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Main dipole circuit simulations Behavior and performance analysis PSpice models Simulation results Comparison with QPS data Ongoing activities Emmanuele Ravaioli LHC-CM 06-04-2011 Main dipole circuit simulations • Main dipole circuit • • • • Components Circuit behavior PSpice model Main parameters • Results • Means for reducing voltage oscillations • Quench Protection System • Conclusions and further work Emmanuele Ravaioli LHC-CM 06-04-2011 2 LHC main dipole circuit Power converter Crowbar Switch2 Filter 77 Magnets Switch1 77 Magnets Emmanuele Ravaioli LHC-CM 06-04-2011 3 Main dipole circuit – Charging of the circuit • A variation of the voltage across the capacitors of the filter causes an oscillation to occur. • The frequency of the oscillations depends on the inductance and capacitance of the filter, L_filter and C_filter. • The damping of the oscillations depends on the resistance of the filter R_filter. Emmanuele Ravaioli LHC-CM 06-04-2011 4 Main dipole circuit – Switch-off of the power converter Emmanuele Ravaioli LHC-CM 06-04-2011 5 Main dipole circuit – Fast Power Abort (Switch opening) Emmanuele Ravaioli LHC-CM 06-04-2011 6 Main dipole circuit – Distinct voltage transients 1. Voltage waves due to the filter ringing • They occur every time the voltage across the capacitance of the filter changes: strong effect when the power converter is shutting down; weak effect when the thyrirstors of the crowbar are already conducting. • Their frequency depends on the inductance and capacitance of the filter, L_filter and C_filter. • Their damping depends on the resistance of the filter R_filter. 2. Voltage waves due to the switch opening • They occur when the switches are opened, due to the sudden change of the voltage across the switches; the magnet string behaves as a lumped transmission line. • Their frequency depends on the magnet inductance L_magnet and on the capacitance to ground C_ground. • Their damping depends on the characteristics of the magnet chain. Emmanuele Ravaioli LHC-CM 06-04-2011 7 Simulated circuit – Complete model Power converter 77 Magnets Switch1 Filter Switch2 77 Magnets Crowbar Earthing point Emmanuele Ravaioli LHC-CM 06-04-2011 8 Simulated circuit - Power converter with output filter Power Converter + 2 Thyristors Grounding point Filter Inductors 6x Crowbars with Thyristors Filter Capacitors Power Converter + 2 Thyristors Grounding point • • • • PC composed of two parallel units 6x Crowbars to allow by-pass of the PC at the shut-down (Thyristor model needed) Filter at the output of the PC PC grounded in the positive and negative branches through capacitors Emmanuele Ravaioli LHC-CM 16-03-2011 9 Simulated circuit – Old dipole model From Methods and results of modeling and transmission-line calculations of the superconducting dipole chains of CERN’s LHC collider, F. Bourgeois and K. Dahlerup-Petersen Emmanuele Ravaioli LHC-CM 16-03-2011 10 Simulated circuit – New dipole model • 19 components 7 components: 1 hour 20 minutes of simulation time • Physically explainable by the effects of the eddy currents • The distribution of unbalanced dipoles in each sector can be simulated assigning a different value to the R_bypass parameter ( and eventually f_bypass2 and R_bypass2 ) of each magnet Standard parameters F_bypass = 0.75 R_bypass = 10 Model of an aperture Model of an aperture (refined for particular dipoles) Model of a magnet Emmanuele Ravaioli LHC-CM 16-03-2011 11 Simulated circuit – Switch model Each switch is modeled by four switches in series to model the different phases of the switch