Power System Engineering Lecture 2

8/3/2019 Power System Engineering Lecture 2

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Introduction to Power System Engineering

Syllabus

Historical Background

Structure of Power System

Power Generation

Power Transmission


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Distribution System

Receives electrical energy from HV/MV levels

Supplies energy to the consumers At MV/LV levels and

Single phase or three phase


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Distribution System Equipments

Distribution transformers

Feeders (overhead or underground cables)

Switches, fuses, etc.

Protective relays

Lightning arrestors

CT/PT


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Demand at Station During a Day


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Overhead vs. Underground Systems

Underground cables are technically advantages than theoverhead lines

Not exposed to environmental conditions

Inductance is lower in cable so voltage drop is less

But cables are much more costlier than overhead lines for

same dispatch capability and voltage

Cost of conductor is high

Cost of insulation is high

Erection cost is lower for cables but high maintenance

cost.


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Overhead vs. Underground Systems

Capacitance is predominant in the cables.

This gives large charging current which limits the length of the

cables.

Therefore long distance transmission overhead lines are preferred.

Cables are mainly used at distribution levels.

Cables are preferred in following conditions:

Public safety involved and low interference is required

Large populated cities

Scenic beauty of city is important

Submarine crossing, and substation and transformer connections


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Comparison of AC and DC Systems

P is the amount of power transmitted over same distance.

Vm is the maximum voltage for AC and DC system.

R1 ,R2 andR3 are the resistances of each conductor for

DC, Single phase AC and Three Phase AC.

DC 2 wire (mid point

earthed)

AC 2 Wire (Single phase)

AC 3 wire (Three phase)


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Comparison of AC and DC System


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From above three equations, we have


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The conductor cross-sectional areas for systems is

Therefore, ratio of conductor weights


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Comparison of AC and DC Systems

Advantages of HVDC system

Lower losses

Lower switching overvoltages, lower tower clearances

No problem of stability and synchronism.

Can interconnect systems with different frequencies.

Disadvantages of HVDC system

Can not supply load directly.

Expensive


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Choice of Working Voltage

The cost of conductor is expensive item in overheadtransmission and this is greatly affected by the choice of

the voltage level.

V1 V2 = mV1

I1 I2 = I1/ m

R1 R2 = m2R1

a1 a2 =a1/ m

2

There will be a saving in cost of conductor material if the

power is delivered at higher voltages.


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But higher system voltages will entail more cost ofinsulation in equipments like transformers, circuit-

breakers, and switches.

Cost of insulation increases rapidly with increasing

voltages.

Therefore for certain length of transmission line, the

voltage level beyond which it becomes uneconomical.

Therefore, the question is how to select the transmission

and distribution voltages?


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If Cis total annual cost as function of conductor crosssectionA and working voltage V

C = f(A, V)

For minimum cost

f /A =0

f /V =0

This will result in most economical conductor cross

section and system voltage, if the functionfcan be defined

accurately.


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C= annual interest and depreciation on capital cost (fixedcost) + annual cost of energy losses (operational cost).

Operational cost depends on conductor cross section,

voltage, power factor, change in load time to time,

harmonics, etc.

Thus, functionfdepends on many factors and complicated

enough to describe it mathematically.

Hence determination based on economic basis is

unsatisfactory.


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Transmission Cost as Function of

Voltage Level.


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100

3

6.15.5

1506.15.5

PLV

kVALV

Vis the line voltage in kV

L is the line distance in km

P is the estimated

maximum power in kW

per phase

Using these empirical relations preliminary estimate is

made.

Then voltage level is chosen by doing complete

economic study of existing interconnected system.

Power System Engineering Lecture 2

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