“Diluting SF6 e.g. with CF4”

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Picture: Transition from SF6 gas filled coaxial cables to RG220 like cable (PS KFA-79)

14/08/2018 ABTEF

T. Kramer

Diluting SF6 e.g. with CF4

• Introduction to TE-ABT kickers application and coaxial

SF6 gas filled HV cables

• Some alternatives to pure SF6 gas

• Other alternatives

• Conclusion

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An overview of alternatives, activities in the sector and initial thoughts

SF6 gas applications in CERN Kicker

Systems Overview

• Pressurized (10 bar) pulse forming lines (PFL cable)

• KFA10, KFA20, KFA14, KFA4, KFA45, KFA71/79, KFH31/32/34,

KFI55, KFE50, KFH120, 867

• Pressurized (4 bar) pulse transmission lines (Tx-cables)

• KFA10, KFA20, KFA14, KFA45, KFA71/79, 867

• SF6 gas filled connection boxes

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Remember: Coaxial HV-cables for kicker pulse

generation and transmission requirements

For fast transient events: wave propagation theory applies hence additional requirements w.r.t. conventional cables:

• 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)

• Need to be radiation and fire resistant, acceptable bending radius etc.

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Geometric restrictions-

precise, reproducible

production method

Limited choice in materials

No lossy materials and

semiconducting layers

SF6 gas filled HV-cable (kickers)

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Construction:

• Dielectric:

• thin PE foil wrapped around inner conductor

• pressurized with SF6 gas

Advantages:

• 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 conventional large diameter

coax cables.

• Low attenuation/losses (large ID, no semiconducting layers needed)

Disadvantages:

• Vacuum and SF6 gas systems needed

• SF6 gas - environmental impact

• Special gas tight connectors (in house production) - no quick disconnect

• Cable relatively stiff and heavy (FAK: 1PFL =2.6 t )

• Expensive production, not produced anymore!

• ~14 km in operation at CERN since the seventies (no issues seen so far)

• Nominal voltages up to 80 kV

A reliable solution to meet the outlined requirements

Superior BDV of

impregnated multilayer

insulation systems

Remember: Voids / cracks in a dielectric

Dielectric with lower εr will take higher stress!

Compare PE with voids (air):

Dielectric constant:

PE = 2.2; Air=1; SF6 =1;

Dielectric strength:

PE = 20-160 MV/m; Air = 3 MV/m; SF6 = 90MV/m @10bar;

Voids filled with pressurized SF6 gas

(instead air) support an up to 30 times

higher stress!

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© A. Küchler, Hochspannungstechnik.

SF6 Properties

• Inert gas• Strong electronegative gas (catches e-)• Dielectric strength ~3 times higher than air

(at 1 bar) • Thermal conductivity (~3x air)• Ɛr ~1 -> high velocity factor• Widely used in electrical installations.

Disadvantages:• Under presence of humidity SF6 can be

transformed into toxic substances by electric arcs.

• Worst greenhouse gas: 1kg 23500kg CO2

(GWP 23500)• More and more stringent regulations for

SF6. Certifications for proper handling etc. needed.

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CERN is doing an effort to minimize it’s emissions including reducing the GHG emissions!

Emissions

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US SF6 gas emissions

TE-ABT 2017: 0,0091 t/yr

SF6 gas replacement

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Alternative gas mixtures

• Since the Kyoto protocol efforts have been made to minimize

emissions and to replace SF6.

• Very successful for “leisure and consumer goods industry”.

• Also very successful due to operational

procedures and renovation in electrical

industry (leak tests, seals, no venting).

• Many alternative gas mixtures have

been developed and studied.

• Be aware that most studies consider

AC or DC applications for GIL or switchgear.

• Loads of information by national body’s, university, industry, CIGRE

etc. The following is an excerpt/summary of the most interesting

articles.

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Boiling point

Alternative Gas Mixtures

Alternative Gases

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Xingwen Li et al 2018 J. Phys. D: Appl. Phys. 51 153001

http://iopscience.iop.org/article/10.1088/1361-6463/aab314/pdf

Industry Efforts: 3M/GE Novec

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DS @1bar 27.5kV

Boiling point (@1bar): -4.7C

Material compatibility?? TDS

vague..

Research Efforts….

Dissertation: Environmentally friendly insulating gases for gas-insulated

transmission lines for HVDC (https://diglib.tugraz.at/download.php?id=576a7c47749f2&location=browse )

Looked at minimum GWP:

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No gas mix with same performance

than pure SF6 gas found.