opening. Emmanuele Ravaioli LHC-CM 16-03-2011 12 PSpice simulation – Main parameters • Number of dipoles 154 • Inductance Lmag of each magnet 98 mH • Capacitance to ground Cg of each magnet 300 nF • Parallel resistance R// of each magnet 100 Ohm • Capacitance C of the power-converter filter 110 mF • Inductance L of the power-converter filter 284 uH • Resistors R in the filter branches (8x in parallel) 27 mOhm • Resistance R_EE of the extraction resistor 147 mOhm Emmanuele Ravaioli LHC-CM 06-04-2011 13 Main dipole circuit simulations • Main dipole circuit • Results • • • • • • I_max=6 kA; dI/dt=10 A/s; No switch opening I_max=6 kA; dI/dt=10 A/s; Delay_s1=0 ms; Delay_s2=0 ms I_max=6 kA; dI/dt=0 A/s; Delay_s1=0 ms; Delay_s2=0 ms I_max=6 kA; dI/dt=10 A/s; Delay_s1=400 ms; Delay_s2=400 ms I_max=6 kA; dI/dt=10 A/s; Delay_s1=400 ms; Delay_s2=560 ms New model of a dipole aperture • Means for reducing voltage oscillations • Quench Protection System • Conclusions and further work Emmanuele Ravaioli LHC-CM 06-04-2011 14 Simulation results – Typical configuration I_max = 6 kA; dI/dt = 10 A/s; No switch opening Max oscillation ≈ 9 V Min voltage ≈ -5 V Magnet 001 Blue Magnet 154 Red Emmanuele Ravaioli LHC-CM 06-04-2011 15 Simulation results – Typical configuration I_max = 6 kA; dI/dt = 0 A/s; Delay_s1 = 0 ms; Delay_s2 = 0 ms Max oscillation ≈ 9 V Min voltage ≈ -1100 V Magnet 001 Blue Magnet 154 Red Emmanuele Ravaioli LHC-CM 06-04-2011 16 Simulation results – Typical configuration I_max = 6 kA; dI/dt = 10 A/s; Delay_s1 = 400 ms; Delay_s2 = 400 ms Max oscillation ≈ 9 V Min voltage ≈ -1100 V Magnet 001 Blue Magnet 154 Red Emmanuele Ravaioli LHC-CM 06-04-2011 17 Simulation results – Typical configuration I_max = 6 kA; dI/dt = 10 A/s; Delay_s1 = 350 ms; Delay_s2 = 600 ms Max oscillation ≈ 9 V Min voltage ≈ -1100 V Magnet 001 Blue Magnet 154 Red Emmanuele Ravaioli LHC-CM 06-04-2011 18 Simulation results – Typical configuration – New model I_max = 6 kA; dI/dt = 10 A/s; Delay_s1 = 350 ms; Delay_s2 = 600 ms Max oscillation ≈ 9 V Min voltage ≈ -1100 V Magnet 001 Blue Magnet 154 Red Emmanuele Ravaioli LHC-CM 06-04-2011 19 Simulation results – nQPS signals – Comparison I_max = 2 kA; dI/dt = 10 A/s; Delay_s1 = 350 ms; Delay_s2 = 600 ms nQPS Measurement Emmanuele Ravaioli Simulation LHC-CM 06-04-2011 20 Voltage waves along the magnet chain - Animation I_max = 6 kA; dI/dt = 10 A/s; Delay_s1 = 350 ms; Delay_s2 = 600 ms Emmanuele Ravaioli LHC-CM 06-04-2011 21 Main dipole circuit simulations • Main dipole circuit • Results • Means for reducing voltage oscillations • Different switch opening delays • Snubber capacitors (13.3 mF) across each switch • Additional resistors (27mOhm 81mOhm) in the PC filter branches • Inversion between the filter and the thyristor branches • Quench Protection System • Conclusions and further work Emmanuele Ravaioli LHC-CM 06-04-2011 22 Simulation results – Snubber capacitors I_max = 6 kA; dI/dt = 10 A/s; Delay_s1 = 350 ms; Delay_s2 = 600 ms Max oscillation ≈ 9 V Min voltage ≈ -15 V Magnet 001 Blue Magnet 154 Red Emmanuele Ravaioli LHC-CM 06-04-2011 23 Simulation results – Additional resistors in the PC filter I_max = 6 kA; dI/dt = 10 A/s; Delay_s1 = 350 ms; Delay_s2 = 600 ms Max oscillation ≈ 7.5 V Min voltage ≈ -950 V Magnet 001 Blue Magnet 154 Red Emmanuele Ravaioli LHC-CM 06-04-2011 24 Simulation results – Inversion between filter & thyristors I_max = 6 kA; dI/dt = 10 A/s; Delay_s1 = 350 ms; Delay_s2 = 600 ms Max oscillation ≈ 3V 17V Min voltage ≈ -850 V Magnet 001 Blue Magnet 154 Red Emmanuele Ravaioli LHC-CM 06-04-2011 25 Main dipole circuit simulations • Main dipole