Sorting Out the Interconnect Standard for Distributed Generation's Future
Editor's Note: The following is excerpted from the article "Intertie Requirements for DGs Connected to Radial Distribution Feeders," which appears in full in the EGSA supplement accompanying this issue of CSE. Distributed generation is attracting more attention as a way to ensure local power supply and add to utility-system resources.
Distributed generation is attracting more attention as a way to ensure local power supply and add to utility-system resources. However, ensuring safe, reliable operation of distributed generators (DGs) requires close attention to this equipment's connection to both utility lines and a facility's internal distribution system. The Institute of Electrical and Electronics Engineers (IEEE) has developed standards for such interconnections, addressing the connection of DGs of various sizes to systems of varying complexity. IEEE Standard 1547-2003, Standard for Interconnecting Distributed Resources With Electric Power Systems, was published in June.
Distributed generation has proven to be a viable alternative to large, centralized generation plants for meeting individual facilities' electricity needs. When such systems are designed, utilities establish intertie requirements to ensure safe and reliable power delivery both within the facility and throughout the facility's utility system.
Basic intertie protection for DGs connected to radial distribution feeders includes preventing islanding—where an area has power but the surrounding area does not—of the DG, by detecting abnormal voltages and frequencies at the point where utility and facility distribution systems meet: the "point of common coupling" (POCC).
As the size of the DG increases in relation to the distribution feeder source and load, the protection required increases to include fault detection for faults on the distribution feeder. In such scenarios, current reversal in the utility-substation breaker can result, making directional protection a possible necessity.
The connection-type of the interconnecting transformer can further complicate protection requirements. Systems with an ungrounded transformer primary will require ground overvoltage protection, while systems with a grounded transformer primary will need additional overcurrent protection.
Finally, where DG power is exported onto the utility system, transfer trip relaying may be required to ensure tripping of the DG before the utility breaker recloses.
As Standard 1547-2003 evolves, with the addition of guides for application, testing and commissioning, and monitoring/information exchange/control, utilities will adjust their standards and make it easier to meet the many challenges associated with distributed generation. However, even with intertie standards becoming more defined, each distributed generation installation will still require the engineer to evaluate these applications on a case-by-case basis.