1. Status of RB circuit modeling PSpice models Simulation results: nQPS & oQPS Comparison with QPS data Ongoing activities
Emmanuele Ravaioli
TE-MPE-TM

2. Status of RB circuit modeling
Main dipole circuit
Main components
PSpice model: New model of a dipole aperture
nQPS
oQPS
Conclusions and further work
Emmanuele Ravaioli TE-MPE-TM

3. Main dipole circuit (RB circuit)
Power converter
Filter
77 Magnets
Switch1
Crowbar
Switch2
77 Magnets
For more information about the PSpice models of the RB circuit
Annex or LHC-CM Ravaioli
Emmanuele Ravaioli TE-MPE-TM

4. Main dipole 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 TE-MPE-TM

5. Main dipole 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 = R_bypass = 10 
Model of an aperture
Model of an aperture
(refined for particular dipoles)
Model of a magnet
Emmanuele Ravaioli TE-MPE-TM

6. Status of RB circuit modeling
Main dipole circuit
nQPS
6 kA, dI/dt = 10 A/s; Old dipole model
6 kA, dI/dt = 10 A/s; New dipole model
6 kA, dI/dt = 10 A/s; With snubbers + additional filter resistors
Comparison with nQPS data
(Animation: Voltage to ground)
oQPS
Conclusions and further work
Emmanuele Ravaioli TE-MPE-TM

7. nQPS – Before shut-down 2010-11 – Simulation: Old model
6 kA, dI/dt = 10 A/s; Delay_s1 = 350 ms; Delay_s2 = 600 ms
Magnet 001  Blue
Magnet 154  Red
Emmanuele Ravaioli TE-MPE-TM

8. nQPS – Before shut-down 2010-11 – Simulation: New model
6 kA, dI/dt = 10 A/s; Delay_s1 = 350 ms; Delay_s2 = 600 ms
Magnet 001  Blue
Magnet 154  Red
Emmanuele Ravaioli TE-MPE-TM

9. nQPS – With snubbers + additional filter resistance – Simulation
6 kA, dI/dt = 10 A/s; Delay_s1 = 350 ms; Delay_s2 = 600 ms
Magnet 001  Blue
Magnet 154  Red
Emmanuele Ravaioli TE-MPE-TM

10. nQPS – Before shut-down 2010-11 – Comparison
2 kA, dI/dt = 10 A/s; Delay_s1 = 350 ms; Delay_s2 = 600 ms
nQPS Measurement
Simulation
Emmanuele Ravaioli TE-MPE-TM

11. Status of RB circuit modeling
Main dipole circuit
nQPS
oQPS
2 kA, dI/dt = 10 A/s; Balance dipoles
2 kA, dI/dt = 10 A/s; Unbalance dipoles
Comparison with oQPS data
Distribution of unbalanced dipoles
Simulation of an outlier dipole
Animation: nQPS and oQPS
Conclusions and further work
Emmanuele Ravaioli TE-MPE-TM

12. oQPS – Simulation: Balanced dipoles
2 kA, dI/dt = 10 A/s; Delay_s1 = 350 ms; Delay_s2 = 600 ms
Magnet 001  Blue
Magnet 154  Red
Emmanuele Ravaioli TE-MPE-TM

13. oQPS – Simulation: Unbalanced dipoles
2 kA, dI/dt = 10 A/s; Delay_s1 = 350 ms; Delay_s2 = 600 ms
Magnet 001  Blue
Magnet 154  Red
Unbalanced
Balanced
Emmanuele Ravaioli TE-MPE-TM

14. oQPS – Before shut-down 2010-11 – Comparison
2 kA, dI/dt = 10 A/s; Delay_s1 = 350 ms; Delay_s2 = 600 ms
Magnet 001  Blue
Magnet 154  Red
Unbalanced
Unbalanced
Balanced
Balanced
Magnet 001  Blue
Magnet 154  Red
QSO Measurement
Simulation
Emmanuele Ravaioli TE-MPE-TM

