978 (PURPA) offered economic and regulatory incentives for electricity generation to non-utility entities whose facilities met certain qualifications. This legislation, which included most small power producers, opened the door to research and development in alternative technologies. Since PURPA implied these small producers would be interconnected with the electric utility grid, it's possible to view it as the birth of DG as we know it today.
If PURPA represents the birth of modern DG, then the discussions of distribution-system impacts and other interconnection topics began well before the christening. Underlying the design and operation of a U.S. electric utility plant is the basic assumption that each feeder is supplied with a single source of electric potential at any given time. Radial distribution feeders emanating from distribution substations bring electricity to most of us. Generation on the customer side of the meter violates that premise directly, giving distribution system designers and operators heartburn when they consider the potential consequences.
Research, development, and demonstration of DG technologies have continued over the past 20 years despite the concerns about the requirements for interconnection with utility distribution systems. Much of the effort has come from advocates of alternative technologies, such as distributed photovoltaic power generation, where distribution system interconnection is the only practical option.
In the last few years, a combination of events has tilted the playing field in DG's favor. Researchers and vendors have done much work to position DG as a viable option for electric generation. Innovative new technologies such as microturbines, breakthrough progress with more familiar concepts such as fuel cells, and well-known products with improved performance (e.g., diesel and gas engines) enhance the market prospects for DG.
Deregulation of the electric power industry in the United States has been a somewhat surprising boon for DG, at least in the public relations arena. High spot-market prices for electricity routinely make the local and national news during times of peak usage, and the recent crisis in California has made electricity a subject even for water cooler discussions. Many times, such stories focus on DG as a possible solution or remedy.
The California crisis has brought the special power needs of the ultra-high-reliability customer — a fast-growing market niche — front and center. Here, DG promises insulation from capacity shortages or rolling blackouts and assistance in providing the high-quality power required on a continuous basis. This has sparked a high level of interest on the part of investors, who sense a major opportunity in the $200 billion annual electric-power market.
Policy makers, from state public utility commissions to the Department of Energy, are also jumping on the bandwagon. They see DG as a strategic technology for the new century — one that has an important role in the overall energy resource mix. Law makers are crafting regulatory policy to remove barriers that discourage the widespread use of DG. Most recognize, however, that technical concerns about the impact of large amounts of DG on utility distribution systems are a real and significant barrier.
It's not surprising that debates over DG interconnection requirements have continued without resolution. If you consider DG technology, including design, protection, and operation, you'll understand the extreme challenge of developing a consensual, detailed policy that applies to all types of distribution systems. Throw in evolving technologies for DG interconnection, such as static power converters, controllers, and communications, and the problem becomes even more complex.
From the utility perspective, there is a strong and economically justifiable feeling that DG equipment should work with the existing distribution system. If changes were required, how would the business recover the costs, or at least allocate them fairly? DG vendors, on the other hand, view even modest incremental costs as onerous — or potential deal killers.
Finally, distribution-system operators face more than the challenge of accommodating DG. An ever-increasing awareness of power quality and reliability by the customer base is already exerting substantial pressure to maintain or improve system operations. The willingness to experiment with the distribution system by interconnecting with appreciable amounts of DG is understandably small. It's a risk that makes very little sense given the other pressures.
To help sort out these conflicting views, the Department of Energy helped organize an IEEE standards group aimed at establishing interconnection requirements and criteria for DG. This effort, known as P1547, has been underway since the latter part of 1998. It includes utility engineers and managers, equipment vendors and manufacturers, researchers, and consultants. As of this writing, a draft document entitled, “Standard for Interconnecting Distributed Resources with Electric Power Systems,” has gone out for balloting. Estimates for an approval date range from mid-2001 to sometime next year.
The document focuses on specifications, performance, and test procedures for the interconnection itself, which is defined as the combination of equipment and devices used to electrically connect the DG device to the power system. The standard strives to consider the greatest possible range of distribution system configurations and practices.
Where Will P1547 Take Us?
Assuming the current draft of P1547 is a good indicator of the approved standard (a relatively safe bet), we can expect specific criteria that explains the purpose and operation of a DG interconnection with the distribution system. The standard will give equipment manufacturers a design target and distribution engineers the primary expectations of DG equipment. It also will contain a reasonable consensus on how DG equipment operates safely and reliably in the typical distribution system environment. Moreover, P1547 will specify performance requirements in the areas of power quality, protection, and response to utility-system events and methods for testing, type testing, and equipment certification. Overall, it should remove a great deal of uncertainty concerning interconnection issues.
Such an IEEE standard should be heavily referenced by DG interconnection policies crafted by state public utility commissions. Policies at this level are different because they require regulated entities to comply with laws rather than simply volunteer to follow guidelines. Industry professionals will welcome the consistency that comes when policy makers refer to a single technical standard, and it should help reduce or remove some market barriers.
Following the adoption of P1547 (with the references to utility commission rulings), individual distribution companies will, for the most part, remain unchanged. These companies need to develop comprehensive interconnection requirements specific for their distribution systems. The requirements must comply with company design and engineering practices as well as relevant public utility commission mandates and policies.
Building the Knowledge Base
By its own admission, P1547 (and the pending final standard) is “not a design handbook nor is it an application guideline.1” Yet, design and application guidelines are crucial for managing the evolution of distribution systems, when and if DG becomes prevalent — and for preventing, anticipating, or resolving problems that may occur.
Although a number of apparently successful pilot programs with various DG technologies have occurred over the years, the lessons learned from these dispersed efforts lack the necessary experience to cope with a sudden influx of DG. And much of the technical discussion focuses on scenarios no one has seen before.
In some ways, the knowledge-base mirrors the industry's experience with power quality about a decade ago. The major impetus for moving us up the power quality curve was marketplace pressure (end-users). But regulatory and political influences also were influential. Led by electric utilities and related groups such as EPRI, the electric power industry, built the knowledge base one piece at a time by:
Measurements and monitoring helped the industry understand the basic electrical phenomena at work in various power quality problems. They also served to quantify the nature, possible origin, and frequency of occurrence for those power system events that led to misoperation or failure of end-user equipment. Efforts sponsored by the Canadian Electrical Association (CEA) and EPRI provided much-needed quantification and characterization on the range of power quality variations important to end-users. Various laboratories and testing agencies contributed detailed characterizations of end-user equipment susceptibilities.
Performing and documenting case studies
The usefulness of anecdotal stories and evidence are limited until someone with an engineering perspective documents a situation fully. Presenting information with sufficient detail so others can learn from the experience is the most effective way to build the collective knowledge of a technically oriented community.
Obtaining and disseminating information
Technical papers and reports, seminars, and substantial attention from industry journals helped spread the word throughout the power industry. All this information contributed to problem solving, and eventually, to avoiding problems entirely.
The same fundamental approach holds much promise for augmenting our distribution system engineering expertise to meet the prospective challenges of DG. The process can be painstaking, however, as no single research or information collecting initiative can by itself meet the need. Rather, it will be through an informal network of multiple forums for technical information gathering, sharing, and dissemination that the required knowledge base will emerge.
Consortium projects are underway to develop and disseminate information and provide resources for developing and implementing interconnection guidelines and design standards that are consistent with technical standards and regulatory mandates and tailored to the specific needs of individual distribution companies.
Other projects focus on DG equipment characterization through laboratory testing, and they will provide critical data needed for specific engineering evaluations and studies. How ever, laboratory testing cannot reliably simulate the important interactions between many DG units and the distribution system itself. This represents the real technical challenge for distribution system engineers. Actual field experience and effective dissemination of lessons learned will still be the best teacher.
1IEEE P1547/D07, “Draft Standard for Interconnecting Distributed Resources with Electric Power Systems,” page 2.