Investigation of High Voltage Insulation Aspects

EUROFUSION WPMAG-REP(21) 31401

S Fink et al.

Investigation of High Voltage InsulationAspects

REPORT

This work has been carried out within the framework of the EUROfusion Con-

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IDM Reference EFDA_D_2PG8DP

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Version/Dates: 1.0

Deliverable MAG-T.02.02-T001-D001

Investigation of High Voltage Insulation Aspects

Work Package MAG

WP Leader Corato, Valentina

References (IMS): Task Specifications (TS), Task Coordinator, Deliverable Owner

TS Title Investigation of High Voltage Insulation Aspects

TS Ref. No MAG-T.02.02-T001

Task Coordinator Wolf, Michael

Deliverable Owner (Benef.) Wolf, Michael (KIT)

Benef(s) involved in Del.

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Reviewer(s)

technical / scientific Bruzzone, Pierluigi

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formal aspects (PSO) Morici, Luigi

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Executive Summary

Review of insulation material suitable for magnets, also outside of fusion environment. Identifications of technical specifications and possible tests on mock-ups to investigate robust insulation for discontinuities and joints

Comments (shortcomings, deviations, etc.)

This report is in an initial description related to the ongoing DEMO magnet design and will require to be updated in an iterative process. As an example, the transient voltage within the magnet system can be influenced by the high voltage component design (e.g. cable properties and length) which in turn may change the test voltage specification for the high voltage component and therefore may cause a subsequent design change for the component.

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Table of Contents

1 Short Introduction and Objectives of Work ........................................................................................ 5

2 Description of Work ............................................................................................................................ 5

3 Materials ............................................................................................................................................. 5

3.1 Vacuum ....................................................................................................................................... 5

3.2 Fluids ........................................................................................................................................... 7

3.2.1 Air ........................................................................................................................................ 7

3.2.2 Nitrogen gas ........................................................................................................................ 8

3.2.3 SF6 ....................................................................................................................................... 8

3.2.4 Helium gas at room temperature ....................................................................................... 8

3.2.5 Demineralized water ......................................................................................................... 10

3.2.6 Liquid nitrogen .................................................................................................................. 10

3.2.7 Liquid helium ..................................................................................................................... 11

3.3 Solids ......................................................................................................................................... 11

3.3.1 Polyimide ........................................................................................................................... 11

3.3.2 Fibre reinforce plastic ....................................................................................................... 12

3.3.3 Mica ................................................................................................................................... 12

3.3.4 Silicone .............................................................................................................................. 13

3.3.5 Teflon ................................................................................................................................ 13

4 Technical specifications .................................................................................................................... 15

4.1 Specification for a cryogenic high voltage instrumentation cable wire feedthrough .............. 16

5 High voltage testing .......................................................................................................................... 17

5.1 Principles for high voltage testing of coils ................................................................................ 17

5.1.1 Paschen tests .................................................................................................................... 19

5.2 Test procedure for initial high voltage testing of a cryogenic high voltage instrumentation

cable wire feedthrough mock-up at room temperature conditions .................................................... 20

6 Limitations of test facilities at KIT ..................................................................................................... 23

7 References ........................................................................................................................................ 23

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List of Tables [if needed]

List of Figures [if needed]

Abbreviations

frp fibre reinforced plastic

tbd to be defined

TFMC Toroidal Field Model Coil

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1 Short Introduction and Objectives of Work

High voltage problems can cause serious delay and cost impact on fusion magnet operation. Hence, it

is necessary to perform studies of the insulation in order to identify potential failures. In most cases

fusion magnet insulation defects occur not on the coil insulation but on transition areas and joint

locations. Therefore, it is important for the DEMO magnet system to develop robust insulation

concepts and techniques for these critical areas. This review should help to identify useful materials

and test strategies in order to develop sound designs for high voltage components.

2 Description of Work

The description of the insulation of fusion magnets in order to identify potential failures starts with a

review of the different materials suitable for magnets. In order to consider potential improvement with

existing technology outside fusion also other potential materials are shortly discussed.

Technical specifications are given in a general view and exemplarily for the cryogenic high voltage

instrumentation cable wire feedthrough. Based on proposed principles for high voltage testing of coils

a test procedure is listed for the initial room temperature test series on a cryogenic high voltage

instrumentation cable wire feedthrough mock-up. Special attention is given to the Paschen test.

3 Materials

Materials used or potentially may be used for the DEMO coil system are listed in the following chapters

according to the physical state. Examples of existing applications and potential problems will be

discussed.

3.1 Vacuum

Vacuum is often considered as a good insulator and numerous high voltage apparatus based on the

dielectric strength of vacuum are well operated outside and inside fusion technology. As an example,

vacuum breakers are well established in conventional power engineering and can also be used for

switching process of the magnet fast discharge unit (counter acting current switch with current zero

transition caused by discharge of a capacitor bank).

In principle vacuum can also be used for insulation purposes of large spaces on fusion magnets e.g. for

ground insulation purposes. Unfortunately, there are two main reasons which prevent such a use.

