Quench Protection for the CLAS 12 Torus Superconducting Magnet

1 LEAS 042 Report | December 2010

Philippe Fazilleau, CEA/Saclay, 91191 Gif-sur-Yvette, France,

(telephone: +33-169086762; e-mail: [email protected])

I. INTRODUCTION

HIS report gives a detailed presentation of the quench protection for the CLAS 12 torus superconducting magnet. The quench analysis, its propagation and consequences on voltages and

temperatures are detailed, including some fault scenarii. A parametric study, with variation of the propagation velocities, detection parameters (threshold voltage and time), RRR of the copper channel is also presented.

II. QUENCH PROTECTION CIRCUIT

A schematic of the magnet electrical power circuit is shown in Fig. 1. During normal operation the resistor is put in parallel with the magnet, with respect to the power supply;

if a major fault occurs (quench, current lead problem), the switch breakers are open disconnecting the power supply and leaving the magnet in series with the dump resistor in a L-R type circuit. A large part of the magnetic energy is then dissipated in the external dump resistor.

Fig. 1. Electrical circuit schematics

The magnet operates at 3.556 kA and its stored energy is 14 MJ.

III. QUENCH ANALYSIS

Quench simulation have been performed to model temperatures, currents and voltages at the terminals of the coils. The quench analysis has been lead with a three dimensional simulation of the quench thermal transient in the magnet, based on the quench propagation velocities and the resistance growth with time. The resistive zone expansion is modeled as a transient thermal conduction problem taking into account the heat source terms arising from normal state-current in conductors and the material property dependences on

Quench Protection for the CLAS 12 Torus Superconducting Magnet

T


temperature, magnetic field and RRR. The magnet terminal voltage, maximum temperature in the quench initiation point and resistance rise in

the winding are calculated numerically with respect to various input variables. A quench is initiated in the mid-plane corner of a coil and starts to expand in the three directions with

velocities vr and vz (transverse propagations) and vφ (longitudinal propagation along the wound conductor). The temperatures of successive quenching volumes are calculated at every step of time. The heat increase

of the dump resistor during the discharge can be included, as it is mostly adiabatic. Heat exchanges between the coils and the winding cases are included in the calculations.

A. Quench back effect

The protection design could take benefit of the quench back effect (magnetic coupling and thermal exchange) due to the eddy currents generated into the coil cases during a discharge.

The eddy currents have been computed with a 3D FEM code in each part of the coil cases (§ Annex 1) and the equivalent electrical circuit has been used to take them into account during the propagation process.

The eddy currents generated into the LN2 shields have also been computed; as they represent effectively only a small proportion of the total energy (approximately a quarter of the Joule power of the eddy currents generated in the coil cases) and they have not direct thermal exchange with the coils, they are not taken into account in the following study.

B. Hot spot criteria

The knowledge of the hot spot criteria temperature – adiabatic heating without any propagation - is a good starting point to design the protection. With an inductance of 2.24 H, a dump resistor of 124 mΩ and a current density of 66 A/mm², the criteria gives a value of 84 K (§ Annex 2). Any of the nominal case computations should be under this value.

C. Discharge Operation

Table I details the protection data during a normal operation; as the normal zone expands, the quench is detected if the resistive voltage at the terminals of the coils is higher than 100 mV during 100 ms. These values should be modified, especially the voltage threshold, to get rid of voltage noise or artifacts. The parametric study, detailed infra in the report, gives matter of choice concerning this threshold.

The quench propagation velocities have been calculated according to the well-known formula depending on the diffusivity, the Joule volumetric energy and the enthalpy. The values for the velocities vr and vz (transverse propagations) and vφ (longitudinal propagation along the wound conductor) are respectively 0.178 m/s, 0.153 m/s and 4.5 m/s at nominal current.

