The operation of an electrical grid is a balancing act – you have to produce *exactly* the same amount of energy as is needed, otherwise bad things happen. I don’t understand how does this rule apply on anything other than the largest scale of things.
I understand that *in general* you can predict a higher load on the grid during Monday evening when everyone is at home, and plan your energy production accordingly. But a power station can’t predict smaller load fluctuations like if I decide to turn on or off my TV at this very moment.
So, how does the electrical grid cope with unpredictable load that differs from the planned & expected one?
In: Engineering
Most of our power comes from synchronous generators spun by a turbine that’s fed hot gasses from some heat source (gas turbine from natural gas, steam turbine for coal/nuclear, gas and steam turbine for combined cyclee.t.c).
You spin a synchronous generator at the frequency of your main supply (or a fraction of and have more coils in the generator, e.g half speed but twice the number of coils). The synchronous generator contains two spinning magnetic fields – one in the rotor (the bit that spins), one in the stator (the bit that doesn’t spin). When generating power, the magnetic field in the rotor is slightly ahead of the field in the stator. This induces a current in the stator and that’s where electricity comes from. The more ahead the rotor is from the stator’s magnetic field the greater the current and the more torque it takes to spin the generator.
So, when you suddenly demand a lot of power – the magnetic field in the stator slips back a bit, this increases the current to match the demand and the torque demand by the generator goes up. This causes the generator to slow down. Fortunately the inertia of the huge generators and turbines used in power generation is so large that there’s a lot of kinetic energy stored up. This means the speed doesn’t drop much and the power station has a chance to catch up by turning up the heat. In a decent grid based power network, many generators are run synchronously across many power stations and they all share the increase in demand. In some countries (including some areas of the USA) a sudden demand change may hit a few power stations very hard and they can’t react fast enough. This can cause huge blackouts as the load starts to rapidly propagate to other power stations taking down a large chunk of the power network. It can rake a while to spin it all back up again.
In the case of you flicking a lightswitch – there will be an immeasurably small decrease generator speed. The control systems in power stations are extremely accurately controlled to ensure an exact number of generator rotations in a day – and as many people turn on lights, the controller will sense that the speed is drifting fractions of a percent and act accordingly.
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