Have you ever wondered what is happening on the grid when your power goes out? Ever wonder why your lights may blink a few times before shutting off? Here is a basic overview of the infrastructure that powers your home, and some techniques that engineers use to mitigate the impact of power outages.
The grid is a network of power sources (generators), power loads (consumers), and power lines to connect them, along with hundreds of other devices for protection, stabilization, and efficiency. The following diagram shows a very simple grid
A generator is a device that produces electric power by spinning a magnet inside a coil of wire. Generators that are connected to the grid are typically large and located far away from consumers (your house). To get to your house, the power needs to be transmitted along a series of wires. However, as you add more wire between your house and the generator, the power lost due to heating of the wire increases. The power lost is directly related to the voltage that the power is transmitted at. Therefore to reduce the transmission losses, transmission systems will be designed at high voltages.
A transformer is a device that can change("transform") the voltage of a system. The transformer will change the voltage of the source(primary), to the voltage of the load(secondary) based on the number of turns in the primary winding and secondary winding. As the voltage is increased, the power loss due to heat will go down, but the physical space required between energized components will go up, leading to larger and more expensive structures.
A fault occurs when a component of the grid fails causing an interruption(open-circuit) or excess(short-circuit) of current. All electrical components have a limit on how much current can flow through them before damage occurs. If too much current is flowing through a component, the circuit needs to be intentionally opened to prevent damage to the equipment. A fuse is a device that will break the flow of current by melting if too much current flows through it. Once a fuse has "blown" it must be replaced to resume normal operation. A breaker is a switch that will break the flow of current by opening itself if too much current is flowing through it. Unlike a fuse, a breaker can be reset after it has tripped.
When a fault occurs on the grid, we want to clear the fault as fast as possible. However, we also want to be as selective as possible to reduce the effect of the outage. Consider a fault between the distribution transformer and the house shown below.
Both the breaker and the fuse sense the fault current. To clear the fault as fast as possible we could have the breaker and fuse trip immediately at the same time as soon as the fault occurs. However this would cause a power outage for all the houses as the breaker would now be open. To be more selective, we could slow down the breaker tripping time and let the fuse operate and clear the fault before the breaker opens. We now have a classic engineering trade-off. This is where the "art" of engineering comes in. Each situation will call for different trade-offs to be made.
Most faults on the electrical gird are transient in nature. For example a strong wind blowing a tree limb against an overhead line. Knowing this, a trade-off can be made to minimize the impact of a fault on the grid. If the fault above occurs, we can open the breaker and then close back in very quickly (reclose). Hopefully this temporary opening will be enough for the temporary fault to clear itself. If the fault has not cleared, then the breaker will enter into a slower mode that will allow the fuse to melt and clear the fault. This scheme is called fuse saving because it "saves" the fuse from blowing on a transient fault. This is advantageous because replacing a fuse can be costly and time consuming.
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