Last month’s article touched on photovoltaic (PV) arrays as a renewable power source. According to SolarBuzz.com, the installation cost of PV modules is still high ($4.31 per watt). The energy costs still exceed grid-power (ranging from $0.35 per kilowatt for residential to $0.20 per kilowatt for industrial), and the unobstructed sun area required is still large (10–15 square inches per watt). With government incentives defraying some of the costs, these finacial barriers have been coming down and will do so at a faster rate as the volume increases. Installations are popping up on residential, commercial and industrial roofs and even on utility poles.
PSE&G in New Jersey is installing 200,000 200W microinverter solar panels as part of its 80 megawatts overall “Solar 4 All” program. Each panel puts out 120 volts alternating current (V AC) nominal, is approximately 5-by-2.5 feet, weighs 60 pounds, and is attached 15–18 feet off the ground on utility poles and light poles. To prevent electrical shock during installation or utility power interruption, no voltage is outputted unless grid power is connected.
The installation and maintenance of PV arrays can pose some unique situations for the electrical contractor. There are several other configurations of PV panels, beside the aforementioned low-wattage microinverter type. Some units are used to power direct current (DC) loads directly, as the DC output voltage of PV panels is proportional to the solar intensity. Individual cells are combined in a series fashion to produce the required DC voltage. Strings of these cells are then combined in parallel to produce the required DC current. Large arrays of PV panels can be interconnected to produce megawatts of power. Hybrid units are used in conjunction with storage batteries and AC inverters to provide power even when there is no solar power source available.
Since the individual cells and even some panels are low-voltage DC, some people think they can be connected however they please. This results in unsafe wiring practices, such as the use of conductors of insufficient current capacity and insulation rating, lack of properly rated connectors, disconnect devices, and overcurrent protection. Too small of a gauge wire and connectors for the current flow will also result in losses that may compromise the required output voltage. The use of AC-rated rather than DC-rated devices is a problem, as the amp rating is not the same in most switches and breakers.
Many AC breakers rely on the voltage passing through zero twice per cycle to extinguish the arc drawn between the contacts by the fault current as the breaker trips. In DC devices, the interrupting capability cannot take advantage of this and must extinguish the arc in a different manner. This usually results in the DC voltage rating and the maximum fault current that the breaker is guaranteed to interrupt being much lower than the AC voltage and fault-current interrupt ratings. According to a white paper, “Democratizing Solar: The AC PV Module” on PetraSolar.com, another potential hazard is that “it is not uncommon in the United States for the DC voltage of PV string arrays to exceed 800V under no-load conditions.” Not only is this a potential hazard to the installer and occupants, it is a problem for firefighter’s conducting search and rescue, fire extinguishment and ventilation tasks on roofs with such panels.
There are regulations, standards and codes with regards to the manufacture and installation of PV systems, including the following, to which all installers must adhere:
• NFPA 70, National Electrical Code (NEC), Article 690—Solar Photovoltaic Systems
• UL Standard 1703, Flat-plate Photovoltaic Modules and Panels
• ANSI Uniform Solar Energy Code
• International Building Code and ASCE 7-05 Minimum Design Loads for Buildings and Other Structures
• IEEE 1547, Standard for Interconnecting Distributed Resources with Electric Power Systems
• UL Standard 1741, Standard for Inverters, Converters, Controllers and Interconnection System Equipment for Use With Distributed Energy Resources
Other relevant articles in the NEC applicable to PV systems include Article 110—Requirements for Electrical Installations, Article 250—Grounding, Article 300—Wiring Methods, Article 310—Conductors for General Wiring and Article 480—Storage Batteries.
On the power quality side, PV sources are similar to other nongrid-connected power sources (such as most backup diesel generators) in that their impedance tends to be higher than the grid. The source of the voltage comes from an unsteady-state source, namely the sun, whose output varies over the course of a day as it passes through more, then less, then more atmosphere, as well as atmospheric interference from smoke and clouds. Variations in loads may impact the voltage regulation. Depending on the regulation of the system, this can result in flicker. One manufacturer rates the output at tolerances wider than traditional limits of 90 to 110 percent of nominal found in most power quality standards and clearly beyond ANSI C84.1. And, since there is a DC-to-AC inverter, such as found in an uninterruptible power supply, there will be harmonics.
While the 40 MW of pole-mounted PV is just 0.3 percent of the generating capacity of PSE&G, it’s a step in the green direction, providing New Jersey with the second largest solar-generating capacity in the country and helping to drive down the costs of subsequent PV-panel projects.
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.