Before we go to the actual discussion, we would like to tell you what is the need for an electrical power system. From the economical point of view, we always construct a generating station where resources are readily available. The consumers consume electrical energy, but they may stay in such locations where the resources for producing electricity are not available. Not only that, sometimes there are many other constraints due to which we can not construct a generating station nearer to the dense consumer’s localities or load centres. We usually have to transmit the generated power to the load centres. We call the entire arrangement from generating plants to consumer ends for delivering electricity efficiently and reliably as the electric power system.
The generation plants produce electrical energy at a low voltage level. We keep the generation voltage at a low level because it has some specific advantages. Low voltage generation creates less stresses on the armature of the alternator. Hence at low voltage generation, we can construct smaller alternator with thinner and lighter insulation. From engineering and design point of view, smaller alternators are more practical. We can not transmit this low voltage power to the load centres. Low voltage transmission causes more copper loss, poor voltage regulations and more installation cost of the transmission system. To avoid these three difficulties we have to step up the voltage to a specific high voltage level. We can not raise the system voltage beyond a certain level because beyond a limit of voltage the insulation cost tremendously increases and also to keep adequate ground clearance the expenses of the line supporting structures also abruptly increase. The transmission voltage depends on the quantity of power to be transmitted. The surge impedance loading is another parameter which determines the voltage level of the system for transmitting an amount of energy.
For stepping up system voltage, we use step-up transformers and their associated protections and operations arrangements at generating station. We call this as generation substation. At the end of the transmission line, we have to step down the transmission voltage to a lower level for secondary transmission and or distribution purposes. Here we use step down transformers and their associated protection and operational arrangements. This is transmission substation. After primary transmission, the electrical energy passes through secondary transmission or primary distribution. After secondary transmission or primary distribution again we step down the voltage to a desired low voltage level to distribute at the consumer premises.
This was the basic structure of an electrical power system. Although, we have not mentioned the details of each equipment used in an electrical power system. In addition to three main components alternator, transformer and transmission line there are numbers of associated equipment. Some of these pieces of equipment are circuit breaker, lightning arrestor, isolator, current transformer, voltage transformer, capacitor voltage transformer, wave trap, capacitor bank, relaying system, controlling arrangement, the earthing arrangement of the line and substation equipment etc.