Among the many small factors which contribute in small ways to the profitability of a solar plant, monitoring and control of photovoltaic systems is one of the more important; it is required for reliable functioning, and for ensuring the maximum yield of any solar electric system.
The simplest monitoring involves simply checking the inverter, and this is done by reading values on the display, which is typically the LCD part of almost each grid-connected inverter.
The variables which can be monitored include PV array power, AC (grid) power, PV array current etc. For sophisticated monitoring and control purposes, environmental data, such as module temperature, ambient temperature, solar radiation, wind speed etc., can also be logged in data loggers, stored, and later analyzed.
Remote control and monitoring can be performed by various remote connections, such as ISDN, GSM, analog modem etc. More important inverter or grid related parameters can hence be made available on-line.
RS232 is commonly deployed for local monitoring. Local PC can be connected to an RS232 connection, for a remote display located in the living room. The connection between inverter and display can usually be made wireless. In some cases, USB connection is also available.
The following parameters are usually monitored:
Array voltage – VDC (V)
Grid voltage – VAC (V)
Array current – IDC (A)
Grid (injected) current – IAC (A)
Array power – PDC (W)
Grid (injected) power – PAC (W)
Module temperature – Tmodule (°C)
Ambient temperature – Tamb (°C)
Solar radiation – G (W/m2)
Daily/monthly solar insolation – H (J/m2)
Wind speed – v
Remote monitoring & communication
Remote control can be realized with RS485 connection or via power-line connection through a grid. Distances up to 1200 meters are easy. Several inverters can be monitored at the same time.
For remote monitoring, different ways of communication, Ethernet, Internet, dial up access, GSM etc., can be used. The system can also send alerts and status messages, by SMS service, GSM, fax machine etc., to the control center or user.
Data can be stored in inverters memory or in external units (data loggers). Different solutions are available on the market. Temperature sensors are usually PT100 or PT1000 sensors. Solar radiation is measured with reference solar cells (Si-mono), analog inputs of the control equipment are usually standard 4-20 mA or 0-10 V inputs. Digital inputs are also available and can be used for net-meter/control equipment interconnection.
PVresources, the source of this article, contains more about PV monitoring systems