TABLE I PROTECTION DATA DURING NORMAL OPERATION

The thermal contact between coils and coil cases is described in table I. The thickness of the insulation is

set up at 3.4 mm. Coil cases are made of 6061 aluminum alloy and cooled down at 4.7 K. The results of computations for a quench, initiated in a coil at the time t=0 s, detected and then followed

by a complete discharge in the dump resistor are presented in table II. The maximal temperature in the coil is 57 K, the average one is 51 K. The gradient temperature, difference between minimal and maximal

Detec tion Detect ion volt age 100 mV Contactor opening de lay 100 ms Pr opagati on Transverse velocit ies vr and vz 18 and 15 cm/s Lo ngitudinal ve locit y vφ 4.5 m/s Thermal contact

Be tw e en w ind ing pa cks and c o i ls Coi l contact area 1.115 m²

Insula tion t hickness 3.4 mm


temperatures inside the coil, is 28 K. The temperature values are acceptable and will not endanger the magnet.

TABLE II COIL QUENCH

The transition is detected within a tenth of second and the switch breakers are opened 100 ms later (at

204 ms). The time constant of the discharge is around 9 s. The total transition time is small (1.84 s) in comparison with the discharge time (35 s).

More than half of the total energy (55 %) is dissipated in the dump resistor. The energy dissipated in the winding cases is small and represents less than 4 % of the total energy.

The maximal voltage to ground during the discharge is 220 V, by means of the grounding circuit which divides by 2 the voltage at the terminals of the magnet.

The quench back effect occurs after several seconds, around 2 s. If this occurrence leads to the quench of the remaining superconducting coils, its impact on the first quenching coil, i.e. where the quench has been initiated, is limited. Moreover, the first quenching coil is totally resistive once the quench back of its winding case becomes effective (at 1.84 s).

TABLE III QUENCH COMPUTATION RESULTS WITHOUT WINDING CASES

It is interesting to compute the discharge without heat exchange between the winding and its cases. As shown in table III, the results are still acceptable. All the energy is dissipated in only one coil and the dump resistor, as the other ones remain superconducting in the absence of quench back effect. The maximal temperature then reaches 73 K.

I in i t ia l = 3.556 k A Di sc har ge Vol tage t hre shol d 0.1 V Delay time 0.1 s

Time constant (I/e ) Di scharge t ime Contac tor opening time

9.5 s 35 s

204 ms Q ue nc h i ni tiation coi l Other coils Trans ition Begi nning 0.00 s 2.07 s End 1.81 s 2.33 s Quench-back time 1.81 s 2.07 s Quenched volume 100 % 100 % Fi nal temperature s Tm ax 57 K 46 K Tm ea n 51 K 45 K ∆T wi thin the windin g 28 K 12 K Dissi pate d Ener gies E i n resis tive part 9.6 % 6.6 % (*5) E in dump res is tor 55 % Voltages V max (a t the ter mi nals of the q- co i l ) 71 V 73 V V max (a t the ter mi nals of the ma gnet) 439 V

I in i t ia l = 3.556 k A Di sc har ge Vol tage t hre shol d 0.1 V Delay time 0.1 s

Tim e constant (I /e ) Di scharge t ime Contac tor openi ng tim e

14.1 s 56 s

210 m s Q ue nc h i ni tiation coi l Other c oils Transit ion Beginning 0.00 s * End 2.3 s * Quench-ba ck ti me * * Quenche d volume 100 % 0 % Fi nal temperature s Tm ax 73 K 4.7 K Tm ea n 65 K 4.7 K ∆T wi thin t he w indin g 33 K 0 K Dissi pate d Ener gies E i n res istive part 21 % 0 % E i n dump resis tor 79 % Voltages V max (a t the ter mi n als of the q- co i l ) 71 V 73 V V max (a t the ter mi n als of the ma g net) 440 V


The role of the winding cases is not essential for the good protection of the magnet; this is mainly due to the low Joule power generated inside them. The insulation between coils and winding cases delays the quench back effect and computations have shown that it occurs once the coils are almost totally quenched.

Its main effect resides in the firing of the remaining superconducting coils; a part of the total joule losses is then transferred to these coils, decreasing the current time constant as well as the maximal temperature in the first quenching coil.

