Generator C.B.


Fig. 28-3 Example of Connection for Parallel Operation with Utility System Not Capable of Standby Operation.

Detailed discussion and negotiation is often required between the local generator and the utility to reach agreement on the components and operation of an interconnected system.

The standard approach to operational difficulty by the utility or an independent system operator (ISO) is simply to drop a generator off line as soon as any problem is detected. Except during peak periods, this is easy for the utility system, due to massive redundancy. In contrast, the on-site generator's goal is to maintain uninterrupted operation and, in some cases, to fulfill power exporting obligations or to avoid backup charges. For this reason, it is often difficult to reach agreement on the required relays and where relay trip limits should be set. A lengthy negotiating process must sometimes be endured to satisfy the needs of both parties.

On-site generators that are large relative to the capacity of the utility feeder present more complex coordination issues and likely will require a more extensive protection scheme. The construction of the relays is also often a contested area. Some utilities require electro-mechanical rather than more modern (and less maintenance intensive) solid-state type. The cost of these more sophisticated relay schemes usually must be absorbed by the local generating facility and can be prohibitive, particularly for small systems of a few MW or less.

Due to all of the protective devices required by utilities for interconnected systems, it is increasingly common for main facility breakers to open for no apparent reason. For example, voltage surges on the utility line are often sufficient to trip a voltage relay and disconnect a facility. From the utility or ISO perspective, this is considered normal operation. From the facility perspective, it is a problematic interruption that is often considered overprotection (and in some cases, regarded as harassment).

By far the greatest concern of all utilities is human safety, particularly for line workers who are exposed to possible electrocution if a power line is not locked out and dead. Strict operating procedures are in place to prevent such an occurrence, and interconnected generators must be included as part of the overall safety procedure to ensure that a facility does not accidentally re-energize a line while work is being done on it. Utilities often insist on the right to inspect an on-site generation facility to ensure that proper procedures are followed. The partnership between the utility or ISO and host facility is never more important than in cases involving line worker protection and other catastrophic potential.

The knowledge and experience of the utility or ISO line engineer is a good resource and should be considered as such by the local on-site generator. Legitimate concerns of the utility or ISO must also be respected and addressed with great care and responsibility. Conversely, the need to eliminate unnecessary power interruptions and facility investment in superfluous protection equipment must also be respected. For these reasons, circuit breaker systems must be carefully designed, extensively tested upon initial installation, and then periodically tested to ensure continued proper functioning.

Connecting with the Utility System Grid

For a self-generator to connect to an electric system served by the utility or ISO, several key factors, such as point of connection, grounding, protective relaying, and system isolation, must be addressed. Essential concerns include:

Utility Transformer

Utility Meter

\ Service Disconnect/ Utility Tie Circuit Breaker

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