The classic way of representing what is happening is to draw what is caled a phasor diagram.
Whenever you make any measurement, you have to have a start point, a referance, and so it is with phasor diagrams.
If you were to take the power in your network that is due to the current and voltage acting together as the referance, then you can add other elements where the current and Voltage do not act together and examine the relationship.
The useful work you get in your transmission line is simply the product of the Current squared* Voltage, and this is our ‘in phase’ referance, or ‘true power’.
Power due to reactance is called ‘out of phase’
We can then add the other elements, capacitance and inductance are opposite ends of the reactance scale and will point in opposite directions on our phasor diagram.
If we take our useful power and use it as our baseline, capacitance works on an axis at right angles to the origin or the true power, inductance works exactly 180 degrees in the other direction as capacitance. Which also means it is at right angles to true power.
If there were, lets imagine, a huge amount of capacitance, such that the reactive power line length were equal to the true power length, we could make a vector sum which would be at 45degrees to both reactive power and true power.
If you had done this to scale, you could measure the length of this new line and you would find it was longer than either true power, or reactive power, and this is the load that the network has to carry.
It is easy using pythgoras to work out the actual lengths.
This longer line is called the ‘apparent power’, and all the network has to be designed to carry it, which could and usually does, mean physically larger components.
Bear in mind that high voltage switchgear can be very large physically, one switch taking up over an acre of land and the scale of engineering means greater cost and maintenece, it is very desirable to reduce the element of reactive power.
In the previous, lets imagine that the reactive power was due to capacitance, how would you counter it?
Add in an element whose action works at 180degrees to it is the answer, so you would switch in an inductive element into a capacitive network(also called a reactor) and in a network that was inductive, you would switch in banks of capacitors.
Unfortunately network conditions do not remain constant, and are continually changing as the load changes. This means that the reactive power correction also has to be able to change.
Another way to change the power to compensate for reactive elements is to change the way the generator operates, it can be made to produce power that can counter for the circuit conditions.