PV monitoring
Since the profitability of photovoltaic installations depends mainly on them being operational, it is essential to ensure that they are permanently functional. The best way of ensuring this is to acquire a monitoring system for the installation. This system should notify all faults immediately and be capable of detecting drifts in output.
Types of monitoring
Several types of monitoring are available for installations:
- Systems which communicate with the inverters and are able to monitor all electrical values relating to output from the installation as well as the condition of the inverters
- Systems without communication protocols for the inverters but fitted with measurement inputs capable of monitoring photovoltaic output. Some systems can include monitoring on the DC side of the inverter having resolution to the combiner box level. Most at least monitor the AC side of the inverter through a kilowatt meter at or very near the point of common connection.
- Hybrid systems supplementing information from the inverters with measurements which are external to the installation such as solar radiation, temperature and wind speed. Information on the modules at the output of the inverters can only be correlated across the installation as a whole by systems capable of measuring solar radiation. Indeed, since output forecasts generally rely on meteorological statistics, it is quite difficult to interpret output data without correlating them to actual solar radiation. An abnormally low level of output may be caused by:
- A low level of solar radiation over a certain period of time (and which is abnormal based on meteorological statistics)
- A problem with the modules (clogging, shade, connection fault, etc.)
- A functional problem with one or more of the inverters
It is only possible to identify these faults by equipping the installation with solar radiation and temperature sensors and comparing the output capacity with actual output.
Depending on the size of the installation, individual monitoring or monitoring by group of photovoltaic module strings may be possible in order to detect abnormal variations in output between strings.
Monitoring systems
These systems may be autonomous or include remote monitoring.
- Autonomous systems (see Fig. P18)
Once data is collected locally, the system sends alerts directly to the maintenance operators as soon as they are generated.
Fig. P18: Example of an autonomous monitoring system
- With remote monitoring (see Fig. P19)
Once the data is collected locally, the system sends output data and alerts as soon as they are generated to a remote monitoring system capable of managing stand-by periods for maintenance work. This enables the installation to be monitored closely, which is essential for multi-site installations or where operators of photovoltaic installations are not necessarily the site occupants.
Fig. P19: Example of a system for remote monitoring
Sensors
Sensors provide data to the monitoring systems and include:
- A sensor for measuring instantaneous luminous flux such as a pyranometer (heat flow sensor used to measure the quantity of solar energy in natural light (W/m2), see Fig. P20). This is the standard reference for the installation. It may be used to identify shifts over time and is recommended to all suppliers wishing to conduct comparative analyses and compile statistics for their installations. At least one sensor is needed at the location however, it is not uncommon for at least two to be installed: one in the global horizontal (GHoz.) position and one in the plan of the array (POA) assuming the system is installed at some angle other than horizontal.
Fig. P20: Pyranometer – Kipp & Zonen
- A temperature sensor – this is an important factor for photovoltaic power supply. This sensor either serves as an external probe, is attached to the back of a module or both.
- A kilowatt hour meter - more often that not this meter is of a "revenue grade" <+/-2% tolerance. This is especially important for companies that engaged in power purchase agreements (PPA) which rely on accurate data for billing purposes.
In the United States of America, the owner of the photovoltaic system may often install their own revenue grade meter to bill the purchaser. At times, the purchaser may also install a meter to verify the output or demand the seller's meter to be calibrated regularly. In other markets it may be the case when selling power, only the kilowatt hour meter operated by the energy distributor purchasing the electricity may be used as a reference.
The other meters fitted within an installation (in the inverter or next to the official meter) are only indicators with their own specific levels of accuracy. Variations of more than 10% may occur between the values given by an installation’s devices and that given by the official meter. However, these variations are not only due to different levels of accuracy. They are also caused by energy lost in the cables and safety devices downstream from the inverter.
It is therefore important to use cables of minimal length and clearly identify:
- The location where the installation will be connected to the network
- The locations where the energy distributor’s meters will be connected
Monitoring the installation
Since modules are expensive and in some cases openly accessible, sites need to be monitored by security cameras.
NB – although this type of surveillance is authorised for private sites, filming of public highways is prohibited.