The first reason to avoid the use of vacuum for insulation purposes of large spaces is the possibility of

degradation of vacuum by appearance of gas leakage. For a certain combination of breakdown length,

temperature and pressure the breakdown mechanism changes from the vacuum breakdown to a gas

breakdown. Such a gas breakdown around the Paschen minimum can happen at voltages of only few

100 V, which is considerably lower than usual operation voltages of DEMO magnets.

The second reason to avoid the use of vacuum for insulation purposes of large fusion magnets is the

appearance or combination of numerous possible reasons that may cause local degradation of the

dielectric strength in case of not well defined conditions. Reasons may be flashovers instead of spark

over engraved with field enhancement on triple points, local gas release or permeation, sliding

discharges caused by impulse voltage waveform, the import of material with a low work function for

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electron release (vacuum breakdown is a surface related breakdown) etc. Consequently, the real

breakdown voltage of huge and not well-defined vacuum spaces is usually much lower than literature

values related to low size experiments with well-defined conditions (polished surface, conditioned,

baked out etc.). In principle it is possible to establish applications for spaces comparable in size to the

vacuum space around a single fusion magnet with voltages far above the magnet voltages (e.g.

MITICA). But it should be considered that this would require a large effort during the complete

operation time. An example for the appearance of a voltage breakdown on large fusion magnet even

under rated vacuum and temperature conditions was the impulse testing of ITER TF Model Coil (TFMC)

with a voltage of 5 kV (see Figure 3-1).

Figure 3-1: Voltage breakdown during impulse testing of ITER TFMC under cryogenic and vacuum condition.

A use of a vacuum space separated from the surrounding vacuum space and with well-defined high

voltage design can be a useful application of vacuum insulation for high voltage components of large

fusion magnets. For ITER TFMC a full size mock-up including cryogenic high voltage instrumentation

cable feedthrough, PTFE high voltage cable and room temperature feedthrough was successfully tested

with a separate vacuum design based on a corrugated and a flat tube (Figure 3-2).

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Figure 3-2: Mock-up for a high voltage instrumentation cable path with separate vacuum within a corrugated and flat tube.

Solid insulation of fusion magnets should be controlled with a “Paschen Test”. For this test, the sample

is embedded in gas with pressure values that lead to a voltage breakdown due to reaching conditions

near the Paschen Minimum in case of existing gaps within the solid insulation.

3.2 Fluids

Fluids are frequently used for high voltage insulation purposes. The disadvantage of fluids are the

usually lower dielectric strength compared to solid dielectrics used in high voltage applications. The

advantage is that they perfectly surround all kind of shapes and they can easily be replaced or they are

even self-healing in case of a disruptive breakdown. Nevertheless too high currents must be prevented

in case of fluid breakdown in order to avoid damaging of adjacent solid conductors or insulators.

3.2.1 Air

The infrastructure on the room temperature side of a fusion reactor is mainly operated in air. High

voltage design in air can follow the traditional rules and standards of power engineering. Air is also an

important insulation material during room temperature high voltage tests of fusion magnets and

components. Room temperature tests must be performed during manufacturing and between several

assembly stages in order to find faults as early as possible. Air is always available and free of cost. Dry

and oil free air is necessary for some applications in order to avoid pollution with water or oil.

Depending on the possibility to clean the surface of objects and local pollution activity the

recommendation for creepage distances for external insulation may be considered [IEC 71-2]. For

pollution level I (light) it is 16 mm / kV. Paschen testing with air is an important method for insulation

diagnostic and fault case localization [Bal19] under room temperature condition. It avoids pollution

with helium which can disturb leak testing. For large scale testing a too fast refilling of the test vessel

with not dry air must be avoided because this can cause condensation.

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3.2.2 Nitrogen gas

Nitrogen gas is used in order to avoid pollution with water, oil etc. compared to ambient or oiled

pressurized air. For room temperature tests nitrogen gas may be selected for replacement of helium

gas because of its higher dielectric strength. This must be especially be considered if there is a risk of

flashovers within the cryogenic axial breaks under room temperature condition.

3.2.3 SF6

SF6 has the highest dielectric strength of all technical gases (in the high pressure side of the Paschen

curve) because of its strong electronegativity. It can therefore be used for room temperature tests

where high dielectric requirements occur (Figure 3-3Figure 3-3: Testing of room temperature high

voltage instrumentation cable feedthrough with cable ends located in a plastic box filled with SF6.).

Figure 3-3: Testing of room temperature high voltage instrumentation cable feedthrough with cable ends located in a plastic box filled with SF6.

In Europe, SF6 falls under a directive which bans or controls its use for several applications. Another

disadvantage is the high price.

3.2.4 Helium gas at room temperature

Helium paths with helium gas on room temperature conditions require special insulation breaks in case

of separation of the high voltage tube system from the grounded side. The dielectric strength of helium

gas is lower than for air at ambient conditions (at 0.1 MPa abs., 20 °C: Ekrit air = 24.5 kV / mm;

Ekrit helium = 3.7 kV / cm [Phi 82]). The design of the insulation break is therefore not trivial if

withstand voltages around 30 kV with an additional safety factor of 2 would be required for a total

component length around 70 cm. The Polo high voltage break is a simple version with an frp tube

screwed between two electrodes (Figure 3-4). According to [Irm 96] this break type had a breakdown

voltage with helium of 50 kVrms for a distance of the electrodes of 45 cm and a pressure of 0.3 MPa.

On the other hand, the high voltage design shall also cover a fault case with a pressure reduction of 0.1

MPa. In addition, the impurity of lab experiments may have been higher than a long duration fusion

reactor operation. Small amounts of electronegative gas (e.g. oxygen) may have strong impact on the

breakdown strength of helium. Furthermore, the flow rate may have an impact (Figure 3-9).

Example of other breaks used or intended for the use of fusion magnets are the polyimide break for

ITER TFMC testing (Figure 3-5, Figure 3-6), an frp type of Bilfinger Noell GmbH (Figure 3-6), an frp type

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from Efremov Institute with an elaborated spiral design (Figure 3-7), and a 3D printed design study

comprising a spiral with flow in up-down-up direction from KIT (Figure 3-8).

