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

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

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

The way through . . . Reference documents: The LHC strategy Some basic questions The HL-LHC strategy 1. “Voltage withstand levels for electrical insulation tests …” EDMS 90327 2. “ELQA Qualification of the superconducting circuits during hardware commissioning” EDMS 788197 3. “Guidelines for the insulation design and electrical tests …” EDMS 1264529

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

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’

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

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

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

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

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)