R.Schmidt, HC 28/5/2009 1

Access to underground areas during powering

Risk: accidental massive helium release (as on 19 September) when superconducting magnets are being powered

• 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 2

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

R.Schmidt, HC 28/5/2009 4

Powering and risk of helium release

• 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

• What is the size of an opening in the helium vessel that could produce such helium release?

• What are the parameters of an electrical circuit (current, stored energy, other parameters) that could produce such opening in case of severe electrical fault?

• 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

• 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

• 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

• The amount of helium release depends on the size of the hole.

• 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.

• 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.

• 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

• 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).

0.1

1

10

0 5 10 15 20 25Ori

fice

are

a [c

m2]

Upstream pressure [bar]

1.9 K5 K10 K15 K20 K

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 volume of 560 mm3, which requires an arc energy of 27 kJ.

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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 L [H] Maximum

current level Energy

[J] Corresponding

powering test step

main dipoles 15.708 0A 0.0E+00 PIC1 main quadrupoles 0.263 760A 7.6E+04 PLI1

arc individually powered

quadrupoles 0.06 900A 2.4E+04

PLI2

600A circuits 0.432

(400A) 400A / 550A 3.5E+04 PNO 120A orbit correctors 2.84 120A 1.4E+04 PNO 60A orbit correctors 6.02 60A 9.1E+03 PNO

Stand alone quadrupole

0.296 600A

5.3E+04 PLI2

Stand alone dipoles

0.052 1000A

2.6E+04 PLI2

L1 = 0.09 L2 = 0.038

inner triplet quadrupoles

(Q1+Q3/Q2a+Q2b) L3 = 0.09 n.a. 5.9E+03 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:• I_POS software limit in FGC configured from I_PNO in LSA

• I_HARDWARE hardware limit

circuits < 600A:• I_POS = I_PNO avec I_PNO = I_5Tev

• I_HARDWARE = set to > a I_7Tev (little difference between I_5Tev et I_7Tev)

circuits > 1000A:

• I_POS = I_PNO with I_PNO = I_PHASE_I for phase I and I_5Tev for phase II

• I_HARDWARE = set to > I_5Tev

circuits RB, RQD et RQF• I_POS = I_PNO with I_PNO = I_PHASE_I for phase I et I_5Tev for phase II

• 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

• I_PNO remains constant in LSA (I_5Tev)

• I_HARDWARE (new property in LSA) remains constant LSA (corresponds > I_7Tev)

circuits > 1000A

• I_PNO changes with the phase of the test: I_PNO = I_PHASE_I for phase I et I_5Tev for phase II

• I_HARDWARE remains constant in LSA (corresponds to > I_5Tev)

circuits RB, RQD et RQF

• I_PNO changes with the phase of the test: I_PNO = I_PHASE_I for phase I and I_5Tev for phase II

• 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

• RPLA - LHC60A-8V : 66A

• RPLB - LHC120A-10V : 52A, 63A, 73A, 84A, 94A, 105A, 115A, 126A

• RPMB – LHC600A-10V : 132A, 264A, 396A, 528A, 660A

• RPMC – LHC600A-40V : 88A, 132A, 330A, 550A, 660A

• RPHGx/RPHH – LHC4-6kA-8V : 4100A, 4650A, 4900A, 6100A

• RPHFx – LHC8kA-8V : 6600A, 6850A, 7300A, 8400A

• RPHE – LHC13kA-18V : Entre [0-14.3kA] à l’aide de la carte d’extension FGC

• RPTE – LHC13kA-180V : Entre [0-14.3kA] à l’aide de la carte d’extension FGC