Upper limits for QPS thresholds for selected 600 A circuits

B. Auchmann, D. Rasmussen, A. Verweijwith kind help from J. Feuvrier, E. Garde, C. Gilloux, R. Denz,

M. Karppinen, Q. King, Y. Thurel, G. Willering

MPE technical meeting, January 24, 2013

Motivation• Request: compute limits for QPS thresholds of the

following circuit types (incl. current thresholds):

Circuit type Uth Δtdiscr

RQTD/F, I < 100 A 2 V ~190 ms

RQTD/F, 100 A < I < 350 A 0.1 V ~190 ms

RQT 0.1 V 10 ms

RQTL 0.1 V 10 ms

RQS 0.1 V 10 ms

RO 0.1 V 10 ms

RSS 0.1 V 10 ms

1.validation of the quench model• propagation• coupling losses• circuit model

2.magnet protection3.busbar protection4.recommendation

Validation quench propagation 1/2

• All concerned magnets use the same strand/insulation/impregnation – wire#3.

• RQTD/F and RQTL circuits are the most critical (assumption later proven by simulation).

• Bloc 4 data of initial voltage rise up to 2 V available at 400 A and 600 A from magnet training, as well as current decays.• Top = 4.2 K, Rcrowb = 27 mΩ, Uth = 50 mV, Δtdiscr = 10 ms

• ROXIE thermal model: • transversal discretization: 1 temperature node per turn.• ROXIE longitudinal discretization: coarse (6cm) to limit computation time.

• QP3 used to compare longitudinal propagation velocity for 1 mm (converged) and 6 cm discretization at different current- and field levels.

Validation quench propagation 2/2

Zoom

400

A60

0 A

400

A60

0 A

Validation quench-back by IFCL• Simulation of FPA at different current levels.• Tune IFCL effective resistivity ρeff and time constant τ to match

observed quench-back.

Quenching mechanisms• RQT, Iq = 400 A

Quenching mechanisms• RQT, Iq = 600 A

Validation R(t)• Comparing MQTL data from Bloc 4 to ROXIE simulation

ROXIE circuit model

voltage regulated power supply Vmax = 10 V, Δtperiod = 80/100 ms

crowbar triggered via QPS or if ΔI > 10 A

EE switch opened by QPS after validation delay and switch delay

External circuit elements; constant Ld, no quenches

Quenching magnet and its R//, Ld(I), quench resistance

QPS triggers on Ures = Rq Imag

R//

Rq

R//ext

Rleads

Rcrowb REE

Ld Ld,ext

RQTD/F, ROXIE vs. PSpice• Use R(t) from ROXIE in PSpice model to validate new ROXIE

circuit model.• Differences due to constant external magnet inductance in ROXIE.

R//

Rq

R//ext

Rleads

Rcrowb REE

Ld Ld,ext

1.validation of the quench model2.magnet protection3.busbar protection4.recommendation

Worst-case protection scenario• Show that magnets are protected by FGC and parallel

resistors alone, i.e., Tmax < 200 K.

• Magnet protection is virtually independent from QPS thresholds; we model for Uth = 1 V, Δtdiscr = 200 ms.

• Determine maximal permissible QPS thresholds for protection of busbars.

• The resulting limits can only improve the protection of the magnets.

RQTD/F, Iq = 600 A• Circuit: 8 magnets, R// = 0.25 Ω, REE = 0.7 Ω, Rcrowb = 0.05 Ω, Rlead = 0.006 Ω

• FGC: Δtperiod = 80 ms, Vmax = 10 V, ΔImax = 10 A.

• QPS: Vth = 1 V, Δtdiscr = 200 ms

Summary magnet protection

Case Tmax

RQTD/F, Iq = 600 A 140 K

RQTD/F, Iq = 400 A 80 K

RQTD/F, Iq = 600 A, R// failure 235 K

RQTL, Iq = 600 A 140 K

RQTD/F, Iq = 600 A, R// failure 146 K

RQTL, Iq = 400 A 110 K

RSS, Iq = 600 A 115 K

RO, Iq = 600 A 120 K

• Due to large cu/sc ratio and low position on the loadline (max ~60%), magnets are very robust.

• FGC and R// provide ultimate protection, virtually independently from QPS thresholds.

