Herman ten Kate and Alexey Dudarev

MCS 600A sextupole circuits(update on study performed in May 2010)

Content: References General observations Electrical circuit Insulation issues Forces Conclusion

(update of status mentioned in this color)

LHC Task Force Splices, Review-1, 19 Oct 2010

References

Based on interviews of Alexey Dudarev and myself with:- Mikko Karppinen- Paolo Fessia- Marta Bajko- Nuria Catalan Lasheras- Antonio Perin- Reiner Denz

Charge: Find issues that may impact safety of the correction coil circuits (not to lose them) main Q and D circuits (arc to Q or D bus) cause severe damage to He circuits, insulation vacuum (arc to

ground)

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General observations

There are about 106k low current splices in the various corrector coils circuits

Obviously we can’t check them all from a-to-z

We studied what is accessible: open magnets in B180 and some documentation

And we tried by interviewing the responsible engineers to find critical areas that may be improved

This focus in this report is mainly on the sextupole correctors MCS,

but many observations hold for all circuits as well

12 Recommendations formulated and status updated

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MCS: how it looks

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Electrical circuit and documentation

MCS coil with internal protection resistor, 154 in series

Connected to HTS leads, dump resistor, cascade of 3 switches and power convertor

Internal protection resistor can take up all energy during fast dump

550A, 0.8mH, 19 kJ, R// = 0.08Ω

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Integration documentation is incomplete:

electrical scheme showing all details, including routing, all welds, bus sections, voltage taps does not exist, hindering fast analysis and will slow down intervention repairs

Cure 1: make integration drawings of all circuits (!)

Update: acknowledged, work started

Electrical insulation: test voltage

In factory: 1000V,

but in SM18 mostly 600V, in tunnel 600V But is this enough?

What is the maximum differential voltage

between the corrector coil circuits/bus mutually and to the Q bus? Following A.Verweij:

- 600A circuits show 0.7Ω at ±600A, thus some ±420V

- Main Q bus shows Ƭ=9.2s at 6kA, thus 190V max Meaning that ΔV_max = ~610V between MCS and bus and 840 mutually! There is no or negative margin in the test voltage and thus the risk of

electrical breakdown is real, in particular by aging with time Normal safety factors are 2-3 at warm and 1.1-1.5 at cold, here <1 Cure 2: Do check all circuits again, make table of maximum circuit

voltages; agree on a sensible higher test voltage for new assemblies, however, not too high in order not to risk damage; say 900 or 1000V

Update: acknowledged, all V’s are verified, new table in progress6

Quench protection, V-taps, leak current

Safety of the 600A bus lines in the case of a bus quench under various cooling conditions not fully analyzed, not proven, nor documented (though there are some indications that it is safe)

Cure 3: Analysis report requested for all spools circuits considering the case of single or combined quenches anywhere in the bus

Update: acknowledged, analysis to start

Today there is no diagnostics on the corrector magnets, identification of quenched unit not possible, have to go into the tunnel

Cure 4: Consider extra voltage taps, wire the taps up to a diagnostic unit

Update: to be discussed, not yet decided

There is no ground insulation current monitoring (only interlock at >50mA in power convertor), no early warnings, no degradation monitoring

Cure 5: Consider to install leakage current monitoring at μA level

Update: to be discussed, no decision yet

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Electrical insulation of bus to ground

The 15m long bus has Kapton wraps and is encased by G10, however, at its terminals, the last 15cm, it has a weak glass-braid insulation and no G10 casing to ground (easy to do), potentially a weak spot, also because conductors move/deflect in field

Insulation is requested on all bare metal surfaces within the end cap & interconnect

Cure 6: insert an insulation plate (or spray coating) between bus and steel, and if possible on entire interior of the end caps

Update:

under discussion,

no decision yet

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Insulation between 600A and Q-bus

Worries: in the case of repair,

1 joint is replaced by 2 and

then not covered by the casing

New insulation and new extended

casing needed to support the wires

(already being done in H180

on the magnets being repaired)