15. oQPS – Distribution of unbalanced dipoles (Sector 81)
2 kA, dI/dt = 10 A/s
(S /06/ )
Balanced
Unbalanced
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 by assigning a different value to the R_bypass parameter of one aperture of each unbalanced dipole
Possible physical explanation: Eddy currents affecting one (or two?) apertures of some dipoles in a different way at different current levels
A set of tests has been proposed in SM18 in order to measure the frequency transfer function of a few dipoles at cold at different current levels
Emmanuele Ravaioli TE-MPE-TM

16. oQPS – Simulation: Outlier dipole (C30R8)
6 kA, dI/dt = 10 A/s; Delay_s1 = 350 ms; Delay_s2 = 600 ms
Adopted parameters
F_bypass2 = R_bypass2 = 500 μ
Emmanuele Ravaioli TE-MPE-TM

17. Simulation results – nQPS and oQPS
2 kA, dI/dt = 10 A/s; Delay_s1 = 350 ms; Delay_s2 = 600 ms
Emmanuele Ravaioli TE-MPE-TM

18. Status of RB circuit modeling
Main dipole circuit
nQPS
oQPS
Conclusions and ongoing activities
Emmanuele Ravaioli TE-MPE-TM

19. Emmanuele Ravaioli TE-MPE-TM 21-04-2011
Conclusions
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 of each aperture model, according to the real behavior observed by the oQPS.
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 (voltage across each dipole) and by the oQPS (voltage difference between the two apertures of each dipole).
Emmanuele Ravaioli TE-MPE-TM

20. Emmanuele Ravaioli TE-MPE-TM 21-04-2011
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 measure the frequency transfer function of a few dipoles at cold at different current levels.
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 TE-MPE-TM

21. Emmanuele Ravaioli TE-MPE-TM 21-04-2011

22. Emmanuele Ravaioli TE-MPE-TM 21-04-2011
Annex
Emmanuele Ravaioli TE-MPE-TM

23. Simulated circuit – Complete model
Power converter
77 Magnets
Switch1
Filter
Switch2
77 Magnets
Crowbar
Earthing point
Emmanuele Ravaioli TE-MPE-TM

24. Simulated circuit - Power converter with output filter
+ 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 TE-MPE-TM

25. Simulated circuit – Switch model
Each switch is modeled by four switches in series to model the different phases of the switch opening.
Emmanuele Ravaioli TE-MPE-TM

26. Main dipole circuit – Distinct voltage transients
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.
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 TE-MPE-TM

27. 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 TE-MPE-TM

28. Main dipole circuit – Switch-off of the power converter
Emmanuele Ravaioli TE-MPE-TM

29. Main dipole circuit – Fast Power Abort (Switch opening)
Emmanuele Ravaioli TE-MPE-TM

30. Main dipole circuit – Main parameters
Number of dipoles
Inductance Lmag of each magnet mH
Capacitance to ground Cg of each magnet 300 nF
Parallel resistance R// of each magnet 
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 m
Resistance R_PC in the power-converter branches 3 m
Resistance R_EE of the extraction resistors 2x147 m
Emmanuele Ravaioli TE-MPE-TM

31. Simulation results – nQPS signals
2 kA, dI/dt = 10 A/s; Delay_s1 = 350 ms; Delay_s2 = 600 ms
Magnet 001  Blue
Magnet 154  Red
Emmanuele Ravaioli TE-MPE-TM

32. Unbalanced dipoles – Measured data (QSO signal)
Same event : FPA at 2 10 A/s (S /05/ )
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 TE-MPE-TM

33. Simulation results – Voltage waves along the magnet chain
2 kA, dI/dt = 10 A/s; Delay_s1 = 350 ms; Delay_s2 = 600 ms
Emmanuele Ravaioli TE-MPE-TM