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GENERATION OF HIGHVOLTAGE
Lecture 8
S-18.3150 High Voltage Engineering
S-18.3146 Suurjnnitetekniikka
https://noppa.aalto.fi/noppa/kurssi/s-18.3150
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Week Date Lecture Topic Exercises
37 10.9 1 General + Safety + High Voltage Lab Tour
38 17.9 2 Electrostatic Fields + FEM 1 + FEM + Seminar tasks
39 24.9 3 Gas Insulation
40 1.10 4 Liquid and Solid Insulation 2 + PD lab
41 8.10 5 Transients 3
42 15.10 NO LECTURE
43 22.10 EXAM WEEK
44 29.10 6 Overvoltages and Insulation Coordination 4
45 5.11 7 HV Testing and Measurements 5
46 12.11 8 Generation of High Voltages Seminar presentations
47 19.11 9 Seminar Presentations Left over seminars
48 26.11 Ensto, Porvoo Surge Arrestor Lab
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10.12.2012S1
14:00 17:00???
EXAM
18.12.2012 S4 09:00 12:00
10.01.2013 S1 13:00 16:00
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DC SOURCES
Van der Graaff
Rectifier Circuit
Cascade Circuit
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DC SOURCES
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VAN DE GRAAFF transporting charges with a moving belt
Charge is sprayed onto an insulating moving belt from corona points (sharp needles)
Charge removed and collected from the belt connected to the inside of an insulatedmetal electrode through which the belt moves
The belt returns with charges dropped and fresh charge is sprayed onto it (belt speed1000-2000 m/min)
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The potential of the HV
electrode at any instant isU= Q/C
Potential of electroderises at a rate of
CI
dtdQ
CdtdV 1
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By 1931 Robert Van de Graaff could charge a sphere to750 kilovolts, producing a 1.5 megavolt differencebetween two oppositely charged spheres.
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Constructed in an unused airshipdock at Round Hill, Massachusetts.Generator was originally used as a researchtool in early atom collisions and high energy X-ray experiments
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Oak Ridge National Laboratory, USA25 MV tandem electrostatic accelerator located inside a 30 m high pressure
vessel
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DC SOURCES
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HALF-WAVERECTIFIER
A single diode isused to pass eitherthe positive or
negative half cycle ofAC while blockingthe other
FULL-WAVERECTIFIERConverts bothpolarities of input
waveform into DC
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DC SOURCES
The supply voltage charges C1to . Duringthe positive half-cycle D2 is conducting andcharges C1. As the AC signal reverses polarityD1 starts to conduct now further charging C1to 2.
With each change in input polarity, thecapacitors add to the upstream charge.
The increase in voltage, assuming idealcomponents, is two times the input voltagetimes the number of stages
= 2n
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CASCADE CIRCUIT converts low level AC to higher level DC using a ladderconstruction of diodes and capacitors
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C2
C1
C2'
C1'
a
b
b
c
c
I
C3'
a
C3d
d
u
2
2
2
2
2
a
b
a
b
c
c
d
d
= 2n = 6
D1
D2
D3
D4
D5
D6
Cockroft-Walton (1932):CW multiplier
Heinrich Greinacher (1919):Greinacher multiplier
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The number of stages n has a large effect onvoltage drop Uand ripple amplitude U
When all of the cascades capacitance C are equal, output
voltage Uis:
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U
C2
C1
C2'
C1'
a
b
cc
I
n = 1
n = 2
b
whereu
t
4
UU
a
b b
c c2U
0
Largest voltage drops occur at lower stages since theyhave to charge the higher stage capacitors
To decrease voltage drop and ripple, lower stagecapacitance could be larger
Voltage drop U and ripple U are smaller withlarger frequency and capacitance
124
3
3
222 23
nnn
fC
IunUUunU
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Doubling capacitance of lowest capacitor in AC column(C1= 2C) Voltage in C1 is only half that of the other capacitors Now voltage drop is decreased and average U becomes:
Increasing the number ofstages nsignificantly decreases efficiency
Most efficient way to decrease voltage drop and ripple is to increase frequency
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5 stages: U = 10140 90 kV = 1310 kV10 stages: U = 20140 700 kV = 2100 kV
= 140 kV, f = 1000 Hz,C = 10 nF, I = 10 mA
Staging of capacitance causes uneven voltage distribution Smaller capacitance at top stages would experience
majority of the voltage stress (requires higher voltage withstand)Differentiation
U
1
2 3
4 5I
2C
C
124
1
3
22 23
nnn
fC
IunU
Stray capacitance also an issue with increasing stages
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1.2 MV Cascade DC Generator
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AC SOURCES
Single-Stage Transformer
Cascade Transformer
Resonant Transformer
Tesla Transformer
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AC SOURCES
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1. Iron core 2. LV winding3. HV winding 4. Field grading shield5. Grounded metal tank/base 6. HV bushing7. Insulating shield or tank 8. HV electrode
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SINGLE-STAGE TRANSFORMER up to 400 kV
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CASCADE TRANSFORMERconnecting HV windings in series
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I
nsulation
300 kVoutput
200 kV
199 kV
1 kV
1 kV
99 kV
100 kV
1 kVinput U2
2U2
3U2
U1
U1
U1
LV primary windingHV secondary windingExcitation winding
1.
