PV systems consist of different components to transfer energy. Measuring the electrical parameters at certain intervals can help gather more information about system operating status and alert users to possible problems. As we discussed earlier, measuring the output of the system is essential for production-based financial incentives offered by federal and local agencies.
The traditional monitoring method entails simply comparing actual energy generation to that predicted from the simulation software. The advantage of this approach is simplicity, affordability, and reliability. There are multiple levels at which a PV system can be monitored. Depending on system size and type, they can be classified as:
Inverter-level AC and DC monitoring offers insights into an inverter’s status, given the strategic location of the inverter to monitor the performance of the PV system. Nowadays, most inverter manufacturers embed their devices with monitoring functionality.
- Relatively low costs (for central inverter)
- Monitoring of DC power being fed into the inverter
- Monitoring the level of AC power being produced on the back end
- The efficiency of power inversion
- Limited level of resolution
- Information gathering, which must be done either manually on-site or via remote Ethernet link established through the inverter’s communications port, can be time consuming and labor intensive.
Going a level deeper into the system, array monitoring involves information from DC circuits located in various sections of a PV array.
- Provides an additional level of data with a relatively small upfront investment.
- It can isolate array level problems to a more specific array segment
- A single shaded or faulty panel is not easily recognized. Several panels will need to fail before a detecting a problem.
A little closer to the modules, string level monitoring narrows the focus even further to individual strings of modules.
- Failure or shading of one panel in a string is easily located
- Additional monitoring complexity
- Increases installation cost
Once we reach the module, Micro Inverter Level Monitoring is installed at the PV module level. They are more common in smaller systems than large commercial or utility scale.
- Provides more information about each individual module in the PV system.
- Not very cost competitive due to the huge number of inverters
An example of monitoring is shown in the video produced by the Northern Mid-Atlantic Solar Education and Resource Center, part of The Pennsylvania State University.
Video: Monitoring Methods for PV Systems (22:47)
SPEAKER: In this recording, we discuss the monitoring of PV Systems. Monitoring often involves data or sensor wires which are separate from the power wires, along with data recording or transmitting devices. So, we will refer to monitoring equipment as a monitoring subsystem of the PV Systems.
When we are monitoring the performance of a PV System, we would like a digital record of the performance of the system as measured at known times. The most important parameters accumulative electricity generated by the system. This can be compared to models that tell how much energy of the system should produce during some period of time to determine whether or not the system is operating properly.
Such measurements are useful even if they are only made monthly or less frequently. Another useful parameter is the AC output power of the system at a given time. This information can be evaluated in combination with weather measurements decide whether the system's output is what it should be under the existing conditions.
The DC voltage and current are also valuable performance characteristics. They can indicate shading or bad connections between or within modules in the array. The DC power coming in from the array, which is the product of the DC voltage and the DC current can be compared to the AC output power to give the efficiency of the inverter.
Monitoring subsystems also sometimes record the grid voltage and frequency. These values tell you how close the grid is to the values at which the inverter is programmed to shut itself down. Lastly, weather measurements are often recorded with PV system data with these measurements you can compare the PV system voltage and current to what they ought to be under the existing weather conditions.
Back when the PV industry was much less developed than it is now, around the year 2000, PV monitoring subsystems were made for specific PV systems by combining off-the-shelf sensors and monitors that were mainly sold for other applications. Measurements were stored in the data logger at some regular interval. Eventually, the measurements were uploaded to a computer every week or less frequently.
The computer might remain connected to the data logger or it might be carried to the PV site and connected to the data logger for each data upload. Some systems also stored measurements from weather sensors. One problem with this approach is that it is very difficult to measure safely the high DC voltages that are common in PV systems.
The currents may also be difficult to measure with the accuracy needed. A good measurement system requires a lot of careful design by someone who understood the PV system, as well as additional sensor wires and system components. But the measurements are also redundant, after all the most important measurements were already made inside the inverter.
These include the DC voltage and current which are needed by the maximum PowerPoint tracker. The inverter also needs to monitor the AC voltage and frequency set by the grid in order to turn off if either one is too high or too low. The inverters control operations, including those of the maximum PowerPoint tracker are all digital anyway.
