Chamonix, Full energy exploitation 24/01/2017, Mateusz Bednarek, TE/MPE-EE
Earth faults during training campaigns
and beam operation
• Diagnostics, tools, procedures
• Other types of faults
Mateusz Bednarek, TE/MPE-EE
1
Introduction
Superconducting magnet circuits cannot operate with earth faults
Chamonix, Full energy exploitation 24/01/2017, Mateusz Bednarek, TE/MPE-EE
•
•
Power converters are equipped with earth fault detection systems
Prior to LHC operation, dedicated insulation tests are carried out by Electrical
Quality Assurance (ELQA) team to ensure that no earth faults are present in LHC
circuits
o
These tests are carried out at warm and at cold
In case an earth fault is detected a Non-Conformity Report (NCR) is created and the
case is studied in detail.
 Multiple fault origins are possible

Warm parts
o DC warm cables
o Power converters
o Detection electronics

Cold parts
o
o
o
o
Bus-bar routing (spiders, lyras etc.)
Instrumentation wires
Coils
Diode boxes
2
Most common earth fault location
At the level of bypass diode connections:
Chamonix, Full energy exploitation 24/01/2017, Mateusz Bednarek, TE/MPE-EE
NCRs: EDMS 871858, 883010, 888746, 745903, 853097, …
These faults were caused by metallic particles
that remain inside many cold masses after the
technological welding process.
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Chamonix, Full energy exploitation 24/01/2017, Mateusz Bednarek, TE/MPE-EE
Fault appearance mechanism
Most faults are provoked by thin and loose
metallic pieces that accumulate on uninsulated
live half-moon connections.
Such particles can move inside the coldmass due
to violent helium movements:
• Flushing (easy to detect during HV tests that
follow)
• Quench
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LHC experience
Chamonix, Full energy exploitation 24/01/2017, Mateusz Bednarek, TE/MPE-EE
Events that occurred in the LHC machine at cold:
• March 2015, NCR: EDMS 1502332
• December 2016, NCR: EDMS 1741891
Courtesy of Aline-Marie Piguiet
Jean-Philippe Tock
Sandrine Le Naour
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Chamonix, Full energy exploitation 24/01/2017, Mateusz Bednarek, TE/MPE-EE
6
Actions in case an earth fault is detected
Cold or warm part?
Where exactly?
• X-ray images
• Cryo condition stabilisation in the affected cell
o
3.3 K, 1.7 bar
2 days
• Earth fault burning
• X-ray images
• Qualification of the circuit
o ELQA HV tests at 2100 V
o Powering tests
1 day
If in the diode box
Chamonix, Full energy exploitation 24/01/2017, Mateusz Bednarek, TE/MPE-EE
o
o
2 days
• Analysis of transient data recordings
• Localisation of the fault
 Total time needed: 5 days
o
Partly in the shadow of the cryo recovery after the quench event
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Analysis and simulations
Courtesy of M. Prioli
Chamonix, Full energy exploitation 24/01/2017, Mateusz Bednarek, TE/MPE-EE
Simulation of the short to ground
Measurements
+ Simulations (dashed lines)
for family PN
A: short appearing
B: fuse blows
PN: before the short
NP: after the short
AB
Quenched before the short
Family PN
Family NP
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Fault finding in the cold part
Chamonix, Full energy exploitation 24/01/2017, Mateusz Bednarek, TE/MPE-EE
Precise localisation of the fault is necessary before the
fault elimination can be attempted.
Method
Precision
Comment
Global AC
Local AC
Local precision AC
Local precision DC
120 m
40 m
3m
0.2 m
Approx. location of faults in a full sector
Points to 2 magnets and a bus-bar
Works well on superconducting bus-bars
Resistive bus-bar sections only
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Earth fault burning
Chamonix, Full energy exploitation 24/01/2017, Mateusz Bednarek, TE/MPE-EE
 The discharge is initiated remotely from the CCC .
 Dedicated application automatically records and stores the measurement
curves from 6 measurement channels.
