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Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05
Superconducting Circuits, a generic view
What is special with superconducting circuits?
What are the specifically dangerous issues?
What can be tested at all and is it worth the effort?
What are the required initial conditions?
What has been tested and does not need a repetition of
the test?
Worst case scenarios.
Can we completely rely on the tests?
Reiner: What remains to be done during hardware
commissioning?
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The basic components:
Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05
Consider a superconductor, already immersed in LHe:
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The basic components:
Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05
Consider a superconductor, already immersed in LHe:
As such pretty useless, but the picture is incomplete, anyhow:
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The basic components:
Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05
Consider a superconductor, already immersed in LHe:
We need: Current leads and all the warm parts
We will have in addition: Inductance, resistance and capacitance
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Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05
A single wire in details
R
C
C
L
R
C
R
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A single wire in detail
Frequency dependence
Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05
Stored magnetic
energy
C
R
C
L
R
C
R
Stored electrical
energy
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Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05
Two Magnets
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Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05
Critical Elements, other than the Superconductor
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Critical Elements, other than the Superconductor
Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05
Diodes to bypass the energy
But the energy must be dumped!
Damping resistors to deal with
voltage transients
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Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05
Symbolic Circuit
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Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05
Symbolic Circuit
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Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05
Current Lead
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Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05
Breakdown at points with high voltage
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Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05
Breakdown at points with high voltage
Typically at the circuits
extremities and at the
voltage taps or
feedthroughs, wherever
the gas can have low
density
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Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05
Symbolic Circuit
Can quench,
Has energy stored
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Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05
Quench - What Went Wrong?
•
Abnormal voltage signals recorded during the provoked quench
Courtesy: A. Siemko
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Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05
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Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05
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What can happen here?
•
In case of a quench:
Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05
– The energy in the magnet is high enough to destroy it.
– The energy must be spread quickly -> Heater
– The energy of the other magnets must be guided around -> Diode
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Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05
Symbolic Circuit
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What can happen here?
• In case of a quench:
Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05
– The energy in the magnet is high enough to destroy it.
– The energy must be spread quickly
» Heater
– The energy of the other magnets must be guided around
» Diode
– The time constant is very, very long
» We have to dump the energy
• To be done with great care, because we have to open a
switch!
• Time constant is still large (in particular for the dipoles).
– Be aware of the transmission line effects during switch
opening.
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Inventory
•
Current Leads
Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05
– 13 kA
– 6 kA
– 600 A
– 120 A in DFB
– 120 A in magnet
Difficult, because CL need a
working cooling environment to run
current. To establish this the load
parameters have to varied, which
in turn requires various currents
through a working magnet circuit.
To be discussed.
– 60 A in magnet
•
Busbars
– Big busbars
Form part of the circuit, but
tested only globally.
– Small busbars
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Inventory
Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05
• Magnets
–
–
–
–
–
13 kA circuits
6 kA circuits
600 A circuits
120 A circuits
60 A circuits
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Inventory
Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05
• Magnets
–
–
–
–
–
13 kA circuits
6 kA circuits
600 A circuits
120 A circuits
60 A circuits
“Easy”, Freddy takes
care.
The 60 A circuits and
most 120 A circuits (
including the current leads
and bus bars) are
protected by the
overvoltage detection of
the powerconverter.
Its AB-PO.
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Inventory
Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05
• Magnets
–
–
–
–
–
13 kA circuits
6 kA circuits
600 A circuits
120 A circuits
60 A circuits
The 120 A MO and the
600 A circuits have a
“global quench
protection”
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Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05
Global Quench Protection
ΔV
ΔV
L dI/dt
DSP
Interlock
24 bit ADC
Fieldbus
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Done at individual system test
Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05
•
Done at the time of HC:
• To be done
– Electronics installed
Establish cooling conditions
– Electronics tested
Establish interlock
– Electrical connection tested
– Fieldbus tested
– Generation of interlock signal
tested
– LdI/dt generation simulated
– Heating of the CL installed and
tested
Test with small current:
-energy extraction,
-current lead cooling,
dI/dt compensation
Interlock reaction
Increase current and repeat
In case of fast ramp down or
quench: Study voltages
carefully
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Inventory
Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05
• Magnets
–
–
–
–
–
13 kA circuits
6 kA circuits
600 A circuits
120 A circuits
60 A circuits
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Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05
6 kA quadrupoles
ΔU
ΔU
Long voltage tap,
Problems to be expected
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Done at individual system test
Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05
•
Done at the time of HC:
• To be done
– Electronics installed
Establish cooling conditions
– Electronics tested
Establish interlock
– Electrical connection tested
– Fieldbus tested
– Generation of interlock signal
tested
– Heater measured
– Heating of the CL installed and
tested
Test with small current:
-energy extraction,
-current lead cooling,
Voltage measurement
Heater firing
Interlock reaction
Increase current and repeat
In case of fast ramp down or
quench: Study voltages
carefully
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Inventory
Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05
• Magnets
–
–
–
–
–
13 kA circuits
6 kA circuits
600 A circuits
120 A circuits
60 A circuits
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Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05
13 kA busbar protection
Courtesy R. Denz
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Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05
Local quench detector for main magnets
Courtesy R. Denz
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Done at individual system test
• To be done
Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05
•
Done at the time of HC:
Establish cooling conditions
– Electronics installed
Establish interlock
– Electronics tested
Test with small current:
– Electrical connection tested
-energy extraction,
– Fieldbus tested
-current lead cooling,
– Generation of interlock signal
tested
Voltage measurements
– Heater installed, measured
– Heating of the CL installed and
tested
Heater test
Interlock reaction
Increase current and repeat
In case of fast ramp down or
quench: Study voltages
carefully
Selective heater test, very
touchy!!