So many of the important values already existed digitally inside the inverter. However, systems like this were built with data loggers and sensors that are connected to the inverter outputs, but otherwise operate independently of other parts of the PV system. Some are undoubtedly still in use.
As PV industry developed, inverters became available with digital connections. When connected to a data receiving device, the measurements from these inverters include values that the inverter needs to measure and digitize anyway and typically some others. Diagnostic information may be available from an inverter when it is not working and in some systems control information can be sent to the inverter over the digital connection.
The inverters digital interface to the outside world may be a standard part of the inverter or it may be an extra added in the factory or in the field. Most inverters have an LCD display on the front, which also shows performance data. The digital output includes the display data and perhaps other data as well.
The digital outputs of some inverters can be read by a computer with the standard connector and software from the inverter manufacturer. More often, the output is intended for a data logger that is also sold by the inverters manufacturer. It can typically read the output from several of the manufacturers inverters at once and also data from a weather monitoring station if there is one.
However, it cannot receive data from another manufacturers inverters. The newest and presently a very popular type of monitoring is by a Web Service. In this type of monitoring, data is sent from an inverter to the manufacturer's website. The system owner can view and download the data from the website using any internet connection.
The data may be password protected or it may be available to the public. The Web monitoring service may be included with the purchase of the inverter or it may be extra. The web interface may be built into the inverter or it may be a separate device.
This type of monitoring requires internet access at the PV site. In a home, the access is over the home owners internet connection using either a wired or Wi-Fi link to the homeowner's router. However, such access is not available in some places, especially for mobile PV system. Some monitoring subsystems can also send data over the cell telephone network.
Since the manufacturers have all of this data from their installed inverters, they can use it to diagnose problems, improve designs and perhaps for promotional or business purposes. Overall, there is no standard for PV monitoring subsystems. So now, we'll simply discuss a few of the systems that have been used for PVR system monitoring.
You should consider the available monitoring process when evaluating an inverter and it may be a factor in choosing between different inverters. This is a picture of the Inverter Independent monitoring subsystem that was made by a company called fat Spaniel. The company was a pioneer in using Web services to monitor PV systems.
It was named after a pet dog which appears in its logo. This product is no longer made and very little information is available about it. The product allowed an independent measure of a PV systems AC output, which was then sent by the communication gateway to the company's website.
These inputs apparently were to measure the DC voltage of the array. The communication gateway can also receive the digital output from a compatible utility grade electric meter as well as from inverters with digital outputs. Fat Spaniel was originally an independent company but it has now been purchased by the inverter manufacturer power one.
There are many PV systems that are monitored by Fat Spaniel equipment. This shows the information on a PV system on the township library in Springfield Pennsylvania. The graph shows the hour by hour production of the system.
In general, the output of the system increases towards midday and then decreases with some fluctuations due to weather even at its peak on this day the system was generating well under its rated capacity due to the weather. And at 3 o'clock daylight savings time, the power output was less than one fourth of the array is rated output. The website can show day by day generation where we will see a lot of fluctuation due to the weather. This month by month graph is much smoother.
We see here a decrease from July down to the minimum in December, when the winter solstice occurs. And then the output begins to increase again. This picture shows a Solectria 1800 watt inverter that PennState uses for some demonstrations.
It's an older product that does not have built in disconnects a DC disconnect and an AC disconnect are mounted next to it. The inverter has an RS232 output that can be connected by a cable to a computer or compatible data logger the inverter does not have any data storage except for its lifetime electricity generation. The data must be collected and stored by a separate unit as it is output by the inverter.
This is output from the inverter on August 10th, 2011 as collected by a laptop computer connected to the inverter. Since the laptop does not have an RS232 connection and RS232 to US peak adapter had to be used. The system was turning on at about 9:23 in the morning.
In the first column, we see the dates and times of the data records. One line of data is output by this inverter about every minute, although many of the parameters change much more quickly. The maximum PowerPoint tracker changes the DC voltage and current at least every few seconds, if not more frequently.
