At the heart of any photovoltaic (PV) system is the inverter, also referred to as the power conversion unit (PCU) or power conversion system (PCS). Its primary function is to convert the direct current (DC) output of the PV array to alternating current (AC) that can be used to power conventional building loads.
This means that the inverter must take the DC power provided by the PV array—which varies with sky conditions, time of day, season, temperature—and provide sufficient AC power of the right voltage and frequency to drive the load.
In addition, unless the building loads served by the PV system are isolated from the rest of the building distribution system or the building is off the grid and powered solely by PV and other alternative energy sources, the inverter must be capable of interfacing with the utility system.
Further, if the PV system is a standalone system with batteries or a hybrid system with both a utility connection and batteries, the inverter must be able to charge the batteries when there is excess PV power being produced. Today’s PV inverters are very sophisticated and electrical contractors who install PV need to understand their features and operation.
Determining the inverter output parameters is usually the starting point for selecting an inverter for a particular application. The inverter continuous power output rating is typically the first output parameter to be determined.
The inverter’s continuous output power is determined by either the potential output of the PV array given the available PV area, planned installation and geographical location or the load that the system will serve if less than the site’s PV potential. Given the planned PV system AC output power and the characteristics of the building service and distribution system, the output voltage and number of phases can be determined.
Article 690 of the 2006 National Electrical Code (NEC) addresses the installation of PV systems. NEC 690.13 requires that a disconnecting means be provided to disconnect the inverter from the building distribution system and NEC 690.17 provides the requirements for this disconnecting means.
The need for this disconnect can either be satisfied by a stand-alone disconnecting device at the inverter output or a disconnecting means such as a circuit breaker that is integral to the inverter. Selecting an inverter with an integral disconnecting means eliminates the need to supply and install a separate disconnecting means.
The inverter DC input parameters will determine the size and circuiting of the PV array given a particular PV module. The maximum inverter DC input voltage adjusted for ambient temperature per NEC Table 690.7 determines the maximum number of PV modules that can be connected in a series string based on the PV module’s rated open-circuit voltage.
Similarly, the maximum inverter DC input current adjusted per NEC 690.8 determines the maximum number of PV module strings that can be installed in parallel based on the PV module’s rated short circuit current.
DC ground-fault protection (GFP) is required for all residential roof-mounted PV arrays per NEC 690.5. However, DC GFP can be installed on any PV system to provide additional protection from fire hazard. If DC GFP is provided, the PV system equipment-grounding conductor size can be reduced in accordance with NEC 690.45.
Like the PV system disconnect, DC GFP is usually either an integral part of an inverter or offered as an option by the manufacturer. Having the DC GFP as an integral part of the PV inverter precludes the need to procure and install a separate DC GFP device. Inverters that are connected to the utility grid through the building’s service entrance are required to shut down automatically upon loss of utility-supplied power per NEC 690.61.
This inverter feature is referred to as “anti-islanding” and its purpose is to protect unsuspecting electricians, linemen and others from voltage that is back fed into the building and utility-distribution system during an outage when troubleshooting or repairing the distributions system. Any PV inverter that is designed to be utility interactive and UL 1741 listed will meet the requirements of NEC 690.61.
Underwriters Laboratories (UL) publishes UL 1741 entitled, “Inverters, Converters, Controllers and Interconnection System Equipment for Use with Distributed Energy Resources,” which provides manufacturing and testing requirements for PV inverters.
In addition to anti-islanding, UL 1741 addresses the construction, performance and other safety features of PV inverters. Any PV inverter installed should be tested and listed in accordance with the latest edition of UL 1741. 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.