Fault disappears before
the EFB is fully discharged
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Final X-ray
Chamonix, Full energy exploitation 24/01/2017, Mateusz Bednarek, TE/MPE-EE
All standard qualification tests passed:
The fault was successfully eliminated
Before elimination
EFB
After elimination
Courtesy of Aline-Marie Piguiet
Jean-Philippe Tock
Sandrine Le Naour
11
EFB design
Chamonix, Full energy exploitation 24/01/2017, Mateusz Bednarek, TE/MPE-EE
EFB is a capacitor bank with triggering and diagnostics electronics
Following parameters are available in the EFB version 2016:
•
•
•
•
V_max = 900 V
C = 7 mF
E = 2.8 kJ
I_max = 400 A
Ongoing development:
• More powerful hardware
• Optimised parameters in accordance with:
o
o
Simulations of various fault scenarios
Lab experiments in more realistic conditions (He)
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Risks associated to RB quenches
Chamonix, Full energy exploitation 24/01/2017, Mateusz Bednarek, TE/MPE-EE
Event class
Damage that can be
repaired without
warm-up
Damage that can be
repaired with partial
warm-up to 3.3 K
Damage requiring
warm-up to 300 K (for
replacement or in-situ
repair)
Considerable
collateral damage,
not limited to one
cold-mass
*
Severity
factor
Probability
(per quench
event)
Repair
time
Quench heater failure without collateral damage
0.001
1d
0.1
Damage to instrumentation/QPS
0.01
0.3 d
0.3
Damage to the 13 kA EE switches
0.001
1d
0.1
Single short-to-ground in the cold part, which can
be removed using the EFB
0.01
100 d
5d
100
5
Single short-to-ground in the cold part, which
requires cold-mass replacement (EFB not applicable)
0.001
100 d
10
Quench heater failure causing coil-to-ground or
inter-turn short
0.001
100 d
10
Coil-to-ground or inter-turn short
0.0001
100 d
1
Diode damage
0.0001
100 d
1
Double short-to-ground (worst case)
0.0001
300 d
3
Opening/arcing of the bypass bus
0.0001
300 d
3
Damage caused by the helium pressure shock wave
0.0001
300 d
3
???
?
?
?
Failure (damage) scenario
= P * R * 100
Courtesy of Arjan Verweij
Estimated number of quench events will be given in the presentation by Ezio Todesco
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Chamonix, Full energy exploitation 24/01/2017, Mateusz Bednarek, TE/MPE-EE
Conclusions
• Electrical faults in the cold magnet chain cannot be
excluded during the training campaign and beam
operation
• Single earth faults resulting from the diode container
issue can most likely be removed within about 5 days
• Further work on the Earth Fault Burner is on-going
o
o
Parameter optimisation
Lab experiments aiming to maximise the success rate of
fault elimination and to better understand potential
limitations of method
• Other types of faults cannot be neglected
o
o
Detailed studies and risk analysis are on-going
Mitigation methods are being evaluated
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Chamonix, Full energy exploitation 24/01/2017, Mateusz Bednarek, TE/MPE-EE
Acknowledgements
Giorgio D'Angelo
Mikołaj Bednarski
Stavroula Balampekou
Lorenzo Bortot
Zinur Charifoulline
Knud Dahlerup-Petersen
Reiner Denz
Manuel Angel Dominguez Martinez
Vincent Froidbise
Piotr Jurkiewicz
Wiesław Kantor
Jaromir Ludwin
Michał Maciejewski
Szymon Michniuk
Joaquim Mourao
Sandrine Le Naour
Stephen Pemberton
Paweł Pietrzak
Aline-Marie Piguiet
Mirko Pojer
Marco Prioli
Felix Rodriguez Mateos
Rudiger Schmidt
Grzegorz Seweryn
Andrzej Siemko
Matteo Solfaroli Camillocci
Krzysztof Stachoń
Jens Steckert
Jean-Philippe Tock
Arjan Verweij
Damian Wojas
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Chamonix, Full energy exploitation 24/01/2017, Mateusz Bednarek, TE/MPE-EE
Spare slides
16
Simulation of a double short-to-ground
Chamonix, Full energy exploitation 24/01/2017, Mateusz Bednarek, TE/MPE-EE
Courtesy of M. Prioli
Quenched before
the shorts
Short 2 Short 1
Sequence of events:
1. Short 1 appears and the grounding fuse is blown
2. Short 2 appears, the circuit is divided into two chains
3. The inductance unbalance generates a current
through the shorts
4. The energy dissipated in the shorts is sufficient to
melt the debris
5. Impulsive inductive voltage is obtained when the
short current tends to extinguish
6. Two arcs are generated that maintain the short
current until full discharge
Assumptions:
1.
2.
3.
4.
Rshort 1 = 1 Ω
Rshort 2 = 0.5 Ω
Edebris 1,2 = 3 kJ
∆Varc 1,2 = 15 V
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Simulation of a double short-to-ground
Chamonix, Full energy exploitation 24/01/2017, Mateusz Bednarek, TE/MPE-EE
Courtesy of M. Prioli
Edebris 1,2 = 3 kJ
Eshort 1,2 = 22 MJ
E EE2 = 80 MJ
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E EE1 = 800 MJ
Remarks:
 The energy dissipated in shorts depends mainly on the position of the two
shorts, the voltage of the arc, and the currents at which the shorts occur.
 10 kJ is sufficient to burn a 10 mm hole in a 2 mm thick steel plate.