1. TE-MPE Workshop 14/12/2010, Manuel Dominguez, TE/MPE-EI 1 Stephen Pemberton, TE/MPE-EI RB Earth Spark Investigation Initial Report. S. Pemberton, G. D’Angelo & M. Bednarek

2. TE-MPE Workshop 14/12/2010, Manuel Dominguez, TE/MPE-EI 2 Stephen Pemberton, TE/MPE-EI Summary  There have been numerous instances of Quench Detectors triggering HDS’s when the ground has been removed from an RB line Current Lead. It has been observed that a spark occurs when un-grounding; this is not only a problem for personal safety but also an undesired phenomenon for magnet protection. The current solution is to switch off all HDS power supplies in the sector that grounding/ un-grounding will take place.  The phenomenon is caused by the slight difference in potential between the groundings of the DFBA’s at either end of the sector:  This causes a current to flow along the RB line.  This current combined with the capacitance of the dipoles to ground slowly charges the RB line.  Peak voltages up to 290V have been recorded when un-grounding one CL.  The RB Spark Test was performed on sector 5-6 (as all HDS’s were off due to the Quench Propagation Test) using:  The PXI’s installed for the QPT.  Keithley DVM and a Tektronix DPO5054 at the level of the Current Leads on the DFBA.  Clip-on ammeter to observe current flow in the grounding leads.

3. TE-MPE Workshop 14/12/2010, Manuel Dominguez, TE/MPE-EI 3 Stephen Pemberton, TE/MPE-EI Summary  Initial tests performed during T.S 9 (9/05/11), then follow-up tests including experimentation with the proposed solution were performed during T.S 10 (4/07/11). We had teams positioned at Point 5 & Point 6 to perform various configurations of grounding/un grounding.  In parallel with our activities Emmanuele Ravaioli has been simulating the phenomenon on a PSpice model. He has also made simulations with our proposed solution.

4. TE-MPE Workshop 14/12/2010, Manuel Dominguez, TE/MPE-EI 4 Stephen Pemberton, TE/MPE-EI T.S 9/10: Spark Test Equipment Configuration.  The schematic below shows the test set-up with the Keithley DVM connected between Current Lead #2 and ground at Point 5.

5. TE-MPE Workshop 14/12/2010, Manuel Dominguez, TE/MPE-EI 5 Stephen Pemberton, TE/MPE-EI T.S 9:The various test configurations.

6. TE-MPE Workshop 14/12/2010, Manuel Dominguez, TE/MPE-EI 6 Stephen Pemberton, TE/MPE-EI Grounding tests which triggered QPS  The table below gives information on the time and configuration of our tests and shows which dipoles were triggered on QPS as a result of the spark phenomenon. Courtesy of Emmanuele Ravaioli TE-MPE-TM

7. TE-MPE Workshop 14/12/2010, Manuel Dominguez, TE/MPE-EI 7 Stephen Pemberton, TE/MPE-EI T.S 9: PXI data  The graph below shows the activity across a single magnet recorded on the PXI installation. We can clearly see the difference in peak voltage levels in relation to the time between grounding/ un-grounding. 5 sec intervals MBA line 5 sec intervals MBB line

8. TE-MPE Workshop 14/12/2010, Manuel Dominguez, TE/MPE-EI 8 Stephen Pemberton, TE/MPE-EI T.S 9: Test with Keithley at Point 5. The Keithley was connected between Current Lead #1 and ground. Screen shot below shows reading when the ground was removed from the Current Lead. 2222ffhhh 290V peak -105V

9. TE-MPE Workshop 14/12/2010, Manuel Dominguez, TE/MPE-EI 9 Stephen Pemberton, TE/MPE-EI T.S 9: Test with Keithley at Point 5 The Keithley was connected between Current Lead #2 and ground. Screen shot below shows reading when the ground was removed from the Current Lead. 270V peak -100V

10. TE-MPE Workshop 14/12/2010, Manuel Dominguez, TE/MPE-EI 10 Stephen Pemberton, TE/MPE-EI T.S 9: Test with Keithley at Point 5 This zoom shot of data taken from the Keithley shows the small voltage (around 10mV in this screenshot) generated in the RB line.

11. TE-MPE Workshop 14/12/2010, Manuel Dominguez, TE/MPE-EI 11 Stephen Pemberton, TE/MPE-EI T.S 10: Further tests and solution.  During the last technical stop we had an opportunity on Sector 5-6 to perform further tests.  We provoked the spark at point 5 and captured the data with an oscilloscope. We had peak voltages of 205V but this would be more if the circuit was left to ‘charge’ for a longer period between un-grounding.  We also used a clip-on ammeter to give us an approximate indication of the current flowing in the grounding leads. Although the ammeter cannot be called a ‘high precision instrument’ it indicated that we had anywhere between 400 – 800mADC flowing in the grounding leads at various stages of the test.  As a solution we used 2 diodes (1N4007) in parallel (forward/reverse configuration) as a voltage limiter between the current lead and ground, thus preventing these peak oscillations when the ground is removed.

