Download PoweringAndAccessMaxI - Indico

Access to underground areas during powering
Risk: accidental massive helium release (as on 19 September) when
superconducting magnets are being powered
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Input from Laurette Ponce, Magali Gruwe, Markus Zerlauth, Matteo Solfaroli, Antonio
Vergara, Boris Bellesia, Gianluigi Arduini, Karl-Hubert Mess and Mirko Pojer
Risk analysis with K.Dahlerup-Petersen, G.Kirby, M.Solfaroli, J.Strait, H.Thiesen, A.Verweij
and R.Wolf
Thanks to L.Bottura, A.Siemko and J.P.Tock
R.Schmidt, HC 28/5/2009
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Two phases during the powering tests
• PHASE I - Low current powering tests: current limited to a value to be
defined, with negligible risk of massive helium release
• PHASE II - High current powering tests: the current in the circuits is
not limited, massive helium release cannot be fully excluded
– Access is closed & all necessary areas (tunnel plus service areas)
patrolled – see Magali Gruwes presentation to hardware
commissioning and LMC
For each circuit (type), it is required to define the maximum current in
Phase I
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nominal current and energy in circuit
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Powering and risk of helium release
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The maximum accidental helium mass release that is compatible with
the safety systems; in the past 1 to 2 kg/s were assumed, here we
assume 1 kg/s
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What is the size of an opening in the helium vessel that could produce
such helium release?
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What are the parameters of an electrical circuit (current, stored energy,
other parameters) that could produce such opening in case of severe
electrical fault?
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The most critical helium release is due to an opening of the envelope by
an electrical arc in the M1, M2, M3 and (less) in the N Line
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The arc is generated by a splice that breaks, or by another interruption in
the circuit inside the cryostat
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Electrical arcs
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The voltage in an arc is in the order of 10-20 V
– as an example, for the circuit with 60A, the power is few hundred to
about 1000 W
Two cables separating, the arc starts between the two cables (and not to
ground). It can later jump over to ground
It is very unlikely that an arc cuts a slice in a tube (as assumed in the
risk analysis for repowering the magnets after the incident)
The power in an arc is limited and depends on the current
Making holes of a given size…..
• requires enough energy in the circuit
• requires a minimum amount of time for the arc to burn, the time depends
on the power in the arc and therefore on the current
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Method to establish Imax for phase I
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The amount of helium release depends on the size of the hole.
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The size of the hole depends on the energy that is used for opening
the hole. The maximum possible hole size depends on the energy
stored in the circuit.
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Energy: We estimate the size of a hole that would lead to accidental
helium release above a critical value. The maximum stored energy in
a circuit that could in the worst case scenario create a hole with such
size is estimated to about 100 kJ. It is suggested to limit the energy in
a circuit during powering phase 1 to this value.
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Power: The voltage across a typical arc is between 10 and 20 V.
Depending on the current, the power in the arc is therefore limited. For
most electrical circuits with large stored energy, the energy is
extracted and the current decreases with a time constant defined by
the extraction system. The arc cannot be sustained for more than,
say, one second.
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Observations from other installations (reported already)
Experience from CERN and other installation with incidents of
superconducting magnets shows that the energy of at least some
hundred kJ is required to create a significant hole
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Sector 34: many MJoule went into the arc opening the pipes
A hole of about 100 mm2 was created in an incident with a LHC dipole magnet
when tested in SM-18 (the energy at full current is 7 MJ).
During tests of an orbit corrector magnet in SM18 an arc created a very small
hole. With a small amount of energy stored in a circuit holes can be created, but
for large holes much more energy is necessary.
A hole of about 10-15 mm diameter was created with 260 kJ in a SSC magnet.
A hole of a few cm diameter was created in an incident with a solenoid at Oxford
Instruments with a stored energy of 10 MJ.
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Experiment using an arc welding machine
Plate
thickness
[mm]
Diameter
of the
hole
[mm]
Voltage
[V]
Current
[A]
Time
[s]
E_diss
[kJ]
Melted
volume
[mm3]
E_melt
[kJ]
E_diss/
E_melt
1
4
14
60
0.5
0.42
12.6
0.069
6.08
1
6
16
100
0.5
0.8
28.3
0.156
5.14
2
10
14
60
10
8.4
157
0.864
9.72
2
14
16
100
10
16
308
1.69
9.45
2
22
18
200
10
36
760
4.18
8.61
• The ratio between E_diss and E_melt is about 9 for the holes in the
2 mm thick plate
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Diameter of a hole and helium mass release
Graph provided by Laurant Tavian: the orifice section required to have a
critical flow of 1 kg/s as a function of the pressure (from 1 to 20 bar) and
for different temperatures (from 1.9 to 20 K).
Orifice area [cm2]
10
1
0
5
10
15
20
25
1.9 K
5K
10 K
15 K
20 K
Upstream pressure [bar]
With 1 kg/s at a pressure difference about 1 bar, the maximum acceptable
area of a hole is ~280 mm2. For a 2 mm thick plate this corresponds to a
0.1560 mm3, which requires an arc energy of 27 kJ.
volume of
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Scaling – arc welding machine and
superconducting magnet
• A superconducting magnet is not designed for
making holes, an arc welding machine is optimised
for melting metal.
• The electrical arc in an incident with a sc magnet will
be much less damaging assuming the same energy
stored in the magnet as during arc welding.
• Magnet circuit:
– The arc starts between the two parts of the cable.
When the distance has become larger than the
distance between cable and pipe, an arc could
develop to ground.
– The arc would be created in liquid helium at low
temperature.
– In a magnet circuit the arc energy will be dissipated on
both ends of the arc (factor 2).
