What is a system impact study in the interconnection queue process?

If you've got a project sitting in an interconnection queue, you've probably already heard the term thrown around in a transmission provider's status update without much explanation. A system impact study, often shortened to SIS, is the stage where the transmission owner or ISO actually runs power flow and short circuit models to see what your project does to the grid once it's energized at your proposed point of interconnection.

It's not a feasibility check. By the time a project reaches the SIS, the queue manager has already done a lighter first pass (feasibility study, where one exists) to flag anything obviously disqualifying. The SIS is the real engineering: thermal overloads on nearby lines, voltage stability, short circuit duty on breakers, and whether your project trips protection settings that were never sized for this much new generation on that bus.

Where it sits in the queue stages

Most queues run through a sequence that looks something like this, though naming varies by ISO:

Feasibility study (optional, lighter scope)
Cluster study or system impact study
Facilities study
Interconnection agreement

A handful of regions, including several that adopted FERC Order 2023 reforms, have folded feasibility and system impact work into a single cluster study that processes a whole batch of projects together instead of one at a time. If your project is in a cluster, the SIS output you get back covers your project alongside everyone else who entered that same cycle, and the network upgrades assigned to you depend partly on what else is in the batch and in what order.

Cluster study versus system impact study

These two terms get used almost interchangeably now, and the confusion is fair. A cluster study is the batching mechanism, a group of projects studied together on the same timeline. A system impact study is the technical analysis performed inside that process. Under the old serial queue model, each project got its own standalone SIS. Under cluster rules, the SIS still happens, just for forty projects at once instead of one, which is part of why restudies and affected systems notices have gotten slower in queues that converted recently.

What comes out the other side

The SIS report tells you two things that matter for underwriting: what network upgrades the transmission system needs to accommodate your project, and roughly what those upgrades will cost and who's on the hook for them. That cost allocation is often the number that kills a project, not the technical finding itself. A project can pass every technical test and still die because its assigned share of a transmission upgrade runs into eight figures.

After the SIS clears, the project moves to a facilities study, which is narrower. The facilities study prices out the specific interconnection facilities, meters, relays, the tie-in equipment, at your point of interconnection, separate from the broader network upgrades the SIS identified. Once that's done and the interconnection agreement is signed, you're through the queue on paper, even if construction of upgrades is years out.

Why the distance to the POI matters before any of this starts

None of these studies happen in a vacuum. The transmission owner sizes your project's impact partly against how far your proposed point of interconnection sits from existing substation capacity and what corridor your tie-line follows across the terrain and rights-of-way in between. Developers who walk into a queue application with a real route and distance figure, checked against current substation and transmission corridor imagery rather than a GIS extract from three years ago, go into the SIS with fewer surprises about which upgrades are coming back assigned to them. That's the gap Grid Proximity Mapping closes, with an annual distance-and-route layer pulled from current high-res satellite imagery for every candidate site you're screening.

If queue risk is part of how you're ranking sites right now, it's worth seeing what a real connection distance looks like before the SIS comes back.