Can We Land?

Means and Methods of PV System Utility Interconnection

When evaluating a site or plan set for the installation of a utility-interactive photovoltaic system, one of the most important steps is to determine how much solar power you can land on a breaker or tap into an existing or planned electrical system. The result of this site evaluation may ultimately limit the size of the PV system that can be cost effectively installed. The evaluation needs to be performed with care to avoid costly mitigation measures or a disappointed client. In this article we discuss methods for determining how much solar power can be safely and legally fed into an existing electrical infrastructure. We begin with identifying and evaluating the existing electrical system, for both residential and commercial PV systems, continue to Code compliance and methods of interconnection, and conclude with strategies for connecting to a utility grid.

SITE EVALUATION

Here we focus only on identifying and evaluating the existing electrical system during a site evaluation, with particular emphasis on existing facilities. If you have an electrical plan set in front of you for a new facility, you are looking for the same general information described. Once the electrical service is evaluated, you can determine the point of interconnection and the method of interconnecting your PV system to the utility grid. Take plenty of photographs during the site evaluation. Pictures of the electrical panel and labels may prove useful later. (Be sure to turn the flash off and the “macro” setting on when photographing metallic labels.)

Electrical service type. In the US we have centralized power generating facilities, mostly coal fired, natural gas and nuclear powered, charging independent grids at high voltages. Transmission voltages are usually 110 kV to 500 kV. Localized substations transform the high voltage feed down to less than 35 kV. Common medium voltages, as described by IEEE, are between 1 kV to 35 kV. The distribution voltage will vary, depending on the client, location and utility provider.

After the utility substation, pad- or pole-mounted transformers located close to businesses and homes knock the voltage down further. It is helpful to know the common voltages provided by the local utility when performing a site evaluation, especially when electrical equipment is not properly labeled. (For more information on electrical service types, see the sidebar in “From kW to MW: System Design Considerations,” October/November 2008, SolarPro magazine.)

Utility-interactive inverters produce the most common voltages in both single-phase and 3-phase configurations: 120, 208, 240, 277 and 480 Vac. In general, 277/480 V 3-phase is used for larger commercial and industrial buildings; 120/208 V 3-phase is used for smaller commercial and large, multi-unit residential buildings; and 120/240 V single-phase is used for residential buildings; but other service configurations are possible. From these common voltages, ac transformers match less common voltages and accommodate electrical transmission over distances.

Utility transformer. The first piece of equipment between the customer and the medium voltage utility grid is the utility transformer. The transformer transfers electrical energy from one circuit to another using magnetic fields. Transformers have electrical and environmental ranges and limitations. They must be clearly and permanently labeled with their electrical characteristics. Utility transformers can be oil-filled or dry, and they come in many sizes, shapes and designs. Most often a utility transformer will be oil-filled and will either be pole mounted near the closest major road or pad mounted in various locations. The key utility transformer characteristics to record at a site inspection are primary and secondary voltages, phase type and power rating in kilovolt-amps (kVA). Also note the transformer number, as marked by the utility, and the approximate location of the transformer on a site map.

Several customers may share residential and small commercial utility transformers. Existing utility transformers may not be able to handle multiple systems installed on any of the shared client’s meters, particularly when loads are very low and PV production is very high. The customer who installs a PV system that exceeds the capacity of the shared transformer will likely be charged for its replacement. This should be handled prior to installation, or it may go unnoticed until the transformer fails and causes a localized outage. Though larger commercial sites generally have dedicated utility transformers, they still have limits. In general, the sum of the PV inverter nameplate ratings should not exceed the kVA rating of the transformer.

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