cross transformer technology (ctt) high voltage power supplies

This Document Contains Confidential Information Proprietary To Kaiser Systems Inc. 1

Cross Transformer Technology(CTT)

High Voltage Power SuppliesPESP 2008

Jefferson LabOctober 2, 2008

Uwe UhmeyerKaiser Systems, Inc.

Beverly, MA


Cross Transformer Technology

• Dr. James Cross Transformer Technology for HV generation based on Insulated Core Transformer (ICT) techniques

• Incorporates several significant innovations– US Patents #5,631,851 & 6,026,004

• Implementations shown from 25kV to 1MV at power levels to 200kW

• Kaiser Systems is exclusive worldwide licensee for the Cross Transformer Technology (CTT) patents

• Technology suitable for SF6, oil or solid insulation


Conventional HV Transformers

• A design issue for HV transformers is the insulation system between the HV secondaries and the transformer core

• Ferrites are conductive (~ 106 Ω/cm) and will draw corona

• A 500 kV HV xfmr with a conventional core would require several inches of clearance between secondary and the core in addition to the size of the windings and core itself

• Greater size, weight and cost• Increases leakage inductance and decreases

efficiency


ICT/CTT Principles of Operation

• HV DC output of the overall system produced by multiple sections wired in series– Each section has its own secondary winding(s) and

rectifiers, usually configured as output doublers• Vdc = 2 x Vpp

– Each section is associated with its own piece of magnetic core material, electrically connected to its rectified output, but insulated from its neighbors

– Core to Winding insulation requirement for each section is never more than the localized Vdc output of that section


Typical ICT Implementations

1st section of 3 Ø Line Freq Design. Disk mounted to base plate

• Conventional ICT designs useline frequency excitation

• Higher power units are 3 phase• Each 20kV (typ) disk contains:

– 3 Series output arc limiting resistors – 6 Rectifiers/Capacitors – 3 magnetic core pieces & secondary windings

• Disk sections are not all the same as higher turns ratio needed on higher disks to keep similar Vout

• Most common regulation technique is motor driven variable transformer to vary primary voltage– Control BW limited to 10s of Hz


Insulated Core HV TransformersICT

Conceptual Diagram(Middle phase ckts not shown)

• In use for many years • Secondary windings in

close proximity to secondary core sections

• Multiple Gap design• Flux leakage occurs at

fringes of gaps.


Effect of Flux Leakage• 25 stage nominal 500kV

HVPS ICT– If 1st stage is limited to 20kV,

the HV output will only go to 420kV

– If the loop is closed and 500kV is set, then the 1st stage needs to go to about 24000V

• To minimize flux leakage, ICT design trade off is to decrease # stages at expense of higher stage to stage voltage

• Often the turns ratio increased with each stage to keep stage to stage voltage the same

Per Stage Voltage for 1.5% flux loss

0

5000

10000

15000

20000

25000

1 3 5 7 9 11 13 15 17 19 21 23 25

Stage Number

Stag

e Vo

ltage

420kV out

500kV out


Advantages of CTT over ICT• Uniform Voltage Per Stage

– Due To Compensation Of Flux Leakage• Extremely Low Stored Energy• Fast Transient Response Time• Greater Efficiency• Straightforward Manufacturability

– Lower Cost To Produce• Compact Size• Higher Reliability

– Corona Free Design– Efficient Operation


Cross Section of CTT Stack

Dome (corona shield)

Ferrite top bar

Grading rings

Section ferrite tiles

12.5kV stack cards (green)

Insulating film (yellow)

Primary winding

Ferrite bottom bar

500kV Stack Shown


CTT Stack Card Building Block

12.5kV, 100mA

16”

17”

0 V

12.5kV



• 32 Identical Circuits

• Each produces up to 400 Vdc

• All in series

• 12,500V per stack card typical



• Zoom in on 4 elements


1 Element of CTT Stack Card

• Output series limiting resistor

• Flux Compensation Capacitor

• Planar secondary xfmr windingsNtyp = 2/5

• Per element fuse

• Voltage Doubler


CTT Advantages

• Low Stored Energy: 50nF/element0.98J / 100kV4.88J / 500kV7.32J / 750kV

• Minimal Voltage Stress across stack board– < 200Vpp from xfmr; Components see <400V– Local E field is no more than 1kV/inch!

• Compare to 10kV/inch in air widely used clearance guideline!– Corona inception voltage never exceeded.

• No gradual degradation of Insulating materials by corona.

