As we know that in the old days there was very little use of electrical power for light and heating purpose only. So for the low consumption of electrical power small power stations were built in between in the load center. However, now in the modern days, the need for electrical power is necessary for a huge amount. So big power stations are now made to fulfill the need of electrical power. These kinds of power stations contain the huge amount of power and occupy a lot of space which cannot be built in the load center due to the little area, high power hazards.

Then these power stations are building away from the people and load center. Here the power system transmission and distribution scheme need in highlighted. Electrical power can be AC or DC. Also, it can be transmitted through overhead or underground transmission system. However, the underground transmission system scheme is expensive for Pakistan. Here we have overhead transmission system. Today we will talk about typical ac system scheme in Pakistan.

          

How AC Power is supplied from Generation Station to Consumers

As the power system scheme is consists of large of number conductors’ network between the powerhouse and consumer. This system is mainly divided into two parts i.e. transmission and distribution system. Both systems are subdivided into primary, secondary transmission system and primary, secondary distribution system. Nowadays power is transmitted in 3 phase 3 wire system but 3 phase 4 wire system is universally adopted for economical proportion.

• Generating station

Generating station represents that location where the electrical power is produced in 3 phase alternators connected in parallel. For convenience electrical power is generated in 11kv and then step up with power transformers up to 132kv.

• Primary transmission

The 11kv generated power is step upped with power transformer to 132kv and then transmitted at 3 phase, 3 wire overhead system to the load center. This forms the primary transmission system.

• Secondary transmission

The primary transmission lines end at the boundaries of the cities receiving station. The voltage is reduced here to 33kv by step down transformers. From this station electric power is transmitted at 33kv by 3phase, 3 wire system to various grid station located at different location in the city. This forms secondary transmission system.

• Primary distribution

The secondary transmission lines terminate at substations. Where the voltage is reduced from 33kv to 11kv, 3phase, 3wire. These 11 kV lines run along the roadsides of the cities. This forms the primary distribution. Often the big consumers having demands more than 50kw are generally supplied power at 11kv directly.

• Secondary distribution

The primary distribution lines 11kv delivered at the distribution substations. These substations are located near the consumer premises and step down the voltage to 400v, 3phase, 4wire for secondary distribution. The voltage between two phases is 400v and between single and neutral is 230v.