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REINHOLD AN D TR UE MPY: FU TUR E HIGH VOLTAGE DC POWER SUPPLIES

FUTURE HIGH VOLTAGE DC P OWE R S UP PL IE S OF TH E SHIELDED DESIGN

G . Reinhold and K . TruempyAccelerator D ep ar tm en t, E mi le Haefely & Co., Ltd.

Basel/Switzerland

Summary

A large number of research labora-tories are now using stabilized 600 kVdc power supplies of th e entirely shiel-ded design, w i th e le ctr on ic ac supplyand feed-back control systems.1 Recently,a new 600 kV power supply with a ratedcurrent of 20 mA has been marketed. At

the pre se nt t i m e , 1 MV dc power supplieswhich will operate i n aluminum vesselsunder pressurized sulphur hexafluoridegas insulation are being constructed. Th estandard models have an overall stabilityincluding ripple voltage and noise of 1

part i n 1 0 0 0 , both fo r continuous andpulsed operation.

Electrical and mechanical layoutof th e high voltage generator

Th e high voltage rectifier in th e600 kV units consists of a 4-stageCockcroft-Walton circuit, which uses oil-insulated ac and dc capacitors andselenium rectifiers. To protect th erectifier circuit from direct short-circuits to ground, a high-ohmic dampingresistor, which together with th e capaci-

tive part of the ohmic-capacitive voltagedivider, forms a f i l t e r , i s connected toth e high voltage terminal. The Cockcroft-Walton circuit i s energized from a highvo lt age t ran sformer at a frequency of

between 7.5 k c / s to 10 k c / s . Th e compo-nents of the cascade rectifier are en-

closed i n a steel tank filled with highquality mineral o i l . To i m p r o v e th e heatdissipation, a large number of coolingpipes are welded to th e tank. No arti-ficial or forced water or ai r cooling i srequired.

Th e high voltage i s brought out by

means of a special lead-through, con-sisting of an araldite socket and a cone-

shaped plug, which fits the socket,attached to one end of th e shielded h i g hvoltage cable. Th e space between th e plugand the socket i s filled with a s p e c i a lthick cable o i l . L e ss than one hour i sneeded for connecting th e high voltagecable to th e rectifier tank.

Th e free end of th e high voltagecable can be equipped with a standard

600 kV dc bushing of th e air-insulatedtype or with another insulating plugterminal, fo r further feeding the outputvoltage into a pressure or oil v es se l.

Power supplies, with an open air

terminal, are used as high voltage dctest facilities i . e . fo r testing cablesor insulators, while the ones with th e

plug termination, serve as supplies forinstallations such as electron accele-rators fo r industrial irradiation or fo r

electron microscopy.

Figure 1 shows a photograph of a

600 kV dc power supply during factorytests. Th e test installation includes a

600 kV dc bushing, a load resistor and

a precision voltage divider used fo r

measuring th e stability of th e outputvoltage.

AC supply and feed-back control systems

Figure 2 illustrates th e simplifiedcircuit diagram of th e cascade rectifier,th e ac supply s ys te m and th e feed-back

control.

Th e high v olt ag e tra n s for mer i s

energized from a class B push-pull power

amplifier which uses two water-cooledtriodes type Eimac 3CW20000Al. Th enecessary 7 kV p la te v ol ta ge i s suppliedby a 3-phase, full-wave, rectifier unitc onnected b etw een th e center ta p of th eprimary of th e high voltage transformerand ground. The high voltage transformerhas been designed so that i t s inductance,together with th e capacitance of th ewinding, forms a resonant circuit at th eintended operating frequency.

An operating frequency in th e rangeof 7.5 k c / s to 10 k c / s w as chosen i n

order to reduce th e s i z e of the co mpo -nents i n th e rectifier stack as well as

th e stored energy of the capacitors,while maintaining a low voltage drop andripple voltage. Th e frequency i s limitedalso by th e selenium rectifiers whichhave a large inverse current at fre-

quencies i n exce ss of 20 k c / s . Moreover,th e design of th e high voltage power

1 3 9

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IEEE TRANSACTIONS ON NUCLEAR SCIENCE, JUNE 1 9 6 7

transformer presents greater problems athigher frequencies.