This is about ½ to ⅛ the stored energy of a Cockcroft-Walton multiplier equivalent


CTT Advantages (cont)

• Lower implementation cost– Simple 2 layer PCB technology

• Planar transformer design– No secondary windings to be individually wound

– Stack cards are identical to build large stacks• e.g. 40 stack cards for 500kV or 60 stack cards for 750kV• Only 1 type spare needed

– Surface mount technology components.• Relatively low cost, especially in volume purchase• Automated assembly on SMT equipment


CTT Advantages (cont)

• High Reliability– Corona free design– Simple construction– Fault Tolerance

• Individual failed elements will not take out the entire system• Typical fault is shorted element as a result of a severe arc.• Fuse for secondary will blow• Shorted element maintains series connectivity• System continues to operate with n-1 output voltage elements


Overcoming Flux Leakage Inherent in ICT

Problem Statement from Patent• “The segmentation of the

magnetic core in the transformer introduces gaps in the magnetic structure with a permeability essentially that of air. This greatly increases the reluctance of the magnetic structure and produces leakage of magnetic flux.

• As a result, the upper sections of the magnetic core carry less flux than the lower sections of the core, which results in lower generated voltage per turn on the secondary windings.”– Page 6, beginning w/ line

63, US Patent 6,026,004

Benefits of flux compensation• Flux compensation restores the

lost MMF per gap.

Resultant Observations• The energy associated with the

‘leaking’ fields may be associated with the value of the ‘leakage inductance” property of transformers.

• Compensating for the leakage flux in effect cancels out the leakage inductance.

• Ideally, this should help the control system by reducing the second order effect of voltage droop.


Derivation of the Cap Value

ZVI

NV

CZ

1

CNCNI 2

CNNIMMF

NIMMF

22

RMMF '

RCN 22

22NRC

• The problem: MMF lost across each gapReconstructing Lost Flux: • Current induced in the secondary will be equal to

the voltage in the secondary over the impedance.• Voltage from a transformer is # of turns times the

first derivative of the time varying flux.:• Impedance created by a capacitance across the

secondary:• …algebra• MMF resulting from the reactive current in the

secondary:

• Set MMF induced in the secondary to MMF lost in Reluctance:

• Solve for the cap value:– This is the total cap value associated with 1 gap

Dr. Cross’ Final equation


Derivation (cont)

• The value of the Flux compensation Capacitor C is a function of only the transformer physical properties and the operating frequency!

• It is independent of the output voltage or output current!

• Further algebraic reduction:Where

l = length of insulated core gapA = Area of insulated core gap

• Substitution:

• Simplification: This is the useful design equation

fAlR

20

ANflC

02224

220 )2( fAN

lC


Control Topology• Ideal drive topology should produce a fixed

frequency sinusoidal voltage waveform at primary of transformer.

• Practical Implementations effectively done with Phase Shift Modulation (PSM)– Allows for Zero Voltage Switching (ZVS)

• Practical systems built by KSI operate at 80 to 90 kHz


PSM / ZVS Efficiency

Losses vs. Output V for Resistive Load

0102030405060708090

100

0 20 40 60 80 100

% Output Voltage

Loss

(nor

mal

ized

) Duty Cycle drives this effect


Building a CTT stack

Section ferrite tiles

2x insulating films

12.5kV stack cards (green)

Grading rings

Primary winding

Ferrite bottom bar



HV Divider Resistors



Clamping bars at top of stack



Dome (corona shield)

Ferrite top bar


A 750kV CTT Stack


KaiserSF6 Vessel


General Specs 750kV, 100mA

• Output Voltage and Operating Range– Continuously variable between 50 kV and 750 kV. – Meets all the efficiency, stability and regulation specifications over

its normal operating voltage range of 100 kV to 750 kV.

• High Voltage Section Insulation. – Pressurized SF6 gas, maximum pressure of 5 atm. absolute (59

psig). • HV Driver

– Separate Cabinet with Control module and Inverter module system.

– Inverters require water cooling.


General Specs 750kV, 100mA• Power Supply Input Voltages.

– Inverter supply: 480V ±10%, 3-phase, 50-60 Hz AC. – Controls and interlocks power: 120 Vac

• Efficiency– > 80% overall. Typically 92% at full voltage

• Line Regulation: < ±0.5% for a change of ±10%• Load Regulation: < ±0.5% for a change of ±10%• Stability & Ripple: < ±0.5% total variation for fixed output

voltage, current and temperature• Temp Coefficient: < 200ppm/°C• Reproducibility: < 0.5% after 1 hour warmup• Operating Temp: 15°C to 40°C


ConclusionsKSI CTT HVPS designs provide many advantages over conventional line frequency ICT designs:• Fault Tolerance• High Reliability due to Corona Free Stage Design• Compact Design

– Easily Integrated Into E-beam Vessel

• Low Stored Energy• Excellent Transient Response• High Efficiency• Scalable Design


CTT Supply

Thanks to:– Matt Poelker for inviting KSI to this conference.– Jefferson Laboratory– David Johns, Yuri Botnar, Ken Kaiser and Steve Swech

for their contributions to this program and presentation

cross transformer technology (ctt) high voltage power supplies

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