By: Adam Baker
SCADA systems can help prevent curtailment, overheated inverters, and tracker malfunctions.
When solar downtime occurs, the last thing owners/operators want to be is in the dark. But I can’t tell you how many times I’ve seen energy loss onsite and thought: This was totally preventable. If only they had installed (or correctly configured) a controls and monitoring system for their solar site.
QF sites less than 5MW rarely implement controls. After all, they’re not required. However, it may be in the owner’s best interest to put one in anyway. I want to talk about five specific scenarios of downtime or energy loss that I’ve personally seen that are completely avoidable through the use of a good SCADA system.
Of course, there are a few downtime scenarios controls can’t fix. For example, when an inverter shuts down because it detects an overvoltage due to a lightning strike. Or, when an inverter detects a ground fault. Those are just the unavoidable risks of owning a utility-scale solar plant, and fortunately don’t happen frequently.
But, for PV downtime issues that are actually preventable, the data collected and analyzed by your controls and monitoring system can help significantly minimize downtime and unnecessary energy loss.
Avoid Utility Discrete Curtailment Through Analog Curtailment
A common reason your site might encounter downtime is due to congestion on the distribution system. If there isn’t enough demand, the utility will open your site’s recloser.
I first ran into this problem in southern California. Imperial Irrigation District (IID), the utility, provides power to the patch of dirt southeast of San Diego. Their grid is full of solar and wind, but hardly any thermal generation. Providing power to farm land means their load is very light. In early spring and late fall, their excess of renewables and lack of load means certain curtailment. The load is so low that they remotely trip off sites up to 20MW on a daily basis.
If the utility has a congestion issue, they think they only have two options. Keep your site on, or cut access. In reality, there’s a third option…if you have the controls for it.
Obviously, it’s in your best interest to deliver some energy (over none). With active power curtailment configured in your site’s control system, you can work with the utility to keep your site running with a reduced output. All you need is the utility’s set point or parameter they use in their curtailment logic.
FYI - smaller utilities, especially co-ops, are much more likely to be on board with this idea than giant utilities, because giant utilities don’t typically have a set point for small sites. Rather, they have enough large units to be able to shed 20 5MW sites and keep the turbines spinning.
If you leave it up to the utility to decide if they want your facility online or not, you have no say in the matter. However, if you have the ability to run at less than full input, they are less likely to trip you offline altogether. After all, a generator that can ramp up quickly is better to have ready than starting up a whole spinning generator from idle.
In fact, in the California example I shared above, IID would shut gas plants off before the solar sites that had variable output. The sites that have the most flexibility stayed on the longest.
Monitor & Alarm On Inverter Internal Temperature
Another common reason an inverter may completely stop is due to a high internal temperature condition. This is a major downtime issue that will send an entire inverter offline and drag output well below acceptable levels.
The good news is, it’s completely preventable with a simple monitoring solution. A decent SCADA monitoring system is able to correlate inverter internal temperature variance with external temperature to determine if there are maintenance activities that would be required.
A clogged air filter, for example. If the air filter is clogged in an air-cooled inverter, the inverter will have reduced air flow and the internal temperature will rise above normal levels. This condition can be created quickly if a nearby farmer is turning a field, or if landscaping is mowing vegetation onsite.
Not only can a SCADA system detect that issue, but its alarming functionality can also warn the operator and help determine when maintenance activities should occur.
Monitor Max Current Load From Tracker Actuators
This situation is something I like to call soft downtime. It’s still negatively affecting energy output, but is less dramatic than curtailment and offline inverter issues.
Say a tracker actuator stops working. The good news is, for at least most the day, you’ll still make energy. The bad news is, you’re not running at optimal. If just one tracker goes bad, you might still be within a normal output range. If several defects affect multiple trackers, you might wind up with a big problem.
Use a monitoring system to gauge the max current load each tracker actuator draws over the course of the day. By trending each tracker’s normalized, max daily current over time, the data will visually indicate when problems start to develop.
If the tracker is binding, or if there is resistance to turning, the tracker with the highest current load will be the first to fail. If the same device is having max current every day, and/or the max current of one tracker is significantly higher than the rest, there’s a maintenance opportunity to avoid a soft downtime event in the making.
There are multiple reasons this could happen. It may be caused by vegetation binding the tracker, bad bearings, or someone leaving a piece of equipment under the tracker. The point is, you now have the data that pinpoints where the problem is so you can send an O&M tech to investigate.
Track Average Current Per String to Detect Blown DC Fuses
Blown DC fuses are yet another less dramatic “soft downtime” issue, but are still very prevalent on PV sites. A blown DC fuse will cause part or all of combiner box to stop delivering energy.
Because they are at such a minute level, blown DC fuses are bit harder to detect.
Rather than tracking the raw value of DC current from a combiner box, I recommend tracking the average current per string from the combiner box. This is what is known as normalized DC data.
This process is very similar to what we do for Solar String Analysis.
If you lose one harness from a combiner box with 20 inputs, you’ll notice the average current of one input will be 5% less than those around it. At this point, you can send someone to examine the fuses inside a combiner box.
A blown fuse is the most likely, but not only, cause. Your issue could also be caused by a bad MC4 connector, cracked cells, or even shading.
Detect Losses From Soiling or Shading
Speaking of shading…a lot of energy loss on solar sites (especially in the Southeast) is due to a completely preventable cause: vegetation.
If your solar site is in the Arizona desert, this is less likely a problem for you. But right now, it’s spring in North Carolina…and the vegetation is growing like mad. I drove by two sites just yesterday with grass so high, the blades are shading the bottom of the modules.
Another related and completely avoidable issue is soiling. When farmers till their fields, or when trees release pollen with a vengeance, layers of particles on your modules can affect energy output.
Luckily, with a controls and monitoring system, you should be able to detect those lower energy output levels. By measuring average current per string, and normalizing the data, your SCADA system should show you how live string data relates to the average output of any given day.
With probable cause (e.g., knowing it’s springtime, knowing vegetation will grow unevenly), you should be able to determine when and where you need vegetation management, or washed panels.
Take Control of Energy Loss
Solar SCADA may seem an unnecessary expense at sites smaller than 5MW, but can actually play a crucial part in mitigating short and long-term energy loss. Any utility-scale site over 1MW should be able to get their money’s worth from the valuable downtime and energy loss data their SCADA system provides.
Need a no-obligation SCADA consultation for your solar site?
Adam Baker is Senior Sales Executive at Affinity Energy with responsibility for providing subject matter expertise in utility-scale solar plant controls, instrumentation, and data acquisition. With 23 years of experience in automation and control, Adam’s previous companies include Rockwell Automation (Allen-Bradley), First Solar, DEPCOM Power, and GE Fanuc Automation.
Adam was instrumental in the development and deployment of three of the largest PV solar power plants in the United States, including 550 MW Topaz Solar in California, 290 MW Agua Caliente Solar in Arizona, and 550 MW Desert Sunlight in the Mojave Desert.
After a 6-year stint in controls design and architecture for the PV solar market, Adam joined Affinity Energy in 2016 and returned to sales leadership, where he has spent most of his career. Adam has a B.S. in Electrical Engineering from the University of Massachusetts, and has been active in environmental and good food movements for several years.