1. Electrical insulation of magnet circuits in sector 7-8
D. Bozzini, V.Chareyre, M. Bednarek, J.Ludwin, A Jacob, O. Desebe AT/MEL-EM
20 December 2005

2. Outline Description of the HV tests Applied voltage levels
History part 1: qualification of sector 7-8
Results
History part 2: Fault location on MB.A78 circuit
History part 3: Qualification after repair
Open issues on sector 7-8
Conclusion

3. Description of the HV qualification
High voltage qualification of the superconducting circuits before starting the cool down
ELQA-TP4-B applies to circuit powered via a DFB
ELQA-DOC applies to dipole orbit corrector circuits locally powered (SSS)
Scope
Measurement of the insulating resistance between circuit and ground
Detection of insulating faults between neighboring circuits
Profile of the test
Ramp-up (steps 200V)
Plateau 15s
Steady state
Measurement of the current
Ramp down

4. Applied voltages
According to specification EDMS 90327, “Voltage Withstand Levels For Electrical Insulation Tests On Components And Bus Bar Cross Sections For Different LHC Machine Circuits”
But:
According to the recommendation made by the EEWG on the 29th of November 2006:
Values at cold (with higher helium pressure) are applied to verify the integrity of the circuit routing through the DFB’s
Compensation of pressure inside helium enclosures is done to be in the higher part of the paschen’s curve.
6 bars in the magnet cold masses
2.1 bars in the DFB helium enclosure
MB.A78 circuit is the most affected circuit. Voltage level is increased from 620 V to 1900 V.

5. History part 1:Qualification of the sector 7-8
Thursday 7 Dec.
Installation of the measuring system, preparation of the test setup
Dry run for calibration
Qualification of the DFBMA, DFBMC circuits 
Qualification of the 6 kA circuits powered via the DFBAO 
Friday 8 Dec
Qualification of the DFBMH 
Qualification of 600 A circuits powered via the DFBAO line N 
Qualification of 600 A circuits powered via the DFBAN line N in parallel with the qualification of the QF,QD and spools from DFBAO 
Partial discharge at 570 V (setpoint 600 V) of circuit RCO.A78B2 
EDMS NC
Short to ground at 1300 V (setpoint 1900 V) of the MB.A78 circuit 
EDMS NC809943

6. Results (1): 13 kA and 600 A spool circuitsNC NC

7. Results (1): MQF.A78 circuit graph

8. Results (1): Short to ground of the MB.A78 circuit
 NC

9. Results (1): Partial discharge of RCO.A78B2 circuit
 NC 

10. Results (2): 6 kA circuits

11. Results (2): RQ8.L8 circuit graph

12. Results (3): 600 A circuits powered via line N

13. Results (4): 120 A circuits powered via DFBM

14. History part 2: Fault location on circuit MB.A78 Phase 1
Monday 11 Dec
Start of the diagnostic of the MB.A78 short circuited to ground NC809943
HV test has not been repeated
Disconnection of external instrumentation ( V_Taps, CL heater)
Separation of the two lines MBA and MBB
Ohmic resistance measurements with multimeter not conclusive, The measured values were floating a lot and not useful for the localization
Powering of each line with a floating current supply. The fault has been localized between half cells 20R7 and 23R7
Measurements are repeated locally accessing from 20R7 and 23R7 and the fault can be better localized between QQBI.B20R7 and QQBI.A21R7 (53 meters)
The fault suddenly disappeared and the resistance to ground went up to several Mohm (measured locally with an multimeter

15. MB.A78 Diagnostic: Fault localization, 1st step
Approximate localization of the short-circuit along the arc
A current of 50 mA is impressed on the MBA line from the current lead on the DFBAO, the MQF circuit is used as the return line
Ground is used as a voltage pick-up due to the short-circuit
The voltage drop measured locally on each voltage-tap next to ground corresponds to the voltage drop on the bus-bar line from the tap to the fault (no current through the ground and the fault).
The zero-volt measurement gives in theory the position of the fault
The localization is limited to the half-cell for a low current

