S-18 3150 Generation of High Voltages

7/27/2019 S-18 3150 Generation of High Voltages

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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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https://noppa.aalto.fi/noppa/kurssi/s-18.3150 2

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


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)

6

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


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


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


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


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

21

~

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


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



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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:

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S-18 3150 Generation of High Voltages