circuit • Results • Means for reducing voltage oscillations • Quench Protection System • nQPS • oQPS • Conclusions and further work Emmanuele Ravaioli LHC-CM 06-04-2011 26 Simulation results – nQPS signals I_max = 2 kA; dI/dt = 10 A/s; Delay_s1 = 350 ms; Delay_s2 = 600 ms Magnet 001 Blue Magnet 154 Red Emmanuele Ravaioli LHC-CM 06-04-2011 27 Simulation results – oQPS signals – All balanced dipoles I_max = 2 kA; dI/dt = 10 A/s; Delay_s1 = 350 ms; Delay_s2 = 600 ms Magnet 001 Blue Magnet 154 Red Emmanuele Ravaioli LHC-CM 06-04-2011 28 Simulation results – oQPS signals – Unbalanced dipoles I_max = 2 kA; dI/dt = 10 A/s; Delay_s1 = 350 ms; Delay_s2 = 600 ms Magnet 001 Blue Magnet 154 Red Emmanuele Ravaioli LHC-CM 06-04-2011 29 Simulation results – oQPS signals – Comparison I_max = 2 kA; dI/dt = 10 A/s; Delay_s1 = 350 ms; Delay_s2 = 600 ms Magnet 001 Blue Magnet 154 Red Magnet 001 Blue Magnet 154 Red QSO Measurement Emmanuele Ravaioli Simulation LHC-CM 06-04-2011 30 Simulation results – oQPS signals – Outlier dipole I_max = 6 kA; dI/dt = 10 A/s; Delay_s1 = 350 ms; Delay_s2 = 600 ms Emmanuele Ravaioli LHC-CM 06-04-2011 31 nQPS and oQPS Simulations - Animation I_max = 2 kA; dI/dt = 10 A/s; Delay_s1 = 350 ms; Delay_s2 = 600 ms Emmanuele Ravaioli LHC-CM 06-04-2011 32 Main dipole circuit simulations • Main dipole circuit • Results • Means for reducing voltage oscillations • Quench Protection System • Conclusions and ongoing activities Emmanuele Ravaioli LHC-CM 06-04-2011 33 Conclusions -1 • The analysis of the voltage transients in the RB circuit after the switch-off of the power converter and during a fast power abort (power converter switch-off + switch opening) has been carried out by means of a complete PSpice model. • The model comprises the power converter and its filter, the dipole chain and its capacitance to ground, the switches and extraction resistors, the paths to ground. • A new model of a dipole aperture has been presented: the model is simpler than the previous one but more accurate in predicting the behavior of the circuit. • The behavior of the unbalanced dipoles, which are oversensitive to any voltage transient, has been successfully reproduced by assigning a different value to one parameter each aperture model, based on the real behavior observed by the QPS. • A slightly more refined model of an aperture has been developed in order to simulate the behavior of the so-called outlier dipoles, whose apertures undergo a strange transient after the opening of the switches. • The simulation results are in very good agreement with the data measured by the nQPS (magnet across each dipole) and by the oQPS (voltage difference between the two apertures of each dipole). Emmanuele Ravaioli LHC-CM 06-04-2011 34 Conclusions -2 • Simulations with different delay of the two switch openings have been performed; in particular, the adopted delay of 400 ms and 560 ms has been investigated in order to assess the advantages of this solution. • The analysis of the circuit highlighted two different kinds of voltage transients occur after a FPA, caused by different phenomena and characterized by different frequency, maximum value and damping. • Oscillations due to power converter switch-off: They happen due to the ringing of the PC filter, thus their frequency is determined by the filter parameters. • Oscillations due to switch opening: They present a much larger peak value (up to ≈1000 V), but since the current decays faster they are damped more quickly; their frequency depends mostly on the characteristics of the magnet chain (inductance and capacitance to ground of the apertures). Emmanuele