Download Main dipole circuit simulations

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
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•
•
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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