Active and Reactive Power.

At this moment, the book I most admire on the Wind Power field, it is this one written by Ackermann. He can transmit dept concepts of power systems by using a clear vocabulary and illustrative examples. In this book, for example, there is an appendix in the chapter three, that explains in a very easy way how the Power System works by using a long bike as an equivalent.

Imagine a long bike with much cyclists running on a flat and straight road. We are the manager of this team and we want to keep the bike at a constant speed (the same frequency) and in an upright position (the nominal voltage equals to 1). How can we achieve this?


On the bike, there are some cyclists who pedal continuously with different force and some cyclists are trying to stop the brake with their brakes (pedaling riders are generators, and braking cyclists are loads). We will reach a constant speed if:

  • Synchronous Machines

Some cyclists have their pedals connected directly to the chain rings (Synchronous Cyclists). These kinds of “riders” are the large power plants like Nuclear and Hydro-Electrical, because they are so “strong” that they don’t have any problem to follow the bike rpm (system frequency). It means that all synchronous cyclists will bike at the same speed.

However, the chains are slightly elastic. This means that it will exist an angle different between the cyclists and their pedals will not be at the bottom position at exactly the same time (the voltages angles, arg (U), are different.)

  • Asynchronous Machines

On the other side, there are cyclists they are weaker and need to use a softer connection between pedals and sprocket (generator with low power production. Eg: Wind power or distributed generation). This means they have to bike slightly faster than the bike rpm. In addition, they are some “braking pedals” with these same characteristics (asynchronous motor loads).

  • Power Electronic Interfaces

Others are “variable cyclists” and use a stepless transmission (power plants with an electronic interface between the generator and the grid. Eg: wind power plant with a double-feed asynchronous generator). This way, the speed of the pedals is more independent of the bike rpm.

  • Frequency Control

There are cyclists who always keep an eye on the bike speed (usually synchronous generators). If the load is too high, they will need to increase the rpm (frequency) and if the load is too low, they will decrease the rpm.

In some bikes a special person who is not biking and is in charge of the speed control (independent system operator in deregulated power systems). His goal is to minimize costs by making changes least expensively.


Bike speed is important, but also to keep the bike in an upright position is (nominal voltage equals 1). There are two different groups of cyclists: Cyclists who sit at the left of the midpoint of the bike (QL, Producers of Reactive Power) and at the right (QR, Consumers of Reactive Power).

  • Asynchronous Machines

The asynchronous cyclists (asynchronous generators and motors) sit always to the right of the bike midpoint (consumers of reactive power).

  • Capacitors

There are some cyclists who never bike or brake. They just sit on the left (capacitors producing reactive power). The best location for them, it is to have them close to asynchronous machines in order to compensate directly their reactive power consumption. Although it is possible to find them at other locations, the distance from capacitors and asynchronous machines increases the losses in the system.

  • Synchronous Machines

They have seats either in the right or in left position. However, it is limited by the system frequency.

  • Power Electronic Interfaces

As the synchronous machines, they can be situated either to the right or to the left. When the Power factor equals to 1, they can achieve the maximum speed.

With this example, it is easy the terms that are usually used in Power Systems. For any further information, I encourage the lecture of this amazing book.


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2 Responses to “Active and Reactive Power.”

  1. blah Says:


  2. Power System Says:

    Power System…

    […]Active and Reactive Power. « Isaac Braña[…]…

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