Breakdown voltage between 600A circuit, bus and ground not known

Cure 7: Qualify the present casing by electrical breakdown tests of this arrangement, engineer a new full-coverage solution for repaired areas

Update: under discussion, see presentation Fessia, new insulation casing under development, extra insulation layer present, new solution to be qualified, check what to do with old splice casing

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Metallic spider between bus and ground

Another worry is the metallic spider with short insulation gaps to 600A circuits, to Q-circuit and to ground

Again, breakdown voltage of present lay-out not known, never measured on many samples

Cure 8:

Do break down test between spider,

all 600A circuits and Q bus

Replace them by non-conductive spiders

or put insulation tube/sheet between metallic spider and the cryostat tube

Update: to be discussed, no decision yet but repair highly recommended !;

see presentation Fessia, small series not tested and some spiders show ~3kV, too low value and worst values to be expected in large series

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Missing clamps on 600A joints

The welded 600A joints have no clamps

Risk of opening of circuits by aging even when

initial weld looks fine

Like in the case of 13kA splices one should

never relay on single welding but always add

a clamp, easy to do,

see the pressure clamps on the soldered joints in the DFB’s

Cure 9: Consider to put clamps on all new joints (like in the DFB’s); not reasonable to do this for all welds; too many and to much work;

Consider best –effort inspection during interconnects opening to reduce the risk

Update: to be discussed, no decision yet for new splices, inspection recommended

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Missing protection resistors on V-taps

In LHC voltage taps do not have a protection resistor, usually 5-10kΩ (incomprehensible !)

Such resistors are required to limit the short-circuit current to ground or other circuits in the case of damaged insulation

If not present the high circuit voltage is in the long cables and on the connectors and everywhere to ground

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The voltage tap cables are very long in LHC,

the risk of long range shorts between circuits

through voltage tap cables, cards and racks is considered substantial

Cure 10: Put in all new assemblies protection resistors

Update: under discussion, no decision yet

Bending of 600A bus wires by Lorentz force

The 600A leads show big open loops exposed to stray field of the 6 and 13kA bus

Quick estimate shows

~ 0.1 kg/dm between 600A wires

~ 0.2-0.3 kg/dm between 600A and 6/13kA The loops will bend in field The wires are not sufficiently supported

Risk of aging effect with time

and finally shorts

Cure 11: improve support

in all new assemblies

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Update: under discussion, no decision

Circuits not yet fully charged

Note!

Correction coils and main magnets were not fully charged in SM18 at the same time, nor in the tunnel

Thus the 600A circuits have not yet seen the full mechanical and electrical load of ±600A in combination with 6 and 13 kA in the bus bars and coils

We have to be careful and may see surprises in terms of training and shorts when charging in full

This argument holds for current and voltage

Cure 12: carefully check readiness for full load and be prepared for surprises, perform additional test before repair shut-down

Update: to be discussed, no decision yet, test program before shut-down planned

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Cabling and joints in the DFBs

So far no surprises found in the DFBs Layout and technical realization well documented Insulation looks solid using right materials, Nomex in stead off

glass braids and spacer spiral wraps to keep distance to ground Joints look fine Clamps on all soldered joints present

Remark:

Techniques, materials choices and methods are different here......

Why not uniform solutions throughout the entire LHC, communication and collaboration!

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Conclusion so far.......

600A circuits, in particular MCS were screened for weak spots, other circuits to follow

Documentation incomplete, in particular electrical installation/integration schemes

Circuit analysis for faults missing Flaws on electrical insulation found Test voltage of 600V is too low to guarantee insulation Quality of insulation not known, breakdown tests to do Not protected for circuit joints that open, clamps missing Protection resistors in voltage taps missing Many 600A wires not well supported, cyclic deflection/bending under

varying loads, risk of weakening/degrading the insulation Correction coils circuits are not yet fully charged!

Update: most recommendations accepted, others still to discuss and decision making is pending, MCO circuit screened as well, next talk

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