Commercial PV System Data Monitoring, Part One: Page 8 of 11
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For a similar reason it is advisable to keep the cables connecting the weather sensors to the datalogger as short as possible to avoid the loss of signal strength.
Energy meters. In addition to solar irradiance and cell temperature, a third measurement is essential to array performance ratio calculations: delivered ac energy, which is recorded by the energy meter.
All revenue-grade energy meters conform to American National Standards Institute (ANSI) Standard C12.20 and are required to provide energy readings to within ± 0.25%. Energy meters collect data from current transducers (CTs) and may also have voltage transducers for high-voltage applications above 600 Vac. The energy meter processes the current and voltage information in a variety of ways and communicates it to the datalogger.
In its most basic form, an energy meter provides a continuous record of the kilowatt-hours that pass through the wires it is monitoring. More advanced meters can also provide additional information about the electrical system—power factor, reactive power, power quality, peak demand and so forth—based on whatever is of interest to the customer, the utility or the entity selling energy to the utility.
Most incentive programs now operating in North America require the use of revenue-grade meters to collect a record of the energy produced and a monitoring services provider to formally vet and report that data. All revenue-grade meters are calibrated against defined standards. The manufacturer or testing facility should be able to provide a calibration certificate from the National Institute of Standards and Technology (NIST), ANSI or an equivalent institution.
Some inverter manufacturers—such as Advanced Energy, Solectria Renewables and others—are now offering revenue-grade metering capabilities within their central inverters. Alternately, a revenue-grade meter can be part of the hardware package supplied by a third-party monitoring provider. Revenue-grade metering also offers the system owner and financers the ability to verify utility-provided metering and billing against a separate, independent dataset.
Note that many energy meters need an external power supply. Others draw power from the voltage connection they are reading. Consult the meter manufacturer’s documentation regarding power supply requirements, terminations and fusing.
Internet gateway. All monitoring services providers use a web-based interface to display, analyze and report data collected at the project site. To get the data from the project site to the Internet cloud, a gateway of some sort is required.
Collected data can be transmitted wirelessly via a cell phone or satellite modem or via a hardwired connection, such as CAT 5, cable, DSL, T-1 or fiber optic lines. Many utility-scale sites have no access to hardwired Internet connections, so cell modem reporting is generally the only option. This requires additional upfront hardware cost for the modem, plus ongoing monthly fees for cell service. However, the simplicity of the connection provided by a cell modem often offsets the time and trouble caused by negotiating with customers and their IT teams to get the necessary permissions established and maintained over the 25-year life of the project.
The frequency of data transmission varies by manufacturer— from about once per minute to once every 15 minutes. If Internet connectivity is temporarily unavailable, some dataloggers may store data in internal memory until reconnected. Dataloggers generally need a separate power source, which might output 120, 240, 277 or 480 Vac depending on the power supply configuration. Consult your vendor’s installation requirements to determine the needed power supply, terminations and fusing.
Web portal. A web portal functions as an access point for information on the Internet. While all currently available commercial and utility-scale PV data acquisition systems have some sort of web portal, their functionality and presentation format varies widely.
Multiple portal interfaces are common. For example, there may be an open-access customer portal in addition to a password-protected O&M portal. Commercial PV systems that serve educational or PR purposes may have yet another web portal.
Multiple portals are essential for utility-scale PV systems, which should have a portal for O&M that is separate from the owner portal. From the O&M portal, it is typically possible to view error alerts and manage work orders for all of the systems being monitored.