Commercial PV System Data Monitoring, Part Two: Page 3 of 6
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Internet access. Since monitoring systems are generally cloud-based, Internet access is essential. If a hardwired Internet connection is used, coordination with the host facility’s IT team is a prerequisite. Communication is key, relates Chuck Wright, principal at PowerDash: “If the customers’ Ethernet is to be used, verify their understanding that it is their responsibility to keep the communication channel open for a period of years, even decades, and that they agree to do so.” Important questions to ask relate to the requirement for static versus dynamic IP settings, firewall setup for inbound traffic and other general network security or compatibility issues. The goal is to gather as much network data as possible so that the monitoring company can preconfigure the datalogger to automatically communicate with the network right out of the box.
If the site is remote or access to the existing network is prohibited due to network security systems and firewalls, a cell modem must be included with the monitoring system to push the data onto the Internet. If cellular communication is necessary, be sure to determine whether there is coverage during the site survey. In some rare cases, a satellite modem must be included. Note that adding a cell modem does not just add a onetime cost for the hardware component; a cell service contract must be included and renewed as long as the system is to be monitored.
According to Solectria Renewables CTO Michael Zuercher-Martinson: “Solectria Renewables recently added a 3G cellular network communication option to its monitoring product line, which has quickly turned into a best seller. Not having to deal with the local IT infrastructure and associated firewalls and not having to run additional communication conduits in many cases pays for the cost of the cell modem and the long-term service contract.”
Power supply. The power requirements for each monitoring package are different. A critical part of the site analysis is identifying available power sources for dataloggers, energy meters, smart combiner boxes and other equipment. While it is possible to purchase small self-contained and self-powered dataloggers and weather stations, they are generally more expensive than tapping into a preexisting load-branch circuit or installing a new load-branch circuit.
In systems covered by the NEC, ac power cannot be pulled directly from the inverter output, and a different circuit must be used. The power source does not have to be a dedicated circuit: Power can typically be pulled from an existing load-branch circuit, if one is available. In the event that there are no existing branch circuits in the vicinity, be sure to size any new ac branch circuit conductors for voltage drop when making long-distance runs to combiner boxes. In systems with inverter output direct to medium voltage, installation of an additional small transformer might be necessary to provide 120 Vac, which can also be used to power on-site service receptacles.
In addition, verify that the monitoring components receive the type of power they require. Smart combiners and data acquisition units might actually require a 24 Vdc power supply. In some cases, a 120 Vac/24 Vdc power supply is integrated into the component enclosure; however, sometimes an external power supply must be provided. The location of power supplies must be incorporated into the total system design and the conductors must be sized for voltage drop.
Note that while NEC Section 300.3(C)(1) allows ac and dc circuits rated 600 volts or less to occupy the same equipment wiring enclosure, cable or raceway, Section 690.4(B) specifies that PV system circuits “shall not be contained in the same raceway, cable tray, cable, outlet box, junction box or similar fitting as conductors, feeders or branch circuits of other non-PV systems, unless the different systems are separated by a partition.”
Depending upon your interpretation—or that of your engineer or the inspector for the AHJ—power system conductors for the PV data monitoring system may be considered non-PV system conductors. If they are considered PV system conductors, then they can share conduits with other PV system circuits; if not, then they cannot. Of course, this is relevant only for the power system conductors.
Communication circuits. Low-voltage monitoring equipment has its own requirements, as described in NEC Chapter 8, “Communications Systems.” All communication cables in PV data monitoring systems carry low-voltage signals, typically under 10 volts. The insulation rating on most communication cables is limited to 150 Vac. Check with the manufacturer or the NEC for specific cable insulation ratings. Cable insulation must be appropriate for the installation conditions—burial rated, UV resistant and so on.
As a result of this low insulation rating, communication cables cannot share conduits, raceways or enclosures with high-voltage power conductors—either ac or dc—unless proper physical separation is provided. Conduits carrying communication cables can share trenches, provided that adequate separation of the conduits is maintained. The additional conduit, wire and installation cost for running lowvoltage communication cables must be considered during system design and construction planning.