2.
3.
First transformer is at ground potential, Thesecond and third transformers are kept oninsulators
The high voltage winding of
the first unit is connected to thetank of the second unit
The low voltage winding ofthe second unit is supplied fromthe excitation winding of thefirst transformer (in series withthe high voltage winding) The rating of the excitation winding
is almost identical to that of theprimary winding.
AC SOURCES
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900 kV600 kV Cascade Transformer
AC SOURCES
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AC SOURCES
22
CL XXRZ RXXRZ CL 22
R
XX CL1tan
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RESONANT TRANSFORMERS Resonance to multiply input
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Re
Im
R
Z
XL XC
Re
Im
R
XL XCLC
1~
I
U
R
UR = IR
UL = IXL(XL= L)
UC= IXC(XC= 1/C)
C
LSeriesRCLcircuit:
XT= 0
R
2R
R
0
U, I
Test Specimen Reactive Power = (Uout
)2 /Xc
whereXc
= 1 / 2fCload
Reactor Losses = Real power dissipated in reactor. Resistive losses in reactorwindings, magnetic losses in reactor core and stray losses in tank structure
Test Load Losses =Real power dissipated in test object. Losses in insulation dueto leakage current, losses in termination equipment, and external stray losses
QualityFactor Q
Test Specimen Reactive Power
Reactor Losses + Test Load Losses
Output Reactive Power
Input Real Power==
CL XX CL XX
0
Uout= Q Uin
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Simplified diagram of series resonance test system
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~
LR
C U2
U1
Transformer secondary winding connected across HV reactor inductanceL and capacitive load C. Resistance Ris the total series resistance ofthe circuit
Resonance:Inductance of reactor L is varied
On-site testing may have fixed L (compact and lighter)
Resonance frequency depends on test object capacitanceFrequencymust be adjustablef = 1 / 2(LC)
Typicallyused for
cable and
capacitortesting
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Resonance is sensitive to partialdischarge Sinusoidal waveform deteriorates Voltage fluctuations
DISADVANTAGES
Clean sinusoidal output
Smaller power requirements Series inductance compensates test objects capacitive reactive
power
No high-power arcing and heavycurrent surges occur
if test object fails Resonance ceases at the failure of the test object
Cascading is also possible (up to 3000 kV)
Simple and compact test arrangement Reactor is considerably lighter than a transformer of equivalent
power
No repeated flashovers occur in case of partialfailures of test object and insulation recovery. It takes Q number of cycles to charge test object to full voltage
ADVANTAGES
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Series Resonance Transformer
k f i ll l
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800 kV Resonance Transformer (Series/Parallel)
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Series Resonance Transformer (On-Site)
Motor 3-Gen. f
FrequencyConverter
Breaker Breaker
ExcitationTransformer
HVReactors
VoltageDivider
TestLoad
AC SOURCES
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AC SOURCES
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TESLA COIL high frequency resonant transformer (high voltage, low current, high frequency AC)
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Circuit consists of a weakly coupled primary andsecondaryoscillatory circuit (only share 10 20% ofmagnetic field)
Large air gap due to HV (avoid inter-winding breakdown)
System is excited to oscillate at high frequenciesby periodic discharge of the primary side capacitor
via a spark gap Primary is fed from a supply through C1, spark gap is
connected across primary and triggered at a desired voltage U1
C1
C2L2L1
M
Sparkgap
Supply U2
U1
Based on circuit parameters and the ratio betweenprimary and secondary windings, voltages in excess of1 MV can be generated (output voltage U2 is a functionof parameters L1, L2, C1, C2 and mutual inductance M)
Voltage gain is proportional to the square root of the ratioof secondary and primary inductances
Secondary winding has same resonance frequency asprimary (windings are tuned to a frequency of 10 100kHz by means of C1 and C2) Voltage gain is proportionalto the square root of the ratio of the primary capacitorC1 to secondary capacitance C2
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IMPULSE VOLTAGE SOURCES
Impulse Voltage Circuit
Marx Generator
IMPULSE VOLTAGE SOURCES
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IMPULSE VOLTAGE SOURCES
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IMPULSE VOLTAGE GENERATOR basic circuit applicable to both LI and SI
1. Surge capacitor C1 is charged and the switch is closedSwitch is typically a triggered (ignitable) sphere gap (trigatron)
2. The charge in C1 is distributed quickly between loadcapacitance C2 so that the voltage over both becomesequal
During this distribution phase some energy is transformed intoheat mainly by damping resistance R1 (determines impulsevoltage front T1)
Once C2 is charged, voltage has reached its maximum value(impulse voltage peak Up)
3. Next, the discharge phase starts. Remaining energy istransformed into heat mainly in discharge resistanceR2 (determines impulse voltage tail T2).