12. TE-MPE Workshop 14/12/2010, Manuel Dominguez, TE/MPE-EI 12 Stephen Pemberton, TE/MPE-EI T.S 10: Test with Scope at Point 5.  The scope was connected between Current Lead #1 and ground with a differential probe. Screen shot below shows reading when the ground was removed from the Current Lead. 205V peak

13. TE-MPE Workshop 14/12/2010, Manuel Dominguez, TE/MPE-EI 13 Stephen Pemberton, TE/MPE-EI T.S 10: Voltage Limiter installed on CL#2.  The schematic below shows the Voltage Limiter installed on Current Lead #2. We performed a series of tests with the oscilloscope to observe its behaviour.

14. TE-MPE Workshop 14/12/2010, Manuel Dominguez, TE/MPE-EI 14 Stephen Pemberton, TE/MPE-EI T.S 10: Test with Voltage Limiter  The scope was connected between Current Lead #2 and ground with a passive probe. The ground was disconnected then the diode disconnected approximately 3 seconds later. Ground applied Lead ungrounded, voltage limited to 904mV Diode removed after 3secs End of Energy dissipation into diode

15. TE-MPE Workshop 14/12/2010, Manuel Dominguez, TE/MPE-EI 15 Stephen Pemberton, TE/MPE-EI Next Steps...........  We will produce and install voltage limiters at each DFBA in the machine.  A full procedure will be written to clarify the steps involved when using the voltage limiter, this can then be used by all departments involving in grounding/ un-grounding the RB lines, especially EN/MEF.  We still have the PXI data from T.S 10 to retrieve and analyze, this will be done with Mateusz.  There will be data within the post-mortem files and possible further data within TIMBER that can be used to support our findings.

16. TE-MPE Workshop 14/12/2010, Manuel Dominguez, TE/MPE-EI 16 Stephen Pemberton, TE/MPE-EI RB Earth Spark Investigation PSpice simulations Emmanuele Ravaioli With thanks to S.Pemberton TE-MPE-TM 14-07-2011

17. TE-MPE Workshop 14/12/2010, Manuel Dominguez, TE/MPE-EI 17 Stephen Pemberton, TE/MPE-EI Emmanuele Ravaioli TE-MPE-TM 14-07-2011 17 RB56 – Dipoles that triggered the QPS during ElQA tests The dipoles that triggered the QPS during the ElQA tests on May 12 th 2011 are those that present the largest oscillations during a fast power abort @ 2 kA, 10 A/s (unbalanced dipoles).unbalanced dipoles

18. TE-MPE Workshop 14/12/2010, Manuel Dominguez, TE/MPE-EI 18 Stephen Pemberton, TE/MPE-EI RB chain – Adopted model Emmanuele Ravaioli TE-MPE-TM 14-07-2011 18 Switch (opened when ungrounding) Resistance of the instrumentation Link to earthing line of the magnets Link to the chain of 77 magnets Resistance of the warm cables U_chain

19. TE-MPE Workshop 14/12/2010, Manuel Dominguez, TE/MPE-EI 19 Stephen Pemberton, TE/MPE-EI Emmanuele Ravaioli TE-MPE-TM 14-07-2011 19 RB chain – Voltages with respect to ground U_chain = 5 mV, R_instrument = 10 mΩ (+0.5 mΩ of the warm cables) Max voltage ~ 250 V

20. TE-MPE Workshop 14/12/2010, Manuel Dominguez, TE/MPE-EI 20 Stephen Pemberton, TE/MPE-EI Emmanuele Ravaioli TE-MPE-TM 14-07-2011 20 RB chain – QSO signals ( Voltage difference between apertures ) U_chain = 5 mV, R_instrument = 10 mΩ (+0.5 mΩ of the warm cables) QPS triggers!

21. TE-MPE Workshop 14/12/2010, Manuel Dominguez, TE/MPE-EI 21 Stephen Pemberton, TE/MPE-EI Emmanuele Ravaioli TE-MPE-TM 14-07-2011 21 RB chain – Voltages with respect to ground With 2 diodes in anti-parallel at the beginning of the chain Max voltage ~ 2 mV

22. TE-MPE Workshop 14/12/2010, Manuel Dominguez, TE/MPE-EI 22 Stephen Pemberton, TE/MPE-EI Emmanuele Ravaioli TE-MPE-TM 14-07-2011 22 RB chain – QSO signals ( Voltage difference between apertures ) With 2 diodes in anti-parallel at the beginning of the chain |QSO| << noise