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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? 1 The basic components: Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05 Consider a superconductor, already immersed in LHe: 2 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: 3 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 4 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 5 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 6 Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05 Two Magnets 7 Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05 Critical Elements, other than the Superconductor 8 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 9 Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05 Symbolic Circuit 10 Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05 Symbolic Circuit 11 Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05 Current Lead 12 Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05 Breakdown at points with high voltage 13 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 14 Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05 Symbolic Circuit Can quench, Has energy stored 15 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 16 Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05 17 Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05 18 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 19 Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05 Symbolic Circuit 20 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. 21 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 22 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 23 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. 24 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” 25 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 26 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 27 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 28 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 29 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 30 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 31 Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05 13 kA busbar protection Courtesy R. Denz 32 Karl-Hubert Meß, AT-MEL, CERN, 1211 Geneva 23, CERN, 11-13.5.05 Local quench detector for main magnets Courtesy R. Denz 33 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!! 34 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?) 35 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. 36 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. 37 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. 38 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. 39 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. 40 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. 41 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. 42