Ravaioli LHC-CM 06-04-2011 35 Conclusions -3 • A set of simulations has been conducted in order to study the proposed (and partly implemented) modifications to the circuit: snubber capacitors across the switches of the extraction system; additional resistors in the PC filter branches; inversion between the PC filter and the thyristor branches. • Delay of the switch openings: The simulations show that delaying the switch opening with respect to the power converter switch-off effectively separates the events, and decreases the voltage differences between electrically-close magnets. • Snubber capacitors across the switches of the extraction system: With this configuration, the maximum voltage observed across the magnets decreases dramatically (≈1000 V ≈15 V). • Additional resistors in the PC filter branches: This modification leads to a quicker damping of the voltage waves, and to a decrease of the oscillation maximum amplitude of about 20%. • Inversion between the PC filter and the thyristor branches: This modification significantly decreases the voltage oscillations due to the power-converter ringing; nevertheless, it does not influence the ringing due to the switch opening, which remains the same with respect to the maximum peak and to the damping. Emmanuele Ravaioli LHC-CM 06-04-2011 36 Ongoing activities • Aperture model: Understanding the cause of the unbalanced behavior of a number of dipoles (hypothesis: eddy currents). A set of tests is foreseen in SM18 in order to obtain information about the frequency transfer function of a few dipoles at different current levels and to verify the initial hypothesis. • Switch model: Refining is required, in particular for smoothing the extremely sharp rise of the switch resistance during the last phase of the opening. • Power converter model: Understanding the reasons why the measured voltage across the PC oscillates at a frequency smaller than the nominal one (28.5 Hz instead of 31.8 Hz) and damps faster. The present model has been corrected according to the measured data. • Quadrupole circuit: Comparing the results of the performed simulations with measured data. Emmanuele Ravaioli LHC-CM 06-04-2011 37 Emmanuele Ravaioli LHC-CM 06-04-2011 Annex Emmanuele Ravaioli LHC-CM 06-04-2011 39 Unbalanced dipoles – Measured data (QSO signal) Same event : FPA at 2 kA @ 10 A/s (S67 20/05/2010 20.53) ONLY BALANCED MAGNETS ONLY UNBALANCED MAGNETS • The amplitude of the voltage difference between the two apertures of the unbalanced dipoles is ~5-6 times larger than that of the balanced dipoles, and in some cases exceeds the threshold (100 mV) • Dipoles oversensitive to any voltage transient • The phenomenon peaks around 2 kA and scales up linearly with the current ramp-rate • 50-60 % of the dipoles in every sector affected • The distribution of unbalanced dipoles is not dependent on the electrical or physical position, or on the manufacturer of the magnets and their cables, or on the date of installation Emmanuele Ravaioli LHC-CM 06-04-2011 40 Unbalanced dipoles – Modelling FPA at 2 kA @ 10 A/s (S67 20/05/2010 20.53) • The behavior of the unbalanced dipoles has been successfully simulated by means of a new simplified model of a dipole aperture • The distribution of unbalanced dipoles in each sector is simulated assigning a different value to the R_bypass parameter of each magnet • Possible physical explanation: Eddy currents ( see Possible cause of quench in B30R7, where U_QSO exceeds 100 mV during fast decay from 7000 A, Arjan Verweij, 2008 ) Emmanuele Ravaioli Standard parameters F_bypass = 0.75 R_bypass = 10 LHC-CM 06-04-2011 41