Generating High Direct Voltages Required voltage levels

Generating High Direct Voltages 

Required voltage levels 

Industrial facilities and components (dust filters, epoxy resin powder coating 

systems): up to about 100 kV 

Medicine technology, diagnostics (x-ray units): several 100 kV 

HVDC: up to 800 kV (China; planned: 1000 kV) 

High-voltage test laboratories: up to ca. 3 MV 

Physical research (e.g. particle accelerators): up to ca. 25 MV 

Fachgebiet 

Hochspannungstechnik 

High-Voltage Engineering / Chapter 3 - 1 -


Required voltage levels 

Specification for insulation coordination 

of HVDV converter stations 

recently published 

IEC Technical Specification 

IEC TS 60071-5: Insulation Co-ordination 

Part 5: Procedures for high-voltage 

direct current (HVDC) Converter Stations" 

... but no standard insulation levels given! 

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Definitions and requirements 

according to IEC 60060-1 

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Definitions (according to IEC 60060-1) 

Amplitude of a direct voltage = arithmetic mean value: 

T 

1 

U= ∫ut ( )dt 

T 

"Ripple": 

0 

1 

δ U = ( û− 

umin 

) 

2 

"Ripple factor": 

δU 

U 

2δU 

u 

+ + + 

- - - 

û 

U 

u min 

t 

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Requirements (according to IEC 60060-1) 

Ripple factor ≤ 3% 

(Note: Therefore measurement of rms value will also give exact results) 

At test durations ≤ 60 s: change in amplitude max. ± 1% 

(Note: not to be mixed up with measuring uncertainty!) 

At test duration > 60 s: change in amplitude max. ± 3% 

Charging of the test circuit in "reasonably short time" 

(Note 1: charging times of several minutes possible! 

Note 2: for dc bushings steady-state conditions after several hours!) 

Polarity reversal (if required) in short time 

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Ways of generating direct voltages 

By rectifying alternating voltage 

By charge separation 

Single phase 

Two-phase 

Band generators 

(after Van de Graaff) 

• One-way rectifier (w – w/o smoothing) 

• Rectifier bridges (w – w/o smoothing) 

• Doubling circuits (w – w/o smoothing) 

• Multiplier circuits 

Three-phase 

6-pulse-bridges 

12-pulse-bridges 

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Generating High Direct Voltages by help of Rectifiers 

Vacuum valve 

„Ideal“ valve 

Reverse voltage up to 100 kV 

Complex (heating!) 

Only in old (still existing) equipment 

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Semi-conductor rectifier 

Selenium (Se) 

Germanium (Ge) 

Silicon (Si) 

Se-Rectifier 

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Se Ge Si 

max. reverse voltage (V) 50 300 2000 

max. current density (A/cm 2 ) 0,5 150 150 

junction capacitance several nF few pF 

reverse resistance kΩ range MΩ range 

variance of reverse resistance 1:5 1:1000 

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Today virtually always applied: Si rectifiers 

Problem with the small junction capacitance and the large 

variance of the reverse resistance ........ 

RC network required: 

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Implementation example: rectifier of 3.4 MV 

maximum reverse voltage (TU Munich) 

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One-way rectifier circuit 

Peak value: û = û T 

Arithmetic 

mean value: 

U 

1 

= û 

π 

Maximum 

reverse voltage: û V = û T 

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One-way rectifier circuit with smoothing capacitor 

Peak value: û = û T 

Arithmetic 

mean value: 

U ≈û−δU 

Maximum 

reverse voltage: û V = 2û T 

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Calculation of ripple: 

Due to: 

V t « T and δU « U 

assumption of a linear capacitor discharge (instead of exponential) 

Then: change of charge 

T 

2δU ⋅ C = ∫ igdt ≈T ⋅I 

δU 

≈ 

I 

g 

0 

1 

2 fC 

g 

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δU 

≈ 

I 

g 

1 

2 fC 

In order to reduce the ripple at given load current: 

• increase C 

• increase f 

--> usual: f up to several kHz 

+ 

~ 

Improvement also possible by two-way rectification: 

- 

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One-way rectifier circuit with/without smoothing capacitor 

Load curve 

U i0 ... imaginary open circuit voltage 

n ... number of rectifiers 

U 1 ... conducting-state voltage of 

one individual rectifier 

( ) g 

U= U −nU ⋅ −k⋅I 

i0 1 

„k“ generator specific 

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Multiplier Circuits 

Villard-Circuit 

• Booster capacitor charged to û T 

• Potential of high-voltage terminal increased to: 

u(t) = u T + U c 

Peak value: û = 2û T 

Arithmetic 

mean value: 

U 

= û 

T 

Maximum 

reverse voltage: û V ≈ 2û T 

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Smoothing of the voltage not possible with the Villard-circuit! 

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Greinacher doubling circuit 


Connection of a smoothing 

capacitor via a second 

rectifier diode! 

