STRUCTURE OF POWER SYSTEM

1). HISTORY OF POWER SYSTEM- 

The commercial use of electricity began in the late 1870s when arc lamps were used for lighthouse illumination and street lighting. The first complete electric power system (comprising a generator, cable, fuse, meter, and loads) was built by Thomas Edison - the historic Pearl Street Station in New York City which began operation in September 1882. This was a DC system consisting of a steam-engine-driven DC generator supplying power to 59 customers within an area roughly 1.5 km in radius. The load, which consisted entirely of incandescent lamps, was supplied at 110 V through an underground cable system. Within a few years similar systems were in operation in most large cities throughout the world. With the development of motors by Frank Sprague in 1884, motor loads were added to such systems. This was the beginning of what would develop into one of the largest industries in the world. 

In spite of the initial widespread use of de systems, they were almost completely superseded by dc systems. By 1886, the limitations of dc systems were becoming increasingly apparent They could deliver power only a short distance from the generators. To keep transmission power losses and voltage drops to acceptable levels, voltage levels had to be high for long-distance power transmission. Such high voltages were not acceptable for generation and consumption of power; therefore, a convenient means for voltage transfom1ation became a necessity.

The development of the transformer and ac transmission by L. Gaulard and J.D. Gibbs of Paris, France, led to ac electric power systems. George Westinghouse secured rights to these developments in the United States. In 1886, William Stanley, an associate of Westinghouse, developed and tested a commercially practical transformer and ac distribution system for 150 lamps at Great Barrington, Massachusetts. In 1889, the first ac transmission line in North America was put into operation in Oregon between Willamette Falls and Portland. It was a single-phase line transmitting power at 4,000 V over a distance of 21 km.

With the development of polyphase systems by Nikola Tesla, the AC system became even more attractive. By 1888, Tesla held several patents on ac motors, generators, transformers, and transmission systems. Westinghouse bought the patents to these early inventions, and they formed the basis of the present-day AC systems.

In the 1890s, there was considerable controversy over whether the electric utility industry should be standardized on AC or AC. There were passionate arguments between Edison, who advocated de, and Westinghouse, who favoured ac. By the turn of the century, the AC system bad won out over the DC system for the following reasons: 

A). Voltage levels can be easily transformed in ac systems, thus providing the flexibility for use of different voltages for generation, transmission, and consumption.

B). AC generators are much simpler than de generators.

C). AC motors are much simpler and cheaper than de motors.

2). MODERN POWER SYSTEM-

A power system consists of several generating stations, where electrical energy is generated, and several consumers for whose use the electrical energy is generated. The objective of any power system is to generate electrical energy in sufficient quantities at the best-suited locations and to transmit it to the various load centers and then distribute it to the various consumers maintaining the quality and reliability at an economic price. Quality implies that the frequency be maintained constant at the specified value (50 Hz in our country; though 60-Hz systems are also prevailing in some countries) and that the voltage be maintained constant at the specified value. Further, the interruptions to the supply of energy should be as minimum as possible.

3). STRUCTURE OF POWER SYSTEM-

An interconnected power system is a complex enterprise that may be 

A). Generation Subsystem 

B). Transmission and Sub transmission Subsystem 

C). Distribution Subsystem 

D). Utilization Subsystem 

           A). Generation Subsystem-

This includes generators and transformers. 

Generators - An essential component of power systems is the three phase ac generator known as synchronous generator or alternator. (Generation Voltage level 11kV to 33kV)

Transformers - The transformer transfers power with very high efficiency from one level of voltage to another. The power transferred to the secondary is almost the same as the primary, except for losses in the transformer. Using a step-up transformer will reduce losses in the line, which makes the transmission of power over long distances possible. 

           B). Transmission and Sub transmission Subsystem- 

 An overhead transmission network transfers electric power from generating units to the distribution system which ultimately supplies the load. Transmission lines also interconnect neighboring utilities which allow the economic dispatch of power within regions during normal conditions, and the transfer of power between regions during emergencies.

Standard transmission voltages are established in the United States by the American National Standards Institute (ANSI). Transmission voltage lines operating at more than 60 kV are standardized at 69 kV, 115 kV, 138 kV, 161 kV, 230 kV, 345 kV, 500 kV, and 765 kV line-to-line. Transmission voltages above 230 kV are usually referred to as extra-high voltage (EHV). 

High voltage transmission lines are terminated in substations, which are called high-voltage substations, receiving substations, or primary substations. The function of some substations is switching circuits in and out of service; they are referred to as switching stations.

fig. Structure of Power System

The portion of the transmission system that connects the high-voltage substations through step-down transformers to the distribution substations is called the sub transmission network. There is no clear distinction between transmission and sub transmission voltage levels. Typically, the sub transmission voltage level ranges from 69 to 138 kV. Some large industrial customers may be served from the sub transmission system. Capacitor banks and reactor banks are usually installed in the substations for maintaining the transmission line voltage.  

           C). Distribution Subsystem- 

The distribution system connects the distribution substations to the consumers’ service-entrance equipment. The primary distribution lines range from 4 to 34.5 kV and supply the load in a well-defined geographical area. Some small industrial customers are served directly by the primary feeders. 

The secondary distribution network reduces the voltage for utilization by commercial and residential consumers. Lines and cables not exceeding a few hundred feet in length then deliver power to the individual consumers. The secondary distribution serves most of the customers at levels of 240/120 V, single-phase, three-wire; 240Y/120 V, three-phase, four-wire; or 440Y/240 V, three-phase, four-wire. The power for a typical home is derived from a transformer that reduces the primary feeder voltage to 240/120 V using a three wire line. 

Distribution systems utilize both overhead and underground conductors. The growth of underground distribution has been extremely rapid.