Figure 3-4: Room temperature insulation break: Polo type with reduced length.

Figure 3-5: Room temperature insulation breaks: industrial frp type from Bilfinger Noell GmbH (background) and ITER TFMC polyimide (Sintimid) type made by KIT (foreground)

Figure 3-6: Leak testing on room temperature insulation break during ITER TFMC testing

Figure 3-7: High voltage breaks from D.V. Efremov Scientific Research Institute of Electrophysical Apparatus [Bur 14]

Figure 3-8: Design study of 3D printed break from KIT. Only a very cheap printer was used. Hence the break failed leak testing as expected

Figure 3-9: Testing of influence of helium mass flow on breakdown voltages for room temperature insulation breaks. Mass flow was up to 4.6 g / s in Waheffca test facility of KIT.

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3.2.5 Demineralized water

Water-cooling of high voltage components (e.g. current lead terminations, busbars) requires

separation to the ground water piping system. Breakdown of water is regarded as starting with

vaporization of the liquid and Paschen curve for water vapor is available in [Sko 11]. Usually, the

breakdown or flashover voltage of water is not a serious problem in fusion facilities because the

separation can be performed by special breaks or simply by low conductive water hoses and such

hoses are often in the length range of 1 m for other practical reasons. An issue is the conductivity of

water even if it is demineralized because these values influence the resistance value measurement

during high voltage DC testing. Quick couplings for the water hoses for temporary disconnection is

therefore an option for more precise insulation resistance measurement of installed magnet systems.

3.2.6 Liquid nitrogen

Liquid nitrogen is the favourite coolant in power engineering applications (high voltage power

transmission cables, superconducting fault current limiters etc.) and hence it is frequently used for

electric insulation purposes. At the present stage of development, it can be considered as unlikely that

the superconductors of DEMO magnets will be operated at liquid nitrogen temperature. Nevertheless,

it should be mentioned that the dielectric strength of liquid nitrogen is higher than the one of liquid

helium under comparable conditions. It can therefore be concluded that designs suitable for liquid

helium are also usable under liquid nitrogen conditions. For design optimization at least of small to

medium scale volumes (up to 2 * 105 mm3) a literature survey and calculation is published by

Hayakawa (Figure 3-10).

Figure 3-10: Breakdown field strength of liquid nitrogen depending on the volume stressed with high electric field (e.g. for T = 77 K and p = 0.1 MPa it is the space where the field strength is higher than 81% of the maximum field strength value within the particular setup). “Intrinsic” revers to not strong boiling conditions of the liquid nitrogen. “Dynamic” refers to (transient) boiling caused by short duration heating of the liquid nitrogen. [Hay 19]

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Case related studies for flashover breakdown are also published but a development of e.g. a cryogenic

axial insulation break with minimum length for specified conditions requires further experimental

efforts.

3.2.7 Liquid helium

Liquid helium is the cooling medium for the large fusion magnets based on NbTi and Nb3Sn

superconductor material. Under operation conditions pure liquid or overcritical helium is used, which

has excellent dielectric properties in the range of transformer oil. Breakdown data for 0.1 MPa are

required for design of high voltage components if a pressure loss is defined as fault case which must be

handled safely. Such helium breakdown data can be found from an overview of different researchers in

[Har88].

3.3 Solids

Solid insulation can reach the highest breakdown field strength of all materials. The critical issues with

solid insulations are imperfections like structure damaging, containment of foreign material or cavities.

For large scale objects like fusion magnets it is not possible to produce solid insulation without

imperfections. For a suitable design such imperfections dominated the dielectric strength of the solid

insulation. Special care must be taken concerning material degradation by neutron and gamma

radiation because most solid insulation is not self-healing. Typical values for DEMO coils are given as

10 Gy for TF coil [Col 19].

3.3.1 Polyimide

Polyimide is a commonly used material for high voltage application also under cryogenic conditions.

Commercial versions with a long term service temperature of 280°C are available [Ens 19]. Mechanical

properties can also be sufficient for manufacturing based on solid parts of pure polyimide. An example

is one type of room temperature high voltage insulation break used for ITER TFMC testing (Figure 3-6).

Derived from the fact that polyimide is also used for seals it was possible to design this break with the

steel flanges directly assembled on the polyimide tube by adjusting the torque of special screws.

Polyimide tapes are frequently used for insulation purposes either alone or in combination with other

material. The ITER TFMC high voltage instrumentation cable is an example for using only polyimide foils

for the main insulation (Figure 3-11, Figure 3-12).

Figure 3-11: Cryogenic high voltage instrumentation cable for ITER TF Model Coil. The main insulation is made from polyimide.

Figure 3-12 Open cryostat extension for ITER TFMC with room temperature high voltage instrumentation cable feedthroughs. Termination of cryogenic polyimide cables are located within the vacuum space.

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It should be noticed that the resistance of polyimide against partial discharge is high. Hence from the

point of operation lifetime of the ITER TFMC a relatively small cable high voltage insulation thickness

would have been sufficient. However, the thickness was selected higher in order to achieve higher

partial discharge inception voltage by the cable. Otherwise, the high partial discharges from the

polyimide cable would have make it impossible to resolve partial discharges that are generated within

the coil body.

An important application of polyimide film in fusion magnet insulation is improvement of fibre

reinforced plastic for high voltage insulation purposes.

