Islands & topology
The key idea
An island is a group of connected elements with no electrical path to the rest of the network. One open switch can create one. An island that contains no source cannot carry any load — and no solver can pretend otherwise.
The idea
Behind every network diagram is a simpler structure: which elements are connected to which. This is the network's topology, and it changes every time a switch changes state.
Close a switch: two groups merge into one. Open a switch: one group may split into two. Each separate group is an island. On a healthy single-supply network there is exactly one island, and the source is in it.
The trouble starts when a switch state in the model does not match the intent. A normally-open point recorded as open when the scenario needs it closed, or the reverse — and suddenly three buses have no path to any source. They are an island without a supply.
A power flow cannot solve that island. Its equations need a slack bus to balance them, and the island has none. The solver reports non-convergence or an island error — and the fix is a switch, not a setting.
Try it
Open SW1 to strand two buses. Then close the ring tie and feed them from the other direction.
islands: 1 · all buses energized
One island, everything energized. Now open SW1 and watch the right half of the feeder strand.
Why it matters
- It is the first thing to check when a solve fails. An island with no slack is the most common cause of "does not converge" — and it takes seconds to see on a topology view.
- Switch states are model data. The same diagram with one different switch state is a different network. Scenarios that change the running arrangement must carry their switch states with them.
- Islands are not always errors. A microgrid running isolated, or a section deliberately split for maintenance, is a legitimate island — it just needs its own source, and its own slack, to be solvable.
See it in Phasor
Phasor's Graph Space strips away the drawing and shows the network as pure connectivity: every island is separated, labeled and listed with its members — including whether it contains a source. It is the fastest explanation for a power flow that will not converge.