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Greinacher doubling circuit 

u V1 = u T + U C1 mit û V1 = 2û T 

⇒ Charging of C 2 to û V1 = 2û T 

Peak value: û = 2û T 

Arithmetic 

mean value: 

U 

= 

2û 

T 

Maximum 

reverse voltage: 

û V1 = û V2 ≈ 2û T 

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Greinacher-Cascade 

(Cockroft/Walton Multiplier) 

(Greinacher 1920, Cockroft/Walton 1932) 

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Greinacher-Cascade: open circuit voltages 

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Booster column 

Smoothing column 

C 1 = C 2 = C 0 /2 

⇒ All capacitors carry the 

same charge! 

(Q = C·U) 

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Greinacher-Cascade: voltages in load condition 

Imaginary direct voltage n·2û T 

decreased by voltage drop ΔU 

Charge of: 

Ripple voltage 2δU has a 

different shape than ripple 

voltage in the doubling circuit 

Fachgebiet 





Voltage drop: 

Ripple: 

3 2 

Ig 8n 3n n 

+ + 

∆ U = ⋅ f ⋅ C 12 

( ) 

g n n 1 

I + 

δ U = ⋅ f ⋅ C 4 

⇒ Depends on number of stages, load current, frequency and capacitance, 

but not on amplitude of the direct voltage! 

Fachgebiet 





3 2 

Ig 8n 3n n 

+ + 

∆ U = ⋅ f ⋅ C 12 

( ) 

g n n 1 

I + 

δ U = ⋅ f ⋅ C 4 

Greinacher-cascades should always be operated as close as 

possible to their upper voltage limits. 

If lower voltages are requested part of the stages should preferably 

be shorted in order to operate the remaining stages at their upper 

voltage limits. 

Optimum stage voltage = 400 kV for cascades of 1 MV ... 2 MV 

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(C = 60 nF, f = 50 Hz, I g = 10 mA) 

600 

40 

Voltage drop / kV 

500 

400 

300 

200 

100 

Parameter: 

I = 10 mA, C = 60 nF, f = 50 Hz 

Ripple / kV 

35 

30 

25 

20 

15 

10 

5 

Parameter: 

I = 10 mA, C = 60 nF, f = 50 Hz 

0 

1 2 3 4 5 6 

0 

1 2 3 4 5 6 

Stage no. n 

Stage no. n 

n = 4: 

Voltage drop ΔU = 157 kV 

Ripple δU = 17 kV 

Requirement of IEC 60060-1: δU ≤ 3% only fulfilled for voltages ≥ 567 kV! 

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Greinacher-Cascade: two-phase circuit 

Booster columns 

Smoothing column 

Considerable improvement 

of voltage drop and ripple 

Consequential further step: 

three-phase circuit 

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Greinacher-Cascade: two-phase circuit 

Protection of top stage 

and of transformer 

by spark gaps or 

surge arresters 

Protection of the whole 

cascade by a damping 

resistor 

Fachgebiet 




Transformer supported cascades 

Currents > 100 mA at low voltage 

drop and ripple possible 

Transformer must be dc resistant 

Cascading of the transformers by 

excitation and coupling windings 

or 

Simple transformers, fed by 

insulated generators 

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Greinacher-Cascades: Physical arrangement 

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Greinacher-Cascades: Physical arrangement 

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Multiplier Circuits - Implementation Examples 

Direct voltage add-on 600 kV/15 mA 

built by TuR for University of Damaskus 

- 2 stages 

- Air cushion base 

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(Note: circuit diagram 

shows 2 stages only.) 

Direct voltage generator 900 kV/40 mA 

built by TuR for Cable Works Budapest 

- 3 stages 

- Automatic change of polarity 

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2 MV (TU Berlin) 

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2 MV (TU Berlin) 

... here during a test on an 800-kV- 

DC voltage divider 

(Source: Omicron, Trench) 

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(CESI, Milan/Italy) 

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(HighVolt, for ABB Ludvika) 

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(CESI, Milan/Italy) 

cable testing 

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Two-phase (symmetrical) 

seven stage 

Greinacher-cascade 3.4 MV 

(Circuit diagram: two-phase (symmetrical) four stage Greinacher-cascade) 

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(built by TuR) 

- 4 stages 

- 500 Hz supply 

- Si-rectifiers 

- Four 450-kV rectifiers each combined in a rhombic frame 

- Frames to be pivoted by motor drives for change of polarity 

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(built by TuR for Toshiba/Japan) 

- 7 stages 

- Fed by two transformers on 2/7- and 6/7-potential 

- Seismic resistant design 

- Air cushion base for high mobility 

- Change of polarity under load within 200 ms (up to 750 kV) 

Fachgebiet 




Booster columns 

Smoothing 

column 

Pulsating voltage test generator 1600 kV (circuit diagram) 

3 stage direct voltage generator, 

3-phase double way rectifier type 

3-phase double-way bridge 

Feeding transformer 

500-kV transformer; series connected to the direct voltage generator 

Insulating transformer 

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Pulsating voltage 

test generator 1600 kV; 

erection in the test lab 

for commissioning test 

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Greinacher-Cascade 16 kV 

(built from electronic components) 

Fachgebiet

Generating High Direct Voltages Required voltage levels

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