3.3.2 Fibre reinforce plastic

Epoxy resin impregnated glass fibre reinforced plastic is a commonly used solid material. The epoxy

resin is often based on bisphenol A diglycidyl ether (DGEBA). Such kind of insulation was selected for

ITER TFMC [Ulb 05]. The radial plates were wrapped by 1.3mm glass / polyimide insulation followed by

an impregnation with DGEBA epoxy resin at about 75 ◦C followed by a curing cycle at about 125 ◦C. The

resin could penetrate through holes in the covers into the turn insulation. An 8 mm thick insulation of

combined glass – polyimide was built up to form the ground insulation of ITER TFMC. This material

combination was also impregnated with DGEBA epoxy resin. It should be noticed that polyimide is not

able to withstand the temperatures of Nb3Sn reaction heat treatment (with e.g. 650 °C for a duration

of 200 h for ITER TFMC). This was the reason for reaction heat treatment in a special reaction mould

before installation of the turn insulation. A more detailed description for resin use on superconducting

magnets can be found in [Eva 20].

Breakdown voltage as a function of the thickness of Kapton, G-10 and stacked dry-glass fabric can be

seen in Figure 3-13.

Figure 3-13: Breakdown voltage as a function of the thickness of Kapton, G-10,G-11 and stacked dry-glass fabric [Mar 14]

3.3.3 Mica

Breakdown field strength values for mica crystals were reported higher than 400 kV / mm. Breakdown

values around 20 kV / mm [Seg] are reported for plates made of mica and resin. Mica tapes are

available for wrapped insulation. Winding of the inner cables of Nb3Sn dipole magnet MSUT was a

folded mica / glass sheet wrapped with a glass fibre tape [Oud 97]. At the beginning and the end of the

turns cable deformation occurred during winding and pressing. Nevertheless, this insulation did not

suffer from any turn-to-turn shorts.

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3.3.4 Silicone

Silicone composites are commonly used for high voltage insulation applications. One possible use is the

moulding of this hygroscope material for high voltage bushings used in not dry air. A more important

application can be the use for high voltage connectors (Figure 3-12) on room temperature side. In this

case the dielectric strength of silicone itself is not the critical issue. The important feature is a high

voltage compatible connection, i.e. absence of flashovers between plug and socket surface. For such an

application it is important that the developed device shows no degradation by specified assembly

related to the foreseen assembly and disassembly numbers.

Figure 3-14: Room temperature high voltage instrumentation cable feedthrough design for POLO and ITER TFMC. Silicon insulation of plug (blue) provided sufficient contact to frp insulation of socket (green) to avoid flashover along boundary surface.

3.3.5 Teflon

Teflon is an important insulation for conventional instrumentation cables. Disadvantage of the material

is the low adhesion for gluing without primer and the low resistance against radiation. This material

should be therefore avoided for DEMO at least in the cryogenic region.

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Table 3-1: Some materials with breakdown field strength. Operation field strengths are usually selected

much lower than breakdown or 1 min withstand field strengths.

Material Breakdown field strength or voltage (peak value)

Electrode arrangement

temperature and pressure

Reference

Vacuum Saturation around 100 kV

Tip (r = 1 mm) to plane for gaps larger than about 2 mm

at 77 K [Ger 98]

Air 24.5 kV / cm AC, peak uniform field 20 °C, 0.1 MPa [Phi 82]

Air 5 kV / cm AC peak, DC positiv

needle to plane

20 °C, 0.1 MPa [Phi 82]

SF6 89 kV / cm AC, peak uniform field 20 °C, 0.1 MPa [Phi 82]

He gas 3.7 kV / cm AC, peak uniform field 20 °C, 0.1 MPa [Phi 82]

Liquid nitrogen

6 kV / mm AC, peak Sphere (20 mm) to plane

77 K, 0.1 MPa [Hay 18]

Liquid helium ±22 kV DC Plane to plane, 1 mm gap

4.4 K, 0.1 MPa [Har 88]

Liquid helium -10 kV DC Plane to plane, 1 mm gap

4.4 K, 0.1 MPa, heater 5 W / cm

[Har 88]

Polyimide > 35 kV / mm DC ISO 60243-1 23 °C [Ens 19]

Polyimide tape Kapton® Type HN

12 µm: 303 kV / mm

60 Hz, 500 V/sec rise

ASTM D-149 1/4 inch electrodes

[DuP 21]

Mica (Phlogopit) resin plate

>20 kV / mm IEC 243 [Seg]

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4 Technical specifications

A large number of high voltage component types are necessary for fusion magnets. Number and kind

of types is depending on fusion magnet and fusion magnet system design. Even the high voltage

specifications may be different for the various high voltage components depending on expected

voltage stress during operation and faults.

Nevertheless, it makes sense to collect some typical values for DEMO magnets. With these values some

specifications for the high voltage components may be derived.

Table 4-1: Overview of some design parameters of POLO coil and DEMO

Polo [Bay 97]

ITER TF DEMO Example

Operation voltage 23 kV 6 kV Insulation breaks

Maximum operation pressure of coil

2.5 MPa Insulation break

Vacuum 10-10 MPa Cryogenic high voltage instrumentation cable

Minimum temperature 4 K Cryogenic instrumentation cable feedthrough

Maximum temperature 300 K Room temperature instrumentation cable feedthrough

Leak rate 10-8 hPa l / s

Axial cryogenic insulation break

Helium mass flow per current lead 0.5 g / s Axial insulation break

It must be considered that some specifications can preferably be used for mock-ups only and hardly

during tests on the coil, e.g. a Schering Bridge test on a completed coil with slightly changed dissipation

factor will hardly cause an intensive search for the origin by Schering Bridge due to a tight time

schedule. In contrast, for testing during component development phase the investigation with a

Schering Bridge can be an interesting issue. Partial discharge measurement is also an interesting

destruction free method of insulation diagnostic. On the other hand, an acceptance rejection of a

complete coil only based on partial discharge data may be replaced by comparison with results for

other coils and potentially more detailed insulation inspection for this coil. It will be difficult to specify

fixed values of partial discharge activity or dissipation factor as acceptance criterion in advance.

