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Technical Aspects of Interconnection
Technical Aspects of Interconnection
A community renewable resource may be made up of a conglomerate of smaller scale installations that are built at different sites but marketed as a package to take advantage of some economies of scale. While smaller installations lose economies of scale in terms of energy and capacity production, the interconnection process can be much simpler. Furthermore, individual installations may be able to take advantage of net metering opportunities.
The relatively small size of small-scale projects makes the process of connecting them to the utility grid simple in many cases. However, while the technical aspects may seem straightforward, particularly to an experienced contractor, the procedural differences between utilities, states, and interconnection type can be confusing.
Technically, small-scale projects are the easiest to interconnect safely to
the grid. Any reputable dealer or contractor will provide customers with
a system design that meets regulations.
System parts that are directly related to interconnection include:
- the DC to AC power inverter
- disconnect switches (often a DC disconnect switch between the generator and inverter and an additional lockable AC disconnect switch)
- distribution panel (a.k.a. circuit breaker box)
- meter (includes the building’s electric service meter and possibly an additional production-side meter).
A grid-tied system that includes battery backup will have additional interconnection components.
The system will include a UL 1741 certified inverter (meaning that it has been tested to meet the Institute of Electrical and Electronic Engineers IEEE 929-2000, recommended practice for safe utility interface of generating systems), and disconnect switches will meet NEC 690 (the National Electrical Code’s Article 690 on solar photovoltaic systems published by the National Fire Protection Association).
The IEEE standards for the inverter, along with system design components such as a lockable disconnect switch, are necessary to prevent “Islanding.” Islanding refers to a situation where the grid power is down and a customer’s generator is still on, creating the potential for power to feed back into the grid. This would cause an unsafe situation for linesmen working on an otherwise non-electrified portion of the power grid. Owners of grid-tied systems should know that their system’s anti-islanding design also prevents them from having power on-site when the grid goes down. A battery backup system, with additional controls for isolating the flow of power from the grid, is needed in order to use power on-site while the grid is down.
Once the power produced from the generator has been converted into AC electricity, it may need to be metered before used to serve any load. Many inverters include metering equipment and displays so that system owners can track the amount of power produced at any given moment, or over certain amounts of time. Some incentive programs that make payments based on power produced (or associated green tags) may require additional production-side metering. A utility-grade meter (one that conforms to ANSI standards) is sometimes required, and can be added at a fairly low cost. Check inverter specs to see if the internal metering equipment meets the standards before making an additional purchase. Examples of programs that require a production meter include Washington State’s Renewable Energy Production Incentive.
