Download KSI`s Cross Insulated Core Transformer Technology

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 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
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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 use
line frequency excitation
• Higher power units are 3 phase
• Each 20kV (typ) disk contains:
1st section of 3 Ø
Line Freq Design.
Disk mounted to
base plate
– 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
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Insulated Core HV Transformers
ICT
• 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
Per Stage Voltage
for 1.5% flux loss
420kV out
500kV out
25000
– 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
20000
Stage Voltage
• 25 stage nominal 500kV
HVPS ICT
15000
10000
5000
0
1 3 5 7 9 11 13 15 17 19 21 23 25
Stage Number
• 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
17”
12.5kV
12.5kV, 100mA
16”
0V
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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 windings
Ntyp = 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
This is about ½ to ⅛
the stored energy of a
Cockcroft-Walton
multiplier equivalent
• 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.
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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
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.
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Derivation of the Cap Value
MMF '  R
I
V
Z
V  N
Z
1
C
I  NC  N 2C
MMF  NI

• 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:
MMF  NI  N 2 2C
R
• Set MMF induced in the secondary to MMF lost in
Reluctance:
• Solve for the cap value:
N 2 2
– This is the total cap value associated with 1 gap
N  C  R
2
Dr. Cross’
Final equation
2
C
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Derivation (cont)
•
C
l
4 2 f 2 N 2  0 A
•
Further algebraic reduction:
Where
l = length of insulated core gap
A = Area of insulated core gap
Substitution:
C
l
0 AN 2 ( 2f ) 2
•
Simplification: This is the useful design equation
R
l
0 A
  2f
• 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
Loss (normalized)
Losses vs. Output V for Resistive Load
100
90
80
70
60
50
40
30
20
10
0
Duty Cycle
drives this
effect
0
20
40
60
80
100
% Output Voltage
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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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Kaiser
SF6 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
– > 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
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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
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