Specifications must be done with a view for the total system because some values make only sense

considering also the properties e.g. of the neighbouring component.

KIT proposes for the present DEMO work package the starting with two high voltage components for

further investigation, namely the cryogenic high voltage instrumentation cable feedthrough and the

axial room temperature insulation break. The cryogenic high voltage instrumentation cable wire

feedthrough is described more detailed in the next section.

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4.1 Specification for a cryogenic high voltage instrumentation cable wire

feedthrough

The routing of high voltage instrumentation wires through the coil solid insulation to the part of the

high voltage cable located within the vacuum space is often reported as source of insulation faults [JT

21], [Wu 10], [Sch 06]. Fiddling of the wires just through the solid insulation is not a technique which

can be considered as compatible for a reliable insulation design. Instead, a special feedthrough design

is one option which must satisfy the following requirements:

Lifetime 30 years

Dimensions tbd

Number of wires (per cable) tbd

Pitch value (if necessary) same as cable

Resistance per wire ≤ 0.3 Ohm

Screen connections available for high voltage and ground

screen of cable

Compressive stress 2 kN (like ITER breaks ITER_D_2NJ8NJ)

(protected or resistant against standing

on it)

Tensile stress 2 kN

Torsion stress 50 Nm (half like ITER breaks

ITER_D_2NJ8NJ)

Mechanical cycles 60000 cycles with 50% of load (see

ITER_D_2NJ8NJ)

Chemical adhesive for gluing with cable and coil

insulation, paintable with (semi-)

conductive varnish to form a Paschen

tight device with all neighbouring parts

Operation temperature 4 K – 300 K (also suitable for room

temperature high voltage tests)

Leak rate 10-8 hPa l / s

Outgassing rate < 1 x 10-9 Pa m3 s-1 m-2, after

conditioning in vacuum

Maximum magnetic field: 12 T

Radiation dose over 20 years 1 MGy

Operation voltage tbd

Voltage during most severe considered fault tbd

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Test voltage, DC tbd

Test voltage, AC, peak value tbd

Test voltage, standard lightning impulse tbd

(Test voltage, chopped tbd)

Test voltage wire to wire or pin to pin tbd

(Partial discharge (PD) inception voltage tbd)

(Maximum apparent charge during PD measurement tbd)

Maximum length on top of the unequipped coil body (without cables) 0.5 m

5 High voltage testing

5.1 Principles for high voltage testing of coils

High voltage testing on DEMO and DEMO components should be arranged according to basic principles

of insulation co-ordination. Principles and rules of insulation co-ordination can be derived from

standard [IEC 71-1] although DEMO is no three-phase AC system.

In a first step a system analysis must be started. This analysis shall deliver the voltages and

overvoltages in amplitude, shape and duration depending on location. Since the DEMO design is

presently (year 2021) under discussion, especially concerning the maximum terminal operation

voltages, no fixed data are presently available. Representative voltages and overvoltages for each class

of the calculated voltages and overvoltages shall be determined. For AC systems typical standard

voltage shapes are AC with sinus waveform, standard switching and standard lightning impulse. Since

the common high voltage equipment is intended for tests with such waveforms one possible strategy is

to divide the voltage waveform classes of DEMO (e.g. discharge with ms front and several seconds or

10th of seconds equivalent discharge time constant) in such a way that tests can also be performed

with already existing test equipment from power engineering. In this case the following waveforms

from power engineering test may be applicable:

DC: a voltage that is time-independent. In reality a test with DC comprises also a ramp up and ramp

down of the voltage. If this ramping is done similar to the equivalent discharge time such a test can be

considered as representative for the fall of a TF coil discharge. An advantage of this waveform is the

low power of the test equipment for high resistive test samples (e.g. no water hoses assembled). This

means that in case of an occurrence of an insulation fault only the capacitive energy of the test sample

is important for the potential material destruction. Hence the DC waveform is usually selected as a first

high voltage test and during Paschen tests. In addition, the (apparent) insulation resistance can be

derived by DC testing.

AC: The test voltage has the sinus waveform with a frequency of 50 Hz or 60 Hz. This voltage can be

used for testing fusion magnets related to waveforms with rise times of few ms (e.g. rise time during

fast discharge). It may be discussed if the usual bipolar voltage can and shall be replace with unipolar

sinus waveform by using a Villard circuit. Bipolar AC waveform can also be used for destruction free

insulation diagnostic with partial discharge measurement and Schering bridge because for this

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waveform the most knowledge exists based on the usual testing from power engineering (bushings,

transformers etc.).

Impulse testing: The test voltage has the waveform of an impulse. In power engineering the waveforms

for switching impulse (SI) and standard lightning impulse (LI) are specified. The LI with its rise time of

1.2 µs and time to half value of 50 µs can be considered as a useful waveform shape for transients

occurring after triggering the switch for discharging the capacitor bank in order to obtain a zero

transition in the vacuum breaker during counter acting current switching. In addition, the rise time of

the LI may be reprehensive for voltage transients generated by insulation faults. It is important that

such impulse tests are analysed in advance for application on large samples like completed fusion

magnets in order to avoid local overstress during high voltage testing.

