INTRODUCTION
The electrical energy is normally generated at the powerstations far away from the urban areas ( where consumers are located). The major source of electric energy in India are fossil fuels and water. The present contribution by different types of plants are : steam plants (58%), Hydro plants(26%), Nuclear plants (2%), Diesel and wind plants (3%), Gas plants (11%) . Generation voltages are 3.3, 6.6, 11 or 33kv. Most usual value adopted in practice is 11kv. Now we have to transmit this power over longer distances economically from generating stations to consumers.At low voltage level, both weight and insulation is less in the alternator, this directly reduces the cost and size of alternator. But this low voltage level power can not be transmitted directly to the consumer end as because this low voltage power transmission is not at all economical. Hence although low voltage power generation is economical but low voltage electrical power transmission is not economical. A large network of conductors is used between the generating stations and the consumers and this network is called transmission and distribution system.
The transmission system is to deliver bulk power from power stations to the load centres and large industrial consumers beyond the economical service range of the regular primary distribution lines where as distribution system is to deliver power from power stations or substations to the various consumers.Most of us don’t live right next to a power plant. So we somehow have to get electricity to our homes.
First electricity travels on long distance, high voltage transmission lines, often over long distances. The voltage in these lines can be hundreds of thousands of volts.
voltage is kept high for longer distances as per Ohm’s law. Ohm’s law describes how the amount of power in electricity and its characteristics voltage, current and resistance are related. as per this law losses are directly proportional to the current flowing. Keeping voltage high lets us keep current, and losses low.
High-voltage transmission lines are big, tall, expensive, and potentially dangerous so we only use them when electricity needs to travel long distances. At substations near your neighbourhood, electricity is stepped down to smaller, lower-voltage power lines.
Although electrical power can be transmitted and distributed by either a.c or d.c but in practice 3-phase 3 wire a.c system is usually adopted for distribution of electrical power.
The transmission network can be classified into following subsystems :
1. Transmission system: The transmission system interconnects all major generating stations and main load system. It operates at higher voltage levels (typically 132kv and above).
2. Sub Transmission system : The operation of sub transmission system is similar to that of distribution system. The difference between them is that of operational voltage. Operating voltage in Sub transmission system is higher than that of distribution system and supplies only bigger loads (Typically between 33KV and 66KV).
3. Distribution system: It represents the final stage in the transfer of power to the individual consumers. The voltages for primary distribution are 11,6.6 or 3.3kv depending upon the requirement of bulk consumers and for secondary distribution useable voltage is 400v.
Faults may occur at different points in power system. faults that occur on a power system are broadly classified as follows
1. Symmetrical faults.
2. Unsymmetrical faults.
To ensure the maximum return on the large investment in the equipment , which goes to make up the power system and to keep the users satisfied with reliable service , the whole system must be kept in operation continuously without major breakdowns. The main idea is to restrict the disturbances during such failures to a limited area and continue power distribution in the balance areas. Special equipment is normally installed to detect such kind of failures (faults) that can possibly happen in various sections of a system and to isolate faulty section so that the interruption is limited to a localised area in the total system covering various areas. The special equipment adopted to detect such possible faults is referred as ‘protective equipment or protective relay’ and the system that uses such equipment is termed as ‘protective system’. It can be well recognised that use of protective equipment are vital to minimise the effects of faults, which otherwise can kill the whole system.
The requirements of protective equipment are as follows
a. Selectivity : to detect and isolate the faulty item only.
b. Stability : to leave all healthy circuits intact to ensure continuity.
c. Sensitivity : to detect even smallest fault and operate correctly before the fault causes irreparable damage.
d. Speed : to operate speedily when it is called upon to do so.
