The Trials and Tribulations of High Voltage Insulators

David Garton Institute for Environmental Research

Overview

• Focus on insulators and insulation used around accelerators to protect people/equipment from High Voltage breakdown

• Will discuss some key insulating materials, their properties, and how we have managed them

• Experiences, good and bad

Conduction Band

Valence Band

Band Gap

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Metal Semiconductor Insulator

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Band Gaps

Dielectrics

• The dielectric strength of a material is its ability to withstand a high electric field before breaking down and becoming conductive. V/length

• Insulators in air: electric field ↑ beyond dielectric strength = corona forms, air becomes partially conductive on surface. Can minimise by using SF6 or similar

• If the electric field continues to rise the air becomes more conductive and eventually avalanche breakdown occurs followed by sparking. The integrity of the insulator is then compromised

• Permanent damage may have occurred

Alumina 16.7 kV/mm

PTFE 50 – 170 kV/mm

SF6 ~750 kV @ 25mm (5 bar)

Air 3 kV/mm

Window Glass 10 kV/mm

Mica 160 kV/mm

Constant conditions Anode Corona and Spark Breakdown Threshold Curves

Glow

dc

volt

age

(kV

)

r d (air gap)

r = 1 cm

Spark

Onset streamers

No ionisation

0 5 10 15 20 25 30 35

Gap spacing (d) (cm)

200

150

100

50

_

_

_

_

I I I I I I I

• Breakdown – Point at which abnormal current levels begin flowing, usually associated with corona discharges or sparking.

• Corona – Also known as corona breakdown is an ionised cloud of gas (plasma). Electrons are being pulled away from the air molecules towards the anode.

• Streamers – Sometimes called leaders, a streamer is an inconsistent flow of electrons between earth and high voltage

• Spark – Point at which air (or other gas medium) becomes a good conductor of electricity.

Dielectric strengths of some common materials

Name kV/mm

Acrylic 15

PTFE 50 - 170

PVC 16 – 51

Peek 18.9

Mylar 295

Polycarbonate (Lexan) 15 - 67

ABS 20 - 25

HDPE 22

Polypropylene 30 - 40

Ertalyte 22

Name kV/mm

Boron Nitride 37

Alumina Al2O3 16.7

Porcelain 35 - 160

Glass (SiO2) 10

Mica 160

Rubber natural 12

Neoprene 12

Quartz 8

Water (distilled) 65 - 70

Transformer oil 110

The Physics Hypertextbook, Dotmar Engineering Plastic Products, DIELECTRIC STRENGTH OF INSULATING MATERIALS - L. I. Berger

Plastics Other materials

• On application of an electric field across an insulator, molecules become polarised within the surface bands of the material.

• The field inside the material is opposite to the applied field. This effectively traps the charge within the material (capacitors).

• Caution must be taken not to use insulators in a way that they can store charge and eventually breakdown. Likewise, the induced electric field may influence the ion beam trajectory.

Capacitance

Managing Electrical Stress with Insulators

Aim to reduce electrical stress in order to minimise breakdown of the insulators.

Consider:

– Is the energy source constant (dc) or transient?

– Is the work environment for the insulator likely to cause early failures of the insulating material?

– Should the insulator’s performance be overrated, that is, have a safety margin?

– Is a secondary insulator required such as an insulating gas?

– Would a control voltage gradient across the insulator help maintain its integrity?

– What is the stored energy and maximum potential energy

– Is the shape and location of the insulator likely to cause breakdown conditions

Corona discharges into the air

Transient conditions

An insulator will have different properties in the presence of a transient discharge.

• In a conductor, high frequency transients can cause currents to flow preferentially closer to the surface (skin effect) due to eddy currents induced in the conductor by the changing field.

• The burst of high energy can jump insulation gaps normally considered to be adequate for dc conditions.

• In some situations like along accelerators tubes/columns transient discharges are a way of life time

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Key points about insulators

• Insulators can be solid liquid or gas

• An insulator will have a specific dielectric strength or rigidity measured in volts per unit length of material

• The creep distance can be extended with fluted edges along the surface

• Most insulators have a finite life. Consider environmental effects

• Insulators must be kept clean. Free of dust, dirt and oils

• Insulators will begin conducting if they become too hot by electrons jumping the band gap

• Sometimes breakdown tracks on insulators are difficult to see. Glassy surfaces don’t allow tracks to grow as easily as matt or rough surfaces

• Insulators can hold some charge. Discharges from the insulator can cause damage on and below its surface

Solid insulators

• Must be homogeneous, that is, the dielectric constant is the same throughout the material

• A variety of insulating materials exist. Modern plastics, rubber, ceramics, glass. Less common now are paper, mineral fibre and bakelite

• Accelerator tubes: ceramic (NEC) or glass (Dowlish type)

• Beam lines: ceramic, Teflon or very modern plastics (Ertalyte, etc)

• At ANSTO we learnt lessons from using bakelite and Perspex as primary insulators. An example is the 846 ion source.

– This caused instabilities in high voltages in the ion source and hence ion beam energies

– Had a rethink about insulting materials and the ways in which high voltage stresses are managed

Gaseous insulators

• Gases such as nitrogen, CO2 and SF6 are commonly used in electrostatic accelerators

• The ultimate breakdown voltage of the accelerator is related to the gas thickness (pressure) in the vessel

• SF6 has amongst the best electrical properties of any gas

• It is a very bad greenhouse gas but it is generally recovered and recycled in accelerator systems

• SF6 is a self healing gas. That is, any by-products produced during spark induced breakdown of the gas eventually recombine back to SF6

• A blend of nitrogen and CO2 are still used in many accelerator facilities but the gas is lost to the atmosphere on each tank opening

Case study 1 – ANTARES stripper gas control rod

• Control rod reaches from the low energy flange on the accelerator through to the HV terminal

• It rotates to switch on and adjust the gas flow through thermal leaks

• Can have up to 8.6 million volts across its length

• Voltage stresses are managed by reducing the voltage gradient at 25 inch centres (635 mm)

2010

2011

No.1

2011

No.2

2011 - No.3

2012 2013

2012

Case study 2 – 846 ion source high voltage deck

• Negative ion caesium sputter ion source, HVEE, model 846B

• Work horse on ANTARES

• Deck voltage can be raised to just above 100kV

• Multiple potentials are managed above and below the deck platform

Old High Voltage deck New High Voltage deck

(less electrical stresses)

Protection of insulators

Summary

• Material selection must be made with consideration to minimising the electrical stress

• Care must be taken not to cause secondary problems such an insulator that becomes a capacitor

• Keep insulators clean

• Look for damage at the macro scale

• Hold your breath and hope for the best!

Lichtenberg Figure with fish scale cleaving