The timestamp is followed by the inverter identification and serial numbers. Here, in the first performance data column, we see the lifetime total electricity that has been generated by this inverter. In this case, 9.0 kilowatt hours have been generated during uses on previous days.
During the six minutes shown here, the lifetime generation went from 9.0 to 9.1 kilowatt hours. In the next column, we see the AC output of the inverter it may be an average value over some time interval to avoid showing rapid fluctuations. Over the six minutes of data shown here, the AC output increased from 916 watts to 957 watts.
The rated power of the array connected to the inverter was 1980 watts. The sky was clear but the array was pointing close to due south while the sun at this time of the day was well east of due south. Next column shows the AC voltage of this grid connected PV system.
208 volts is a common line to line voltage in a low voltage three phase system. The computer connection can be used to set this inverter for an AC voltage of either 208 volts or 240 volts. The next column shows the AC output current of the inverter.
The last that a column shows the DC voltage of the array. This is the maximum PowerPoint voltage as found by the maximum PowerPoint track. Note that it does not always go up, even when the output power goes up.
After the DC voltage is a column for other information. The demonstration on this day ran until about 1 o'clock in the afternoon with another line of data collected about every minute. By then, the lifetime generation was at 12.8 kilowatt hours.
So on those four hours the system generated about 3.8 kilowatt hours. Note that the DC voltage of the array went down because the array got hotter as the day progressed. This graph shows the AC output power as a function of time.
For the first few hours, the sky was clear and the output increased fairly smoothly as the position of the sun changed to be closer to due south where the array was pointed. Then, scattered clouds appeared in the sky. When a cloud covered the sun the system's output dropped to between 250 and 500 watts.
Between clouds, the output of the system was up near where it would have been if the sky had been clear. A few times, the inverter actually exceeded its rated output of 1800 watts. The recorded outputs were as high as 1,893 watts and there were several greater than 1,880 watts.
Collecting this data required keeping a laptop computer operating for the four hours that the inverter was operating that day this is not very practical for collecting data over a long period of time. A simpler data logger could also be used as long as it is able to collect the data in the format that the inverter outputs it. Solectria also sells a Datamonitor which can collect data from up the 16 Solectria inverters and transmit it through a router and the internet to the manufacturers server.
It can then be viewed and displayed from anywhere with an internet connection. The Datamonitor can also receive data from a revenue grade and meter and transmit it along with the inverter data. The company charges for the Web Service.
The inverter manufacturer Fronius, sells a PV monitoring subsystem shown in the schematic diagram. Multiple Fronius inverters can be connected to the same data logger using communications cables. Simultaneous weather measurements can also be recorded by connecting the weather transducers to a sensor box which is also part of the data network.
A communications card needs to be added to each inverter to allow it to communicate with the data logger, this card can be installed in the field or by the vendor. The data logger can store the information sent to it for a period ranging from several days to several weeks, depending on how many inverters or other devices are sending data to it and how often they're sending it. Data can be uploaded from the data logger using an RS232 connection to a computer.
As shown, that computer needs to be near the data logger. However, there are options for transmitting the RS232 signal for longer distances. For example, if there is an internet connection near the data logger than an RS232 internet interface can be used.
And then a computer anywhere else on the internet can communicate with the data logger. Newer versions of the Fronius data logger use an ethernet connection either directly to a computer or to a local area network. When the data logger is connected to a local area network, it can be read by any other computer on the network.
Fronius also has a Web Service, which can receive data from the new version of the data logger. Accessing data about a PV system from the company's website generally requires a password but the graphs above are available to the public. These graphs were available on March 20th, 2013 the upper graph shows the total electricity generation for each day of the current month.
The lower graph shows the output power for the current day and two previous days. The jagged lines are caused by fluctuations in cloud cover. It was mid-afternoon in Europe when these graphs were seen on the website, which is when the data ends.
The inverter manufacturer SMA has two different PV monitoring subsystems. The first provides a local display but is not linked to the internet, and the second sends the information on the PV array to a company Web Service but does not have a local display. SMA may also sell so whether sensor interface that sends weather data to the Web Service.
