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Design of a power converter robust to grid perturbations

Design of a power converter robust to grid perturbations. Stefano Rossini. TE-EPC-MPC. February 2014. Outline. Grid perturbations and LHC Topology for a new power converter Choice of the modulation technique Simulation results Conclusions. Grid perturbations and LHC.

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Design of a power converter robust to grid perturbations

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  1. Design of a power converter robust to grid perturbations Stefano Rossini TE-EPC-MPC February 2014

  2. Outline • Grid perturbations and LHC • Topology for a new power converter • Choice of the modulation technique • Simulation results • Conclusions

  3. Grid perturbations and LHC • Grid perturbations and LHC • Electrical grid • Grid perturbations in LHC • Beam trips statistics • Concerned circuits and power converters • Requirements for a new power converter

  4. RTE/EDF 400kV Electrical grid 90 MVA 110 MVA 90 MVA 110 MVA 90 MVA 66kV 18kV 18kV 18kV LHC 2,4,6,8 LHC 1 5 x 38 MVA 70 MVA 18kV 18kV … P.C. 2 MVA P.C. 0.4kV Permanent operating conditions • EDF 400kV grid: • CERN 18kV grid: • CERN 0.4kV grid: 4

  5. Electrical grid • Grid transients (both 18kV and 0.4kV) as demanded by CERN electrical grid specification (EDMS #113154) • Over-voltages (voltage swells) • Slow: during 10ms • Surge: (peak) during 0.2ms • Under-voltages (voltage drops) • during 100ms • Most voltage drops due to lightning strikes ( summer). 5

  6. Grid perturbations in LHC Warm magnetstrip during grid perturbations With respect to nominal grid 18kV grid • Data taken from • “Power converters of circuits monitored by FMCM”, H. Thiesen, 03.04.2012, \\cern.ch\dfs\Departments\TE\Groups\EPC\Projects\LHC\Consolidation_RPTG\Hugues\1. LHC FMCM Power Converters(1).pptx. 6

  7. Beam dumps made by FMCM • Most (24 over 29) beam trips due to grid perturbations • More than half of these (14 over 24) are only causedby FMCM(Fast Magnet Current Change Monitor) Data taken from (based on measurements in year 2012) MPP Workshop March 2013, http://indico.cern.ch/conferenceOtherViews.py?view=standard&confId=227895, Ivan Romera Ramirez, http://indico.cern.ch/getFile.py/access?contribId=34&sessionId=7&resId=1&materialId=slides&confId=227895 7

  8. FMCM beam trips • RD1 and RD34 magnets are responsible for most of the beam trips occurring in LHC •  RPTGpower converters replacement is necessary • RD1.LR5 is the most problematic(90% of the EL FMCM trips due only to RD1/RD34) Data taken from (based on measurements over 2010 - 2012) MPP Workshop March 2013, http://indico.cern.ch/conferenceOtherViews.py?view=standard&confId=227895, Ivan Romera Ramirez, http://indico.cern.ch/getFile.py/access?contribId=34&sessionId=7&resId=1&materialId=slides&confId=227895 8

  9. Concerned circuits and power converters Point 5 Beams collision Point 3 Point 7 Point 4 RF Point 6 Beam dump Beams cleaning Beams cleaning Point 2 Point 8 Point 1 Beams collision 9

  10. RPTG power converter • RPTG power converter not able to ensure constant output current during grid perturbations •  New switch-mode power converter in order to continue operation during grid perturbations 10

  11. Requirements for the new power converter • Requirements • Robust to grid perturbations • Transients lasting up to 100ms • Transients of UN voltage step (1ph or 3ph) • Lifetime expectancy: ≥ 15 years of operation • Current ripple in the magnet: ~ 1 ppm (10% of resolution) • 0.9UN • 0.8UN • 100ms 11

  12. Lifetime requirements • 15 years of operation • Estimated 40’500 slow thermal cycles (~ hour) • IGBT/Diode modules suited for “traction” applications • AlSiC baseplate (~10x more cycles capability than Cu) • Only available in 1700V (or higher) •  Or avoid baseplate (Semikron SKIIP) 12 Source: Mitsubishi

  13. Technical requirements RPTG RD1.LR1 cycle at 4TeV • Unidirectionnal current: • Minimal output current: (better than RPTG) •  no energy recovery required • , 13

  14. Topology for a new power converter • Topology for a new power converter • General structure • Input stage • Output stage • 1Q converters • 4Q converters • Effect of grid perturbation on the DC-link • Choice of the topology 14

  15. Structure of the converter The structure of the converter is widely used in the MPC section and has proven to work well. 15

  16. Analysis methodology • Decoupling of grid perturbations rejection from the load. • Synthesis of the converter split in two: • Input stage  ensure minimum DC-link voltage • Output stage  perturbation rejection & magnet supply Input stage Output stage 16

  17. Input stage of the converter • Input stage of each module constituted by • 50Hz transformer • AC/DC converter • DC-link coil • DC-link capacitor • AC/DC converter will be a 3-phase diode bridge rectifier • Reliable • Simple • Passive •  Ensure sufficient voltage margin on DC-link capacitor  12-pulse diode rectifier 17

  18. Output stage of the converter • Output stage of each module constituted by • DC-link capacitor • DC/DC converter • HF coil • HF capacitors and damping resistor • Equivalent load (warm magnet) • Several “classical” (Hard-switching) topologies are possible • 1Q/2Q converters (series or parallel) • 4Q converters (series or parallel) 18

