Now, as we learn few things about basic electricity, it's time to move to generation transmission and distribution of electric power. Let's start with substations and transformers. Electrical energy is transmitted and distributed via overhead lines, and underground cables, operating at different voltage levels. It is necessary to mesh networks to ensure dependable supply for consumers, even if individual transmission paths fail. Points in such a network, where two or more lines meet, are called buses. Present at each of these buses are switching devices, which isolate affected lines, and/or equipment, for the purpose of eliminating faults or performing maintenance and repairs. The special arrangement of the facilities necessary for measurements, monitoring, protection, and ancillary tasks, is known as a switch gear. If the arrangement includes transformers for changing voltage levels, one speaks of a substation. Let's talk about a transformer. A transformer is a device that transforms AC voltage from high to low, that would be step down transformer, or from low to high, that will be step up transformer. A transformer is simply made of two sets of wire coils wrapped around a soft iron core. The coil on the side where the voltage comes in is called the primary coil. The coil on the side where the voltage goes out is called the secondary coil. Winding on each coil change the magnetic field within the iron core, which in turn, changes the voltage. The voltage is changed in proportion to the number of windings on each coil. If the primary coil has more windings than the secondary coil, the voltage is decreased. This is referred to as a step down transformer. If the secondary coil has more windings than the primary coil, the voltage is increased. This is referred to as a step up transformer. Transformers are very important in power systems. Voltage generated at power plants is an order of six to 20 kilovolts. To deliver vast amounts of generated power over long distances, it should be stepped up, and here, step up transformers do their job by converting generated voltage to high levels such as 115 to 750 kilovolts, or sometimes even more. This is very economical for several reasons. Let us recall the power wheel presented earlier. Since resistance of a line connecting generating plants with load centers is constant for a given line, the higher the voltage across the line, the lower the current through the line. You remember that I equals V over R, that's the Ohm's law. At the same time, if current is flowing through the line, it creates power losses that are dissipated as heat, and you recall that P equals I square R. Since power is not free and costs money, the lower the losses, the higher are the savings. Therefore, it is much more economical to transmit electric power at large distances, at higher voltages. Also, recall that resistance of a line is directly proportional to the length of the line. So that, the longer the line, the higher its resistance. Now, as we know a little bit about transformers, let's move to substations. As mentioned before, transformers, one or more, and switch gear, comprise a substation. Substations are used to branch from one or several incoming lines, into several outgoing lines that have their paths and connect with other substations at the same or different voltage levels. This figure illustrates substations at various voltage levels and branching of power lines. In this figure, high voltage transmission line enters a substation on the left. You can see three long insulators through circuit breakers and switches, which we will discuss later, connections are made to a step down power transformer in the middle. Finally, through another set of switches and circuit breakers, connections are made to three outgoing transmission lines, at lower voltage level on the right side of the figure.