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Worst case scenarios 1
•
What can go wrong?
Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05
– Missed quench can result in:
• Overheating
– Melting
– Pollution
• Overvoltage
– Can destroy large fractions of a sector
• Quench avalanche
– Backward Voltage
•
We are told that the theoretical probability for a missed quench is
small (~once per lifetime of the LHC?)
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Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05
Worst case scenarios 2
•
•
The system is designed failsafe.
A single fault should not be dangerous. <= to be seen.
•
•
Double (maybe correlated) failures:
Assume:
– The UPS fails
– During the fast ramp down, a quench happens. No detection, no
heating.
– Maybe, a switch can not open, but can not tell it.
– No Post Mortem.
•
The result will be severe damage and confusion.
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Worst case scenario 3
Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05
•
Assume a splice (interconnection) breaks. Has been tested successfully
but the repetitive forces lead to fatigue.
– The fluctuation in the arc-voltage and arc-current lead to
overvoltage at vulnerable positions.
• May lead to loss of voltage taps
• May lead to destruction of a diode
• Maybe even winding short with quench and destruction of the
coil.
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Worst case scenario 4
Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05
•
•
Assume a high contact resistance in a diode connection.
Together with a switch open failure and a break of the direct heater
signal.
– Bypass busbar will overheat.
– Resistance will even grow, the diode may be overheated, …..
– …see above
•
The very worst: Negligence and Sabotage
•
To put up an exhaustive list is inconceivable (for me). We have to keep
our eyes wide open. And we have to train/tell people in the field
continuously. This requires a certain information bandwidth. Given the
limited amount of experienced people, which would act as information
node, the number of simultaneous fronts is limited.
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Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05
Summary I
What is special with superconducting circuits?
Large inductance, large stored energy, low resistance,
long time constants, extremely high current density
What are the specifically dangerous issues?
Shorts, opening connections, high voltage, high energy
density, hydraulic problems
What can be tested at all and is it worth the effort?
Functioning of the safety systems (at that time and
against simple failure scenarios)
It is worth it, because the existence of the Lab can
be at stake.
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Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05
Summary II
What are the required initial conditions?
Finished installation, cold machine, electrically OK
Electronics tested as far as possible.
What has been tested and does not need a repetition of
the test?
Except for trivial things, which will be tested in the
shadow anyhow, everything is new or could have been
altered since the last test.
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Summary III
•
Worst case scenarios
Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05
– Single faults are supposed to do no harm
– Combined faults can happen due to noise, interference, network
overload or failure, power supplies… you name it.
– Whenever a quench is not detected: we have a problem
– Whenever the switches do not work properly: we have a problem
– Whenever the signal distribution does not work: we have a problem
– Whenever the readout does not work: we are blind
– There are certainly more reasons, why it may not work properly.
– Not in all, but in many cases, a careful test will tell us in due time
about problems. To make use of this: we need a careful analysis of
each test, “successful” or faulty, to exclude mistakes and faults.
This requires patience, experience, communication and above all:
time.
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Summary IV
Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05
Can we completely rely on the tests?
No, never. All systems could fail at any time. Also, the
beam can produce a quench with a voltage distribution
in the coil, which can not be tested without beam.
We need a good hardware, a good software and
experience to make the failure probability small.
Experience can only be gained, slowly.
The reliability of the tests depends on the quality of
the tests. Quality does not come for free.
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