What is Reactive Power....???

Reactivevpower is an essential component of power all pervading in electrical system. When you look around, you will find reactive loads at work, supplying reactive power to the system and consuming reactive power from the system everywhere around you.

There are innumerable examples of reactive power phenomena that change the face of the world’s energy sector. Reactive power is required for transmit the active power, control the voltage and system and normal operations of power systems. Therefore, reactive power managing service is one of the most important ancillary services in electricity market currently.

What is reactive power? This is the first question we were asked at the Kothmale power station while we were having our in-plant training at CEB. Why we calculate average power for the active part and maximum power for the reactive part? Is it a strange concept? These are some of answers come to our mind. It is the imaginary part of the apparent power. It is a manoeuvre to explain the energy conservation. If one follows any textbook, the concepts of active and reactive power are as the product of the voltage and current measured at the same point. The active power based on the cos part and the reactive power based on the sin part wher

e the angle is taken in between the voltage and current. Is this classic explanation smart enough to sense you something about reactive power? That is why I thought about sharing something ease your mind regarding this manner.

Reactive power refers to the circulating power in the grid that does no useful work. This results from energy storage elements in the grid mainly from capacitors and inductors. Since it is a main part of voltage, it has a strong effect on the system voltages. Therefore, it must be well balanced in the system properly to prevent voltage issues and collapses. As we know, reactive power is present when the voltage and current are not in phase, which measured in volt-ampere reactive (so called VAR). Most of textbooks say it like this.

So most of us imagine the rectangled triangle and then hypotenuse to represent the apparent power and other two legs represent the active and reactive power. Can we explain even the basic behaviours and differences of the active power and reactive power from this hypothetical representation?

Here is a one mathematical model to understand the reactive power and its physical behaviours at a glance. Considering the instantaneous voltages and currents,

,

Using trigonometric we can modify this as,

In generally we call above two parts as instantaneous active power and instantaneous reactive power. As you can see in the graph, the active power oscillates around an average value while the reactive power oscillates around the zero axis. But it is not practical to work with instantaneous values as far as we can’t measure it. So they introduced the average power of the instantaneous power with rms values of V and I as,

This is the so-called active power equation that we have been already taught. Then what about the average power of reactive power? If you observed the equation carefully there is only a single sin term in the reactive power function. So the average reactive power is obviously zero. Then what is the reactive power measured in the industry as,

This is the maximum of the instantaneous reactive power that transfer to the net work. For our convenience, we have dropped the sin term of time here. This quantity measures the maximum reactive energy that flows through the device during a cycle and therefore we can get a good estimate of how much of energy is moving through the circuit even if the average reactive power is zero. What I am emphasising here is, there is no symmetry in the active power and reactive power terms as inferred in many textbooks. So P and Q have different meanings while P denotes the average active power and Q denotes the instantaneous maximum reactive power. So, P and Q cannot be treated on equal foot as they are not similar.

For the active power there exists a net flow from one point of the network to another. However, keep this in your mind. The zero average does not mean that no energy is flowing. The actual amount that is flowing for half a cycle in one direction is coming back in the next half a cycle.

Then what about the reactive power loss in the transmission lines? Well, as you already know, we cannot talk about a gain or loss in the reactive power scenario. This is just a loss in the amplitude of the instantaneous reactive power, which partially shares by the line impedance and other network elements. Therefore, the so-called loss in reactive power is not a real loss but rather a loss in the amplitude of the sending end reactive power. Reactive power does not travel very far. That’s why they ask to produce it close to the place where it is needed.

Therefore the so-called loss in reactive power is not a real loss but rather a loss in the amplitude of the reactive power as no reactive energy is lost. Shunt capacitors, Synchronous condensers, Synchronous generators and Static VAR compensators are some reactive power supply mechanisms.