1146 Institute of Electrical and Electronic Engineers IEEE Standards Coordinating Committee

When parallel operation is considered, issues of concern are raised by both the electric power system (EPS) and distributed resource (DR) owner. Among these issues are the continued quality and reliability of power and the safety of personnel and equipment. In addressing these concerns, EPS owners have established protective relaying schemes that must be met by the DR owners. DR owners claim that these schemes are restrictive, cost prohibitive, and unnecessary. In between these diverse interests sits the State Public Utility Commission given the task of establishing the real requirements. Consequently, there is a need to establish a single standard defining the requirements for proper protection of both the EPS and the DR at the point of interconnection. The IEEE has taken on the task of creating and publishing such a standard.

The IEEE Standards Coordinating Committee 21 (IEEE SCC-21) is attempting to develop this standard. This is probably the most qualified organization to develop such a standard. For decades, the IEEE has developed recommended standards addressing the safe and proper configuration of equipment for generation, transmission, distribution, and utilization of electric power. Through their many societies and sponsored conferences in various disciplines, IEEE members are kept current with emerging technologies and trends in electric power. Clearly, the IEEE is preeminently qualified to draft a standard defining the appropriate configuration of electrical equipment at the point of interconnection. At this writing, a working draft of the standard, Draft 02, has been created and is circulating for comments and input. This draft is still missing some important sections. However, there is a significant body of text and content to indicate the specification's intent. Bear in mind that anything discussed in this commentary may well be superceded or overridden by the final draft of the standard.

Those unschooled in the science and art of electric power production, distribution, and utilization would prefer to have a standard that clearly defines all of the requirements for every combination of electric power equipment. Unfortunately, any such standard would be too large to carry. The standard must provide for the infinite permutations of EPS and DR ratings and capacities at the point of interconnection and common coupling. Accordingly, as found in the current draft of the standard, several factors are considered in the determination of the protective and control schemes. While some attention is still paid to size and ratings, this standard defers to the more important determinants of interconnect protection. These are defined as:

• Stiffness ratio — a term developed for this study that compares the utility system's available fault current at the planned interconnection point of the DR unit with rated output of the DR unit (looking at the DR)

• Contribution ratio — a term developed for this study that compares the DR's fault contribution to the power system at the planned interconnection point of the DR unit to that available from the utility system (looking at the system)

From the standpoint of the system and equipment, available energy must be determined at every point to ensure that the response to anomalies is correct and adequate. In selecting switching devices, if the device is to interrupt current, it must be capable of interrupting the maximum potential current available at the location of the device in the circuit. Similarly, if the device is to remain closed during a fault so that another device may interrupt the fault, it must be capable of withstanding the maximum potential current available at the location of the device in the circuit. For example, at any given fault location, circuits downstream of the DR must be capable of handling the maximum potential current of the EPS plus the DR. Circuits upstream of the DR need be capable of handling the maximum potential current of the DR. The significance of this approach is that consideration is given to the contribution capacity of the DR, as is the case with every other generator on the system. When this distinction is made, the nature of the generation process becomes transparent and proper protection and coordination are achieved.

Distributed resource generation will be comprised of a number of technologies. Among these will be PVs, fuel cells, wind turbines, and induction and synchronous generators. Those processes providing power through inverters will not have the same transient performance as synchronous generators. Since the transient performance determines the protection profile, differing protection packages are required for different generation processes. Protection packages are intended to respond to the abnormal condition. Normal conditions are handled by the process controls. It is not difficult to design a system to operate under normal conditions. The difficulty comes in designing the system to provide for normal operation and respond to the transient situations that arise. For example, when the DR operates in parallel with the EPS, controlling its output flow of power is readily achieved. Adjusting for increases and decreases in power flow as a function of normal variations in available fuel, (e.g., as the angle of the sun's rays varies the output of a PV system) is relatively easy. Determining what happened and initiating the appropriate response to a transient condition is another issue.

When a fault occurs on the distribution grid to which the DR is connected, the protection scheme is required to identify the nature, location, and proper action to take. Taking the case of a fault on a distribution circuit, if it is a single-phase fault (a power pole knocked down by a car, a power line broken due to ice loading, a tree blown over in a storm knocking the lines down, etc.), the response must be to isolate the affected circuit to provide continuity of service to the surviving circuits. A fault on a circuit appears as both a voltage reduction and current increase. The distinguishing features that differentiate the event as a transient or normal occurrence are the magnitude and quantity of changes. The protection scheme must be able to make the distinction.

The various generation processes will have different responses to transient conditions. The output of an inverter will not provide as much initial fault current as it would were it equipped with a battery on its DC bus. The output of an induction generator will not provide as high a contribution to fault current magnitude and duration as will that of a synchronous generator. As the present draft of the proposed standard indicates, it is necessary to take the measure of the DR in stiffness and contribution at the point of connection to the EPS to develop an appropriate protective scheme. The outcome of the final design is to leave the EPS system no less reliable nor lower in power quality than what existed before the DR was integrated. While the draft as it now stands is still only a work-in-progress, it is headed towards becoming a standard that will be a useful guide in defining interconnect requirements for DR.

References

Hirst, E. and Kirby, B., Creating Competitive Markets for Ancillary Services. ORNC/CON-448, Oak Ridge National Laboratory, Oak Ridge, TN, 1997.

Oklahoma Electric Company, Guidelines — Operating, Metering and Protective Relaying for Interconnection of Cogenerator, Small Power Producers, and Other Non-Company Sources of Generation to the OG&E System, 1993.

National Standard of Canada, Wind Energy Conversion systems (WECS) — Interconnection to the Electric Utility, CAN/CSA — F418-M91, January 1991.

NY State Department of Public Service, New York State Standardized Interconnection Requirements, Application Process, and Contract for New Distributed Generators, 300 kVA or Less, Connected in Parallel with Radial Distribution Lines, staff proposal, July 1999.

Sandia National Laboratories, Quarterly Highlights of Sandia's Photovoltaics Program, vol. 3, 1998.

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