Cross Transformer Technology CTT High Voltage Power Supplies

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Cross Transformer Technology(CTT)High Voltage
Power Supplies

• PESP 2008
• Jefferson Lab
• October 2, 2008
• Uwe Uhmeyer
• Kaiser Systems, Inc.
• Beverly, MA

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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

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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 O/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

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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

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Typical ICT Implementations

• 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

1st section of 3 Ø Line Freq Design. Disk mounted
to base plate
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Insulated Core HV TransformersICT

• In use for many years
• Secondary windings in close proximity to
  secondary core sections
• Multiple Gap design
• Flux leakage occurs at fringes of gaps.

Conceptual Diagram (Middle phase ckts not shown)
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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

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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

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Cross Section of CTT Stack

• 500kV Stack Shown

• Dome (corona shield)
• Ferrite top bar
• Grading rings
• Section ferrite tiles
• 12.5kV stack cards
• (green)
• Insulating film (yellow)
• Primary winding
• Ferrite bottom bar

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CTT Stack Card Building Block
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12.5kV
12.5kV, 100mA
16
0 V
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CTT Stack Card Building Block

• 32 Identical Circuits
• Each produces up to 400 Vdc
• All in series
• 12,500V per stack card typical

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CTT Stack Card Building Block

• Zoom in on 4 elements

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1 Element of CTT Stack Card

• Output series limiting resistor
• Flux Compensation Capacitor
• Planar secondary xfmr windingsNtyp 2/5
• Per element fuse
• Voltage Doubler

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CTT Advantages

• Low Stored Energy 50nF/element
• ?0.98J / 100kV
• ?4.88J / 500kV
• ?7.32J / 750kV
• Minimal Voltage Stress across stack board
• lt 200Vpp from xfmr Components see lt400V
• 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 ? the stored energy of a
Cockcroft-Walton multiplier equivalent
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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

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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

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Overcoming Flux Leakage Inherent in ICT

• 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.

• 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

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Derivation of the Cap Value

• The problem MMF lost across each gap
• Reconstructing 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
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Derivation (cont)

• Further algebraic reductionWhere l length
  of insulated core gap A Area of insulated core
  gap
• Substitution
• Simplification This is the useful design
  equation

• 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!

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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

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PSM / ZVS Efficiency
Duty Cycle drives this effect
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Building a CTT stack

• Section ferrite tiles
• 2x insulating films
• 12.5kV stack cards
• (green)
• Grading rings
• Primary winding
• Ferrite bottom bar

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Building a CTT stack

• HV Divider Resistors

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Building a CTT stack

• Clamping bars at top of stack

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Building a CTT stack

• Dome (corona shield)
• Ferrite top bar

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A 750kV CTT Stack
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KaiserSF6 Vessel
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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.

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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
• gt 80 overall. Typically 92 at full voltage
• Line Regulation lt 0.5 for a change of 10
• Load Regulation lt 0.5 for a change of 10
• Stability Ripple lt 0.5 total variation for
  fixed output voltage, current and temperature
• Temp Coefficient lt 200ppm/C
• Reproducibility lt 0.5 after 1 hour warmup
• Operating Temp 15C to 40C

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Conclusions

• KSI 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

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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