Page 314 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Primary distribution

At a sub station, the level of secondary transmission voltage (132KV, 66 or 33KV) is reduced to 11KV by step
down transformers.
Generally, electric supply is given to heavy consumer whose demands is 11KV, from these lines which carries
11KV (in three phase three wire overhead system) and they make a separate sub station to control and utilize
this power.

In other cases, for heavier consumer (at large scale) their demand is about 132 KV or 33KV they take electric
supply from secondary transmission or primary distribution (in 132KV, 66KV or 33KV) and then step down to the
level of voltage by step -down transformers in their own sub station for utilization (i.e for electric traction etc).
Secondary distribution
Electric power is given to (from primary distribution line (i.e.) 11KV) distribution sub station. This sub station is
located near by consumers area where the level of voltage reduced by step down transformers is 415V. These
transformers are called distribution transformers, in
3 phase four wire system (3 phase - 4 wires), there is 415 volts (Three phase supply system) between any two
phases and 240 volts (single phase supply) between neutral and any one of the phase (lives) wire.
Residential load (i.e. Fans, light, and TV etc) may be connected between any one phase and neutral wires, while
three phase load may be connected directly to the three phase lines.
Elements of distribution system
Secondary distribution may be divided into three parts.
1 Feeders
2 Distributors
3 Service lines or service mains

Those electric lines which connect generating station (power station) or sub station to distributors are called
feeders. Remember that current in feeders (in each point) is constant while the level of voltage may be different,
the current flowing in the feeders depends on the size of conductor.
Distributors
Those tapings which extracted for supply of electric power to the consumers or those lines, from where consumers
get electric supply is called distributors
Current is different in each section of the distributors while voltage may be same. The selection of distributors
depends on voltage drop and may be designed according to voltage drop. It is because consumers get the rated
voltage according to the rules. (Fig 2)
Service lines or service mains
The normal cable which is connected between distributors and consumer load terminal are called service line or
service mains. A complete typical AC power supply system scheme is in Fig 3.
Comparison of DC and AC transmission
The electric power can be transmitted either by means of DC (or) AC. Each system has it’s own merits and
demerits. Some technical advantages and disadvantages of two systems are stated below.
AC transmission
Some years ago, the transmission of electric power by DC has been receiving of the active consideration of
engineers due to it’s appreciable advantages.
Advantages of DC electric power transmission
1 It requires only two conductors
2 There is no problem of inductance, capacitance and phase displacement which is common in AC transmission.

301

CITS : Power - Electrician & Wireman - Lesson 106-116