Evaluation Efforts…

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• Clear recommendation to

improve operational

processes and

equipment.

• Says SF6 gas

alternatives have been

studied

• Doesn’t give any clear

recommendation on that

• Further development,

needed…

Statements…

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Short summary:

Despite intensive scientific

research no SF6 gas

equivalent with the same

electrical/physical parameters

has been found.

Statements

CIGRE Study Committee A3 update 05/17 on

“Recent development of alternative gases to SF6 for

switching applications”

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CIGRE: Conseil International des Grands Réseaux Électriques

Back to less exotic solutions: N2

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• Used since awhile

• N2 material

compatibility well known

• Dielectric strength (DS)

compromised.

• Could be interesting

where DS is not

critical.

DS important for our applications!

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• See also next ABTEF presentation by Dimitrios.

Opera sim for 80kV

• Cables would not even work with pure SF6 at 1bar!

• Not enough margin for compromising DS without detailed study and test program.

• Limit max. voltage?

Pulsed Applications

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Important:

• Most studies are done for AC

or DC 50/60 Hz

• Electrode shape plays a role

as well

• There are indications that BDV

in pulsed systems profit from

high SF6 gas levels.

• This behavior introduces a

significant spread in

performance and needs to be

studied in detail. AC/DC

studies are hence not more

than a rough 1st indication.

CF4: • GWP 6500 (100yrs)

• atmospheric lifetime 50,000 yrs

Engineering Letters, 15:1, EL_15_1_22

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Other Alternatives: Modified Heliflex cables

Basic Idea: OTS Heliflex cables with modified dielectric

E.g. fill with oil – e.g. Midel 7131 (Er of 3.2) or Theso (Er 15)

Adjust Er (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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as discussed during ABTEF in autumn 2017

Figure 6. Comparison of the same ‘triple point’ in air (top) and

in oil (bottom) – Heliflex cable simulation

Other Alternatives: SF6 free extruded

cables for >40kV

Advantage:

• “Clean” solution, no gas/hydraulics system

needed (maintenance cost!)

Disadvantage:

• Still needs big diameters for attenuation

reasons. Not many companies have

production lines for that.

• Bulky

• Difficult to manufacture (tolerances)

• No semiconducting layers allowed

• Connectors to be developed as well

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• The CLP30 cable will be an

alternative for the KFA71/79 SF6

gas filled TX cables.

• Performance evaluation (BDV/DS) is

needed to consider it also for PSB

cable replacement (60kV).

I

Other Alternatives:

New Generator Technologies

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As outlined in several ABTEF’s the MARX and IA generator technologies

might replace SF6 gas filled PFL cables on the long-term.

e.g. KFA71 40kV terminated load

e.g. AD/KFA45 40kV short circuited load

Summary (I)

• CERN operates unique SF6 gas filled HV cables.

• 40 yrs Old – not produced anymore (very expensive and long

process to relaunch production)

• Only a few spares and no alternative technology ready for operation

yet

• Any intervention on the SF6 cable systems can therefore be

considered to carry a significant risk.

• Any new insulating gas must be 100% material compatible (long-

term)

• Any new insulating gas must achieve the same DS (system) (or

alternatively operational voltages are to be lowered (very unlikely).

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Summary (II)

• SF6 gas alternatives are heavily studied by industry,

governments, universities, working groups

• Some alternatives have been developed and might find a niche

where DS can be compromised.

• GWP for these alternative gases is often not zero (up to ~12000)

• Operating conditions are challenging and long term decomposition

as well as material compatibility needs further studies.

• Issues with toxicity for some alternative gas mixtures

• Available studies are mostly for AC/DC applications – would need

detailed long-term studies for pulsed operation.

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Conclusions• W.r.t the world wide SF6 emissions the CERN contribution caused by kicker

systems is marginal.

• GWP of alternatives is often significant too.

• Balancing risks and benefits, any change for operational systems must be excluded at the moment (missing detailed studies for our pulsed power systems).

• Budget could be allocated to study and examine alternatives on the 867 test platform for KFA71/79 (additional spares available, mid-term/long-term project).

• Given the total GWP balance and situation concerning DS the focus of efforts should be to suppress SF6 gas by alternative design.

• It is therefore recommended:1. Even if not economic, to continue to proactively contribute to CERNs efforts in

reducing the emission of green house gases.

2. Continue to minimize SF6 gas emission in operating systems by operational procedures / maintenance actions.

3. Successively reduce the installed volume by prioritizing replacement strategies concerning new conventional cables and new solid state technologies.

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Thanks for your attention!

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