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U2C2R2
R1
C1U0
Rv
U
t
i l i l h k RR
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Single stage impulse generator reaches ~ 100 kVFor higher voltages basic circuits are constructed on top ofeach other to create n stage generators
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C2R2
R1
C1
Rv
Typically 100 250 kVper stage
Can reach tens of stages(not limited by voltagedrop)
Indoors: 400 4000 kV
Outdoors: 10 MV
Typical energy 10 20 kJ
Marx Generator Erwin Marx (1923)
1. Capacitors are charged in parallel to desired voltage and first spark gap is triggered
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p g p g p g p gg
2. The rapid change in potential causes the subsequent gaps to ignite causing the stages to beconnected in series
3. Output voltage is the product of charging voltage and the number of stages U0 = n Uc
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RC
RC
CS
CSRD
RE
RC
CSRD
RE
RD
RE
RD
CB
UC
RC
RC
CS
CSRD
RE
RC
CSRD
RE
RD
RE
RD
CB
UC
UO
3 Stage ImpulseGenerator CHARGING DISCHARGING
RC
RC
CS
CSRD
RE
RC
CSRD
RE
RD
RE
RD
CB
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~
Charging
Discharging (T1)
Discharging (T2)
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Impulse generators are usually designed C1 >> C2 so that energy,
is sufficient to achieve desired pulse shape
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Efficiency
Time to peak
Voltage over test object
RC
RC
CS
CSRD
RE
RC
CSRD
RE
RD
RE
RD
CB U0
UC
21
2111
CC
CCR
2122 CCR
1212
21
21
00 )(
tt eeCR
Utu
1
2
12
21 ln
pT
21
1
0
0
CCC
Uu
U0 = nUC
C1 = C s / n
C2 = Cb + Ctest object
R1 = nR D + R D
R2 nR E
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RC
RC
CS
CS
RD
RE
RC
CS
RD
RE
RD
RE
RD
CB U0
UC
1.0
0.9
0.5
0
U
t
Tp
T2
Damping resistance R1 and load capacitanceC2determine front time T1 and time to peak Tp
Discharge resistance R2 and surgecapacitance C1determine time to half value T2
Charging resistors R C limits current to protectsource
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IMPULSE CURRENT SOURCES
Surge Currents
Rectangular Pulse
IMPULSE CURRENT SOURCES
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Standard surge currents used in testing:
0.1
0.5
0.91.0
i
tT2
T1 0.1
0.91.0
i
tTt
Td
< 0.1
Td+20 %, Tt 1.5Td=
500 s, 1000 s, 2000 s, 2000 3200 s
Testing of surge arrestor ability to dischargecharges with different cable lengths
T1/T2 10 % =
1/20 s, 4/10 s, 8/20 s, 30/80 s
Simulate lightning current stress
Rectangular PulseImpulse
B i i it f i l t t
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Basic circuit for impulse current generator:
Current impulse should be exponentiallydecaying or strongly attenuated (damped)in case of oscillations (b = imaginary, i)
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btebL
Uti t sinh)( 0
3
2
1
i
t
1. Exponential over-damped pulse
2. Weakly damped oscillating pulse
3. Undamped oscillating pulse
R
L
CU0
iTest
object
LR2
LCL
Rb 14
2
2
Basic circuit for long rectangular current generator:
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In theory, could be done by charging a cable to a desired voltage(dependant on current) and discharging into test object using a switch
In practice, cable would need to be 75 km long for a 1000 s pulse
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RU0
i
Cn
Ln
Cn-1
Ln-1 Ln-2
Cn-2
L1
C1
Practicalsolutionis a LC
chain
LCn
nTLC
n
nT td
12
12
n = number ofLCunits
Pulse peak durationand total duration
Required total capacitanceand total inductance
2
)1(2CRL
nRnTC d
n = 8 is optimal
Basic circuit for long rectangular current generator: