For several years now, members of IEEE's P1547 working group have been developing a draft document to provide engineers with uniform guidelines pertaining to the performance, operation, testing, safety, and maintenance of distributed generation (DG) equipment. Writing the “Standard for Interconnecting Distributed Resources with Electric Power Systems,” however, hasn't been a cut-and-dried endeavor. To date, the working group has rejected two versions of the proposed standard. Members are now working on a significant revision for the next round of ballots. Read on for some insight into this standard's rocky road to adoption.
The difficulties faced by the working group should not be underestimated. Distributed generation encompasses a wide range of technologies (e.g., synchronous and induction generators, photovoltaics, fuel cells, and microturbines) and sizes (e.g., a few kilowatts to several megawatts or more). Utility system designs, along with their operating and maintenance practices, vary from company to company. A standard that addresses all possible technologies, installation scenarios, and distribution system variants is a tall order indeed.
From the perspective of DG vendors and project developers, however, uniform interconnection guidelines are a commercial necessity. Variations in requirements pose a serious barrier to DG projects because the costs undesirably segment the market into smaller, potentially unreachable pieces. The consequence for DG users then becomes higher capital costs and longer installation times.
The fundamental challenge is that most distribution systems in the United States are designed, protected, and operated on the notion that there is a single source of electric potential active at any given time.
Still, many utility systems can currently accommodate some DG without negative consequences. The problem is that — for any given system — there will be a point where too much DG will undermine the performance, safety, and reliability of existing equipment. (see Fig. 1 and Fig. 2, below, and Fig. 3, on page 39). This leads to yet another question: How do you determine the exact point or proper balance where DG systems and the utility grid work in concert?
In our opinion, it would be useful to begin tackling this seemingly intractable problem by breaking it into a number of smaller issues. If you segment DG technologies into large and small (without regard for type of technology, just nameplate capacity), and low penetration and high penetration, you can define four distinct, but more manageable, problems (see the table). This tactic also provides some insight regarding the challenges faced by DG advocates and utility engineers.
For some time, Electrotek Concepts has been accumulating valuable field experience in the low-penetration area. Many utilities already have small amounts of customer-owned generation connected to and operating in parallel with their systems. In most cases, these tend to be large, more conventional DG units.
Small DG systems operating in parallel with the utility are much less common. However, a number of utility companies in the United States have participated in pilot projects (going back as far as the late 1980s) with residential DG applications. Although these projects are small (in the aggregate) relative to feeder capacity, they have shown that many small DG installations can coexist with the utility system without compromising performance, reliability, and safety.
With high penetrations, all bets seem to be off. A utility with large amounts of connected generation would require a complete redesign in terms of protection, voltage regulation, automation, and operating procedures. Few of the conventional practices employed for radial distribution design would apply. The interface between the DG and utility systems is no longer the primary issue because the behavior of the composite system has become a function of both the aggregate generation and the utility infrastructure.
At least with large DG systems in the high-penetration boundary, the impact on the distribution feeder is obvious, and engineers could allocate their resources to study the situation well in advance.
However, small DG systems in high-penetration areas could move in under the radar (i.e., a few kilowatts per week or month) until a feeder suddenly goes haywire when its penetration level becomes too great. In such cases, any uniform interconnection guidelines would severely reduce or eliminate the utility's ability to perform technical evaluations and intervene when action is needed. This is why the “plug-and-play” attribute of DG causes concern among utility engineers (see Fig. 4, on page 40).
Low penetration characterizes the present state of most DG applications. The accumulated experience to date and our engineering intuition tell us that some streamlining in the interconnection process is possible. But because it's impossible to address all of the variants within one document, utility-specific guidelines should still remain the last word. Defining the largest possible “common denominator” to reduce the burden on equipment manufacturers and project engineers is a worthy goal, and one that Standard P1547 will most likely address.
For small, high-penetration DG to become prolific, a “killer app” would be required, one where the overall cost would be so attractive that home improvement stores would stock home-generation kits for parallel operation with the utility. No such application is yet on the horizon, which means that engineers probably won't be dealing with this scenario any time soon.
With larger DG installations, higher penetrations could come sooner, as long as power markets continue their movement toward allowing standby and emergency generators to bid into various energy and capacity markets.
In the meantime, all stakeholders in DG must move forward in an incremental fashion, accumulating field experience and making appropriate modifications to technical, business, and institutional policies — all in an effort to facilitate better use of DG technology. Efforts like the forthcoming IEEE standard are of value to all. Watch for the new, leaner draft standard late this year or early next year. And while you're at it, be on the lookout for that killer app.
Robert Zavadil manages distributed generation and wind energy projects at Electrotek Concepts in Knoxville, Tenn. You can reach him at [email protected].
Mark McGranaghan directs power quality projects and product development at Electrotek Concepts. He can be reached at [email protected].