To avoid excessive plate dissipatiori n th e power amplifier, i t i s necessaryto tune th e operating fre quency e xac tly

to the ringing frequency of the highvoltage t ra ns fo rm er a nd th e associatedrectifier circuit. This i s achieved bytuning the RC oscillator in such a w aythat no reactive currents are drawn bythe high voltage transformer from thepower amplifier. Experience has proven,

that the operating frequency of theoscillator and the r es on an t frequency of

th e entire circuit will remain constantover v ery long periods of t i m e , so thatno resetting i s required. Th e frequencyadjustment i s a simple procedure whichcan be made w ith an oscilloscope.

The power dissipation of the anodesi s given by th e following relationship:

E I E Ip bb ppm ( 1 )

p - lcos 5 4

whereP =p

E b bI -pm

E -pm

1 i

plate dissipation

dc plate voltage

peak plate current

peak plate voltage

9 9 = phase shift due to incorrecttuning

Because of the high power gain of

these t u b e s , a s i m p l e p r e a m p l i f i e r , withrelatively low power output can be usedfo r d r i v i n g .

Th e operational data of th e poweramplifier are as follows:

dc plate voltage

dc grid voltage

7000 V

-1300 V

zero-signal dc plate current 1.5 Amax-signal dc plate current 7.0 A

peak ac grid driving voltage 1300 V

d r i v i n g power

o utp ut p ow er

O W

29000 W

The power amplifier i s p ro te cte d b yan over-current r e l a y . Other protectivecircuits will prevent damages which maybe due to lack of cooling w ater or to

failures in the n e g a t i v e grid s u p p l y .

To achieve a high stability of thedc output voltage two feed-back loops areused. Th e operation of these feed-backloops can be b ette r understood from th eblock diagram of figure 3 and from th e

simplified circuit diagram of figure 2 .

Th e control signal of the firstfeed-back loop i s obtained from anadditional measuring winding on theprimary of the high v olt ag e tra n sfor mer.This system includes a rectifier andfilter u n i t , th e modulator and th e power

amplifier. Primarily, i t i s designed i norder to minimize the influence of th erelatively high impedance of th e highvoltage transformer and th e fluctuationsof the mains supply voltage.

High voltage stability with respectto variations of the load i s achieved by

th e second feed-back l o o p . Th e dc o utputvoltage i s measured by means of an ohmic-capacitive vo lt age d ivid er. The o utputvoltage of th e secondary unit of the

voltage divider i s compared with an ad-justable reference voltage. Any devi-ations between the reference voltage andthe secondary voltage, i . e . error signalsare amplified i n separate ac and dc

amplifiers and applied to th e modulatorthrough a mixer amplifier. Th e modulatorserves to regulate the amplitude of themean frequency, so that fluctuations ofth e dc output voltage will be outbalancedby th e amplitude variations of the ac

input voltage. Obvi ou sl y, th is systema ls o c om pe ns at es fo r variations of th e

mains supply voltage.

Th e reference voltage s ource us es

s p e c i a l glow discharge tubes enclosed i n

a thermostate. Two helipots are providedfo r th e coarse and fine adjustment of thereference voltage.

The relationship between th e highvoltage dc output and th e referencevoltage i n p u t i s g i v e n by th e formula:

G G2 G 3v d c R Xl+H G + H G G G

1 2 2123 ( 2 )

where G and H are th e circuit parameters,as shown i n f i g u r e 3 . This formula im-plies t h a t , fo r high gain regulation, th edenominator must be much bigger thanunity. Thus, formula ( 2 ) becomes

= V Rdc H 2

R .where H2 = 1

m

( 3 )

1 4 0

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IEEE TRANSACTIONS ON NUCLEAR SCIENCE, JUNE 1 9 6 7

F i g . 1 . Test i n s t a l l a t i o n f o r 6 0 0 kV d o p o w e r s u p p l i e sw i t h l o a d r e s i s t o r , h i gh v o lt a ge b u s h i n g a n dp r e c i s i o n v o l t a g e divider.

P L A E O T . A G E

F i g . 2 . S i m p l i f i e d c i r c u i t d i a g r a m o f t h e 6 0 0 kV d cp o w e r s u p p l y .

F i g . 3 . B l o c k d i a g r a m o f t he f e e d - b a c k c o n t r o l o f t h e6 0 0 kV d c p o w e r s u p p l y .

1 4 2

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