Commercial PV System Data Monitoring, Part One: Page 6 of 11
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Finally, a third arrangement is a peer-based network using TCP/IP and CAT 5 or CAT 6 cables. This system is installed much like a local area network for computers. However, each device would be an energy meter, inverter, combiner box, datalogger and so on.
Wireless devices could replace any or all of the cables in these sample networks and generally run their own bus drivers and protocols to simulate or replace those used on hardwired connections.
In conversation, Ethernet and Internet are sometimes used interchangeably, like PV panel and module, but they are not the same thing. An Ethernet network is any network built with TCP infrastructure in mind. The Internet is the collection of computers, servers and sites that we call the web. The ubiquitous “www” refers to an earlier nomenclature: World Wide Web.
While it is possible to create an Ethernet network that is separate from the Internet, the Internet cannot exist without Ethernet networks. An example of just such a separate network is a Modbus TCP network in a utility-scale PV plant. This network uses Ethernet devices—cables, connectors and switches—but uses Modbus at the application layer instead of HTTP or other protocols.
Communication cables. Generally speaking, communication cables use one or more twisted pairs of stranded (7x30 or 7x32) small-gauge wires, usually 18 to 24 AWG. Using twistedpair cable minimizes radiated and conducted electromagnetic interference (EMI).
Different bus driver standards allow for longer or shorter transmission distances. RS-485, the most commonly used bus driver, is specified to transmit data for up to 4,000 feet. This distance is achieved in part by the standard itself, which uses positive and negative voltages for 1s and 0s, as well as the EMI resistance offered by the cable type. RS-232 specifies positive voltages for 1s and no voltage for 0s. A bus driver that uses RS-232 is inherently more sensitive to line noise and interference. Even with the same EMI resistance, this specification allows connection only up to 50 feet.
CAT 5, CAT 5e and CAT 6 cables—which consist of four twisted pairs of wires and terminate with RJ-45 connectors— were created to carry larger amounts of data. While additional carrying capacity is needed as networks become more complex, CAT 5 or CAT 5e cables are sufficient for the transfer rate needed for most PV monitoring applications. The bus driver behind TCP/IP-based communication provides for a transmission distance of only 300 feet between networked devices. Devices like routers, switches and hubs may be used as repeater stations to increase the distance between devices.
Like other types of electrical cable, the wires in a data monitoring system are used to carry electricity. In this case, voltage and current signals are used to communicate data within the monitoring network. Longer cable runs reduce the force or amplitude of data monitoring signals in a process similar to voltage drop in power conductors. This gradual loss of intensity is known as attenuation.
The reason that distance limits are defined for each bus driver has to do with the ability of devices to differentiate between 1s and 0s as the signal is dissipated across the cable. Most communication protocols have error detection routines that identify data corrupted by signal attenuation, but this only forces the master device to request the same data multiple times. In most situations, it is best to keep cable runs as short as possible and to minimize the amount of EMI-producing equipment in the vicinity.
On a final note, be aware that cable manufacturers may have multiple cables with the same basic specifications—for instance, one twisted pair, 18 AWG, shielded, with drain—that are differentiated by insulation properties like oil resistance, burial rating, UV resistance and so forth. As when choosing power conductors, make sure that you are selecting communication cables with the proper environmental resistance properties for your application. Installing indoor-rated cable in a buried conduit results in a malfunctioning monitoring system in short order.
Datalogger. A datalogger, also called a data acquisition unit, is an electronic digital processing device that resides in the physical layer. This device records data over time and has internal memory for data storage. Some dataloggers can also function as analog-to-digital signal converters or Internet gateways, or have other capabilities.
In a network, a “master” device initiates communication between the “slave” devices and itself. In a PV monitoring system, the master device is usually the datalogger. Slave devices, which all have unique network addresses, may include inverters, weather station equipment, energy meters of revenue grade or lower accuracy, building load or net-energy meters and so on.