1. Initial thoughts re prospects for alternatives to SF6 cables
T. Kramer
24/09/2017
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2. Content Coaxial HV cables Introduction SF6 gas filled HV cables
Alternatives
Complete Pulse generation alternatives
Overview
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3. High Voltage Coaxial Cables for Kicker Systems
Transition from SF6 gas filled coaxial cables to RG220 (PS KFA-79)
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4. Coaxial HV-cables for kicker pulse generation and transmission
For fast transient events: wave propagation theory applies hence different requirements:
Matched and homogenous impedance (to avoid a loss of kick strength and reflections along the line)
Low attenuation / losses (to avoid droop and pulse distortion)
High dielectric strength (to support voltages high enough to drive the required current)
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5. Coaxial Cables Requirements
Coaxial cables play a major role in kicker systems!
Some important requirements :
Need to transmit fast pulses, high currents.
Should not attenuated or distort the pulse (attenuation < ~5.7dB/km for RG220 and <3dB/km for SF6 filled both at 10 MHz).
Need to insulate high voltage (conventional 40kV, SF6 filled 80 kV).
Precise characteristic impedance over complete length mandatory (<0.5% tolerance).
Need to be radiation and fire resistant, acceptable bending radius etc.
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6. Cross-section of coaxial cable
Coaxial Cables Basics
Cross-section of coaxial cable
Dielectric (permittivity εr)
Capacitance per metre length (F/m):
Inductance per metre length (H/m):
Characteristic Impedance (Ω):
(typically 20 Ω to 50 Ω).
Delay per metre length:
(~5ns/m for suitable coax cable).
Material and diameters can be selected
(b-a) needs to withstand Unom
Where:
a is the outer diameter of the inner conductor (m);
b is the inner diameter of the outer conductor (m);
is the permittivity of free space (8.854x10–12 F/m).
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7. Attenuation / losses Resistive losses skin effect, proximity effect
Losses in the dielectric
Radiated losses for high frequencies only ( less important for our applications)
material conductivity
diameter
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8. Why often 30/50/75 Ω?
Because for each dielectric an attenuation minimum exists:
PE (er=2.2): the optimized impedance is ~52Ω
Air: 75 Ω -> extensively used in radio transmitters
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9. Coaxial HV-Cables Applications: Pulse Generators Overview
For energy storage and pulse shaping pulse forming lines (PFL) or artificial pulse forming networks (PFN) can be used.
A power switch is needed to switch the charged “energy storage” to the load. Spark gaps (not anymore at CERN), Thyratrons, Ignitrons, Solid state switches etc. are frequently used.
HV-Capacitor
HV-Coaxial Cable (PFL)
Artificial pulse forming network (PFN)
“Distributed” energy storage and switching
Marx Generator
Inductive Adder
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10. Pulse Generators HV-Capacitor LHC MKD:
FHCT stack with
trigger transformer
LHC MKD:
Pulse generators using capacitor discharge for pulse generation.
Advantage:
(in principal) fairly simple circuit.
Disadvantage:
Droop due to capacitor discharge. Droop compensation needed.
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11. Pulse Generators Pulse Forming Line (PFL) Low-loss coaxial cable
Fast and ripple-free pulses
Attenuation & droop becomes problematic for pulses > 3 μs
Above 40 kV SF6 pressurized PE tape cables are used at CERN
Bulky: 3 μs pulse ~ 300 m of cable
Reels of PFL used at the PS complex (as old as the photograph!)
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24/09/2017
SPS extraction kicker (MKE) PFN (17 cells)

12. SF6 gas filled HV-cable (kickers)
Dielectric: thin PE foil wrapped around inner conductor, pressurized with SF6 gas - fills all voids
Superior dielectric strength
Lower velocity factor due to low density PE core
No issues with surface discharge of spacers used in large diameter coax cables.
Low attenuation/losses (large ID, no semiconducting layers)
~14 km in operation at CERN since the seventies (no issues seen so far)
Nominal voltages up to 80 kV
Disadvantage:
Vacuum and SF6 gas systems needed
Special gas tight connectors (in house production)
No quick disconnect
Cable relatively stiff and heavy (FAK: 1PFL =2.6 t )
Not produced anymore!
