Eli5 – what is MVAR (Reactive Power)


Why do we need it / measure it ?


In: 1

Your transmission lines have to carry it, but you don’t have to generate it. If you hook a capacitor up to the mains, current will flow back and forth through it, but the energy it stores will be put back into the grid.

It’s undesirable because you still have resistance losses, and need bigger conductors. It’s current flowing without doing anything useful.

The classic example is a beer, apparently power is the whole glass, real power is the beer and VARs are the head.

I prefer to think of it like water. If electricity is water, the wires as pipes and reactive elements as tanks (capacitors and inductors) . It is like the water in the tanks and pipes, when you turn on the system everything has to fill up before the water gets to its destination.

The grid has to deliver electricity to the end user. The grid has to be able to handle the shipping of energy from power plant to user.

Some users (electronics and motors, but even power lines themselves can do this) act a bit weird. They order too much electricity then have to send back the excess immediately.

The grid not only has to ship the extra power to the user, it also has to ship it back. So, this causes extra grid congestion, but the power plant gets its energy back so it doesn’t actually have to work harder or burn more fuel.

MVARs are a measure of how much power is shuffling back and forth doing nothing useful clogging up the grid.

The grid has to be able to supply enough MVARs, just like it had to be able to supply enough MW. If it doesn’t have enough, then there will be power problems like brown outs.

MVARs can be created by power plants, but they can also be created by other equipment like capacitors. Motors need VARs, but this can come from the grid, but it is common in industrial motors to fit the motor with capacitors, this way the capacitors create the VARs that the motor needs, so the VARs don’t need to come from the grid.

Thanks guys super helpful, surely unplanned wind / solar just adds to the congestion? Or is the grid dynamically moving to minimise the loss (assumed to be turned into heat somewhere) I haven’t noticed heat sinks in substations, so do the congested transmission wires just get warm ? – thanks