  19. Output stage: 1 Quadrant topologies (1Q) 1Q series (2 x 350V @ 2 x 405A) 1Q parallel (700V @ 4 x 203A) 350V 700V • Ripple frequency seen by the load: • Not possible because of minimal conduction time of IGBTs • @ 18 A  Worse than RPTG 19

  20. Output stage: 4 Quadrants topologies (4Q) • Minimal DC-link voltage • ’ (load requirements) • & • Transformer • Voltage drop in input coil • Switching effect of diodes (empiètement d’anode) • Voltage drop in transformer windings  20

  21. Output stage: 4 Quadrants topologies (4Q) 4Q parallel (700V @ 2 x 405A) 4Q series (2 x 350V @ 810A) 350V 700V at nominal grid: 610 V at nominal grid: 860 V at low grid: 765 V at low grid: 540 V Voltage margin: 158 V (~ 25%) Voltage margin: 12 V (~ 2%) • Not sufficient by itself • Need AFE / Boost / ”UPS” •  Simplest working solution 21 (low grid = grid at nominal - 10%)

  22. Simulation method for grid analysis • Transformer replaced by the no-load voltage and short-circuit impedance • Magnet &H-bridge replaced by a current sourcecontrolled to absorb a constant power from the DC link • Evaluate DC-link stability • Evaluate the voltage on the DC-link during grid perturbations 22

  23. DC-link stability • Operation at constant power can be source of instability • Power converter acts as a negative impedance on DC link • Grid perturbation causes oscillations of the input filter • RL damping branch in parallel to main DC coil • HIGHER DC-link voltage  Better damping •  Reduced undershoot •  Better DC-link stability 23

  24. Effect of grid perturbations on DC-link voltage 0.9 UN 0.8 UN DC-link voltage for 4Q series configuration 10% voltage drop on the 3 phases from LOW grid Steady state: Transient (100ms, 3 phases): U20 Critical point 24

  25. Output stage choice: 4Q series Limits of continuity of operation ( / ) 1.10 UNpermanent 1.10 UN 3ph • 100ms 0.80 UN 1.10 UN 1ph • 100ms 0.00 UN 0.90 UN 0.90 UN permanent 3ph • 100ms 0.75 UN 0.90 UN 1ph • 100ms 0.55 UN 25

  26. Output stage choice: 4Q series • Large DC-link voltage margin continuity of operation • Grid perturbation may even last longer than 100ms •  Choice of modulation technique 26

  27. Choice of the modulation technique • Choice of the modulation technique • 2-Level PWM • 3-Level PWM with frequency doubling • Comparison & ripple on/in the load 27

  28. 2-Level modulation State 2 State 1 Output voltage 28

  29. 3-Level modulation with frequency doubling State 4 State 1 State 2 State 3 Output voltage 29

  30. Modulations comparison • 2-Level modulation • Each diagonal of IGBTs is controlled by the same signal • The output voltage is or • The output frequency is • 3-Level with frequency doubling modulation • The output voltage is , 0 or • The output frequency is 30

  31. Gain [dB] Modulations comparison 0 Frequency [Hz] LC output filter Cutoff frequency of the filter ) -40dB / decade For a given filter ( , ) 31

  32. Output filter Magnet modeled as Model established after impedance measurements made on the the of RD1.LR1 / RD34.LR3 circuits Output voltage ripple of the power converter (FSW = 2.5kHz) 10 kHz 32

  33. Output filter • 3L2FSW modulation technique • True output voltage ripple frequency for each sub-converter (interleaving is not perfect) • For a given output filter cutoff frequency the ripple isgreatly attenuated without modifying the dynamic response of the converter.Switching losses not increased! • Zero current stability (small spikes) • Common mode currents (EMC / CEM) 33

  34. Simulation results • Simulation results • Some notes on control • Ripple current in the magnet • Grid perturbations rejection • Contributions of the differents control loops • Insufficient DC-link voltage 34

  35. Control structure Control used to validate grid perturbation rejection 35

  36. Control structure • DC-link compensation: real-time measurement of the DC-link voltage & correction of the duty cycle • Damping loop: (partial) state feedback of the current flowing in the output capacitor in order to simplify the synthesis of the voltage controller & damp oscillations • Voltage loop: PI controller • No offset • Good closed-loop bandwidth (~ 500Hz) 36

  37. System simulation results Ripple current in the magnet Nominal grid in steady-state Assumption before measurements Magnet: 1.8H / 0.8Ω / 10nF 37

  38. System simulation results Grid perturbation rejection Low grid with voltage drop Magnet: 1.8H / 0.8Ω / 10nF 38

  39. System simulation results Grid perturbation rejection High grid with voltage swell Magnet: 1.8H / 0.8Ω / 10nF 39

  40. System simulation results Grid perturbation rejection DC-link FF DC-link FF + VLOOP 40

  41. Severe perturbation: insufficient DC-link voltage 0.9 UN 0.7 UN Severe 3 phase undervoltage U20 Might continue operation If more severe  FMCM trip 15mA Continuity of operation  sufficient DC-link voltage 41

  42. Conclusions • RPTG replacement in order to improve reliability of LHC • Priority for RD1.LR5 (@CMS) • Switch mode power converter • Continuity of operation requires • Sufficient DC-link voltage  Series configuration • DC-Link compensation + VLOOP controller • Low output voltage ripple  3L2FSW modulation 42

  43. Questions Thank you for your attention Questions? 43

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