How a substation works, from the lines in to the feeders out
Power arrives at 115,000 volts and leaves at 12,470 on the wires along your road. In between, a ring of breakers, two transformers and a house full of relays keep one problem from becoming everyone's. Run a day through a model substation, set off a fault, and take a breaker out for work.
Right now
- Line A, 115 kV
- 16.0 MW in
- Line B, 115 kV
- 16.0 MW in
- Transformer 1
- 21.1 MVA (53% of 40) · oil 63°C, hot spot 74°C · fans off · tap +6
- Transformer 2
- 12.0 MVA (30% of 40) · oil 61°C, hot spot 65°C · fans off · tap +6
- Bus 1, 12.47 kV
- 122.6 V on a 120 V base (12.74 kV)
- Bus 2, 12.47 kV
- 124.2 V on a 120 V base (12.91 kV)
- Feeder 1, Route 101
- 11.2 MW · 2,100 customers · 117 V at the far end
- Feeder 2, Village
- 8.8 MW · 1,850 customers · 118 V at the far end
- Feeder 3, Mill Road
- 12.0 MW · 2,600 customers · 118 V at the far end
- Capacitor bank
- In, 3.6 MVAR
- Customers without power
- 0 of 6,550
SCADA event log
No events. Pick something to happen, or click a breaker in the drawing.
Four voltages between the grid and your kitchen.
Each step down happens at a transformer. The substation is the one in the middle, where the grid that crosses the state meets the wires that run past houses and businesses.
- 115,000 volts
Transmission
High voltage carries power long distances with little loss. Two lines bring it to this substation from the bulk grid.
- 12,470 volts
The substation
Transformers step it down, breakers protect every piece, and three feeders take it out along the roads.
- 7,200 volts
The feeder
Each wire on the poles is 7,200 volts to ground. Branch lines with fuses run off it down side streets.
- 240 and 120 volts
Your service
The can on the pole by your house steps it down one last time to what your outlets and panel use.
Any one piece can fail or come out for work.
The ring bus joins the two lines and the two transformers in a loop, with a breaker between each pair. Power can reach every transformer two ways round. When a line has a fault, the two breakers beside it open and the other line carries everything. When a breaker needs maintenance, it opens and the ring carries on as a string.
On the low side it works the other way. Each transformer feeds its own bus and the tie between them stays open, so a fault on one bus can't take down the other. If a transformer is lost, the tie closes and the other one picks up both.
The catch shows up on a hot afternoon. Each transformer is sized to carry most, but not all, of the peak by itself. Planners call it the N-1 test: lose any one piece and the rest still serve the load, if only for a few hours.
Relays decide in milliseconds.
- 0 ms
A fault: a limb, an animal, a lightning flashover. Current jumps to many times normal.
- 5 to 20 ms
The relay measures the current and voltage and decides which breakers have to open.
- 50 ms
The breaker's contacts part and the arc goes out at the next zero of the current. About three cycles.
- 0.5 to 2 s
On a feeder or a line, the reclosing relay tries again. Most faults have cleared by then.
- 15 to 30 s
If it trips again, another try or two. After the last one, it locks out and waits for a crew.
That's why your lights blink in a storm and come back: the blink was a reclose. Lights that blink three times and stay off mean the feeder locked out.
Six things that happen to a substation.
Each one plays out on the drawing with the breakers, the relays and the SCADA log, at the speed it really happens.
Lightning hits Line A
No one loses power. The ring stays closed through B2 and B3, so both transformers keep feeding from Line B. Lights dip for a moment.
A limb falls on Feeder 2
The 1,850 customers on Feeder 2 lose power for two seconds. The limb burned through and fell clear, so the reclose holds and their lights come back. The clocks on their microwaves blink.
A car hits a pole on Feeder 3
The 2,600 customers on Feeder 3 blink three times, then stay out. Wires down are dangerous: stay at least 35 feet away and call 911.
A squirrel on the 12.47 kV bus
Feeders 1 and 2, about 3,950 customers, go out. A bus fault doesn't reclose automatically, because if something is damaged, closing back onto it makes it worse.
Transformer 1 fails
Bus 1 goes dark for about three seconds. Then an automatic scheme closes the bus tie and Transformer 2 picks up both buses. On a hot afternoon that's more than one transformer can carry for long.
A heat-wave afternoon
Everyone has power, and the voltage at the end of each feeder stays inside the range customers' equipment is built for (114 to 126 volts on a 120-volt outlet).
Keep your distance.
Stay outside the fence
Equipment in a substation can kill without being touched: the current can jump to a person who gets too close. Never climb the fence or go after a ball, a drone or a pet that ends up inside. Call the utility.
Wires down: 35 feet and 911
Treat every downed wire as live. Stay at least 35 feet away, keep others back, and call 911. If a wire is on your car, stay inside unless it's on fire.
Report damage
A gate left open, a cut fence, missing ground wires or smoke from a substation: call the utility's emergency number. Copper thieves are killed in substations every year.
Kites, balloons and ladders
Keep them away from any overhead line. Metallic balloons that drift into a substation cause outages.
Where AI helps around the substation.
The relays and breakers already work in milliseconds. The slow parts are the people around them: the phones in a storm, the paperwork before planned work, the logs after an inspection. That office work is where software helps a small utility or a contractor, with a person approving anything that touches the grid.
Outage calls in a storm
When a feeder locks out, two thousand people call at once. A phone agent tells each caller what the outage system knows, takes reports of wires down and sends those to a person first, and logs the rest so the dispatcher sees where the damage is.
AI voice agentsSwitching orders, checked twice
An agent drafts the switching order for planned work from the one-line diagram and the work request, and flags steps in the wrong order, like a switch before its breaker. A person reviews and approves every order. The agent never operates anything.
Custom AI agentsInspection rounds from photos
The monthly substation check is gauges, counters and oil levels. The crew photographs them, and the office learns to have AI read the dials into the log and flag what changed: a fan that didn't run, a breaker counter that jumped.
AI Cowork training
This page shows a made-up 115/12.47 kV substation with a four-breaker ring bus, simplified for learning, with the spacing between equipment shortened to fit the model. Loads, temperatures, voltages and relay times are typical values from a simple model, not measurements of any real substation, and real utilities set their protection and switching their own way. It is not a guide to operating electrical equipment: substations are dangerous, and only qualified utility workers go inside the fence.
