1. 1 CSCM-7TeV Powering Implementation and ELQA Third LHC Splice Review 12 - 14 November 2012 M. Bajko, M. Bernardini, B. Bordini, k. Brodzinski, J. Casas-Cubillos, Z. Charifoulline, G. D’Angelo, R. Mompo, A, Perin, M. Pojer, F. Savary, M. Solfaroli Camillocci, J. Steckert, H. Thiesen J.P. Tock A. Verweij, G. Willering, D. Wollmann,

2. Initial objective The Initial objective of the CSCM project (before splice consolidation) was a possible increasing of the LHC energy up to 5 TeV in 2012. The current and the time constant of the main circuits were limited at 9 kA - 68 s for the RB circuits and 9 kA – 15 s for the RQ circuits The specification for the power converter was [6kA/300V] 2 Third Splice Review, 12 – 14 November 2012 RB circuit – 5 TeV / 68 s

3. Objective after LS1 After LS1, the objective of the CSCM project will be to qualify the entire “busbar system” at 7 TeV. The currents and the time constants of the main circuits will reach their nominal values 12 kA – 100 s for the RB circuits and 12 kA – 30 s for the RQ circuits The specification for the power converter is now [12kA/400V] 3 Third Splice Review, 12 – 14 November 2012 RB – 12 kA / 104 sRQ – 12 kA / 30 s 360 V 12 kA 300 V  = 90 s  = 18 s

4. Powering options Several options have been studied to power the main circuits during CSCM test Install new 5 MW high current power converter Connect in series the both RB power converters of the even points Reconfigure the RB power converters Only the last option is realistic specially for the type test at the beginning of the LS1. The topology of the RB power converter is two thyristor bridges [6.5kA/200V] connected in parallel to reach [13kA/200V] The proposed configuration for the CSCM test is to connect in series the two [6.5kA/200V] bridges to reach 400V. It is possible to operate thyristor bridges at over current but it is not possible to increase the output voltage of the thyristor bridges. This maximum voltage is fully fixed by the input transformers. 4 Third Splice Review, 12 – 14 November 2012 L L L L

5. RB power converter reconfiguration for CSCM 5 Third Splice Review, 12 – 14 November 2012 The reconfiguration of the RB power converter consists mainly of the removal of two copper plates and the installation of six 240 mm 2 DC cables between the new free connections Output voltage sensor must be also adapted (resistor bridge) Bridge over current protection level must be also increased

6. 6 RB power converter in EPC test hall U_out I_out RB power converter reconfiguration for CSCM The CSCM configuration has been tested last year in the EPC test hall but only at 6.5 kA Result of the test at nominal voltage (400V) 400 V 90 A Challenge = run at 12kA Third Splice Review, 12 – 14 November 2012

7. 7 Run the thyristor bridges at 12 kA The new CSCM objective requests to run the thyristor bridges at 12 kA instead 6.5 kA (factor 2) This over current operation concerns not only the power converter but also the full “powering chain” AC distribution (18 kV MCB and AC cables between MCB and power converter) 18 kV transformers Thyristor bridges Output filter chokes Third Splice Review, 12 – 14 November 2012

8. 8 18 kV AC distribution The factor 2 on the output current of the thyristor bridges implies the same factor for the AC line current: 200 A rms instead 100 A rms. At CERN, the minimal current of the 18 kV cells is 630 Arms The rating of the TIs which protect the AC cables is 200 A rms (200:1) At CERN the minimum cross-section of 18 kV cable is 50 mm 2 No major issue with the AC line (to be confirmed by electrical service). Only the protection thresholds of the 18 kV cell must be increased for the CSCM test Third Splice Review, 12 – 14 November 2012

9. 9 RB power converter For the 18 kV input transformers, the factor 2 on the current with 100 s decay time constant is not a major issue. Thermal time constant of the transformer >> minute No saturation effect Concerning the output filter chokes The output filter chokes are water cooled: thermal time constant of the chokes >> minute Saturation effect which can increase the current ripple inside the chokes and the output capacitors (must be check during the tests in EPC test hall). The major issue concerning the power converter is the cooling of the thyristors. Thyrsitor bridges have been designed to operate at 7 kA (6.5 kA with margin) But they have to support the decay of the output current w/o water Time constant = 100 s Maximum semiconductor temperature = 100 o C Third Splice Review, 12 – 14 November 2012

10. 10 RB power converter Third Splice Review, 12 – 14 November 2012 Half bridge + Half bridge - FWT Three separated circuits to cool each SCR bridge Power dissipated in the bridge is mainly linear with the current Estimate temperature of semiconductor at 6.5 kA is about 70 o C and maximum operating temperature for the thyrsitor is 125 o C The thermal characteristics of the water plates have to be identified and validated before to run at 12 kA. T_water_in = 28 o C Flow_water = 10 l/mn I_bridge (kA)6.5101112 P_SCR (kW)2.484.344.945.56 Tj_max (oC) (1) 69100109117 Tj_max (oC) (2) 76112124133 Rhw = 5 o C/kW (1) or 8 o C/kW (2)

11. 11 RB power converter Third Splice Review, 12 – 14 November 2012 Thermal characterization of RB power converter cooling system 1kA/s – 12 s 2 s at 12 kA  = 90 s Rjh Cjh Rhw ? Chw ?P TjTh Tw_out = Tw_in + 15 o C*P(kW)/flow(l/mn)

