Reactive power is both obscure for non-engineers and important in the design of electricity systems, especially at the distribution level. While understanding reactive power requires knowledge of integral calculus, the basic intuitions can be understood without rigorous mathematical study. As distribution systems become more complex with distributed energy resources and demand automation, industry participants need a common grasp of the implications of “imaginary power” for system efficiency and stability.
Electrical power (P, in Watts) is composed of voltage (V, in Volts) and current (I, in Amps). The formula is P = V × I. A good analogy to describe the relationship between voltage and current is water flowing down a river. Current is the speed of the water, while voltage is the inclination of the river. When it becomes steeper, this river behaves oddly. The speed of the current remains the same, however the water becomes denser and the flow is heavier as a result. The ability of the flow to push you downriver—the speed of the current times the density of the water (voltage)—is the power of the river.
The apparent power of the river—if you simply measure it—includes both forward motion and downward pressure on the riverbed. While the forward motion is useful to do work (say, run a small hydro turbine), the pressure on the riverbed only serves to support the flow. That is the difference between real power (P, in Watts) and reactive power (VAr, in imaginary Watts). The ratio of reactive power to apparent power (real power2 + reactive power2)1/2 is called the power factor. Consider the example of a horse pulling a railcar.
Source: Consolidated Edison
As shown in the above image, picture a horse that is pulling a railcar from the side of the track. Although the horse is tied diagonally, the railcar can only move along the rails. The strength of the pull on the rope is the apparent power; only a portion of this power is “working” (real) power that pulls the railcar forward. Due to the angle of the horse’s pull, some of the energy expended is wasted as “non-working” (reactive) power. As this angle becomes larger, the ratio between real power and reactive power declines until the horse is pulling directly away from the tracks, not moving the railcar at all. This ratio is often calculated as the power factor: real power divided by apparent power (real + reactive).
Enormous blackouts have resulted from reactive power failures
Reactive power is essential to power flow because it helps to regulate voltage. Referring back to the river analogy, without a riverbed to push against for forward motion there could be no water flow. Increasing reactive power may be described as making the riverbed steeper while “squeezing” the water forward. This “squeeze” increases the density of the water and allows it to travel further. Similarly, reactive power is crucial in transmission lines to increase voltage upstream and“squeeze” flow downstream.
Producing reactive power, sometimes referred to as imaginary power, requires power plant capacity while yielding no direct economic value—think of the horse pulling the railcar diagonally. For integrated monopoly utilities, running power plants to produce reactive power is compensated through the rate base. For merchant generators, reactive power takes away from plant capacity that could produce real power instead. As such, reactive power needs to be compensated as an ancillary service.
On July 14, 2003 a historical power outage occurred in the Northeastern US and Canada that affected an estimated 55 million people in eight states and one province. Among the reasons for this enormous system failure, a severe shortage in reactive power has been cited as an important factor. In the hours leading to the blackout, demand for reactive power was particularly high due to large volumes of long-distance transmission streaming through Ohio into Canada. At the same time, the supply of reactive power was dangerously low in part because there was a lack of incentive to produce reactive power. Reactive power failures have also contributed to blackouts in the West (1996) and in France (1978).
In a direct current (DC) circuit, the power is of constant intensity and can only flow in one direction. Current and voltage in alternating current (AC) circuits, on the other hand, fluctuate rapidly and power appears to flow in all directions. The speed of fluctuations is referred to as the frequency and the delay between two “frequencies” is their phase angle. The phase angle is important both at a single location and between two points. For example, delay in the voltage frequency between the starting and ending points of a wire produces power flow. An important consideration in AC circuits is the delay between voltage and current fluctuations at any single point. When current and voltage at a single point are perfectly in phase with each other, thus having the exact same timing, all of the power resulting from the flow is real power. As the delay between current and voltage increases, so does the amount of reactive power—the horse is pulling further away from the railcar. Reactive power is present whenever current either “lags” or “leads” voltage.
Impediments to power flows on a power line are called impedances. These impedances can either be resistance or reactance. Resistance is friction of electrons with the atoms inside electrical conductors and affects both current and voltage equally, converting a small amount of power to waste heat. Reactance can either refer to electric fields or magnetic fields. Electric fields, which affect voltage, are created when two electrically charged metal plates are placed close to each other without touching. These capacitors build up voltage without a flow of current, thus effectively storing and delaying voltage fluctuations relative to current. Magnetic fields, on the other hand, cause current to make a “detour” relative to voltage. Electrical lines themselves are constantly storing and retrieving alternating current in a magnetic field that spirals around the wire. “Inductors” are specially-designed coils of wire that are meant to store current in magnetic fields. Some appliances such as electric motors and refrigerators have inductive properties.
When current lags voltage, there is positive reactive power in the circuit. The most important cause of positive reactive power is the reactance of power lines themselves. All along the line, a portion of the current takes a “detour” in a spiraling magnetic field around the line. Transformers, which rely on inductors, also inject positive reactive power into the lines. At the grid edge, inductive appliances such as electric motors and refrigerators contribute positive reactive power as well.
Because higher reactive power equates to higher voltage, too much positive reactive power in one part of the network can cause voltage to drop precipitously. In order to compensate for the reactance of power lines, transformers, and inductive appliances, there needs to be a sufficient injection of negative reactive power. This service can be provided by power plants, albeit at the expense of real power production and constrained by transmission capacity. Alternatively, negative reactive power can be used downstream to improve power flow. For example, capacitors placed downstream near transformers and inductive loads can be used to mitigate voltage drops where most needed. Some electrical devices such as smart inverters can also stabilize reactive power locally.
Although reactive power is essential for voltage stability in transmission, too much positive reactive power in the distribution system affects energy efficiency. Referring back to the example of the horse and the railcar, increasing the angle of the pull lowers the amount of real power applied to the railcar. In 2011, Consolidated Edison in New York introduced a reactive-power charge to penalize large electric customers with inefficient induction equipment. The utility recommends that large customers install capacitors near inductive loads, cycle the operation of inductive equipment, and retrofit their plants with more efficient equipment to keep their power factor above 95%.
Reactive power—the delay between voltage and current at a given point—is subjected to transmission constraints. As a result, it is often necessary to produce reactive power close to the location where it is needed. Additionally, some appliances such as electric motors need negative reactive power to run their magnets properly. Supplying reactive power locally is thus much more effective than producing it from afar. This is where distributed energy resources may bring significant benefits to reactive power regulation.
According to SDG&E, smart inverters can effectively regulate reactive power with little incremental cost. In January 2014, the California Public Utilities Commission issued a technical report recommending standards on smart inverter capabilities. PJM has also issued strong statements supporting smart inverters for the regulation of reactive power. According to documents from the IEEE 1547 working group, “results from […] simulations show that real and reactive counter-flow is not a significant issue and that no significant changes to feeder operation need to be made at high levels of [smart] inverter penetration.” In April 2014, FERC published a staff report outlining methodologies to compensate reactive power as an ancillary service. With the addition of new “smart grid” capabilities such as automation, predictive analytics, and local coordination, reactive power may be one horse that we can tame.