the diode lead resistance ‘issue’

The diode lead resistance ‘issue’

A. Verweij, TE-MPE, CSCM workshop 7/10/2011

Contents:

Diode geometry

Measurements performed in the past

Measurements in S56 on 6 dipole diodes

Measurements in S56 on 4 quad diodes

Origin of the large resistances / discussion / conclusion

Dipole magnet(1.8 MJ @ 6 kA)

Diode

Joint

Investigate in situ in the machine if the heat developed in the magnet & diode & diode leads will propagate towards the 13 kA joint, eventually causing it to quench.

The goal of the test

I

I


Diode

‘half moon’ contacts

‘heat sink’ contacts

21 cm

Similar for a quadrupole



Diode

Joint

Ulead,A

Ulead,C

I

I

While performing these tests, unexpectedly large voltages were measured on the diode leads

Udiode


The dipole diode

Rc,moon

Rc,hs

Diode box, Helium contents : 5 liter

Lower diode busbar

Rc,diodeLower heat sink

Upper heat sink

Voltage taps on the diode


Upper diode busbar(partially flexible)‘Half moon’ contact

Main busbars

towards diode

Voltage taps

Dipole to diode connection


The quadrupole diode

Rc,CL

Rc,hs

Diode 1

Diode 2

Rc,CU

Ansys model from S. Izquierdo


Diode Quad

Upper diode busbar About 410 mm, RRRspec=100R4K=0.23 mW, R40K=0.6 mW, R100K=5 mW

About 1.6 m, RRRspec=100R4K=1.8 mW, R40K=4.4 mW, R100K=40 mW

Contact resistances between upper and lower diode busbars (Rc,moon or Rc,CU and Rc,CL)

Contact surface=1600 mm2

Dipole side: Ag coated (5-10 mm)Diode side: Ni coated4xM6, 10 Nm, 4-5 CuBe spring washers


Dipole side: Ag coated (5-10 mm)Diode side: Ni coated2xM5, 8 Nm, 3 CuBe spring washers

Lower diode busbar Maximum 480 mm, RRRspec=100R4K=0.3 mW, R40K=0.7 mW, R100K=6 mW

135 to 370 mm, RRRspec=100R4K=0.15-0.4 mW, R40K=0.4-1 mW, R100K=3.3-9 mW

Contact resistance between lower diode busbar and heat sink (Rc,hs)

Contact surface=950-1070 mm2

Both sides are Ni coated4xM6, 10 Nm, 4-5 CuBe spring washers

Contact surface=440-640 mm2 (?)Both sides are Ni coated3xM5, 8 Nm, 3 CuBe spring washers

Heat sink RRRspec=100, R4K<<0.1 mW, R100K<0.1 mW,

Contact resistance between heat sink and diode (Rc,diode)


Apparent resistance from inductive voltage

<1 mW, assuming a few mV at 6 kA decay

Apparent resistance from thermal voltage <1 mW, assuming a few mV for DT=200 K

The diode lead resistance is the sum of:


The bolted contacts in the diode leads have been discussed many many times in the EEWG in the years 2003-2006

(see: http://lhcp.web.cern.ch/lhcp/tcc/powering/eewg/eewg.htm). The minutes of 18/9/2003 state: “…the baseline design leaves the possibility for potential dangers.”

Large values of Rc,moon were reported in the early production, both for ‘cold’ and ‘warm’ measurements.

This lead to a slightly modified design of the half moon, and to more stringent procedure for the cleaning and assembly of the diodes, with increased torques.

This resulted in a clear reduction of the contact resistances to a few mW.

Testing at warm in SMA18, and tests at cold in SM18 were added to the magnet reception test program.


http://lhcp.web.cern.ch/lhcp/tcc/powering/eewg/eewg.htm

050

100150200250300350400450500

0 50 100 150 200 250

R c,m

oon

[mW

]

0

100

200

300

400

500

600

700

0 50 100 150 200 250

R c,m

oon

[mW

]

0

20

40

60

80

100

120

140

0 50 100 150 200 250

R c,h

s[mW

]

0

20

40

60

80

100

120

140

160

0 50 100 150 200 250

R c,h

s[mW

]

About 250 diodes have been repaired at CERN. The plots below show the resistances Rc,moon and Rc,hs before the repair. After the repair all resistances were below 5 mW.

anode

cathode

cathode

anode


What was measured in the past?

