As I mentioned before, one event that we know can bring down a grid is a voltage collapse. A voltage collapse can occur in large parts of the grid where the load is greater than the generation available in that area. The problem is that when electric power is delivered over long distances (for example, twenty-five to more than fifty miles), one property of alternating current becomes very important. This property is called reactive power. Reactive power is one of those complex technical concepts that I said I wouldn’t get into, but I must explain it, as this concept is crucial to the delivery of electricity. In order to deliver real power in our alternating current grid, there must be reactive power. Reactive power does not provide real power or energy, but it is a necessary ingredient for the grid to deliver real energy. Think of it as the oil in an automobile engine. The oil doesn’t provide any energy, but without it the car will not go very far. Another analogy that I have heard many times is that reactive power is like the froth at the top of a glass of beer; it doesn’t amount to much, but who wants a flat beer?
Reactive power can be produced by generators or by devices called capacitors that utilities install on the transmission system. Unfortunately, unlike real power, reactive power cannot be transmitted very far. The impact of this reactive-power requirement is that only so much real power can be imported into an area (think of this area as a large city, such as Cleveland). In effect, the area in question would need to have its own reactive-power sources in order to maintain the delivery of real power. What this means is that the overall grid can be balanced with frequency at a very comfortable 60 Hz, but the area without adequate reactive resources (such as nearby generators or capacitors) can be at great risk of a voltage collapse. This is one issue that makes the control and operation of the grid very complex and challenging.