A power substation changes the voltage of electricity, either raising it for long-distance travel or lowering it before it gets to homes and businesses. The electricity you use has passed through at least one power substation to get to you.
How a power substation works
The electricity leaves a generating station at a relatively low voltage. At that voltage, sending it hundreds of miles would waste huge amounts of energy as heat. So the first stop is a step-up power substation that raises the voltage to anywhere from 115,000 to 765,000 volts. The higher the voltage, the lower the current. And the lower the current, the less heat loss over long distances. When electricity arrives at its destination, a step-down power substation lowers the voltage gradually, from transmission level to distribution level, and then down again at lower voltages in smaller substations, until it gets to the 120 or 240 volts your appliances use.
The transformer is the actual device within a power substation that changes voltage levels, using electromagnetic induction. This is what determines whether the voltage goes up or down. The ratio of the number of turns of wire in each coil.
Types of power substation
The transmission substation is a junction between large generating plants and the high-voltage network. It covers the transmission of large amounts of power over long distances.
Most people live near distribution substations. It steps down voltage from transmission lines to levels local lines can carry, usually 4,000 to 35,000 volts.
Wind and solar farms have a collector substation. It gathers the output of turbines or solar panels, boosts the voltage, and delivers power to the transmission grid.
A converter substation converts alternating current to direct current and vice versa. These are found where undersea cables or long-distance DC lines connect to the main grid.
A mobile substation is a power substation mounted on trailers. They are used by utilities when a permanent substation fails or needs maintenance.
Equipment inside a power substation
The power transformer is the biggest and most expensive item. A transmission-class transformer can weigh 400 tons, take 18 months to build, and cost several million dollars.
Circuit breakers: isolate sections of a power substation in case of fault. They stop current in milliseconds protecting equipment from damage. A fuse is used only once. A circuit breaker can be reset once the fault is removed.
Disconnect switches: enable workers to physically isolate sections for repair. They only open once a circuit breaker has already cleared the load.
Surge arresters take the energy from a lightning strike and absorb it before it can damage transformers or other equipment inside the power substation.
Capacitor banks: improve the power factor and help the power substation deliver energy more efficiently.
Busbars are thick copper or aluminum bars that distribute electricity from incoming to outgoing circuits.
Instrument transformers generate reduced versions of current and voltage to supply protection relays and meters without exposing equipment to full grid voltages.
The control building contains computers, relays, and communication systems that allow operators to monitor and control the power substation locally or remotely.
How a power substation handles faults
Protection relays are constantly monitoring current and voltage. If the readings are outside the normal limits, the relay trips the corresponding circuit breaker in less than 100 milliseconds.
Backup protection sees the same fault conditions. Backup relays will close if primary protection fails to operate.
Differential protection compares the current flowing into and out of a transformer. Any large difference means an internal fault and makes the transformer trip immediately.
Ground grids buried below the power substation safely carry fault current into the earth. This protects workers from dangerous voltages during a fault.
What is the minimum distance from homes to an electrical grid station?
Distribution power substations are already found inside many residential neighborhoods; the fenced-in enclosures you pass along suburban streets are often the last step in voltage before power gets into homes. Normal distances have not been shown to pose health risks in studies.
Large transmission power substations are situated away from residential areas. Setback distances depend on country and voltage level. Property values near a large power substation sometimes run lower depending on how visible and audible the facility is.
How long does it take to build a power substation?
Once permitting is done, a small distribution power substation is 3 to 6 months. The construction of a large transmission power substation takes one to three years. Usually the longest step is the main transformer (12 to 18 months delivery after ordering). And then you have one to three years of permitting and environmental review before you even start building.
What happens when a power substation fails?
Customers on the system lose power. Operators are trying to shift electricity through substations in other areas, but only as much as those substations can take. In the event of a major site failure, a mobile substation can be brought in and energized in a matter of hours. But swapping out a large power transformer may take many months.
Power substations and renewable energy
The development of each large solar or wind project involves the construction of a collector power substation. Battery storage systems also interface to the grid through dedicated power substation equipment.
Some utilities are building digital substations that replace copper control wiring with fiber optic cables and use standardized communication between devices. These are faster to commission and easier to upgrade than conventional designs.