Initial thoughts re prospects for alternatives to SF6 - PowerPoint Presentation

Slide1

Initial thoughts re prospects for alternatives to SF6 cables

T. Kramer

24/09/2017

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Content

Coaxial HV cables IntroductionSF6 gas filled HV cablesAlternatives Complete Pulse generation alternatives

Overview

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High Voltage Coaxial Cables

for Kicker Systems

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Transition from SF6 gas filled coaxial cables to RG220

(PS KFA-79)

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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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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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Coaxial Cables Basics

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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).

Cross-section of coaxial cable

Dielectric (permittivity

ε

r

)

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).

(b-a) needs to withstand

U

nom

Material and diameters can be selected

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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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Why often 30/50/75 Ω?

Because for each dielectric

an

attenuation minimum exists:

PE (

e

r

=2.2): the optimized impedance is ~52

Ω

Air: 75

Ω

-> extensively used in radio transmitters

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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.

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HV-Capacitor

HV-Coaxial Cable (PFL)

Artificial pulse forming network (PFN)

“Distributed” energy storage and switching

Marx Generator

Inductive Adder

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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

c

apacitor discharge.

Droop compensation needed.

Pulse Generators

HV-Capacitor

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Pulse

Generators

Pulse Forming Line (PFL)

Low-loss coaxial cable

Fast and ripple-free pulses

Attenuation & droop becomes problematic for pulses >

3

μ

s

Above 40 kV SF6 pressurized PE tape cables are used at CERN

Bulky: 3

μ

s pulse ~ 300 m of cable

Reels of PFL used at the PS complex (as old as the photograph!)

SPS extraction kicker (MKE) PFN (17 cells)

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SF6 gas filled HV-cable (kickers)

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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!

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

CO

2

More and more stringent regulations for SF6.

Certifications

for proper handling needed.

Fairly complex

and costly cable

production.

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Remember: Voids / cracks in a dielectric

Dielectric with lower

e

r will take more 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 SF6 (instead air) support a ~30 times higher stress!

RG220 @ 35kV

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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.

Siemens GIL

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

(<

e

r

).

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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?

p

rocess to get and keep impedance

BDV – spacer surface discharges?

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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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Pulse

Generators

Pulse Forming Network (PFN)

Artificial coaxial cable made of lumped elements

For low droop and long pulses > 3

μ

s

Each cell individually adjustable: adjustment of pulse flat-top difficult and time consuming.

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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.

FAST PFN Project?

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Inductive Adder

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.

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pulse capacitor

SC-switch

stalk

(secondary)

stacked layers

magnetic core

primary winding

insulation

parallel branches

PCB

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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Marx generator concept, results &

challenges

Marx generator concept: n

capacitors charged in parallel by relatively lowvoltage power supply Udc, through Tc switches and diodes Dc, subsequently

Tp

switches connect all C capacitors in series with the load, applying approximately

n

Udc

. F

or

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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Overview and Outlook

Replacement of KFA45 would require 1MCHF

What could be done with 1MCHF?

And who is available?

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