May 15, 2007

DG interconnection standards remain elusive

Charles J. Mozina, Beckwith Electric Co., Inc.

This April, when the U.S. Department of Energy (DOE) released a report titled "The Potential Benefits of Distributed Generation and Rate Related Issues That May Impede Their Expansion," the agency noted that, "in some cases, [interconnection] rules and requirements are excessive, arbitrary, time-consuming, and add unnecessary costs to [distributed generation (DG)] projects." The report, prepared by the DOE in consultation with the Federal Energy Regulatory Commission (FERC) as mandated by the Energy Policy Act of 2005, identifies the potential benefits of cogeneration and small power production and the barriers that may prevent developers of DG projects from achieving them.

 

According to the report, interconnection of a DG plant to a utility's grid remains shockingly problematic in many regions of the U.S. nearly 30 years after the passage of the Public Utility Regulatory Policies Act of 1978. Due to the lack of standards among utilities, many owners of small (10 MW or less) grid-connected DG plants are subjected to excessive costs and discriminatory practices that leave them with little recourse but to pay the bill when it comes. Some owners also complain about overly burdensome technical requirements that exceed those normally used to protect both a plant and the grid it supplies. The report also found that "there are no standard models, tools, or techniques for utilities to evaluate DG and incorporate DG resources into electric system planning and operations."

Pick your poison

Several model interconnection standards have been proposed, by:

• The U.S. Environmental Protection Agency's Combined Heat and Power Partnership (www.epa.gov/chp/).
• The National Association of Regulatory Utility Commissioners. NARUC's "Model Distributed Generation Interconnection Procedures and Agreement" was published in 2002.
• The Interstate Renewable Energy Council (2005).
• The National Rural Electric Cooperative Association (2006).

The most recognized and accepted standard is IEEE Standard 1547, titled "Standard for Interconnecting Distributed Resources with Electric Power Systems" (2003). Standard 1547 (the "Standard") leads the pack because it was vetted through the peer review process of the American National Standards Institute and includes a realistic framework of rules and requirements. Its stated goal is to serve as a single document of standard technical requirements for DG interconnection with which all plant owners and utilities must comply. Because adoption of any standard is the prerogative of individual states, acceptance of the Standard has been slow. California, with its Rule 21, and New York, which has set statewide specific technical standards, are notable exceptions.

All of the proposed standards listed above recognize that the major function of interconnection protection is to prevent system islanding by detecting asynchronous DG operation. In other words, it's critically important to determine when the generator is no longer operating in parallel with the utility grid. The system protection design also:

• Disconnects the DG when it is no longer operating in parallel with the utility system.
• Protects the utility grid from damage caused by connection of the DG. Among the main causes of such damage are transient overvoltages and the fault current supplied by the DG, which can damage utility equipment and/or cause loss of relay coordination, resulting in unnecessary utility customer outages.
• Protects the DG from damage from the utility system, especially through automatic reclosing.

Although the focus of all of the standards is clear, a number of holes remain in the Standard that a robust interconnection design must address. DG developers should seriously consider the technical suggestions that follow. Be proactive and develop well-thought-out solutions that address these gaps in the Standard before approaching utility planners with an interconnection plan for your proposed project. To its credit, the IEEE is moving forward with a number of standards-making working groups to address the holes in IEEE 1547.

Protection: A two-way street

Most 10-MW or smaller distributed generators are connected to the local utility grid at the distribution and subtransmission level. These utility circuits are designed to supply radial loads. Distributed generators need to be protected not only from short circuits but also from abnormal operating conditions that the utility system can impose. Examples of such abnormal conditions are overexcitation, overvoltage and undervoltage, unbalanced currents, abnormal frequencies, and excessive torque on turbine rotors resulting from utility automatic reclosing.

If a generator is subjected to these conditions, it may be catastrophically damaged in a matter of seconds. This is obviously a major concern of DG owners. Utilities, on the other hand, are more concerned that the interconnection of DG will damage their equipment or that of their customers.

The Standard provides limited specific guidance on damage prevention other than calling for over/underfrequency and over/undervoltage interconnection protection. However, it does specify that the protection be installed at the Point of Common Coupling (PCC) between the DG and the utility system. The installation can be either at the secondary of the interconnection transformer (Figure 1, top) or at its primary (Figure 1, bottom), and the choice will determine who incurs the transformer losses. The circuit to protect the generator itself is typically installed at its terminals.

 

 

Most DG plants of 10 MW or less are connected to a utility's distribution grid. In the U.S., most distribution systems are multigrounded four-wire designs whose voltage ranges from 4 kV to 34.5 kV. This design allows single-phase, pole-top transformers (which typically constitute the bulk of the feeder load) to be rated at line-to-neutral voltage. Thus, on a 13.8-kV system, single-phase transformers would be rated at 13.8 kV/1.73 = 8 kV. Line-to-neutral-rated transformers and lightning arrestors can be subjected to damaging overvoltages depending on the choice of DG interconnection transformer. Five different kinds of transformer connections are widely used to interconnect dispersed generators to the utility grid (Figure 2, bottom). Each has advantages and disadvantages, as noted.

 

 

The Standard cites obvious requirements for interconnecting DG but offers insufficient methods, solutions, or options for meeting them. The guidance is at such a basic level that not enough detail is provided to complete an interconnection design. Following are six major areas where the Standard definitely requires refinement.

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