11423 Utility Protection and Control Guidelines and Requirements

Most utilities have published guidelines and requirements for operating, metering, and protective relaying for the interconnection of small power generators to the utility network. The fundamental requirements of utilities for small generators (less than about 200 kVA), assuming that the ratio of installed generation capacity to minimum load requirements on the feeder is 10% or less and including protection and control requirements, can be summarized as follows:

• The power supplied must be 60Hz alternating current.

• Basic designs must meet applicable minimum electrical standards as adopted by, but not limited to, national, state, and local governing bodies. This includes the National Electrical Code (NEC) and others.

• A manual and lockable disconnecting device must be installed at the point of interconnection in series with a protective fuse and a fused disconnect.

• A line voltage relay/contactors must be installed to disconnect the generator from the de-energized feeder and to prevent its reconnection until the line is re-energized by the utility. Undervoltage, overvoltage, underfrequency, and overfrequency sensors must be installed and connected to the relay. This is the anti-islanding requirement.

• All reactive power requirements for induction generators or power inverters are supplied by the utility. This is intended to reduce the possibility of self-excited operation in the event that the feeder is de-energized.

Many utility guidelines were developed with the expectation that few interconnected generators would be installed on a given feeder and that the penetration, in terms of the ratio of the capacity of the generators to the minimum load demand on the feeder, would be very small (typically less than 10%). Therefore, it was assumed that there would be no appreciable effect on relays and breakers designed and set up for current flow in one direction only, and also no effect on the overall stability of the feeder. Furthermore, where penetration was small it was assumed that, should the feeder be isolated from the network for any reason, the distributed resources could not, under any circumstance, continue to supply the load, the voltage would collapse, and the distributed generators would shut down automatically (on an undervoltage trip).

However, the most comprehensive utility guidelines recognize the need for more protective devices and other requirements as the penetration increases. These guidelines, along with standards already in place and being developed, form a very good basis for protection and control specifications for distributed resources. See, for example, the proposed New York State standardized interconnection requirement (N.Y. State Department of Public Service, 1999) and the Oklahoma Gas and Electric interconnection guidelines (Oklahoma Electric Company).

For many utilities, distributed resource installations, particularly small PV and small wind power generators, are already relatively common, and substantial operating experience has been gained. Nevertheless, many owners of these smaller distributed generators have found that obtaining the necessary interconnection approvals can be costly. In general, this cost barrier still exists for smaller generators. Furthermore, standardization of requirements is not yet an accomplished fact. The wind power industry has been a leader in establishing standards for the interconnection and operation of distributed wind power generators (National Standard of Canada, 1991). Many of the standards were already in place in the early 1980s.

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