• Failure of R// • is VERY unlikely – robust design!• would not be fatal.

• R// is dimensioned to absorb the energy of an entire RQTD/F circuit with Tmax = 400 K.

• Also studied:• HF/LF quench locations;• different quench timing w.r.t.

FGC regulation period;• neither has significant impact.

1.validation of the quench model2.magnet protection3.busbar protection4.recommendation

Protection of the 600 A busbar

Worst case scenario: Quench starts in a thermally insulated part of the bus.Quench propagates not or slowly outside the insulated part (so good cooling to LHe).Global voltage built-up is small even though local hot-spot can be high.

Helium

6 cm long plug

busbar

Parameters used: Cross section bus: 2 mm2 with Cu/Sc ratio=9 Bus insulation: 0.2 mm kapton Cooling: 600-1200 W/K/m2

Lcircuit=0.248 H Rcircuit=0.23-0.7 W

Worst case scenario combined with:• high QPS threshold: 0.8 and 1 V, • long QPS discr time: tdiscr=190 ms, • Rcircuit=0.23 W.

0

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100

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200

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400

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200 250 300 350 400 450 500 550 600

Hot

spo

t tem

pera

ture

[K]

Quench current [A]

1 V, 1200 W/K/m^21 V, 600 W/K/m^20.8 V, 1200 W/K/m^20.8 V, 600 W/K/m^2

Conclusion: 0.8 V, 190 ms is safeup to 600 A, for all busbars with tEE<1.1 s.

1.validation of the quench model2.magnet protection3.busbar protection4.recommendation

Recommendation• The busbar protection defines the upper limit

for QPS thresholds of all studied circuits:

Uth ≤ 0.8 V, Δtdiscr ≤ 190 ms

• Respecting these limits, set thresholds to the lowest practicable values.

Caveat?

RPMBB-type has no DC cont.

Validation MIITs vs. ROXIE• ROXIE temp. slightly below MIITs estimate (propagation).• Calculated MIITs: 0.037 kA2s, Tmax = 235 K, Bpeak = 4 T

RQTD/F Iq = 600 A

RQTD/F Iq = 400 A

RQTD/F, Iq = 600 A, R// failure

M. Karppinen: Failure VERY unlikely – robust design!

R// is designed to absorb the energy of an entire RQTD/F circuit with Tmax = 400 K.

RQTL• RQTL9: 2 magnets, R// = 0.2 Ω, REE = 0.7 Ω, Rcrowb = 0.05 Ω,

Rlead = 0.002 Ω

• RQTL7-8/10-13: 1 magnets, R// = 0.2 Ω, no EE, Rcrowb = 0.05 Ω, Rlead = 0.002 Ω

• FGC: Δtperiod = 100 ms, Vmax = 10 V, ΔImax = 10 A.

• QPS: Vth = 1 V, Δtdiscr = 200 ms

• In the Δtdiscr = 200 ms / POWERING_FAILURE scenario, REE is not instrumental. We study RQTL9 as a worst case.

RQTL, Iq = 600 A

Effect of 10V PC output voltage during 1 period before POWERING_FAILURE, PSpice

RQTL, Iq = 400 A

RQTL, Iq = 600 A, R// failure

RSS, Iq = 600 A• Circuit: 4 magnets, R// = 0.15 Ω, REE = 0.7 Ω,

Rcrowb = 0.05 Ω, Rlead = 0.003 Ω

• FGC: Δtperiod = 100 ms, Vmax = 10 V, ΔImax = 10 A.

• QPS: Vth = 1 V, Δtdiscr = 200 ms

RO, Iq = 600 A• Circuit: 13 magnets, no R//, REE = 0.7 Ω, Rcrowb = 0.05 Ω,

Rlead = 0.003 Ω

• FGC: Δtperiod = 100 ms, Vmax = 10 V, ΔImax = 10 A.

• QPS: Vth = 1 V, Δtdiscr = 200 ms

IFCL induced quenches during FPA in RQTD/F

• Preliminary study: to avoid quenches up to 600 A, REE would need to be reduced to 0.1 Ω.

• Any reduction in REE reduces the number of quenches and the test voltages.

• Busbar protection calculation assumed REE = 0.23 Ω.