# What is a Single-Line Diagram?

Brit Heller

The single line diagram, or SLD sheet, is an essential component of the electrical drawing set for a utility scale solar power plant. It’s a simplified schematic diagram that illustrates the overall electrical system from the panels all the way out to the point of interconnection.

Tune into this excerpt from Understanding Utility Scale Solar Construction Drawings taught by Andy Nyce to learn about the very basics of single-line diagrams. You can watch the video or read the transcript below. To dive deeper into the details of solar construction drawings, enroll in Understanding Utility Scale Solar Construction Drawings today!

This sheet includes information on the major components of the solar power plant, including the main substation, inverters, switchgear, transformers, and other electrical equipment. It shows how the equipment is interconnected and the direction of power flow. The SLD sheet also provides details on the electrical distribution system, such as the number and size of feeders, and the locations of panels, junction boxes, and other devices. The sheet may also include information on protective devices, such as fuses and circuit breakers.

Here’s the single line diagram from our sample set.  Now, on first glance, this can look a little overwhelming, but let’s take a closer look and I think you’ll start to see that it’s actually fairly straightforward.

Let’s start here at the solar panels. As you’ll remember from our electrical flow lessons, electricity gets generated at the panels, is transported to the combiner boxes via DC home runs,  then to the inverter via DC feeders, then from the inverter to the point of interconnection via AC feeders.

That’s exactly what this diagram is showing. From the panels, electricity flows via home runs in a cable tray to the combiner boxes here. From the combiner boxes, electricity continues through to the inverter via electrical fuses for safety purposes.

Now, it’s worth noting here that everything within this dashed line is a component that’s on the PCS, or power conversion station, and that tells us that all these parts and pieces are located at the same location on the site.

Now that the conversion of electricity has occurred from DC to AC at the inverter, it’s on to the step-up transformer. This is where the voltage is stepped up from 1500 volts all the way up to 15,000 volts or 15 kV.

Then, onwards to the AC disconnect switchgear.  And then from there all the way out to the point of interconnection.

The remainder of the details shown here on the single line is actually showing the electrical lines required for the various tracker controls, sensors, and communication boxes.  All are obviously critical, but I think the more important piece of the puzzle to grasp is being able to connect the dots and see how the flow of electricity moves through the system, from the panels all the way out to the utility lines.

There’s plenty of other useful information on this sheet as well. With module specifications, inverter specifications, and additional electrical data – all called out in these tables.

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#### Brit Heller

Director of Program Management @ HeatSpring. Brit holds two NABCEP certifications - Photovoltaic Installation Professional (PVIP) and Photovoltaic Technical Sales (PVTS). When she isn’t immersed in training, Brit is a budding regenerative farmer just outside of Atlanta where she is developing a 17-acre farm rooted in permaculture principles. She can be found building soil health, cultivating edible & medicinal plants, caring for her animals or building functional art.