4613Protection Requirements

Protection systems are required to protect the local distribution utility's distribution system and service. The protection of the generation system's equipment is the responsibility of the owner/operator of the distributed generation. This guideline does not purport to provide appropriate protection for either the generator or distribution system, but identifies minimal areas of protection that are needed to provide safe and reliable operation. The local distribution utility will not assume any responsibility for protection of the generator or of any other portion of the generation system's equipment for any operating conditions. The generation system owner/operator is fully responsible for protecting distributed generation equipment in such a manner that faults or other disturbances on the utility system do not cause damage to the generation system equipment. The generation system must be protected in such a way that ensures the separation of the generation source from the faulted area of the electrical distribution system. The protection system shall be designed so that the generator shall not energize a de-energized circuit owned by the utility. The owner/operator of the generation system is solely responsible for properly synchronizing the distribution generator with the utility. Protection requirements differ with the technology.

The owner/operator of the distributed generation shall provide a "standard" one-line diagram depicting the electrical system under normal and contingency operations. The one line diagram should include, at a minimum, all major electrical equipment that is pertinent for understanding the normal and contingency operation of the generation system including generators, switches, circuit breakers, fuses, protective relays, and instrument transformers. The diagram should include transformer connection configuration (i.e., Delta, wye, and grounding) where a transformer is required.

A document displaying protection threshold and coordination studies shall also be provided to the host utility on request. At the host utility's option, it may specify settings for protective relays in the interconnection scheme.

For packaged generating equipment where the protective devices are part of a manufactured assembly that has been certified (including necessary relay settings) for use by the utility, each subsequent installation of the manufactured assembly shall be deemed certified for use by the utility.

Many utility distribution systems use breaker-reclosing schemes in order to provide a high degree of reliability to the distribution system. The schemes may use instantaneous reclosing, timed reclosing, or some combination of the two. To safeguard against any misoperation where reclosing is used, isolation within 8-10 cycles may be required. In other cases, isolation will be allowed to occur over a longer period to avoid excessive operations and/or sympathetic trips for faults on feeders. In most cases, the utility will not be able to remove reclosing schemes.

Basic functionality would require that the generator be equipped with adequate protection to trip the unit off-line during abnormal system conditions. For units greater than two megawatts in parallel with the utility, this functionality should include but not be limited to single phasing of utility supply, system faults, equipment failures, abnormal voltage, or frequency.

Some common, basic protective schemes required at the interconnection interface might include the following (numbers in parentheses represent device numbers used to perform the stated function):

• Undervoltage (27) and overvoltage (59) protection

• Frequency (81) protection for over- and underfrequency sensing

• Synchronizing relay (25)

• Phase and ground over-current relays (50/51 and 50/51N)

• Ground overvoltage (59N) for delta transformer connection interfaces

• Phase directional overcurrent relays (67) (if not selling to utility)

• Devices required to initiate transfer tripping utility transfer trip protection

Protection may be obtainable through components of integrated products (hardware and/or software) that meet independent testing facility standards and production line testing. The relays listed here may not always be the best solution. New technologies may achieve the required outcome more efficiently.

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