D. Fault scenario

It is always of great interest to calculate the main parameters in case of a fault scenario; the non detection of the quench or the non opening of the dump switches will lead to the same scenario, the total discharge of the energy in the resistive part of the superconducting coils.

The results of the computation are shown in table IV. A quench is initiated in a coil at the time t=0 s, starts to expand but is not detected. This simulation takes into account the quench back effect. The maximal temperature increases in comparison with the normal discharge operation with a value of 100 K. The temperature gradient also increases with a maximal value of 33 K.

TABLE IV FAULT SCENARIO RESULTS

97 % of the total energy is dissipated in the main magnet, with about half of the total energy in the first

quenching coil. The three remaining percents are dissipated in the winding cases. It is still interesting to compute the fault scenario without heat exchange between the winding and its cases. As shown in table V, the results still remain acceptable. The maximal temperature reaches 137 K and the maximal temperature gradient stays below 40 K.

I in i tia l = 3.556 k A No det ect ion Quenching coi l Discharge Time const ant (I / e) Discharge time Swi tch breakers openi ng tim e Trans ition

14.9 s 38.6 s

*

Begi nning 0.00 s End 2.18 s Quenched volume 100 % Fi nal temperature s Tm ax 100 K Tm ea n 88 K ∆T wi thin the windin g 33 K Voltages V max 149 V Dissi pate d Ener gies E i n res is tive part 49 % E i n other que nching c oils 48 % E i n coil cases 3 %


TABLE V FAULT SCENARIO RESULTS WITHOUT WINDING CASES

IV. PARAMETRIC STUDY

A parametric study has been lead with variation of the propagation velocities, detection parameters (threshold voltage and time), RRR of the copper channel.

A. Propagation velocities

As presented supra in the report, the propagation velocities have been computed with the usual formulas; however, we decided to set them manually in order to detect a possible influence on the main parameters, i.e. maximal temperature, maximal temperature gradient and voltage.

Fig. 2 and 3. Dependence of the main parameters with the propagation velocities

Fig. 2 and 3 show the non-dependence of the maximal temperature and maximal temperature gradient with the propagation velocities; this is mainly due to the quench-back effect that helps to spread the quench all along the magnet.

Moreover, the propagation velocities have no influence (or a so very small one, less than 0.1 V on the maximal value of the voltage) on the voltage as its maximal value arises at the very beginning of the discharge when the switches open.

B. Detection thresholds

The voltage threshold Ud and action time td are two values that usually need to be set during the test

I in i t ia l = 3.556 k A No det ect ion Quenching coi l Discharge Ti me c onst ant (I/ e) Discharge tim e Swi tch bre akers openi ng time Trans ition

19.4 s 50.2 s

*

Begi nning 0.00 s End 2.18 s Quenche d volume 100 % Fi nal temperature s Tm ax 137 K Tm ea n 120 K ∆T wi thin t he w indin g 34 K Voltages V max 340 V Dissi pate d Ener gies E i n resistive part 100 % E in other quenching c oils 0 % E in coil cases 0 %

58,6

57,4

57

56,6

29,2

28,3

27,8

27,4

0 10 20 30 40 50 60 70

5

10

15

20

DTmax Tmax

Tra

nsve

rse

velo

citie

s (c

m/s

)

vlongi = 4.5 m/s

57,7

57

56,7

29

28

26,5

0 10 20 30 40 50 60 70

1

3

10

DTmax Tmax

Lon

gitu

din

al v

eloc

ity (

m/s

)

vtransverse = 15 cm/s

Temperature (K)


phase of the magnet, in order to get rid of unwanted discharge due to voltage noise or artifacts.

Fig. 4. Dependence of the main parameters with the propagation velocities

Their influence is very low on temperatures and gradients as shown in Fig. 4. (only the td dependence is

represented as the Ud one is too small, less than 0.1 K for Ud between 0.1 and 1 V).

C. RRR of the copper channel

The RRR of the copper channel, even if specified at 100, could be modified by the soldering and winding processes ; it is then of great interest to study its influence on the quench results.

