02/07/2016
WHAT is REACTIVE POWER, for those who still don't understand, worth reading. Will clear the concepts for most of you.
Consider there is a small river dividing two cities and you need to build bridge over it to connect both the cities. After building bridge, you also need to construct a house on the other side of the river.
For the whole work including bridge and house, you've got only 100 wooden planks of which 10 are used to create bridge over the river. The workers will use this bridge to cross the river and transport items and construct a house on the other side. Here the main work is to build the house on the other side for which we've created a bridge to move from one point to another. In this case, are the wooden planks used in constructing the bridge directly contributing to the building of house ? NO. We've used 10 wooden planks for bridge and that cannot be used for building the house.
And this - THE BRIDGE is what reactive power does.
100 wooden planks is the total complex power of which some amount is used to create bridge (10 planks) and remaining active power (90 planks) does the useful work. This is why we say reactive power does not do useful work.
In transmission lines, reactive power maintains the voltage level of the line so that active power can flow to do useful work and this is why we compensate reactive power by external means. It is very much necessary for the line to operate, for active power to flow.
And for how it is done, a capacitor (to supply reactive power to increase voltage profile) or an inductor (to absorb reactive power to decrease voltage profile) is connected in parallel to transmission line.