BASIC INTRODUCTION POWER SYSTEM

Power system

The generation, transmission and distribution of electric power is called power system. A power system has the following stages.

Generation of electric power
Transmission of electric power
Distribution of electric power

Layout of power system

Generation/power system
Primary transmission
Secondary transmission
Primary Distribution
Secondary Distribution

1. Power system :- The bulk power is generated at the power station, by 3-phase, 3-wire system employed several alternators in parallel. The usual generation voltage is 11 Kv . Due to economic consideration the generation voltage is stepper upto 220 kv or 132 kv at the power station with the help of step-up transformer.

2. Primary system :- The electric power at 220 kv or 132 kv is transmitted by overhead lines at this high voltage. this forms the primary transmission system. In primary transmission lines, spans between tower are large and the conductor are of aluminium strands for tightness and 3-phase, 3-wire system is always employed.

3. Secondary transmission :- On outskirt of the city, there are sub-stations which step down the primary transmission voltage to 66 kv or 33 kv and power is transmitted at this voltage in this three phase 3-wire system is used.

4. Primary Distribution :- At suitable places near the city or load centers there are sub-station which step down the secondary transmission voltage to 11kv and power is deliver at the voltage.

5. Secondary Distribution :- At each distribution sub-station the voltage is steeped down to 400 v and the power is distributed by 3-phase, 4-wire system the voltage between any two phase is 400 v and voltage between phase and neutral is 230 v. Domestic load such as lamps, refrigerator, fans, heaters, etc. Operates at low voltage and therefore, they are connected between any one phase and neutral.

Commonly used voltage for different distances

DISTANCE	VOLTAGE
15-30	11 KV
30-60	33 KV
60-100	66 KV
100-200	132 KV
200-300	220 KV
300-400	400 KV

Advantage of high voltage of transmission

It has been discussed above that if electric power is transmission at high voltage it has certain advantage such as..

Efficiency of transmission is increased.
For given power cost of conductor required is less.
Percentage line voltage drop is reduced.

Limitation of increase in transmission voltage

The high transmission voltage has the following limitation..

More insulation :- For high transmission voltage, the insulation required between the conductor and the earthed tower is more.
Higher tower :- The clearance required between the conductor and ground depends upon the voltage of transmission line.
Long cross arms :- The clearance between the conductor also depends upon the transmission voltage, the longer are the cross-arms required.

Various system of transmission of electrical power

D.C SYSTEM

Dc two-wire
Dc two-wire with mid-point earthed
Dc three wire system

SINGLE PHASE AC SYSTEM

Single phase two wire
Single phase two-wire with mid-point earthed
Single phase three wire

TWO-PHASE AC SYSTEM

Two-phase four-wire
Two-phase three-wire

THREE PHASE AC SYSTEM

Three phase 3-wire
Three phase 4-wire

COMPARISON BETWEEN AC AND DC TRANSMISSION LINE

BASIC FOR COMPARISON	AC transmission Line	DC transmission Line
Number of conductor	Three	Two
voltage drop	High	Low
Skin effect	Occurs	Absent
Need of insulation	More	Less
corona loss	Occurs	Don;t Occur

MAIN COMPONENTS OF OVERHEAD LINES

The main components of an overhead line are enlisted as below;

The supports :- Poles or tower depending upon the working voltage and the region where these are used.
Cross-arms and clamps :- These are either of wood or steel angle section and are used on pole structure to support the insulation.
Insulators :- Pin, strain or suspension types as the case may be, for supporting the conductor and taking strain or suspending the conductor respectively.
Conductor :- Copper, aluminium or A.C.S.R or any other composition depanding upon the current to be carried and the span of the line.
Phase plates ;- In order to distinguish the various phase.

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