The Trials and Tribulations of High Voltage Insulators. David Garton Institute for Environmental Research

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1 The Trials and Tribulations of High Voltage Insulators David Garton Institute for Environmental Research

2 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

3 Electron Energy Band Gaps Metal Semiconductor Insulator + Conduction Band e - Band Gap e - e - e - Valence Band _

4 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 Air 3 kv/mm Alumina 16.7 kv/mm Window Glass 10 kv/mm PTFE kv/mm Mica 160 kv/mm SF6 ~750 25mm (5 bar)

5 dc voltage (kv) r Constant conditions Anode Corona and Spark Breakdown Threshold Curves r = 1 cm d (air gap) Spark Glow Onset streamers No ionisation I I I I I I I Gap spacing (d) (cm) 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.

6 Dielectric strengths of some common materials Plastics Name kv/mm Acrylic 15 PTFE PVC Peek 18.9 Mylar 295 Polycarbonate (Lexan) ABS HDPE 22 Polypropylene Ertalyte 22 Other materials Name kv/mm Boron Nitride 37 Alumina Al 2 O Porcelain Glass (SiO2) 10 Mica 160 Rubber natural 12 Neoprene 12 Quartz 8 Water (distilled) Transformer oil 110 The Physics Hypertextbook, Dotmar Engineering Plastic Products, DIELECTRIC STRENGTH OF INSULATING MATERIALS - L. I. Berger

7 Capacitance 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.

9 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

10 Corona discharges into the air

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

12 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

13 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

14 Gaseous insulators Gases such as nitrogen, CO 2 and SF 6 are commonly used in electrostatic accelerators The ultimate breakdown voltage of the accelerator is related to the gas thickness (pressure) in the vessel SF 6 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 SF 6 is a self healing gas. That is, any by-products produced during spark induced breakdown of the gas eventually recombine back to SF 6 A blend of nitrogen and CO 2 are still used in many accelerator facilities but the gas is lost to the atmosphere on each tank opening

15 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)

16 No.1

17 2011 No.2

18 No

19 Case study 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

20 Old High Voltage deck New High Voltage deck (less electrical stresses)

23 Protection of insulators

24 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!

25 Lichtenberg Figure with fish scale cleaving

DEPARTMENT OF ELECTRICAL ENGINEERING DIT UNIVERSITY HIGH VOLTAGE ENGINEERING

DEPARTMENT OF ELECTRICAL ENGINEERING DIT UNIVERSITY HIGH VOLTAGE ENGINEERING UNIT 1: BREAKDOWN IN SOLIDS 1.) Introduction: The solid dielectric materials are used in all kinds of electrical apparatus and devices to insulate current carrying part from another when they operate at