Commercial PV System Data Monitoring, Part One: Page 5 of 11
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Basic Monitoring Concepts and Components
PV system designers, integrators and installers are rarely IT experts. However, it is important that they understand basic IT concepts and can identify the major components in a data monitoring system. PV system installers are often expected to install and supply power to data monitoring system components, as well as to source and install the necessary conduit, cable and connectors.
To the extent that data monitoring is part of the scope of work, it needs to be included in the plan set—and not just as a separate, vendor-supplied, single-line item. Ideally, PV system designers call out the conduits and receptacles that the data monitoring system requires within the electrical plan set for the PV system itself. After all, this is what the installers are working from.
To ensure successful execution in the field, the best practice is for PV system integrators to coordinate or partner with a monitoring system provider during the project planning stage. The concepts and components detailed in the following pages assist PV system designers with this process. Note that all communications systems should comply with Chapter 8 of the NEC.
Physical layer. The physical layer of a data monitoring system includes all of the hardware: sensors, meters, conduit, cable, loggers, wireless transmitters and receivers, combiner boxes, inverters and so on. The physical layer is what is generally represented on the plan sets and is the basis for connecting all of the necessary components of the monitoring system together. (The relationship between the physical layer of a data monitoring system and the programming layers is described in the sidebar above.)
Bus driver. A bus driver is a method of data transmission that defines how voltages or currents on a serial communication bus should be interpreted. Examples of bus driver standards are RS-485, RS-232, and RS-422.
Protocols. A protocol is a program language that electronic devices use to transmit data to one another. One common example is the Modbus protocol, published by Modicon in 1979. Modbus is one of the most widely used protocols in the US because it is an “open” protocol. An open protocol is one that manufacturers can build their equipment to use without paying royalties or license fees to the publisher. TCP/IP (transmission control protocol/Internet protocol) and DNP3 (distributed network protocol 3.0) are also widespread. Proprietary protocols come in a variety of flavors from various equipment manufacturers.
Ideally all devices in your monitoring system network use the same bus drivers and communication protocols. This is not always necessary, however: some dataloggers are capable of speaking more than one protocol and may even be able to do so simultaneously. When incompatibilities arise—generally this occurs with proprietary protocols—translation devices, or protocol converters, can be deployed.
“Putting in a little bit of time on the front end will save it tenfold in the field,” advises Whitley of Southern Energy Management. “Monitoring systems have so many options and possible configurations, and the installation manuals often leave something to be desired,” he warns. “If all the components are not sourced from one supplier, confirm that they are all using compatible protocols and have worked together on other sites. Otherwise, you run the risk of wasting time later chasing phantom alarms.”
Network. A monitoring network can be connected in a variety of ways—point to point, daisy chain or peer to peer— depending on the bus driver and protocols used. The simplest network consists of two devices connected directly to each other. The devices could be connected by a two-pair twisted cable, such as Belden 9842 cable, and use the RS-232 bus driver to transmit data via the Modbus protocol from one device to the other.
Using a similar type of cable, Belden 9841 or 3601A, it is possible to connect several devices in a daisy chain. In this case, an RS-485 bus driver can be used with the Modbus protocol to collect data from all the devices. Data is collected at a single point, usually a dedicated special-purpose datalogger.