Chopped voltage: No standard waveform is specified in [IEC 71-1] for fast transients with front times ≤

100 ns instead it is referenced to specifications of the relevant apparatus committees. For transformer

bushings and transformers chopped wave lightning impulse tests (LIC) where specified as lightning

impulses cut after a time to chopping between 3 μs and 6 μs. In power engineering the reason for fast

transients is different as for fusion magnets. Hence it may be preferable to chop other waveforms like

DC. It is very important that such impulse tests are analysed in advance for application on large

samples like completed fusion magnets in order to avoid too high local overvoltages during testing.

Rules for selection of testing of completed coils are described in [Fin 04]:

The proposed ideal test voltages derived from the following considerations:

The standard rule for voltage tests related to single fault events is proposed to be

2 * Ufault + 1 kV. The value is rounded up to integer values (e. g. 2*8.1 kV + 1 kV = 17.2 kV =>

18 kV).

The test voltage for values < 5 kV is taken as 5 kV if there is no special reason to select a lower

voltage.

In case of a radial plate design: For radial plate and conductor insulation the higher value is

taken for both.

The (terminal) test voltage for impulse voltages with rise times in the range of some µs may be

reduced to avoid too high stress of radial plate and conductor insulation.

According to the standard for power transformers ([IEC 76-3], chapter 9) repetition of tests on

new power transformers are to be performed with 100% and the level is reduced to 80% for

most voltage tests of refurbished or serviced transformers, unless otherwise agreed upon. (The

80% level may be DEMO relevant for e. g. PD testing after several years of operation.)

The test voltages for the first test series should be increased with a certain factor if the manufacturer

or the DEMO community prefers a higher safety margin for the first test compared to later tests. If the

manufacturer wants to have added a certain safety margin for the manufacturing tests the margin

must be added to the specified test levels.

In power engineering, the devices were commonly subjected to an initial standardized test series and

then operated for 30 years or longer without further test series. The situation is different for fusion

magnets. A large number of high voltage test series is useful before first full fusion reactor operation in

order to notice a potential problem as early as possible. In addition, a Paschen tight design generates

large size systems where parts cannot be tested separately from each other. Multiple testing for

manufacturing control, fault localization or after potential repair activities must be covered by the

dielectric design without starting of degradation. Hence for achieving a sufficient long-term high

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voltage reliability a voltage application for 30 years with the specified test voltages should be

predefined in order to avoid limitation in test time or number of tests. This does not mean that

individual tests take a long time or longer than needed or that all test are performed with full specified

test voltage but it is not necessary to care about potential limitation of test number and time or

voltage value (as long as it is equal or lower than the specified test voltage level). The cost impact of

this specification can be expected as relatively low for a reliable design. It is also in the hand of the

manufacturer to specify sufficient high test voltage levels when the insulation is incomplete, e.g., no

vacuum impregnation, to eliminate fabrication faults as early as possible and allow repair.

The increasing of the test voltage may be performed in defined steps related to the maximum test

voltage. A possible sequence is 10%, 30%, 50%, 70%, 90%, 100%. Especially before polarity reversal

with impulse tests it is not allowed to start with a 100% impulse. Instead at least one initial 30%

impulse is required before testing with full test voltage amplitude of reversed polarity.

Type and routine high voltage tests should follow the same level as the coil tests. Tests at higher levels

are to be performed on high voltage components during development on mock-ups and prototype

samples in order to analyse the safety factors.

5.1.1 Paschen tests

The proposal for Paschen testing is mainly derived from [Fin 09]. A reliable high voltage operation even

under vacuum breakdown in the cryostat requires an entire encapsulation with solid insulation and

covering of this insulation with conductive material (e.g. conductive paint). Such a “Paschen tight”

concept must be verified by a Paschen test where the sample is subjected to a transition of the

Paschen curve.

A vacuum vessel is necessary for such a test. Every test sample must be prepared to be Paschen tight.

For superconducting coils an appropriate time of a Paschen test is after connection of the high voltage

instrumentation path at the manufacturer site. But this makes only sense in case the fusion reactor

facility site allows to let have connected the cryogenic cable paths during coil installation in the

cryostat. Otherwise open ends must be either encapsulated in a Paschen tight manner or guided in a

Paschen tight manner to the ambient.

Because it is more time consuming to achieve a good vacuum in a large vessel which had been filled

with air than filling an “empty” vessel with air up to ambient pressure it is proposed to start Paschen

testing from vacuum.

One method is to apply permanently the DC test voltage while increasing the pressure continuously

from vacuum till ambient pressure. Another method is to increase the pressure in steps. The steps

should consider likely gap lengths of the fault location in the ground location. But even taking into

account a minimum thickness of a fusion magnet for the ground insulation of a certain length, shorter

gap lengths are not impossible (e.g. by invasion of conductive paint in a gap). The following table shows

a set of interesting pressure values and the belonging gap length for Paschen Minimum conditions with

a Paschen Minimum value in air at room temperature of p * d = 7.3 bar µm:

Table 5-1: Pressure steps for Paschen test considering the belonging gap lengths in air for reaching

Paschen Minimum conditions.

p / hPa d / mm

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0.001 7300

0.01 730

0.1 73

0.3 24

0.5 15

0.73 10

0.91 8 (often selected for ground insulation thickness)

1 7.3

5 1.5

10 0.73

100 0.073

1000 0.0073

A combination of the two Paschen test methods mentioned before may a one minute test arround the

values listed in the table and in between a “fast” pressure change with applied high voltage.

