Why does a power grid need to be synchronized?



For example, apparently the grid on one side of Japan uses 50 Hz and the other side uses 60 Hz, and this means they can’t interconnect, but… why? Why isn’t it the case that current is current is current is current? I’ve heard some super vague explanation about how deviations in the line frequency can damage electrical equipment, but *how* does it do that?

In: Engineering

The voltage in the power grid changes with time. It forms a wave – it moves back and forth between around -230 to +230 volts (for a single phase, but that requires a bit more complex explanation). The current doesn’t flow in a straight line either. It essentially flows back and forth continously. The frequency tells you how often this cycle happens in a second. If you connect multiple power stations that don’t have the same frequency (or even the same position in the AC wave), it would cause a massive current to flow from one power station to the other, since one power station would be at a different voltage at the same point in time, and current flows from high voltage point to low voltage point. The power stations must be synchronized, so that they output the same voltage and there is no net flow of energy bewteen them.

The frequency is the rate at which the current alternates. In order to have power flowing you need a positive and negative wire. Current flows from positive to negative. With alternating current you swap the wires around at 50 or 60 Hz. But all the power stations need to do this at once. If not then you end up with a short circuit where the positive wire from one power station connects to the negative wire on the other power station and vice versa.

It is technically possible to connect two such power grids together. However it requires huge expensive switches that can undo all the switching on one side and then redo it on the other side at the different frequency. But these are not very practical.

You ever see in a cartoon where there’s a bunch of people going “heave ho” and all pulling or pushing in unison on something?

Synchronizing is like that. You want all the people pulling at the same time and pushing at the same time, not pushing and pulling at the same time.

Additionally the power grid essentially provides power in a sine wave. Deviations to a perfect sine is survivable by some stuff (like an electric motor driven by VFD, it’s rare or very expensive to get a completely accurate sine off them) but some electronics can be very sensitive to “dirty power” that isn’t roughly a sine wave. If the power stations aren’t synced instead of a perfect sine you get all kinds of whacko wave patterns.

AC is alternating current, being synchronized means it’s alternating in the same direction at the same time. If you connect two grids that are the same voltage and frequency, but 180 degrees out of phase, it’s like short circuiting double the voltage, because the positive of one grid is aligning with the negative of the other.

A 50hz and 60hz grid will go in and out of phase 10 times a second, if you connected them when they were in phase, the 60hz grid would try to speed up the 50hz grid, and the 50hz would try to slow down the 60hz grid. If the connection can’t handle enough current to hold them synchronized, stuff goes bang.

Japan uses 100 volts ac. Appliances and devices are designed around using 100 volts. Because of the slight frequency difference the Two power grids can not be connected together, if they were the whole power grid would oscilate between 200 and 0 volts.

As one frequency is Higher than the other the Phase Angle between the powerline Vortages shifts. The Voltage would be chaos, Anywhere from 200 to 0 volts. Sudden Voltage Spikes and drops would kill your devices.