In our first article, we took a general overview of solar monitoring and why it is far more than just watching the energy production meter “spin” and counting dollars of revenue. Yes, production is important, but it is not the only metric we should be interested in as operators of large scale PV systems. In this article, let’s dig deeper into some of the components that can and should be monitored to ensure the operational and the financial health of the PV system.
This article was authored by Keith Davis, Director of Solar Services at
Affinity Automation
I think we all would agree the PV system inverters are the most acutely important pieces of hardware on the site. After all, they sit squarely in the critical production path of converting high voltage DC into a useable form of AC energy that the grid can absorb. A non-functioning inverter means no energy delivery which means no revenue from that portion of the array. And no one wants that to happen.
Inverters of all types are manufactured by many different firms, large and small, and each machine has its own strengths and weaknesses, advantages and disadvantages. The system designer must make good decisions to use the best machine to meet the electrical and operational challenges each site creates. And once they are installed and working, the system operator cannot just ignore them. On the contrary, a large portion of the site’s O&M effort is focused on the inverter due to the possibility of it becoming a single point of failure with huge consequences.
Therefore monitoring the inverter remotely makes it the very first item to connect a data logger to read and record how well the machine is running and to gauge its “health” by looking at certain operational parameters. While we tend to look at AC energy output as the most important metric, the DC voltage input levels, DC current flow, inverter component temperatures, power factor, grid voltages and ground fault currents are all critical as well. Each data point gives a savvy system operator great insight into how well the machine is performing beyond total AC energy output. Higher than normal currents and temperatures are disastrous for the long-term life of any inverter so watching them and setting maximum tolerances is vitally important to their survival over the many years a machine is expected to perform. Lower than expected voltages and current signify trouble in the array itself or blown overcurrent devices in the DC section, all with a loss of energy output and revenue. Keep a close eye on these machines first and foremost.
Depending on the owner’s desired level of granularity into the plant’s operation, there are several other critical factors that can be monitored and compared to an expected norm. On the DC side, DC feeder currents are often monitored at the re-combiner or DC disconnect level. Some think that monitoring string voltages and currents is important as well which yields a tremendous amount of data, so be careful what you ask for.
On the AC grid side, voltage, current, phase angle, kVARS, power factor and the like can all be monitored and recorded by the revenue grade meter in addition to production. Personally I believe monitoring the many transformers on the site is quite critical as well due to their long lead time if a replacement is needed. Many transformer manufacturers will install optional sensing devices that monitor transformer temperature, oil levels and pressures, which being within specifications is extremely critical to the device’s longevity and performance. How much better it would be to shut down the connected inverter(s) and the transformer itself well before a meltdown occurred? A good monitoring and control system can do this.
Environmental measurements are also quite important so the site’s overall expected performance can be compared to the reality of real time production. Good weather instruments are not inexpensive, but they are well worth the extra costs over the long-haul. It only takes a very minor configuration mismatch between the pyranometer and the DAS to invalidate irradiance readings or to skew the calculated performance quite widely. Ambient and back-of-module temperatures also need to be highly accurate and recorded as a part of the energy production calculation as well as wind speed and direction, so install and maintain good instruments to give the system owner confidence in the systems expectations.
When the system is running and performing well, monitoring is important. But what is more critical is knowing when something is wrong (or better yet–about to go wrong!) and what needs to be done about it. And there are levels of severity that a good monitoring and control system can provide to the site operator so their response is tailored to the situation. But there must be a well-designed and maintained monitoring system in place, fully functional and dependable in order to have 100% faith in what the system is reporting. I have heard on more than one occasion that the biggest system failure on many PV sites is the monitoring system itself, so go the extra distance upfront to include the right hardware and software to provide the level of data detail required with a 99.9% system availability. A tall order, I know, but this will be the requirement in the very near future as systems grown in size, cost and complexity.
In our next article we’ll look at the monitoring hardware itself and talk about communications protocols. We’ll also look at how these choices are critical to the correct information gathering and data logging of the site’s data stream, and then it’s export across the Web to the monitoring portal. So check back next month!