Usually the Paschen tests are performed at room temperature because otherwise the vacuum vessel

would need an additional vacuum shield.

5.2 Test procedure for initial high voltage testing of a cryogenic high voltage

instrumentation cable wire feedthrough mock-up at room temperature

conditions

As an example a possible test procedure for the room temperature test sequence of a cryogenic high

voltage instrumentation cable wire feedthrough mock-ups is described.

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Procedure WPMAG CrWiFe 211125:

1. Procedure WPMAG CrWiFe 211125-01-01: DC 5 kV, wire to wire

1.1. Connect first wire of feedthrough with DC test apparatus and ground all other wires, screens and

cases

1.2. Conditions around socket and plug: air, 0.1 MPa, RT

1.3. Check grounding by continuity tester

1.4. Make a photo and include the photo in the test report

1.5. Apply a DC voltage of 5 kV on the first wire

1.6. Note the current after 1 min testing period

1.7. Ground the test circuit after each test

1.8. Repeat the test with the other wires

2. Procedure WPMAG CrWiFe 211125-01-02: AC 3.5 kVrms, wire to wire

2.1. Connect first wire of feedthrough with AC test apparatus and ground all other wires, screens and

cases




2.5. Apply an AC voltage of 3.5 kVrms on the first wire

2.6. Note the current after 1 min testing period

2.7. Ground the test circuit after each test

2.8. Repeat the test with the other wires

3. Procedure WPMAG CrWiFe 211125-01-03: DC 35 kV (? TBD), 5 min

3.1. Connect all wires and high voltage screen with DC test apparatus. Ground the ground screen.




3.5. Voltage steps for all samples: 10 kV, 20 kV, 30 kV for 1 min; and 35 kV (TBD ?) for 5 min

3.6. Note the current after testing period

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3.7. Ground the test circuit

4. Procedure WPMAG CrWiFe 211125-01-04: AC 24.7 kVrms (? TBD), 5 min, eventually with partial

discharge (PD) measurement

4.1. Connect all wires and high voltage screen with AC test circuit. Ground the ground screen




4.5. rms voltage steps for all samples: 3.54 kV, 7.1 kV, 14.1 kV, 21.2 kV for 1 min; and 24.7 kV for 5 min.

The step duration is 3 min instead of 1 min in case of partial discharge measurement in apparent

charge vs. high voltage phase angle mode.


5. Procedure WPMAG CrWiFe 211125-01-05: Impulse test 35 kV

5.1. Connect all wires and high voltage screen with the impulse test circuit. Ground the ground screen.




5.5. Crest value is 35 kV, some impulses at lower voltages are necessary to adjust and confirm the

parameters of standard LI (1.2 μs virtual front time, 50 μs time to half value)

5.6. 10 impulses for both polarities without breakdown are necessary to fulfil the test, additionally one

impulse with lower crest value (about 30%) after polarity change


6. Procedure WPMAG CrWiFe 211125-01-07: Paschen test 35 kV (? TBD)

6.1. Arrange a sample in a vacuum vessel and connect a DC test apparatus to all active wires and the

high voltage screen. Connect a vacuum and a pressure measurement.

6.2. Conditions around plug: air, 0.1 MPa, RT, Evacuate the area around the socket for at least 3 h

(preferably longer e.g. over night or week end)

6.3. Check grounding by continuity tester, control the vacuum with a target value of p ≤ 0.002 hPa

6.4. Adjust zero point of manometer scaling


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6.6. Increase the voltage to 35 kV (? TBD)

6.7. Increase the pressure, at hold points keep the pressure for at least 1 min. It is not necessary to

ramp the voltage down and up between the steps if pressure adjustment duration if it can be done

electrically safely. Take at least the following hold points: ≤0.002 hPa, 0.01 hPa, 0.1 hPa, 0.5 hPa, 1 hPa,

10 hPa, 100 hPa, ambient pressure

6.8. Note the current at each hold point pressure at the end of the test minute


It is indispensable that the sample is subjected to thermal cycling, too, and also afterwards tested of

the dielectric strength. A test series under cryogenic conditions is also recommended. The Paschen test

may be skipped for the test procedure under cryogenic conditions because it is a large effort to build-

up an appropriate test setup.

“Long duration” high voltage testing (10 h) should be added for the final test series after all (3 up to 5)

thermal cycles and mechanical stress tests. Then tests at higher voltages will be added to find the

dielectric limits of the mock-up.

6 Limitations of test facilities at KIT

KIT proposes the following maximum lengths for the mock-ups:

Cryogenic high voltage instrumentation cable feedthrough: maximum length in cryostat only: 0.7 m;

minimum cable length outside cryostat: eventually 4 m, or suitable bushing, tbd (also depending on

test voltage and used cable)

Room temperature insulation break for testing in Waheffca: 0.75 m (including steel pipes with KF-

flanges)

Longer mock-up lengths may be tested on request.

