Name :- Smit Shah -140410109096

T.Y Electrical 2 Sem 5

Subject:-EPS-1

Topic :-AC Supply system, Comparison Between AC-DC System, Advantage HV transmission..

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Contents

Supply Systems:

Electric supply system, Typical AC power supply Scheme, Comparison of DC and AC transmission, Advantages of high transmission voltage Various system of power transmission,

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Electric supply system

• The conveyance of electric power from a power station to consumers’ premises is known as electric supply system. An electric supply system consists of three principal components viz., the power station, the transmission lines and the distribution system.

The electric supply system can be broadly classified into, 1. d.c. or a.c. system2. overhead or underground system.

• Now-a days 3-phase, 3-wire a.c. system is universally adopted for generation and transmission of electric power as an economical proposition.

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Typical AC power supply Scheme,

The network can be broadly divided into two parts viz.,1. transmission system and2. distribution system.

Each part can be further sub-divided into twoprimary transmission and secondary transmission and primary distribution and secondary distribution.It may be noted that it is not necessary that all power schemes include all the stages.

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• Main Parts :

• (i)Generating station : • (ii) Primary transmission.• (iii) Secondary transmission.• (iv) Primary distribution. • (v) Secondary distribution

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COMPARISON OF DC AND AC TRANSMISSION ,

DC Transmission:– Some times ago. The Electric power transmission was in DC due to the

following advantages.Advantages:

– There are two conductors used in DC transmission while three conductors required in AC transmission.

– There are no Inductance and Surges (High Voltage waves for very short time) in DC transmission.

– Due to absence of inductance, there are very low voltage drop in DC transmission lines comparing with AC (if both Load and sending end voltage is same)

– There is no concept of Skin effect in DC transmission. Therefore, small cross sectional area conductor required.

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– A DC System has a less potential stress over AC system for same Voltage level. Therefore, a DC line requires less insulation.

– In DC System, There is no interference with communication system.– In DC Line, Corona losses are very low.– In High Voltage DC Transmission lines, there are no Dielectric losses.– In DC Transmission system, there are no difficulties in synchronizing

and stability problems.– DC system is more efficient than AC, therefore, the rate of price of

Towers, Poles, Insulators, and conductor are low so the system is economical.

– In DC System, the speed control range is greater than AC System.– There is low insulation required in DC system (about 70%).– The price of DC cables is low (Due to Low insulation)– In DC Supply System, the Sheath losses in underground cables are low.

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– DC system is suitable for High Power Transmission based on High Current transmission.

– In DC System, The Value of charging current is quite low, there fore, the length DC Transmission lines is greater than AC lines.

Disadvantages:Due to commutation problem, Electric power can’t be produce at High

(DC) Voltage.For High Voltage transmission, we can not step the level of DC Voltage

(As Transformer can not work on DC)There is a limit of DC Switches and Circuit breakers (and costly too)Motor generator set is used for step down the level of DC voltage and

the efficiency of Motor-generator set is low than transformer.so the system makes complex and costly.The level of DC Voltage can not be change easily. So we can not get

desire voltage for Electrical and electronics appliances (such as 5 Volts, 9 Volts 15 Volts, 20 and 22 Volts etc) directly from Transmission system.

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AC Transmission:Nowadays, the generation, transmission and distribution mostly is in AC.

Advantages:AC Circuit breakers is cheap than DC Circuit breakers.The repairing and maintenance of AC sub station is easy and inexpensive

than DC Substation.The Level of AC voltage may be increased or decreased step up and Step

down transformers.

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Disadvantages:

In AC line, the size of conductor is grater than DC Line.The Cost of AC Transmission lines are greater than DC Transmission lines.Due to Skin effect, the losses in AC system are more.In AC Lines, there is Capacitance, so continuously power loss when no load

on lines or Line is open.Other line losses are due to inductance.More insulation required in AC SystemAlso corona Losses occur In AC System,  There is telecommunication interference in AC System.There are stability and synchronizing problems in AC System.DC System is more efficient than AC System.There are also re-active power controlling problems in AC System.  

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Advantages: With increase in the transmission voltage size of the conductors is

reduced (Cross section of the conductors reduce as current required to carry reduces).

As the reduction in current carrying requirement losses reduces results in better efficiency

Due to low current voltage drop will be less so voltage regulation improves

ADVANTAGES OF HIGH TRANSMISSION VOLTAGE

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Disadvantages: With the increase in the voltage of transmission, the insulation required

between the conductors and the earthed tower increases. This increase the cost of line support

·With increase in the voltage of transmission, more clearance is required between conductors and ground. Hence higher towers are required.

·With increase in the voltage transmission, more distance is required between the conductors. Therefore cross arms should be long

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Various system of power transmission,

It has already been pointed out that for transmission of electric power, 3-phase, 3-wire a.c. system is universally adopted.  The different possible systems of transmission are :

1. D.C. systemD.C. two-wire.D.C. two-wire with mid-point earthed.D.C. three-wire.

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2. Single-phase A.C. systemSingle-phase two-wire.Single-phase two-wire with mid-point earthed.Single-phase three-wire.

3. Two-phase A.C. systemTwo-phase four-wire.Two-phase three wire.

4. Three-phase A.C. systemThree-phase three-wire.Three-phase four-wire.

 

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