This is a picture of the SMA Sunny Beam. It communicates by Bluetooth to up to 12 inverters within its range and it can store data for at least 90 days. The display here is showing power output of 2.5 kilowatts and electricity generation for the day of 24 kilowatt hours and a lifetime electricity generation of 1,178 kilowatt hours by the inverter it is linked to. The SMA a Sunny WebBox also communicates by Bluetooth and can receive data from up to 50 inverters.
The WebBox then communicates by an ethernet connection either to a local computer or to a router connected to the internet. It also has local memory. A version for large PV systems has wired connections to the inverters.
This graph shows some of the data available from the companies monitoring website for a 6.11 kilowatt PV system in Bryn Mawr, Pennsylvania. The blue line is the output power of the inverter and the red line is accumulative electricity generated up to various times in the day. This graph is also available on the monitoring website and shows the electricity generated on each day during the month.
The variations were due to the weather on different days. Other graphs and information are also available about various PV systems on the company's website. The last monitoring subsystem we will discuss here is by the micro inverter manufacturer Enphase.
In a system using micro inverters, there is a micro inverter for every module. Each one needs to be monitored individually. A residential system typically has only 10 to 40 micro inverters, which is not a large number to communicate with.
However, large systems have over 100 micro inverters and communicating with all of them can be more of a challenge. The Enphase monitoring subsystem uses a data device called the Envoy. The Envoy communicates with the micro inverters over the AC power wires.
In a house, the Envoy can often be plugged into a convenient outlet, which it will then use for both power and communication. The unit also needs an ethernet connection to an internet router. This is a picture of the front of the Enphase Envoy.
The display shows the present AC power output of the system and also the lifetime electricity generated by the entire system. But the display is not able to show the data from individual micro inverters. A local computer can be connected to the convoy to read some system diagnostic information.
This includes system events such as a shut down for the night or a shutdown because the grid frequency or voltage is too high or too low. A local computer can also read the information on the front display. However, a local computer connected directly to the convoy cannot read data from individual micro inverters.
Thus, even though data from individual micro inverters pass through the Envoy to the company intranet service it can only be read over a connection to the company's website. This means that in places where an Internet connection is not available, it is not possible to get data on performance of individual micro inverters. This is a disadvantage of this system in some places.
An electrical filter is sometimes needed with the Envoy and micro inverters on one side and the grid on the other. One reason that such a filter might be needed is electrical noise and the power wires. This can be the situation in an industrial building with large electrical equipment.
Another reason might be interference from other nearby Envoy that are also communicating over the power lines. There may be multiple on voice and a large PV system using micro inverters. This is Enphases webpage about a system in Philadelphia.
The diagram shows the layout of the array. The odd shape is due to various obstructions on the roof. The little rectangles and the drawing represent individual PV modules and the color is related to the most recent output transmitted to the website.
The system has 444 micro inverters and a rated output of 81 kilowatts. When this image was taken, the power output was 19.3 kilowatts and mostly cloudy conditions. And the maximum power output for the day so far has been 22.7 kilowatts.
We also see the electricity generated so far in that day, week, month. And over the lifetime of the system. Various reports are also available but they require a password that the system owner has. Two issues are indicated, possibly underperforming micro inverters but access to this information also requires the password.
Various graphs can also be displayed. This one shows the system's output for the previous 24 hours. The image was taken in the morning so it shows the previous afternoon generation followed by nighttime with no generation and then output beginning again in the morning.
This graph shows a week of generation. We see that the system generated a lot more electricity on some of these days than others due to the weather. There were also some relatively large fluctuations within the days.
Public information on many other systems with Enphase micro inverters is available on the monitoring website. In summary, we have seen monitoring subsystems that are available from different inverter manufacturers. These subsystems are not compatible with inverters from other manufacturers. It is good to consider monitoring when choosing between different inverters.
Is micro inverter monitoring level suitable for utility scale PV systems?
ANSWER: No, due to the large number of modules. String and Inverter level are more feasible.