Dipole diodes:

Dipole diodes Quad diodes

Diode acceptance tests in FRASCATI at 4.3 K

8-10 endurance tests were carried out with 13 kAUdiode , Ths and Rc,diode were measured on all diodesRc,diode was decreasing with number of tests, reaching <1 mWNo contact resistances were measuredAll reports are available

Diode tests at CERN at 4.3 K Similar testing as in FRASCATI, but only on about 76 diodes

Warm measurements of the resistance of the diode leads (with I=5-20 A)

691 diodes were measured at CERN aver=2.5 mW, s=1.1 mW, max=11 mW

At Accel and CERNSpec: Rc,hs<5 mW, Rc,CU and Rc,CL<2 mW

1.9 K measurements in SM18 of the resistance of the diode leads during a 1 s long transient following a heater provoked quench at 3 kA

677 diodes measured(aver=3.3 mW, s=1 mW, max=9.8 mW)

None


Measurements on 6 dipole diodes in S56

- Diode voltages at 6 kA- Diode lead voltages/resistances at 2 kA- Diode lead voltages/resistances at 6 kA


Measured magnets

Dipole Training during reception tests

in SM18 *

Training quenches during

HWC in 2008

Tests during TS in May

2011

Tests during TS in July 2011

A15R5-3188 12.3, 12.4 10.5(5.2, 4.6) 2, 6

B15R5-3353 12.2 (6.3, 7.4, 6.8) 2, 6 2, 5, 0.76, 4, 6, 6, 3

C15R5-3338 12.2, 12.7 (2.4)10.9, 10.7 2, 6

A16R5-3204 11.6, 12.2 11.2 2, 0.76, 4, 6, 3, 5

B16R5-3361 12.5, 12.4 (7.4, 2) 2, 5, 0.76, 6, 3, 4

C16R5-2246 11.5, 11.8, 12.4, 12.8 (0.6, 5.2) 2, 6, 4

All numbers in kA

In total 28 heater induced quenches.


Diode voltages for 6 kA quenches

Magnet not yet fully s.c.,all current in magnet

Magnet s.c.,all current in magnet, U=L*dI/dt

Diode blocks

Diode cooling down

Conclusion:Forward voltage (and hence the heating) over the 6 diodes is very uniform.

(s<10 mV)


Diode lead voltages for 2 kA quenches

}At 1.5 kA

Note the large difference between voltages of B15R5-run2 and run1.


From voltage to resistance

The current in the diode lead is not known, but should be equal to the current in the power converter (IPC), except for:

- t<t1, because the current is still diverting from the quenching magnet into the diode.

(t110 s for 2 kA, t13 s for 6 kA)

- t>t2, because the magnet starts recovering, and the current is slowly transferring back from the diode into the magnet.

(t230-40 s for 2 kA, t2120 s for 6 kA)

So the effective resistance of the diode lead is obtained by dividing the voltage by IPC, valid for t1<t<t2

Validity range

t1 t2


Diode lead ‘resistances’ for 2 kA quenches

1.8-3.6 mW: measured at cold reception in SM18

Conclusion:- Resistances constant in the 10-40 s range.- Resistances up to a factor 2 larger than measured at cold in SM18.

}At 1.5 kA


Diode lead voltages for 6 kA quenches

Conclusion:- Large spread among the 12 leads.- ‘Steps’ occurring in first 15 s.- Significant difference between the voltages of B15R5-run1, run2, and run3.


Diode lead ‘resistances’ for 6 kA quenches

1.8-3.6 mW: measured at cold reception in SM18

These curves cannot be explained by ‘normal’ Joule heating in the resistive busbars and in the contacts.


Diode lead ‘resistances’ for B15R5 AnodeThe results are not reproducible!!!- The two 2 kA curves differ a factor 3-4.- The three 6 kA curves differ a factor 2.


Summary dipole diodesA15R5 B15R5 C15R5 A16R5 B16R5 C16R5

C A C A C A A C C A C A

R1 (mW) 3 3 2 2.7 3 2.9 3 1.8 3.6 3.2 3.1 2.4

R2 (mW) 1.5 2.3 2.76.7

1.99.7

3.3 1.9 8.8 3.8 1.6 2.2 1.6 2.2

R3 (mW) 2.2 9 241210

222017

7.9 5.3 37 21 6 11 4.2 15

Rmax (mW) 2.2 9 24 22 7.9 5.3 48 21 6 11 4.2 15

Umax (mV) 9 38 102 86 36 21 230 114 27 47 66 18

C=Cathode, A=Anode

R1: resistance measured during cold reception in SM18R2: maximum resistance measured during the 2 kA quench(es)R3: maximum resistance measured during the 6 kA quench(es)Rmax: maximum resistance measured during all quenchesUmax: maximum voltage measured during all quenches


Measurements on 4 quad diodes in S56

Main differences w.r.t. dipole diodes:

Circuit time constant is 9.2 s instead of 50 s.

Resistance of a quadrupole aperture is about 6x smaller than a dipole (for T>10 K).

Opening of the diode after a quench takes longer.

Current transfer from a quenched magnet into the diode is therefore slower.

The upper diode busbar is much longer (about 1.6 m instead of 0.4 m).

3 bolted connections (2xM5, 2xM5, 3xM5) instead of 2 (4xM6, 4xM6).

