Before we move on to study frequency control of a power system, we will study about another phenomena known as voltage instability. To get a better understanding of frequency control and voltage instability, we will first briefly review the topic of feedback control systems.

What does one mean by a feedback control system and why/when is it necessary? To understand this let us take a simple example.

Let us suppose that you wish to buy 1 kg of rice from your grocer. If all rice grains were of the same weight, it would be possible (although by a very tedious process!) to calculate how many grains are required. However, not only will this be time consuming, but it is also true that there are uncertainties in the grain weight. Therefore, rather than follow this procedure the grocer will put some rice into his weighing pan (counter balanced by 1 kg standard weight) and depending on the extent of imbalance, he will incrementally add or reduce grain in the pan. A seasoned grocer may even hold some grain in his fist and gradually pour it by monitoring the position and rate at which the pan moves. By doing this, he can speed up the process of weighing the rice. This is a manual feedback control system. The major components are shown below: