I'm trying to understand exactly how an electrical grid works, and I'm having one simple question that is challenging me. What happens when a new load appears on the grid? I know in practice that any single small load doesn't matter, so let's say it is either a small grid or a huge load. I'm assuming whatever is done it is enough to affect the grid in a measurable way.

My thought is that the new load will attract more current on the power lines, going all the way back to the electricity generator. In order to provide the same power out, the voltage will effectively drop slightly. Measuring the current level of the voltage along several lines is a good way to determine the load, which will let the grid know if power plants need to operate at higher levels, or turn on additional power plants. When a new plant comes on, it will come on slowly, which will allow for more current to pass through the devices, and increase the voltage on the system somewhat. If the same load turns off then later, then less current is running through, which will increase the voltage. Is this understanding even remotely close?

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    \$\begingroup\$ Typically the frequency will droop, the power plant measures that and will adjust the speed of the generators to bring the frequency back into range. This is done automatically, the plant really doesn't like it if the frequency drops too much, so heavy industrial customers may call the power company ahead of adding the load so that they can have enough "spinning reserve" to handle the load. \$\endgroup\$
    – Ron Beyer
    Feb 13, 2020 at 1:15
  • \$\begingroup\$ So basically a heavy load will cause the turbines to speed up a bit, producing more power? Trying to really understand this. \$\endgroup\$ Feb 13, 2020 at 1:17
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    \$\begingroup\$ The load will cause the generator to slow down, whatever controls it will speed it back up to maintain frequency on the grid. BTW, the opposite is also a problem, if a big load is suddenly disconnected, the generator will speed up and the control system will have to slow it down. This can be a bigger problem because of generator inertia. \$\endgroup\$
    – Ron Beyer
    Feb 13, 2020 at 1:22
  • \$\begingroup\$ Gotcha. That makes a lot of sense, thanks! \$\endgroup\$ Feb 13, 2020 at 1:40
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    \$\begingroup\$ @relayman357 great resource, however it points to caution with this exercise , while certainly yes a load coming online will shift the frequency, at grid scale for consumer load this effect is miniscule and accommodated by the grid without active frequency response , according to the publication you linked, the grid scale droop (most commonly from generators coming on/offline) in the eastern US is almost 3GW per 0.1Hz! \$\endgroup\$
    – crasic
    Feb 13, 2020 at 4:43

1 Answer 1


When the new load comes onto the grid, a large load current will flow through it.

This load current has to be sourced by the generators. The increased current in the generators throws an increased torque requirement onto the engine or turbine that's driving the generators.

As the engine control at that instant has no way of knowing that more torque is needed, then the turbine+generator combo will start to slow down. The increased output power is delivered from the fall in kinetic energy of the large rotating mass. It's this energy stored in the rotating machinery that is the buffer that allows the grid to handle unplanned load variations.

Spread out over the grid, there is a lot of rotating machinery, so not much change in speed is required to source the power. However, the grid is not infinitely stiff, so the machines local to the load will slow down more quickly. This creates a phase shift between the local machines and the remote grid, which creates a power flow into the local grid area. This distributes the load across the grid, and maintains phase synchronization between the local and remote areas.

The interconnects between parts of the grid have to be big enough to handle these transient power flows without overloading. For a really big power requirement, say a generating station suddenly goes offline, then the interconnect can drop out, leading to cascading blackouts.

Eventually the engine or turbine controllers will detect the fall in speed, and increase fuel or water flow to match the output power required. The grid controllers will use the instantaneous grid frequency as a proxy for its relative loading, and bring other sources of power online if the frequency is too low.

Over the course of 24 hours, most grids will control the frequency so that there will have been 24 * 3600 * grid_frequency cycles, so that motor-driven electric clocks keep good time from day to day. During the course of a day however, their hour to hour timekeeping could be out due to the load-related frequency variations.

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    \$\begingroup\$ There are also websites that show the current grid frequency, this shows how close they keep the frequency adjusted to the proper values: mainsfrequency.com (for Europe's grid) \$\endgroup\$
    – Ferrybig
    Feb 13, 2020 at 8:54
  • \$\begingroup\$ If a generator is spinning slightly faster, won't that lead to a slight increase in voltage, and visa-versa as well? \$\endgroup\$ Feb 13, 2020 at 10:51
  • \$\begingroup\$ @PearsonArtPhoto While the voltage will change, it varies so much in the network with IR drop in cables, that it's regulated separately anyway. It's principally the phase difference between parts of the grid that control which way the power flows. \$\endgroup\$
    – Neil_UK
    Feb 13, 2020 at 11:48
  • \$\begingroup\$ Check out fnetpublic.utk.edu \$\endgroup\$ Feb 13, 2020 at 17:40

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