Upload
nnvpratap
View
220
Download
0
Embed Size (px)
Citation preview
8/3/2019 Power System Engineering Lecture 2
1/17
Preview
Introduction to Power System Engineering
Syllabus
Historical Background
Structure of Power System
Power Generation
Power Transmission
8/3/2019 Power System Engineering Lecture 2
2/17
Distribution System
Receives electrical energy from HV/MV levels
Supplies energy to the consumers At MV/LV levels and
Single phase or three phase
8/3/2019 Power System Engineering Lecture 2
3/17
Distribution System Equipments
Distribution transformers
Feeders (overhead or underground cables)
Switches, fuses, etc.
Protective relays
Lightning arrestors
CT/PT
8/3/2019 Power System Engineering Lecture 2
4/17
Demand at Station During a Day
8/3/2019 Power System Engineering Lecture 2
5/17
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.
8/3/2019 Power System Engineering Lecture 2
6/17
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
8/3/2019 Power System Engineering Lecture 2
7/17
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)
8/3/2019 Power System Engineering Lecture 2
8/17
Comparison of AC and DC System
8/3/2019 Power System Engineering Lecture 2
9/17
Comparison of AC and DC System
From above three equations, we have
8/3/2019 Power System Engineering Lecture 2
10/17
Comparison of AC and DC System
The conductor cross-sectional areas for systems is
Therefore, ratio of conductor weights
8/3/2019 Power System Engineering Lecture 2
11/17
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
8/3/2019 Power System Engineering Lecture 2
12/17
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.
8/3/2019 Power System Engineering Lecture 2
13/17
Choice of Working Voltage
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?
8/3/2019 Power System Engineering Lecture 2
14/17
Choice of Working Voltage
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.
8/3/2019 Power System Engineering Lecture 2
15/17
Choice of Working Voltage
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.
8/3/2019 Power System Engineering Lecture 2
16/17
Transmission Cost as Function of
Voltage Level.
8/3/2019 Power System Engineering Lecture 2
17/17
Choice of Working Voltage
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.