01 electric fields (elham hejrani's conflicted copy 2011-11-01) (in konflikt stehende kopie von...
TRANSCRIPT
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Significance of electrical insulationOverview of high voltage applicationsHigh voltage in electric power engineeringCourse syllabus
Content
Significance of electrical insulation
Reliability of energy systems Safety Efficiency
Applications of high voltage insulation– transmission lines, – cables, – transformers, – capacitors, – electrical machines
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High Voltage Applications
Light Engineering
Voltage up to 6 kV
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High Voltage Applications
Cathode-Ray Tubes:
TV sets, computer monitors, oscilloscope, etc.
(up to 25 kV)
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High Voltage Applications
Rail Transport:
subway, railway
(up to 27 kV)
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High Voltage Applications
Technology: electrostatic precipitation, separation, painting, electrohydraulic stamping, water cleaning, electroerosion electropulse
machining
(10-100 kV)
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High Voltage Applications
X-ray equipment
for medicine and industry
(up to 200 kV)
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High Voltage Applications
Scientific research
(millions Volts)
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High Voltage Applications
Electric power engineering
3, 6 – 1150 kV
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History of electric power engineering
1870 – invention of the DC generator 1882 – first large-scale use of electricity –
Edison's Pearl Street System; provided electric lighting for Lower Manhattan
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First DC power system
1) Current in transmission line: IL ≈ PLoad / V2
2) Voltage decay: V = V1 – V2 = IL Rline
3) Power loss: P = IL V = IL2
Rline = (PLoad / V2)2Zline
G
Transmission line
Rline V2V1
PLoad
Generator Load
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Ways to increase perfomance
P = (PLoad / V2)2Rline
1. increasing the operating voltage of a system
2. decreasing the impedance of a line
Impedance (DC): Rline = r L / S, where
r – specific resistance of the wire,
L – length of the line, and
S – cross-sectional area of the wire
already optimal
not effective
not effective
G
Rline V2V1
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Modern AC power system
1884 – invention of a transformer 1890 – 28 miles 10 kV line Deptford – London
Power transfer capability (PLoad ≈ V22 / Zline)
V(kV) 400 700 1000 1200 1500
P(MW) 640 2000 4000 5800 9000
G
Transmission line
ZlineV2V1
PLoad
Generator
LoadTransformerTransformer
V3
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Major AC power systems
10 50110
220287
380
525
735
1150
0
200
400
600
800
1000
1200
1400
1880 1900 1920 1940 1960 1980 2000
Year of installation
AC
vo
lag
e, k
V
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Application of the HVDC systems
Connection of Asynchronous AC Systems Long and Submarine Cables
Transmission lineDC
ACInverterRectifier
AC
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Major DC power systems
0
100
200
300
400
500
600
700
1950 1960 1970 1980 1990 2000
Year of installation
DC
vo
lta
ge
, kV
Demands to electrical insulation
Reliability Safety Efficiency
Ways to achieve: Design Manufacturing Maintenance
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Three questions to be answered
What is the electrical stress distribution that the materials will experience within the device?
What are the processes in insulation under these stresses that influence breakdown strengths of the materials?
How does the reliability change through the life of the device as a result of a reduction in the material breakdown strength (aging) and a change in the electrical stress distribution?
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Course structure
THEORY Electrical discharge in gas, liquid and solid dielectrics
(physics, theory, factors affecting discharge voltage, breakdown, corona discharge, flashover, partial discharge, treeing).
Design of high voltage insulation (transmission lines, cables, transformers, capacitors, electrical machines).
Testing techniques for insulation maintenance.
EXAM
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Textbooks
High Voltage Engineering Fundamentalsby E. Kuffel, W. S. Zaengl, and J. Kuffel
High Voltage Engineering by M.S. Naidu
Hochspannungstechnik. Theoretische und Praktische Grundlagen by M. Beyer, W. Boeck, K. Möller, and W. Zaengl
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ELECTRIC FIELD IN INSULATORS
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Contents
Electric field concept Electrical discharge and breakdown definitions Electric fields classifications
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Electric field related equations
Potential induced by charge in the medium
= q / (40 R)
q – charge
– permittivity of the medium
0 – electric permittivity of vacuum (8.85 10-12 farad/m)
R – distance
Voltage between the points 1 and 2
V = 1 – 2
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Electric field related equations
Electric field strength
E = –grad = –V / x
= – ∫l E dl
The force of electric field
F = q E
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Electrical discharge and breakdown
Electrical breakdown – loss of insulating ability
Electrical discharge – process (the stage) of breakdown
Dielectric strength of insulator: the maximum electric field strength, which the
material can withstand without breakdown the voltage at which the current starts increasing
to very high values and breakdown occurs
Breakdown voltage – applied voltage at the moment of breakdown
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Lightning
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Multiple Lightning
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Examples: spark discharge
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Examples: arc discharge
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Examples: long arc discharge
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Parameters affecting breakdown voltage
electric field distribution, pressure, temperature, humidity, nature of applied voltage, imperfections in dielectric materials, material and surface conditions of electrodes, time of voltage application,and others
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Electric field classification
Dependence on time of application: constant alternating pulsed
oscillating impulse aperiodic or unipolar impulse
t
E
t
E
t
E
t
E
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Electric field classification (space)
Е
0 D
Е
uniform
Е
0 D
Е
Е
0 D
Е
strongly non-uniformslightly non-uniform
Non-uniformity factor КN = ЕMAX / EAV
EAV = V / D; ЕMAX = f(voltage, shape, size, distance)
КN = 1 КN ≤ 3КN > 3
E = V / D E ≈ V / D E = f(x,y,z) E = f(x,y,z)
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Methods for estimating the potential and electric field distribution
the direct measurement of potential distribution
analytic calculations the numerical methods of calculations using
digital computers
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Calculation of maximum electric field
Planes
Concentricspheres
Sphere vs. plane
Twospheres
Coaxial cylinders
Cylinder vs. plane
Parallelcylinders
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Electrodes symmetry
Symmetric and asymmetric electrodes Symmetric electrodes are two electrodes of the
same shape and size and there is no electrode grounding
Asymmetric electrodes are two electrodes of the different shape or size, or one of them is grounded
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Solve the next problem
Sphere vs. plane
Given: r = 1 cm; a = 10 cm; U = 100 kVCalculate1. non-uniformity factor;2. at a given r the value a that provides slightly non-uniform field;3. at a given a the value r that provides slightly non-uniform field