Fig. 5 and 6. Influence of the RRR of the copper channel on the quench temperatures and gradients As shown in Fig. 5 and 6, this influence is quite low, whatever the case, nominal one or fault scenario.

The best value of the RRR for the protection results (lowest temperature and gradient) is obviously the higher one (RRR=200).

D. Operating temperature

The influence of the operating temperature has been studied with a range between 4.5 and 5 K; it does not play any major role for the protection results with maximal temperature and gradients remaining at the same values.

57

58

59

62

28

29

30

32

0 10 20 30 40 50 60 70

0,1

0,3

0,5

1

DTmax Tmax

Temperature (K)

td(s

)

Ud = 0.1 V

60

57

55

31

28

26

0 10 20 30 40 50 60 70

50

100

200

DTmax Tmax

Temperature (K)

RR

R

Nominal case

100

100

102

37

33

32

0 20 40 60 80 100 120

50

100

200

DTmax Tmax

Temperature (K)

RR

R

Fault scenario


V. CONCLUSION

A detailed quench analysis has been performed. The resulting hot-spot temperature and voltages show the proposed protection system will be adequate to protect the superconducting torus with:

• maximal temperatures lower than 150 K; even for the worst case, fault scenario without coil cases,

the maximal temperature is 137 K (all the magnetic energy is dissipated in only one coil), not endangering the magnet; for the nominal case the maximal temperature is only of 57 K;

• maximal temperature gradients lower than 50 K; the maximal temperature gradient is kept below 30 K for the nominal case, the maximal gradient arising for the fault scenario without coil cases with a value of 34 K; this will not induce any issues for mechanics;

• maximal voltages lower than 500 V; the maximal voltage at the terminals of the magnet during

normal operation arises when the switches open and only depends on the resistance and current values at this time, which is trivially always below 440 V. A study has also been lead to check the voltage to ground all along the magnet at any time and it shows that the maximal voltage to ground is 340 V during a fault scenario.

The impact of the winding cases has also been enlightened: even if only a small part (less than 4 %) of

the total magnetic energy is dissipated inside them, they play a passive role of protection, initiating and spreading the quench all along the magnet by thermal exchange, then decreasing the maximal temperatures and gradients.


A NNEX 1 – EDDY CURRENT COMPUTATION RESULTS

All the computations have been performed for a current variation of 178 A/s (linear discharge of the current from nominal to zero in 20 s, conservative case). They assume a uniform current density in the thickness of the models as the calculations are performed with thin shells.

Current function T (total current = 3.46 kA) and current density J (Jmax = 3.06 A/mm²) in the cover plate (e = 3.9 mm)

Current function T (total current = 1.64 kA) and current density J (Jmax = 1.64 A/mm²) in the internal side plate (e = 20 mm)


Current function T (total current = 2.22 kA) and current density J (Jmax = 1.88 A/mm²) in the external side plate (e = 23.7 mm)

Current function T (total current = 14 kA) and current density J (Jmax = 2.22 A/mm²) in the central plate (e = 26 mm)


A NNEX 2 – A DIABATIC HOT SPOT CRITERIA


A NNEX 3 – QUENCH NOMINAL CASE

Imesh1 is the current in the magnet. Imesh2 is the total current in the winding cases. Tmai1 are the maximal temperatures of the six coils. Tma12 is the maximal temperature in the winding cases (their temperature is considered uniform). Vmai1 are the maximal voltages at the terminal of each of the six coils. Vma12 is the maximal voltage in the winding cases (purely virtual). Rmai1 are the resistance of each of the six coils. Rma12 is the total resistance of the winding cases.


A NNEX 2 – QUENCH NOMINAL CASE WITHOUT WINDING CASES


A NNEX 3 – QUENCH FAULT SCENARIO


A NNEX 4 – QUENCH FAULT SCENARIO WITHOUT WINDING CASES

Quench Protection for the CLAS 12 Torus Superconducting Magnet

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