Utility-interactive PV systems are classified by IEEE Standard 929 as small, medium, or large (ANSI/IEEE, 1999). Small systems are less than 10 kW, medium systems range from 10 to 500 Kw, and large systems are larger than 500 kW.
Each size range requires different consideration for the utility interconnect. In addition to being able to offset utility peak power, the distributed nature of PV systems also results in the reduction of load on transmission and distribution lines.
Normally, utility-interactive systems do not incorporate any form of energy storage — they simply supply power to the grid when they are operating. In some instances, however, where grid power may not be as reliable as the user may desire, battery backup is incorporated to ensure uninterrupted power.
Since the output of PV modules is DC, it is necessary to convert the module output to AC before connecting it to the grid. This is done with an inverter, also known as a power conditioning unit (PCU).
Modern PCUs must meet the standards set by IEEE 929. If the PCU is connected on the customer side of the revenue meter, the PV system must meet the requirements of the National Electrical Code® (NEC®) (National Fire Protection Association, 1998). For a system to meet NEC requirements, it must consist of UL listed components. In particular, the PCU must be tested under UL 1741 (Underwriters Laboratories, 1997).
But UL 1741 has been set up to test for compliance with IEEE 929, so any PCU that passes the UL 1741 test is automatically qualified under the requirements of the NEC. Utility-interactive PCUs are generally pulse code modulated (PCM) units with nearly all NEC-required components, such as fusing of PV output circuits, DC and AC disconnects, and automatic utility disconnect in the event of loss of utility voltage.
They also often contain surge protectors on input and output, ground fault protection circuitry, and maximum power tracking circuitry to ensure that the PV array is loaded at its maximum power point. The PCUs act as current sources, synchronized by the utility voltage. Since the PCUs are electronic, they can sample the line voltage at a high rate and readily shut down under conditions of utility voltage or frequency as specified by IEEE 929.
The typical small utility-interactive system of a few kilowatts consists of an array of modules selected by either a total cost criterion or, perhaps, by an available roof area criterion. The modules are connected to produce an output voltage ranging from 48 V to 300 V, depending upon the DC input requirements of the PCU.
One or two PCUs are used to interface the PV output to the utility at 120 V or, perhaps, 120/240 V. The point of utility connection is typically the load side of a circuit breaker in the distribution panel of the occupancy if the PV system is connected on the customer side of the revenue meter.
Connections on the utility side of the meter will normally be with double lugs on the line side of the meter. Section 690 of the NEC provides the connection and installation requirements for systems connected on the customer side of the revenue meter. Utility-side interconnects are regulated by the local utility.
Since the cost of PCUs is essentially proportional to their power handling capability, to date there has been no particular economy of scale for PV system size. As a result, systems are often modular. One form of modularity is the AC module.
The AC module incorporates a small PCU (≈300 W) mounted on the module itself so the output of the module is 120 V AC. This simplifies the hook-up of the PV system, since NEC requirements for PV output circuits are avoided and only the requirements for PCU output circuits need to be met.
Medium- and large-scale utility-interactive systems differ from small-scale systems only in the possibility that the utility may require different interfacing conditions relating to power quality and/or conditions for disconnect. Since medium- and large-scale systems require more area than is typically available on the rooftop of a residential occupancy, they are more typically found either on commercial or industrial rooftops or, in the case of large systems, are typically ground-mounted.
Rooftop mounts are attractive since they require no additional space other than what is already available on the rooftop. The disadvantage is when roof repair is needed, the PV system may need to be temporarily removed and then reinstalled. Canopies for parking lots present attractive possibilities for large utility-interactive PV systems.
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