How Does Electricity Flow Through a Utility-Scale Solar Site?

The utility-scale solar segment installed 7.6 GWdc in Q2 2024 – a whopping 59% jump from last year, according to SEIA’s latest Solar Market Insight Report. If you’re one of the newer folks joining in the industry to help get solar projects online, welcome! It may be helpful to dive into something fundamental to getting more solar energy onto the grid and that’s how electricity actually flows through these massive PV systems.

Understanding this process is crucial for solar professionals regardless of their role in the industry. In this excerpt from Utility-Scale Solar PV Fundamentals, HeatSpring instructor Andy Nyce breaks down the journey from photons in sunlight all the way to electricity flowing through high-voltage transmission lines.

Want to dive deeper? Check out Andy’s Utility-Scale Solar Essentials course bundle. It’s packed with foundational concepts and practical insights that’ll give you a serious edge in the utility-scale solar segment.

I wanted to take the time to make sure you all totally understood the concept of electrical flow through the PV plant. The following slides show one specific setup in regards to power conversion and electrical flow.

Firstly, sunlight strikes the PV modules. The sunlight is converted into electricity within the PV module. 

As we know from previous slides, the electricity that’s generated is DC electricity – direct current. Modules are wired together in strings. Remember that a string is just the technical name for a group of modules all wired together in series. 

This string of PV modules is basically collecting all the electrical current. And all of this collected up electricity is then transported via a home run of DC cabling to a combiner box as shown.  

You now have the electricity from multiple strings all feeding into a single combiner box, where additional electrical protection, such as fuses, will be located.  

The combiner box is pretty much doing exactly what it says – combining or consolidating all this electricity. There’s a few obvious benefits here. The DC feeders are higher gauge cables, so they’ll have reduced electrical losses. And since each of them can carry more current, there’s simply less of them, which makes for easier management and less terminations once we get to the inverter stage. 

The DC electricity then needs to be transported from the combiner box to the next key component in line – the inverters – shown here. This is achieved using what are commonly known as DC feeders. There’s a number of ways to do this, either in underground trenching or above ground cable trays. 

Once the DC electricity has been transferred to the inverter, the inverter does its job, which is to convert this DC electricity into AC electricity.  Remember that the key reason we need to do this is that the electricity on the utility grid is high voltage AC, and we have to match that.  

Now we have AC electricity, which has been produced by the inverters, and we need to transport this electricity to the next step in the chain, which is the transformer. 

Now we’re at the transformer level, and the transformer’s job is to step the voltage of the AC electricity up. The reason for this is that as we should remember back from our power and voltage slides much earlier in the presentation, and our discussion around Ohm’s Law – resistance equals voltage divided by current. 

Now, remember that power can be calculated by multiplying voltage by current, but Ohm’s law states that voltage can also be expressed by current multiplied by resistance. This means that you can calculate power by squaring your current and multiplying it by the resistance. Since the resistance of the cabling from the transformer to the interconnect won’t change, as that’s a function of the material that’s being used, to minimize the power losses in the system, we want to minimize the current as much as possible. 

The only way to do that, again, going back to Ohm’s law, is to increase the voltage. So, step the voltage up, reduce the current required, and reduce the power losses in the system. 

We then need to transport this AC electricity from the location of the transformer out to the point of interconnect. This is done via AC cabling and is commonly done via underground trenching.  The point of interconnect, or POI, is simply where the power plant ties into the utility grid. 

Now, once you’re at the point of interconnect, depending on the size of the plant or the transmission lines that you’re tying into, there may be another transformer to step the voltage up again to match what’s on the existing lines. So you can go from 34.5 kV up to 69 kV or even higher. 

And there you have it. You’ve then got high voltage AC electricity onto the utility lines. So, what started as sunlight hitting an individual PV module, which created low voltage DC electricity, has gone through all these various steps and conversions, through home runs, combiner boxes, DC feeders, inverters, transformers, AC cabling, and is then connected into the utility lines as high voltage AC.