The quad diode busbars have 2x smaller cross-section


Measured magnets

SM18 LHC 2008 Aug 2011

D14R5

(SSS251)

Defocusing11279 A

223556633

Focusing

D16R5

(SSS150)

Defocusing-

Focusing

All numbers in kA

In total 9 heater induced quenches in the 4 apertures in parallel.


All resistances in mW

Current D14-c D14-a F14-c F14-a D16-c D16-a F16-c F16-aD-HM1 D-HM2 F-HM1 F-HM2 D-HM3 D-HM4 F-HM3 F-HM4

2000No diode opening

2.7 2.8 3.7 3.9

2000 2.5 2.6 3.6 3.8

3000 5.2 6.3 6.1 5.6 3.3 3.6 4.5 4.7

5000 7.8 8.4 9.3 7.5 3.8 43.3 5.9 6.9

5000 9.1 8.6 9.3 8.2 3.8 38.6 6.3 7.4

6000 11.4 9.4 11.9 28.3 4.0 45.1 9.6 13.0

6000 12.2 9.8 11.8 25.0 4.0 45.6 10.3 14.1

3000 9.4 7.6 9.2 18.6 3.4 26.4 9.3 11.1

3000 9.3 7.5 9.1 18.7 3.4 25.9 9.3 11.2

Summary quad diodes (preliminary, analysis on-going)

3000 4.2 1.3 3.1 13 0.1 22 4.8 6.4

“Difference in resistance between 1st and 2nd/3rd quench at 3000 A


D16R5 anode: 2 consecutive quenches at 5 kA

Probably a movement in one of the bolted connections during the first tests, resulting in an permanent increase in the contact resistance.

Step in Dt<50 ms


What could be the origin of these excessive resistances?

Lorentz force causing a reduction of the force on the contacts, possibly resulting in (micro)movement of the contact.

Thermal gradients, especially between the heat sink and the lower diode busbar.

Local heating at the microscopic contact points.


2. Interface contact resistance: Rc=(r2pH/4F)0.5

r=resistivityH=Vickers hardnessF=applied force

Rc can change strongly with temperature because r=f(T) and H=f(T).T can increase strongly with current because P = I2Rc.

1. Interface temperature: Tc=(Tbulk2+V2/4L)0.5

The microscopic contact spots can reach very high temperatures at high currents and may then soften/deform or even melt, altering the contact resistance from quench to quench.

Ni-Ni contacts with 4 M6 bolts

2 rules of thumb:

Tbulk=10 K


Comsol output for the final temperature of the dipole diode after a 6 kA quench with Rc,moon=40 mW (adiabatic conditions)

95 K

90 K

180 K

Electro-thermal simulations are ongoing with QP3 and Comsol. However, there are many unknown parameters, microscopic effects, and irreproducible results.

At present we are unable to simulate what is going on in the diode lead and, more important, what would happen at 12 kA….

Simulations

D. Molnar


A working group is looking into detail into the functioning of the diode and the leads. Analysis and tests are on-going/planned to find out what is happening

Cold tests in SM18 on several diodes (planned for Nov) As similar as possible to the machine but with additional instrumentation. Currents up to 12 kA. Tests at 1.9, 4.3, and maybe 20 K.

Tests on a few diodes at 300 and 80 K (ongoing) Small current (10-1000 A). Applying a force on the diode busbar simulating the Lorentz force in the machine.

Electro-thermal computations using QP3 and Comsol (ongoing)

Mechanical computations using Ansys (ongoing)

Mechanical measurements (ongoing) Torque preload characteristics of the bolts, behavior of the washers, …

4th series of quench tests in the machine (to be discussed if useful)

Tests in SM18 on a dipole + diode (not before 2012, but probably not possible due to stability issues in the cryogenic feed box)


Voltages over 12 dipole diode leads and 8 quad diode leads have been measured at currents between 0.76 and 6 kA.

Resistances are up to 15 times larger than measured during cold reception tests in SM18, and seem to strongly increase with the current.

The observed spread in the resistance of these leads is very large (factor 20), indicating that much larger resistances are likely to be present in some of the other 4000 diode leads of the machine.

There seem to be at least two different phenomena at the origin of the excessive increase in resistance.

The results are irreproducible, and correct simulation is presently not possible due to the large number of unknowns.

Conclusion 1/2


Large, and rather erratic high resistances in the diode leads are very worrying, because safe operation at 12 kA cannot be guaranteed.

But on the positive side, we have not experienced any problem with the high current training quenches during the 2008 hardware commissioning.

Many tests and simulations are on-going to understand the origin of the excess resistance, and investigate the diode lead behaviour at higher currents.

The CSCM test is a relatively fast method to map all the diode leads of one (or more) sectors at several current levels. Note that this test would very likely permanently increase the resistance of many bolted connections.Proper setting of the thresholds is important and not at all trivial.

Conclusion 2/2


the diode lead resistance ‘issue’

Documents

diode connectiona

magnet diode diode leads

diode leadsudiodea

lower diode busbars

cscm workshop

diode geometry measurements

hs diode box

moon rc