2. High Voltage Withstand Levels
F. Rodriguez Mateos, TE/MPE-EE
on behalf of the HVWL Working Group
MQXF Workshop – Electrical QA

3. Outline Introduction HL-LHC HVWL Working Group General Strategy
MQXF case study
Discussion

4. The HVWL WG Regular meetings, bi-weekly in average Members
Based on experience from LHC, the aim is to engineer a common strategy for HL-LHC circuit components
On design of insulation and voltage withstand levels for main components and ancillaries
On electrical measurements : dielectric, transfer functions, instrumentation, etc
Common understanding and application of ElQA programme from manufacturing to operation through testing and commissioning
Adequacy of test infrastructure in agreement to the agreed strategies
Hugo Bajas TE-MSC, Secretary
Marta Bajko TE-MSC
Amalia Ballarino TE-MSC
Mateusz Jakub Bednarek TE-MPE
Jean-Paul Burnet TE-EPC
Giorgio D'Angelo TE-MPE
Paolo Ferracin TE-MSC
Christian Giloux TE-MSC
Juan Carlos Pérez TE-MSC
Jose Vicente Lorenzo Gomez TE-MSC
Félix Rodríguez Mateos TE-MPE, Chair
Frédéric Savary TE-MSC
Ezio Todesco TE-MSC

5. The way through . . . Reference documents: The LHC strategy
Some basic questions
The HL-LHC strategy
1. “Voltage withstand levels for electrical insulation tests …” EDMS “ELQA Qualification of the superconducting circuits during hardware commissioning” EDMS “Guidelines for the insulation design and electrical tests …” EDMS

6. General strategy
Definition of worst case voltages from quench modeling
Definition of conditions under which worst case voltages will show up:
ambient conditions (gas, liquid), pressure and temperature
Scaling factor ƒ
Definition of test conditions
Definition of strategy:
i) Validation that the worst case voltages will be endured with no degradation
ii) Looking for insulation defaults

7. Strategy i) Calculate worst case quench voltage Uq
Apply safety margins to it U=2*Uq+ 500 [V]
Give worst case voltage conditions C
Define test conditions C’
In order to be in “equivalent” ambient dielectric conditions, apply scaling factor ƒ and get new voltage levels U’
Apply the new calculated values U’ under test conditions C’

8. Strategy ii) Calculate worst case quench voltage Uq
Give worst case voltage conditions C
Define test conditions C’
Define minimum distance between electrodes (active part and ground): assume missing insulation failure at a certain stop which creates a certain creep path length l
Get dielectric strength for environment under conditions C [E in kV/mm]
Find factor ƒ to get voltage to be applied under test conditions C’
U’= E × l × ƒ will be the voltage to be applied under C’ test conditions

9. Main ElQA test steps Step: –n to –1 : In fabrication process
Step 1 in SM18
Step 2 in SM18
Step 3 in SM18
From modelling, the worst case in operation is obtained: Vq
Vee
Vpa
E.g. Vmax r =
Vq+Vee+Vpa
Rated level
V rated= f1 * Vmaxr + X
f1 and X are safety factor and margin
(for LHC : f1= 2 and X= 500 V)
1st test level at arrival to test bench
V test air = f2 *
Vrated
f2 is an equivalence between 75 K He and 300 K air
1st test level at cold in the test bench
V test LHe = f3 *
Vrated
f3 is an equivalence between 75 K and
4.2 K in He
1st test level after cold test in air but possible with He gas
V test LHe = f4
Vrated
f4 is an equivalence between 75 K and
300 K in He
75 K, 1 bar , He (gas)
75 K, 1 bar , He (gas)
300 K, 1 bar , AIR
4.2 K, 1 bar, He (liquid)
300 K, 1 bar , He (gas)
Test station is limited to 3000 V
This test is not revealing the default as we are in air applying gas conditions . This can be corrected by pressurised He.
This are not easily reproducible conditions, therefore values haveto be scaled to test in
air, LHe, GHe

10. Dielectric strength for 0.5 mm gap
VAir, 275 K, 1 b
2500
VLHe, 4.2 K, 1 b
7000
VGHe, 275 K, 5 b
700
VGHe, 75K, 1 b
600
VGHe, 275 K, 1 b
280
C.R. Huffer
f2= 2500/600 = 4.2
f3= 7000/600 = 11.7
f4= 280/600 = 0.47
VLHe, 4.2 K, 1 b = 140 kV/cm

11. Case: worst case voltage QXF
Worst case voltages as presented by Emmanuele/Gianluca:
Coil to ground = 620 V
Heater to coil =
Applying the previous rules to these numbers:
Test in dry air: 2604 V
Test in liquid helium: 7254 V
Test after warm up: 291 V

12. Discussion
Should we consider that in fully potted coils bubbles can appear
What will be the p/T conditions in those cracks/fissures?
By testing at the levels indicated, still one could detect absence of insulation as levels are above the dielectric given by air/helium for the defined distance
Limits given by the hardware (test benches): 3 kV to ground in SM18
Work on overall plan (including manufacturing)