– In total, factor 3-4: the limit is about 100 kJ.
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Burning time of an arc
• For all circuits with a stored energy above some 10 kJ, there is a
systems to extract the energy
• Extraction switch with resistor in parallel for 600 A circuits, plus resistor
in parallel to the magnets
• Quench heaters for all magnets with higher current
• After activating the energy extraction, the current will decrease to zero in
a fraction of a second. During this time the power is insufficient to open a
large hole
– for a quadrupole operating at 5 kA, the power will be max 100 kW. If the
decay time is 0.2 s (typical value), the energy in the arc will be in the order
for 20 kJ
• The correct functioning of the extraction systems will be validated at low
current
• Exception: RB and RQ, where the current decay takes much longer
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Suggestion
Circuit type
main dipoles
main quadrupoles
arc individually
powered
quadrupoles
600A circuits
120A orbit
correctors
60A orbit
correctors
Stand alone
quadrupole
Stand alone
dipoles
inner triplet
quadrupoles
(Q1+Q3/Q2a+Q2b)
Maximum
current level
L [H]
15.708
0.263
0A
760A
Energy
[J]
0.0E+00
7.6E+04
Corresponding
powering test step
PIC1
PLI1
2.4E+04
0.06
0.432
(400A)
400A / 550A
3.5E+04
PNO
2.84
120A
1.4E+04
PNO
6.02
60A
9.1E+03
PNO
0.296
0.052
L1 = 0.09
L2 = 0.038
L3 = 0.09
900A
600A
1000A
n.a.
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PLI2
5.3E+04
2.6E+04
5.9E+03
PLI2
PLI2
PCC
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Commissioning circuits in parallel
Powering several circuits in the same cryostats increases the energy stored
and the risk to generate a large hole and to have a massive helium
release.
To limit the risk the powering conditions during phase I are:
• In the same cryostat, no more than one main circuit (RQ, IPD, IPQ or IT)
simultaneously.
• No restriction for the 60 A, 80-120 A and 600 A circuits.
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Summary
• We propose a limit of 1000 A (by hardware or software), except RB.
• For the 600 A circuits, the maximum stored energy will be substantially
below (35 kJ). Since the last test step is PLI2 for many circuits, the
energy in other circuits is also far below 100 kJ.
• A limit that is slightly higher would not simplify much the powering tests.
Most test steps can be done under these conditions.
This approach that should be reviewed after we are confident in the quality
of the splices between magnets.
Powering tests in sector 23 will start in about 10 days time in phase I
Request from QPS to do some limited powering of RB and RQ in June
(last weekend?)
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Limit current during powering phase I
Sector 23
– RB locked – what about PIC 1?
– RQ limited to 800 A by hardware
– All other high current magnets, current limitation to 1 kA
Other sectors
– RB locked
– RQ limited to 800 A by hardware
– procedure goes only to current less than 1 kA
– limitation by FGC software to 1 kA
– software interlock switching off converters if current exceeds 1 kA
• for the start (when interlocks not yet validated) some options for hardware
limitations
– power converters can be only controlled from the CCC when starting
powering tests
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Limit current during powering phase II
Initial operation with high current
• RB and RQ limited to 2 kA
– RQ limited to 800 A by hardware
– All other high current magnets, current limitation to 1 kA
Other circuits
– limited to current for 5 TeV operation
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end
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Limit current during powering phase I
The LHC power converters have two options to limit the current OVER_I:
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I_POS software limit in FGC configured from I_PNO in LSA
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I_HARDWARE hardware limit
circuits < 600A:
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I_POS = I_PNO avec I_PNO = I_5Tev
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I_HARDWARE = set to > a I_7Tev (little difference between I_5Tev et I_7Tev)
circuits > 1000A:
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I_POS = I_PNO with I_PNO = I_PHASE_I for phase I and I_5Tev for phase II
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I_HARDWARE = set to > I_5Tev
circuits RB, RQD et RQF
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I_POS = I_PNO with I_PNO = I_PHASE_I for phase I et I_5Tev for phase II
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I_HARDWARE = 1.05*I_PHASE_I for phase I et 1.05*I_5Tev pendant la phase II
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Consequences
circuits < 600A
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I_PNO remains constant in LSA (I_5Tev)
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I_HARDWARE (new property in LSA) remains constant LSA (corresponds > I_7Tev)
circuits > 1000A
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I_PNO changes with the phase of the test: I_PNO = I_PHASE_I for phase I et I_5Tev for
phase II
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I_HARDWARE remains constant in LSA (corresponds to > I_5Tev)
circuits RB, RQD et RQF
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I_PNO changes with the phase of the test: I_PNO = I_PHASE_I for phase I and I_5Tev for
phase II
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I_HARDWARE changes with the phase of the test: I_HARDWARE = 1.05*I_PHASE_I for
phase I et 1.05*I_PHASE_II for phase II
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Hardware options
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RPLA - LHC60A-8V : 66A
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RPLB - LHC120A-10V : 52A, 63A, 73A, 84A, 94A, 105A, 115A, 126A
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RPMB – LHC600A-10V : 132A, 264A, 396A, 528A, 660A
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RPMC – LHC600A-40V : 88A, 132A, 330A, 550A, 660A
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RPHGx/RPHH – LHC4-6kA-8V : 4100A, 4650A, 4900A, 6100A
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RPHFx – LHC8kA-8V : 6600A, 6850A, 7300A, 8400A
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RPHE – LHC13kA-18V : Entre [0-14.3kA] à l’aide de la carte d’extension FGC
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RPTE – LHC13kA-180V : Entre [0-14.3kA] à l’aide de la carte d’extension FGC
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