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In electrical grid systems, reactive power is the power that flows back from a destination toward the grid in an alternating current scenario.
In a direct current system, the voltage and load is static, and to put it simply, the direction of energy is "one way," but in alternating current, there are different phases having to do with elements of the system like capacitors and inductors.
Reactive power gets energy moving back into the grid during the passive phases.
Reactive power is also known as: phantom power.
Another way to explain this is that reactive power is the resultant power in watts of an AC circuit when the current waveform is out of phase with the waveform of the voltage, usually by 90 degrees if the load is purely reactive, and is the result of either capacitive or inductive loads.
Actual work is done only when current is in phase with voltage, such as in resistive loads. An example is powering an incandescent light bulb; in a reactive load energy flows toward the load half the time, whereas in the other half power flows from it, which gives the illusion that the load is not dissipating or consuming power.
Reactive power is one of the three types of power present in loaded circuits.
The actual amount of power in watts being dissipated by the circuit
The dissipated power resulting from inductive and capacitive loads measured in volt-amperes reactive (VAR)
The combination of reactive and true power measure in volt-amperes (VA)
Reactive power is also called "phantom power" because it is not apparent where it goes. It is general knowledge that reactive loads such as capacitors and inductors do not actually dissipate power in a sense that it is not used to power them, but measuring the voltage and current around them indicates the fact that they drop voltage and draw current.
The power dissipated through this voltage drop and current draw is in the form of heat or waste energy and is not done as actual work; hence engineers have sought ways to lessen this. Because of this phantom power, conductors and generators must be rated and sized accordingly to carry the total current including the waste and not just the current that does the actual work.
Some energy experts talk about reactive power as part of a capacitor’s movement that resembles the movement of a clock pendulum from its zenith to its nadir. In this analogy, as the pendulum swings up, the alternating current is supplying active power to a destination device. As the pendulum swings back down reactive power is moving back into the grid to be absorbed.
In these types of definitions, experts would say that reactive energy is energy circulating back and forth between the source and the load, specifically, that reactive power “fades” back toward a source. In a sense, this related to the delay between current and voltage. In addition to capacitors, static VAr compensators and synchronous condensers can be used to handle reactive power in a system.
The key is to put reactive current equipment close to power loads. This reduces the amount of reactive current that the delivery system has to carry a particular distance.
In order to deal with the realities of alternating current and changing energy paths, planners make sure to put voltage control measures in place. Energy experts point out that even a 5% change in voltage over a given system can cause blackouts and other problems.
To this end, many elements of an electrical system, such as transformers, can switch from delivering to absorbing reactive power in phases. But those close to the industry stress that this is going to become even more important as we switch over portions of the American electrical grid to renewables.
Reactive power is also very important in the context of our changing energy grids.
For many important reasons, renewables like solar and wind are replacing traditional energy sources like coal and natural gas. But that may have ramifications for the electrical grid as a whole.
“A surge of renewables onto a grid without sufficient rotating mass could cause serious problems: power being cut in certain areas in an effort to bring demand back in line with supply; and large power plants getting disconnected from the grid to prevent them becoming overloaded,” writes Archie Robb at Renewable Energy World, describing the principle of “grid inertia” and how that applies to handling reactive power in a system that is changing over to the renewable build.
As renewables deliver energy to the grid differently, there will be more of a demand to micromanage active power and reactive power accordingly.