16. MB.A78 Diagnostic: Fault localization, 2nd step
The same principle of measurement is applied by accessing the near voltage and current taps
A higher current is impressed via the current taps of the magnet protection diode for measuring higher voltage drops
Up to 6 A are injected, the voltage drop over the bus bars can be measured with good precision

17. History part 2: Fault location on circuit MB.A78 Phase 2
Tuesday 12 Dec
Repetition of the high voltage tests on the MBB and MBA lines up to 250V.
MBB line OK, MBA break down at 70V
Local diagnostic focused on MBA line. Repetition of voltage drops to ground measurements
Calculation of main bus bars resistances
Localization of the fault in the interconnect QBBI.B21R7

18. MB.A78 Diagnostic: Local measurements
V1 = VEE111 - VEE ≈ mV with I = 6 A V2 = VEE213 - Vf ≈ mV V3 = VEE111 - Vf ≈ mV
The developed length of bus-bar between the V-taps EE111 and EE213 has been computed based on specification drawings and is about mm
V1 gives the voltage drop per unit of bus-bar length: mV/mm
V2 gives the distance to the fault Lf from v-tap EE213 and is about 1300 mm
Fault localization using ground as voltage pick -up

19. MB.A78 Diagnostic: Measurements accuracy
Resolution of the DMM used for voltage measurements: 0.02 mV that corresponds to a bus-bar length of around 54 mm
The accuracy of the measurements mainly depends on the configuration of the circuit and related instrumentation as well as the measurement setup
The measurements are less accurate if:
Impressed current is too low
Voltage measurements are performed across two voltage pick-ups separated by a distance of a few meters (40 m in this case)
Voltage drops are measured across non-homogeneous circuits (diode connections for instance)
Fault localization
Fault localization based on less-accurate measurements (low current, diode connections)

20. History part 3: Qualification after repair
Qualification of the MB.A78 circuit after repair of the lyra
High voltage test performed independently on the two lines MBA and MBB
External instrumentation connected (CL heaters, voltage taps
Cold masses and DFB pressurized with helium respectively at 6 and 2.1 bars
Maximum applied voltage 600V
NC solved

21. Pending ELQA issues in sector 7-8
NC : Partial discharge at 570 V of circuit RCO.A78B2 has not been investigated
What shall we do?
Leave it as is the resistance to ground at 400V (last plateau measured is good 814 Mohm)
Qualify the circuit at cold at 600 V. It the problem is still there we will probably localize it in the helium gas environment (DFB)
ELQA-DOC has been performed only on few dipole orbit corrector circuits. Resources were allocated to the diagnostic of MB.A78 non conformity
Circuits of the inner triplet 8L have not been qualified. Last test done was at the end of the assembly, after closure of the internal lines
Final installation of current lead clamps not yet done on the 600A circuits of DFBAN
How to apply the EEWG recommendation of the 28th of November?

22. Conclusion ELQA at warm before cool down: procedure and tools ok ✔
NC on MB.A78: Real exercise to acquire experience on diagnostic methods and also to train/form specialists ✔
4 engineers and 2 technicians are needed to do diagnostic measurements along the arc. Three people on each DFB side
Measurable shorts can not be “easily” localized ✔
(overall score for ELQA activities: 4 out of 4)
Organization and collaboration between departments and groups ok ✔
Methods and tools for partial discharge faults are in preparation. Experience is low !
Diagnostic is time/resources consuming and this time is never foreseen (extra hours are not counted but are a reality). Management priorities defines what the ELQA team will do first, this causes impact on other ongoing activities (DFB qualification on surface, qualification of DFL cable installation,…) !
3 out of 4 engineers participating to the diagnostic have short contracts (2 from HNINP will terminate in August, Vincent end of June) 