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13. SF6 Properties Electronegative gas (catches e-)
Dielectric strength ~3 times higher than air (at 1 bar)
Insulating gas penetrates into little gaps and cracks
Ɛr ~1 -> higher velocity factor
Pure SF6 gas would give 1/3 longer PFL
Mixed PE structure with SF6
Wrapped PE to avoid surface discharges
Disadvantages:
SF6 can be transformed into toxic substances by electric arcs and under presence of humidity
Worst greenhouse gas 1kg 23000kg CO2
More and more stringent regulations for SF6. Certifications for proper handling needed.
Fairly complex and costly cable production.
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14. Remember: Voids / cracks in a dielectric
35kV
Dielectric with lower er will take more stress! Compare PE with voids (air): Dielectric constant: PE = 2.2; Air=1; SF6 =1; Dielectric strength: PE = MV/m; Air = 3 MV/m; SF6 =
Voids filled with SF6 (instead air) support a ~30 times higher stress!
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15. Alternatives to SF6/PE insulated Cables: GIPFL – Gas Insulated Pulse Forming Line
GIL Used for energy distribution (up to 500kV/5kA) e.g. installed below Palexpo, extensively used in the alps for caverned hydropower stations.
Advantage:
Simple and robust.
Long life time (~50yrs).
No maintenance (gas enclosed).
Largely self healing.
Disadvantage:
Spacers are critical for surface discharges.
Not (yet) designed for pulse transmission.
Not flexible.Bigger diameter than SF6 cables.
High velocity factor due to gas insulation (<er).
Siemens GIL
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16. Alternatives: Modified Heliflex cables
Basic Idea: Take OTS Heliflex cables and modify the dielectric
E.g. fill SF6 or oil –
since SF6 to be supressed take e.g. Midel 7131 (Er of 3.2)
or Theso (Er 15)
Adjust Er (hence impedance!) with Nanoparticle additives?
Advantage:
OTS,
Versatile (one fits all),
Perfect impedance match possible,
Disadvantage:
Oil needed, bulky
Complex? process to get and keep impedance
BDV – spacer surface discharges?
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17. Alternatives: SF6 free extruded cables for >40kV
Advantage:
“Clean” solution
Disadvantage:
Still needs big diameters for attenuation reasons. Not many companies have machines for that.
Bulky
Difficult to manufacture (tolerances).
No semiconducting layers allowed.
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18. Pulse Generators FAST PFN Project? Pulse Forming Network (PFN)
Artificial coaxial cable made of lumped elements
For low droop and long pulses > 3 μs
Each cell individually adjustable: adjustment of pulse flat-top difficult and time consuming.
SPS MKP PFN working at 150ns with MKPS magnet.
Would need a PFN which is more than twice as fast (and still delivers within flat top spec.)
Feasible?
Advantage:
Known technology.
Disadvantage:
Challenging front cell development and tuning.
Construction is a challenge compared to “ordering” a cable and assembly done in house (manpower).
Needs prototype to finally answer feasibility.
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19. Inductive Adder Advantages: Disadvantages: Modularity;
pulse capacitor
SC-switch
𝐼 sec
stalk (secondary)
stacked layers
magnetic core
𝐼 prim
primary winding
insulation
parallel branches
PCB
Complete pulse generator concept.
Energy stored in distributed capacitors.
Capacitors are partially discharged via SiC MOSFET switches in parallel branches.
Several layers add up to the required output voltage.
Advantages:
Modularity;
Short rise and fall times;
Output pulse voltage can be modulated -> excellent flat top quality.
Switches and control electronics are referenced to ground.
Disadvantages:
Output transformer maximum pulse length limited to typically ~5-10 μs (depends on application and magnetic core);
Still needs some R&D
80kV stack currently challenging (size and tr) R&D: IA in SC mode could be very attractive.
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20. Marx generator concept, results & challenges
Marx generator concept: n capacitors charged in parallel by relatively low
voltage power supply Udc, through Tc switches and diodes Dc, subsequently Tp switches connect all C capacitors in series with the load, applying approximately nUdc. For fast rectangular pulses MOSFET technology is required.
Important results to date include: 3 kV operation  3 kA pulses with 65 ns rise and 35ns fall.
Challenges to be studied:
Long-term reliability – concepts to avoid a single-point failure
Droop compensation;
Operation with a short-circuit load.
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21. Overview and Outlook What could be done with 1MCHF?
Replacement of KFA45 would require 1MCHF
What could be done with 1MCHF?
And who is available?
24/09/2017
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