It hasn’t been that long since humans figured out how to create power grids that integrated multiple generators and consumers. Ever since AC won the battle of the currents, grid operators have had to deal with the issues that come with using AC instead of the far less complex DC. Instead of simply targeting a constant voltage, generators have to synchronize with the frequency of the alternating current as it cycles between positive and negative current many times per second.
Complicating matters further, the transmission lines between generators and consumers, along with any kind of transmission equipment on the lines, add their own inductive, capacitive, and resistive properties to the system before the effects of consumers are even tallied up. The result of this are phase shifts between voltage and current that have to be managed by controlling the reactive power, lest frequency oscillations and voltage swings result in a complete grid blackout.
Flowing Backwards
We tend to think of the power in our homes as something that comes out of the outlet before going into the device that’s being powered. While for DC applications this is essentially true – aside from fights over which way DC current flows – for AC applications the answer is pretty much a “It’s complicated”. After all, the primary reason why we use AC transmission is because transformers make transforming between AC voltages easy, not because an AC grid is easier to manage.
What exactly happens between an AC generator and an AC load depends on the characteristics of the load. A major part of these characteristics is covered by its power factor (PF), which describes the effect of the load on the AC phase. If the PF is 1, the load is purely resistive with no phase shift. If the PF is 0, it’s a purely reactive load and no net current flows. Most AC-powered devices have a power factor that’s somewhere between 0.5 to 0.99, meaning that they appear to be a mixed reactive and resistive load.
PF can be understood in terms of the two components that define AC power, being:
- Apparent Power (S, in volt-amperes or VA) and
- Real Power (P, in watts).
The PF is defined as the ratio of P to S (i.e. `PF = P / S). Reactive Power (Q, in var) is easily visualized as the angle theta (Θ) between P and S if we put them as respectively the leg and hypotenuse of a right triangle. Here Θ is the phase shift by which the current waveform lags the voltage. We can observe that as the phase shift increases, the apparent power increases along with reactive power. Rather than being consumed by the load, reactive power flows back to the generator, which hints at why it’s such a problematic phenomenon for grid-management.
From the above we can deduce that the PF is 1.0 if S and P are the same magnitude. Although P = I × V gets us the real power in watts, it is the apparent power that is being supplied by the generators on the grid, meaning that reactive power is effectively ‘wasted’ power. How concerning this is to you as a consumer mostly depends on whether you are being billed for watts or VAs consumed, but from a grid perspective this is the motivation behind power factor correction (PFC).
This is where capacitors are useful, as they can correct the low PF on inductive loads like electric motors, and vice versa with inductance on capacitive loads. As a rule of thumb, capacitors create reactive power, while inductors consume reactive power, meaning that for PFC the right capacitance or inductance has to be added to get the PF as close to 1.0 as possible. Since an inductor absorbs the excess (reactive) power and a capacitor supplies reactive power, if both are balanced 1:1, the PF would be 1.0.
In the case of modern switching-mode power supplies, automatic power factor correction (APFC) is applied, which switch in capacitance as needed by the current load. This is, in miniature, pretty much what the full-scale grid does throughout the network.
Traditional Grids
Based on this essential knowledge, local electrical networks were expanded from a few streets to entire cities. From there it was only a matter of time before transmission lines turned many into few, with soon transmission networks spanning entire continents. Even so, the basic principles remain the same, and thus the methods available to manage a power grid.
Spinning generators provide the AC power, along with either the creation or absorption of reactive power on account of being inductors with their large wound coils, depending on their excitation level. Since transformers are passive devices, they will always absorb reactive power, while both overhead and underground transmission lines start off providing reactive power, overhead lines start absorbing reactive power if overloaded.
In order to keep reactive power in the grid to a healthy minimum, capacitive and inductive loads are switched in or out at locations like transmission lines and switchyards. The inductive loads often taken the form of shunt reactors – basically single winding transformers – and shunt capacitors, along with active devices like synchronous condensers that are effectively simplified synchronous generators. In locations like substations the use of tap changers enables fine-grained voltage control to ease the load on nearby transmission lines. Meanwhile the synchronous generators at thermal plants can be kept idle and online to provide significant reactive power absorption capacity when not used to actively generate power.
Regardless of the exact technologies employed, these traditional grids are characterized by significant amounts of reactive power creation and absorption capacity. As loads join or leave the grid every time that consumer devices are turned off and on, the grid manager (transmission system operator, or TSO) adjusts the state of these control methods. This keeps the grid frequency and voltage within their respective narrowly defined windows.
Variable Generators
Over the past few years, most newly added generating capacity has come in the form of weather-dependent variable generators that use grid-following converters. These devices take the DC power from generally PV solar and wind turbine farms and convert them into AC. They use a phase-locked loop (PLL) to synchronize with the grid frequency, to match this AC frequency and the current voltage.
Unfortunately, these devices do not have the ability to absorb or generate reactive power, and instead blindly follow the current grid frequency and voltage, even if said grid was going through reactive power-induced oscillations. Thus instead of damping these oscillations and any voltage swings, these converters serve to amplify these issues. During the 2025 Iberian Peninsula blackout, this was identified as one of the primary causes by the Spanish TSO.
Ultimately AC power grids depend on solid reactive power management, which is why the European group of TSOs (ENTSO-E) already recommended in 2020 that grid-following converters should get replaced with grid-forming converters. These feature the ability absorb and generate reactive power through the addition of features like energy storage and are overall significantly more useful and robust when it comes to AC grid management.
Although AC doesn’t rule the roost any more in transmission networks, with high-voltage DC now the more economical option for long distances, the overwhelming part of today’s power grids still use AC. This means that reactive power management will remain one of the most essential parts of keeping power grids stable and people happy, until the day comes when we will all be switching back to DC grids, year after the switch to AC was finally completed back in 2007.