Electric supply is always equal to Electric demand
or why the electric grid is the greatest machine
The electric grid is one machine. We might not think of it as one. But it is.
I live in Ahmedabad and the nearest power plant is the coal-powered Sabarmati Thermal Power Plant. When I was a kid, my father used to work for the Torrent Group which operates and owns the plant. His office was in the same complex as the power plant, even though he was not employed in a technical role. The power plant is on the banks of the Sabarmati - the river that divides Ahmedabad in more than one way. Every time we would drive over the river, my dad would point out the chimneys with great pride. He would make sure to use the technical word for chimney - chimney stack. Of course, my mother and I knew he had a less than working-knowledge of the sciences. Now that he has left that job, we turn it around and gush about the chimney stacks every time we drive by.
I thought for a long time that the electricity that powered my computer and fridge came from that particular power plant. But I know now - after four years of electrical engineering that that is not true.
There are two reasons for this confusion:
we think of 'electricity' as if it were a tangible thing. We pay for it like we pay for newspapers, on a monthly basis
often the companies producing electricity are also responsible for transmitting & distributing it. For ex.
Torrent power is in the business of generating power;
transmitting it (those towers you see on the highway) and
distributing it (the final stage of power delivery - where electricity reaches the individual consumer). This last stage includes getting the power to your homes, meter reading and all that
But really, generation to consumption happens via, and in a system called the electric grid. It is basically magic.
I still can't wrap my head around this - but the demand for electricity is instantly matched to supply. Every time you turn on a light that much more electricity is generated somewhere in the system. This is how it happens.
Power generation
Most of the electricity in the world is produced by steam passing over turbines. That's it. So much human effort through history has been spent figuring out ways to create steam, and use steam to turn heavy steel whirls. We burn coal, burn natural gas, hit atoms, use heat trapped under the earth, collect water behind huge concrete walls just to get turbines to spin.
In India, all turbines that are connected to the grid operate between 49.5 - 50.5 Hz. This is fancy talk for "turbines spin 50 times in a second, +-5%". Now it is essential that this 'rate' is maintained to keep the system 'in-sync'.
The greater the variance in 'load' the greater the chances of system falling out of sync. Here is how power engineers handle this.
System Inertia
Power engineers and other kinds of responsible, smart people account for variance in electric demand. We know people will turn on their ACs in the summers, so you account for that.
But the little fluctuations in demand - when I switch on the light, when I suddenly feel cold and turn off the AC - we don't need to account for that.
The system has inertia and these small fluctuations are handled by the system. Here is the best way I know to explain that.
Consider this painting that my mother made a couple of decades ago, and that hangs in our dining room.
If someone were to pour additional liquid in the vessel while the lady was turning the plunger. What would happen? Well, the plunger would slow down a little, until the lady expends that extra energy to compensate for the extra liquid. But the plunger does not stop completely. This is system inertia.
When I turn on a light, a turbine slows down every so slightly. When this happens, the automated control system accounts for this and increases steam flow through turbines. The reverse happens when I turn off a light, a turbine speeds up ever so slightly; and the control system reduces steam flow. Now, the bigger the turbine, the more variation it can ride through.
Of course, this is a greatly simplified explanation. A single power plant is complicated enough ... an interconnected system that spans an entire sub-continent is nightmarishly complicated. Literally. I still get fever dreams about my Power Systems exam.