A utility-interactive photovoltaic (PV) system is defined in Section 690.2 of the National Electrical Code (NEC) (NFPA 70-2005) as a “photovoltaic system that operates in parallel with and may deliver power to an electrical production and distribution network.”
NEC Article 690 covers Solar Photovoltaic Systems. Utility-interactive PV systems—also commonly referred to as grid-connected PV systems—allow bi-directional power flow to and from the building through the building’s service entrance.
The Public Utilities Regulatory Act of 1978 (PURPA) requires utilities to purchase power from independent power producers such as a building owner with a PV system at the utility’s avoided cost. PURPA provided an incentive for developing distributed generation (DG) resources while interconnecting those DG resources with the utility grid.
However, utility-required metering and relaying requirements for interconnection were usually geared toward industrial cogeneration and were impractical for small residential and commercial PV installations. This has changed in recent years with the development of new standards for small-scale utility-interactive PV systems, advances in PV-system technology, changes to the NEC and changes in utility regulatory requirements.
Power quality is an important concern for utility-interactive PV systems because PV system operating characteristics can have an impact on the utility-distribution system operation and affect other nearby utility customers. The inverter is the key to power quality because it transforms the direct-current (DC) power produced by the PV array into alternating-current (AC) power that matches utility power requirements.
The Institute of Electrical and Electronics Engineers (IEEE) publishes the IEEE Recommended Practice for Utility Interface of Photovoltaic (PV) Systems (IEEE Std 929-2000). This standard provides power-quality guidelines for PV system voltage, flicker, frequency and distortion. A companion standard to IEEE Std 929-200 is IEEE Standard for Interconnecting Distributed Resources with Electric Power Systems (IEEE Std 1547-2003), which addresses inverters for all types of DG sources including PV and has identical inverter performance requirements.
The power-quality requirements and the test procedure for inverters contained in IEEE Standard 929 serves as the basis for Underwriters Laboratories’ (UL) testing standard UL 1741. Inverters used in utility-interactive PV systems should be UL-listed and meet the serving utility’s power-quality requirements if they are more stringent than UL 1741.
Safety is also a major concern for utilities when customers have utility-interactive PV systems. When utility power is lost to the residential or commercial building, the PV system may be producing power that will be fed back into the utility-distribution system and pose a danger to unsuspecting linemen working to restore power.
To prevent back feeding PV-generated power into the utility-distribution system during an outage, PV inverters listed for use on utility-interactive PV systems are designed to automatically disconnect from the utility system when an outage or other abnormality is detected. This feature is often referred to as “islanding protection.” A PV inverter with this feature is referred to as a “nonislanding inverter.”
Both IEEE Standards 929 and 1547 require islanding protection for utility-interactive PV systems and provide testing procedures that are incorporated into UL 1741. NEC Section 690.60 requires inverters used in utility-interactive PV systems to be listed for the purpose and NEC Section 690.61 requires that “an interactive photovoltaic system shall automatically de-energize its output to the connected electrical production and distribution network upon loss of voltage in that system and shall remain in that state until the electrical production and distribution network voltage has been restored.”
Therefore, the NEC requires islanding protection for grid-connected PV systems.
As noted above, a grid-connected PV system offers building owners the opportunity to sell surplus power to the utility. Metering can be complicated, but many states and utilities are moving toward net metering that is simpler and more cost-effective than other metering methods.
With net metering, the building owner pays for the power delivered to the building by the utility less any PV power supplied to the utility as recorded by the meter. To find out if net metering is available in a particular location, contact the serving utility, the state commission governing utility operations or the Database of State Incentives for Renewable Energy (DSIRE) at www.dsireusa.org. EC
GLAVINICH is an associate professor in the Department of Civil, Environmental and Architectural Engineering at The University of Kansas and is a frequent instructor for NECA’s Management Education Institute. He can be reached at 785.864.3435 or firstname.lastname@example.org.