By: Adam Baker
Why DC design balance is crucial to solar power plant output.
I wanted to talk a bit about module mismatch, and how it affects energy and power production at a solar plant. Module mismatch isn’t super obvious for those who don’t design and build module string configurations for solar power plants. It’s difficult, but very important, to appreciate why things are put together the way they are.
One of these things is not like the other
Normally, strings of modules are wired together in a series. Let’s say we have 20 325 watt modules in a string. 20x325 is 6,500 watts of power from this particular string.
What if I replace one of our 325 watt modules with a 300 watt module? Unlike you may think, the difference here isn’t just a 25 watt loss in power. The 300 watt module actually brings all the other modules down to its level.
Now we have 20 modules x 300 watts, or 6,000 watts. That’s a 500 watt loss, even though there’s only a 25 watt difference in just one module in the entire string!
This is why it’s so important to keep modules of the same bin class together in strings. One little tiny error will have a significant impact in the amount of power that comes out.
Adding more modules may actually produce less power
Often, extra modules are added onto the end of strings. Owners and site designers might think: “I have a bunch of strings with 19 modules, but on this particular row I have more room. I’m going to tack on another module, to make a total of 20 modules on this string.” Unfortunately, I see this a lot.
Here’s what happens.
Everything comes together at the combiner box. The inverter will force the voltage to the point in which the maximum amount of power comes out of the combiner box. It’s usually in the neighborhood of 750 volts. Each string will contribute between 5 and 6 amps.
If you add a couple extra modules to the end of the string, the inverter will try to push the voltage for that string up, but the combiner box overall will not allow it. So it actually forces the current to go lower, and the voltage is forced higher. This means the area under the max power curve is smaller.
A smaller curve means the total power produced is actually less than if those extra modules hadn’t been tacked on to the string in the first place.
So, adding another 325 watt module to your 6,500 watt string of 20 325 watt modules won’t increase the power by 325 watts. It might actually DECREASE the output from that string.
Module mismatch isn’t the only reason for dramatic underperformance
Module mismatch from 325 to 300 to 325 is a rather dramatic difference from one to the next. However, it’s not the only reason for significant underperformance. Underperforming modules could result from causes as small as a cracked module, hydraulic fluid that got left behind in the construction process, or a soiling condition.
The reason it can be so dramatic? Just how module mismatch brings all the surrounding modules down to the lowest performer’s level, the condition (soiling, MC4 connection issues, etc.) affects the entire string in the same manner.
One final thought about looking at normalized average current per string from each combiner box. If you have a very small difference from one to the next, the difference may not be super impactful as you look at it.
However, these tiny differences will add up. Something as small as one wrongly-sized module or cracked module will have an effect on the total combiner box output. Why? That one small change is trying to move everything in that combiner box around on its maximum power point.
These issues are hard to troubleshoot and detect with a normal SCADA system because you’re only looking at small differences of 195 amps or 200 amps at a time. Data normalization is really the only way to see very small differences at a macro level. Unless, of course, you put individual instruments on each individual string to try and quantify exactly what their current contribution is back to the combiner box.
All these examples show why it’s critically important to get all modules in balance in the DC design of the power plant.
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.