Comparing Low Voltage Vs. Medium Voltage Equipment on Solar Sites

When solar professionals transition from residential and commercial projects to utility-scale, one of the most obvious differences is in the electrical components and system architecture. While the fundamental principles of solar design remain consistent across project scales, the equipment itself changes dramatically as you step up from low voltage systems to medium voltage utility-scale projects.

This shift can be jarring at first for those making the leap, but understanding these equipment differences is crucial for anyone working in utility-scale solar. In this clip from the “Utility-Scale Solar Design Overview” class, instructor John Selby explains several key differences you can see when it comes to low voltage and medium voltage components. To learn more about the basics of utility-scale solar design, don’t miss enrolling for free in the “Utility-Scale Solar Design Overview” course! 

Transcript below.

 Let’s work our way backwards from the interconnection point and compare some equipment. 

First off, we have our system disconnect, which is almost without exception required by utilities for any distributed generation project, with the possible exception of some small residential projects. For low voltage projects, we typically have a blade disconnect. For utility-scale systems, especially overhead connected projects, we’ll see what’s called a GOAB or gang operated air brake switch. 

Next up, we have fuses, which can be a really helpful item to have for blanket protection from the utility, which often has a high available fault current. For low voltage projects, we typically have a fuse disconnect. For utility-scale systems, especially overhead connected projects, we might see a set of three fuse cutouts.

The next protective device might be a main circuit breaker, which can provide a bit more sensitive protection features than a fuse, and could also be connected to a shunt trip circuit to be opened by a remote input. For medium voltage systems, a recloser sort of takes the place of a main circuit breaker, but with a whole lot more bells and whistles. A pole-mounted recloser is separated into two sections – a switching unit at the top of the pole and a control relay usually mounted at ground level. The relay senses what’s happening in the switching unit and tells the switching unit to open or close according to the settings in the relay. Most low voltage circuit breakers are not set up to trip on voltage or frequency, but this is a standard function of a recloser. 

In almost every PV system nowadays, you’re going to have some kind of revenue metering. In the low voltage world, this could be a CT cabinet like the one shown here. For overhead medium voltage systems, you’re likely to see a primary metering rack – though a good percentage of projects still meter each subarray at the low voltage side, either in addition to or instead of a medium voltage. 

Transformers are not always used in commercial systems, but they’re also not totally uncommon either, especially when you encounter a 208 volt service, but you want to use a 480 volt string inverter for rooftop stringing efficiencies. If they’re used, they typically are dry type transformers, less than 300 kVA. One of the key differentiators of utility-scale projects – in addition to being connected in medium voltage – is that they use a step up transformer, so you’ll have one or more of these on every project. 

Lastly are inverters. While you might see 120 kW string inverter – like this one here – on a commercial project, utility-scale projects sometimes use single inverters that are 2 MWs or more and are often combined into skids of 4 to 6 MWs and paired with a transformer.

Fortunately, however, solar modules are still solar modules… kind of.