Transcript TenKate_-_LF_Review-1__MCS-600A_Tu_19Oct10

MCS 600A sextupole circuits
(update on study performed in May 2010)
Herman ten Kate and Alexey Dudarev
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
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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
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
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Electrical insulation: test voltage
 In factory: 1000V,
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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 progress
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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
 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
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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
Update: under discussion,
no decision
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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
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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
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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
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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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