7 References

[Bal 19] J. Baldzuhn at al., “Spark Detection and Search for High-Voltage Paschen Leaks in a Large Superconducting Coil System”, transactions on plasma science, Vol. 47, No. 11, pp. 5125-5138, 2019

[Bay 97] M. Darweschsad, G. Friesinger, R. Heller, M. Irmisch, H. Kathol, P. Komarek, W. Maurer, G. Nöther, G. Schleinkofer, C. Schmidt, Ch. Sihler, M. Süsser, A. Ulbricht, F. Wtichner (1995), ”Development and test of the poloidal field prototype coil POLO at the Forschungszentrum Karlsruhe”, Fusion Engineering and Design 36, pp. 227-250, 1997

[Bur 14] A. S. Bursikov, N. M. Voronin, S. M. Gavrilov, V. A. Grinchenko, Yu. A. Klimchenko, V. A. Korsunskiy, O. A. Kovalchuk, A. A. Lancetov, E. L. Marushin, A. A. Mednikov, I. Yu. Rodin, and A. V. Safonov,“ Novel cryogenic high voltage insulation breaks with spiral channel” AIP Conference Proceedings 1573, pp. 1700-1706, 2014

[Col 19] M. Coleman, Y. Hörstensmeyer, F. Cismondi, “DEMO tritium fuel cycle: performance, parameter explorations, and design space constraints,” Fusion Engineering and Design, Vol. 141, pp. 79-90, 2019

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[DuP 21] DuPont, “DuPont Kapton summary of properties,” datasheet, p. 10, EI-10142 (5/21)

[Ens 19] Ensinger Sintimid GmbH, “TECASINT 1011 natural - Stock Shapes (rods, plates, tubes)”, datasheet, Version: AC, 2019

[Eva 20] D. Evans, “Resins for superconducting magnet construction – an overview of requirements, processing and properties”, IOP Conference Series.Materials Science and Engineering, 756(1), 2020

[Fin 04] A. Fink, W. H. Fietz, A. M. Miri, X. Quan, A. Ulbricht, “Study of the transient voltage behaviour of the present ITER TF coil design for determination of the test voltages and procedures,” Forschungszentrum Karlsruhe, Scientific Report FZKA 7053, 2004

[Ger 98] J. Gerhold, “Properties of cryogenic insulants,” Cryogenics, Volume 38, Issue 11, pp 1063-1081, 1998

[Har88] M. Hara, T. Kaneko, K. Honda, “Thermal-Bubble Initiated Breakdown Characteristics of Liquid Helium and Nitrogen at Atmospheric Pressure,” IEEE Transactions on Electrical Insul., Vol. 23, No. 4, 1988

[Hay 18] N. Hayakawa, K. Ishida, M. Mimbu, H. Kojima, S. Isojima, M. Kuwata, "Volume Effect of Dynamic Breakdown Strength in LN2 for Insulation Design of Resistive Superconducting Fault Current Limiters," Transactions on Applied Superconductivity, vol. 28, no. 4, pp. 1-4, 2018

[Hay 19] N. Hayakawa, M. Mimbu, H. Kojima, S. Isojima and M. Kuwata, "Dynamic Breakdown Characteristics of Pancake Coil Model for Resistive-Type Superconducting Fault Current Limiters," IEEE Transactions on Applied Superconductivity, vol. 29, no. 5, pp. 1-6, Aug. 2019

[IEC 71-1] IEC standard IEC 60071-1, “Insualtion co-ordination – Part 1: definitions, principles and rules”

[IEC 71-2] IEC standard 60071-2, “Insulation co-ordination - Part 2: application guidelines”

[IEC 76-3] Power Transformers - Part 3: Insulation levels, dielectric tests and external clearances in air

[Irm 96] M. Irmisch, A. Ulbricht, A. “Hochspannungskomponenten für die supraleitende Poloidalfeld-Modellspule POLO”, Wissenschaftliche Berichte, FZKA-5568, 1996

[JT 21] JT60 SA, “Integrated Commissioning Status on 09.07.2021”, Retrieved from: www.jt60sa.org/wp/category/news/

[Mar 14] R. P. Reeda, N. N. Martovetsky, “Electrical Insulation Systems for the ITER CS Modules,”AIP Conference Proceedings 1574, p. 357, 2014

[Pha 21] N. S. Phan, W. Wei, B. Beaumont, N. Bouman, S. M. Clayton, S. A. Currie, T. M. Ito, J. C. Ramsey, G. M. Seidel “A study of DC electrical breakdown in liquid helium through analysis of the empirical breakdown field distributions”, Journal of Applied Physics 129, 083301, 2021

[Phi 82] Eugen Philippow, Taschenbuch Elektrotechnik, Vol. 6, pp.279 ff and pp. 291

[Oud 97] A. den Ouden, W. Wessel, H. Krooshoop, ; H. ten Kate, “Application of Nb3Sn superconductors in high-field accelerator magnets”, transactions on applied superconductivity, vol. 7, no. 2. pp. 733-737, 1997

[Sch 06] H. Scheller et al., "Paschen Testing on W7-X Coils and Components in the BNN Test Facility," in IEEE Transactions on Applied Superconductivity, vol. 16, no. 2, pp. 759-762, 2006

[Seg] Segliwa GmbH, “Sewitherm P“, data sheet

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[Sim 94] N. J. “Cryogenic Properties of Inorganic Insulation Materials for ITER Magnets: A Review” (NISTIR--5030), United States, page 151, 1994

[Sko 11] N. Skoro, D. Maric, G. Malović, W. G. Graham, W. Z. Petrović, “Electrical breakdown in water vapor”. American Physical Society, Phys. Rev. E, volume 84, iss. 5, pages 055401, 2011

[ulb 05] A. Ulbricht at al., “The ITER toroidal field model coil project,” Fusion Engineering and Design, Vol. 73, Iss. 2–4, pp. 189-327, 2005

[Wu 10] Y. Wu, "Experience in Operating Safety of EAST Superconducting Magnets," Transactions on applied superconductivity, vol. 20, no. 3, pp. 431-437, 2010

Investigation of High Voltage Insulation Aspects

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