12. 12 RB power converter Third Splice Review, 12 – 14 November 2012 Today the characteristics of the water plates are unknown and they are showstoppers to run at 12 kA. It is not a major issue for the type test. We have a good feeling to be able to reach 10 kA or above but we need time to characterize the water plates

13. 13 RB power converter Third Splice Review, 12 – 14 November 2012 The second challenge for the powering is the current control in the circuit specially during the start up of the power converter Each of the 154 dipole magnets (47 or 51 for the RQ circuits) can be modeled by R=0.3 m  (xx m  for MQ) and L=100 mH (5.5 mH for MQ) series impedance with diode in parallel 2x240mm2 30 m  100 mH

14. 14 RB power converter Third Splice Review, 12 – 14 November 2012 At low level (< 100 A), the current passes through the magnets and the impedance of the load is approximately 5  and 15.5 H (xx  and 0.3 H for the RQ circuits) When the diodes conduct, the impedance of the circuit is close to 0 This variation of impedance can generate control instabilities which can be amplified by the diode threshold which changes with the temperature (of the diodes) At 20 K, the threshold of the diodes is 2.5 V (estimation) and only 0.7 at 300 K 2x240mm2

15. 15 RB power converter Third Splice Review, 12 – 14 November 2012 One solution is to use a “diode warm up” power converter to solve partially the stability issue, but this solution introduce other problems (free wheeling) and for the type tests we prefer to avoid this solution. 2x240mm2 30 m  100 mH

16. RB power converter 16 TE-TM – CSSM Status – 04 September 2012 Test of 4 x RQ diodes in SM18 I = 12.8 kA / tau = 50 s didt = 10kA/s 80A Results of the diode qualification test in SM-18

17. 17 ELQA Third Splice Review, 12 – 14 November 2012 The current can generate damage (over heating) but also the voltage across the circuits Instrumentation Insulation As the magnets are not superconducting and the energy extraction systems are short circuited during the CSCM test, only the power converter can generate voltage across the circuit No voltage generated by the quench No voltage generated by the opening of the 13kA-EE switches Maximum power converter output voltage = 400 V Totally define by the input transformers Maximum expected voltage across the circuits during the test 360 V for the dipole circuits 300 V for the quadrupole circuits Electrical properties of the gaseous He are strongly dependent of the pressure. Temperature (between 20 K and 100 K) has less influence (< 10%)

18. 18  From Standard ELQA procedure (Edms:788197)  From SM18 Test benches:  Proposed ELQA Test for CSCM  Pressure in the cold masses of the ARC: P > 5.5 bar (> 4.0 bar is acceptable)  Pressure in the DFBAs: P > 1.8 bar (same condition as TP4-C) Circuit@ 300 K, p=1 bar@ 1.9 K p=1 bar Main Dipole vs GND600 V600 V, 1900 V and 3100 V CircuitTP4-C, T=80K, p=6+/-0.5 bar TP4-E, T=1.9K, p=1 bar Main Dipole vs GND600 V1900 V CircuitMax Voltage expected ELQA HVQ, T=20 K, p> 5.5 bar Max leakage current after 300s. Main Dipole vs GND400 V600 V (standard value) 50 µA ELQA – RB circuits Third Splice Review, 12 – 14 November 2012

19. 19  From Standard ELQA procedure (Edms:788197)  From SM18 Test benches:  Proposed ELQA Test for CSCM  Pressure in the cold masses of the ARC: P > 5.5 bar (> 4.0 bar is acceptable)  Pressure in the DFBAs: P > 1.8 bar (same condition as TP4-C) Circuit@ 300 K, p=1 bar@ 1.9 K p=1 bar Main Quad vs GND180 V180 V, 900 V CircuitTP4-C, T=80K, p=6+/-0.5 bar TP4-E, T=1.9K, p=1 bar Main Quad vs GND120 V240 V CircuitMax Voltage expected ELQA HVQ, T=20 K, p> 5.5 bar Max leakage current after 300s. Main Quad vs GND300 V400 V (safety margin = 20%) 20 µA ELQA under this condition has not been done before, but it should not be a problem! ELQA – RQ circuits Third Splice Review, 12 – 14 November 2012

20. 20 What can be wrong? Third Splice Review, 12 – 14 November 2012 The main risk concerning the powering aspect is to forget something and/or the behavior of the system is not as expected Local damage The CSCM test implies a full reconfiguration of the powering system Cryogenic Circuits protection system Powering system The reconfiguration risks can be managed by procedure and by increasing the current step by step Individual system tests Short circuit tests (w/o current inside the cold part of the circuit – short circuit at the level of the DFB) Different current level with approval criteria between two current levels

21. 21 Could we imagine permanent installation for RQ circuit? Third Splice Review, 12 – 14 November 2012 The CSCM test requests important modifications of the RB and RQ systems (powering and protection) These modifications imply heavy commissioning campaigns before and after the test. It is difficult to imagine to realize CSCM during Xmas break for the 8 RB circuits and 16 RQ circuits, but… It is possible to realize full CSCM campaign after each LS. It is possible to test one sector during Xmas break in case of doubt (bad quench, magnet replacement)

22. 22 Next steps and conclusion Third Splice Review, 12 – 14 November 2012 The next step is to identify the thermal characteristics of the thyristor bridge cooling system and to test one bridge at 12 kA with 90 s decay time constant. For the powering system, is it not mandatory to realize the type test at full current. 6.5 kA is enough to extrapolate the results for the test at 12 kA. Decision for the CSCM campaign at the end of LS1 depends of the results of the type test.