(n)QPS performance and RRR measurements

(n)QPS performance and RRR measurements

Andrzej Siemko with contributions of:

J. Steckert B. Auchmann, R. Denz, E. Ravaioli, A. Verweij, M. Koratzinos,K. Dahlerup-Petersen, R. Schmidt, M. Solfaroli Camillocci

QPS team, MP3 team

Topics• nQPS cabling problem• Effects of transient oscillations on QPS– Energy extraction oscillations– Power converter oscillations– Event of Feb 24 (50 magnets quenched)– Impact on old QPS (11 magnets quenched)– Measures taken

• nQPS as powerful diagnostic tool• RRR measurements • Conclusion

MAC – 26th April 2010 Andrzej Siemko, TE-MPE 2

Origin of the nQPS cabling problem

• Inadequate workmanship quality• Specification of the cables not particularly forgiving the

possible assembly errors

F. Formenti - HCC meeting January 12th, 2010 4

Measuring, cutting, crimping, soldering, checking, … reconnecting of 4248 nQPS Harting connectors

Solution to the nQPS cabling problem

Quench detectors for dipoles - working principle

• Symmetric quench detector compares 4 electrically adjacent dipole magnet voltages• If differences between these voltages are larger than threshold (200mV), the magnets’

heaters get fired• dUmag max is the biggest of 6 differences (Ua –Ub; Ua –Uc; Ua –Uas; Ub-Uc; Ub-Uas, Uc-

Uas)

• Old analog quench detector compares the two apertures of one magnet in a measurement bridge

1 3 42

Ua Ub Uc Uas

Plots courtesy J. Steckert, R. Denz TE-MPE

Andrzej Siemko, TE-MPE 6

Problem with transient oscillations

• Event of February 24 – 50 dipole magnets quenched by firing the quench

heaters by nQPS

• Event of March 11 – 11 dipole magnets quenched by firing the quench

heaters by old QPS

MAC – 26th April 2010

Transient oscillations - opening of EE switches

EE from 2kA plateau, power converter at ~0V boost voltage

Violent signals during EE switch opening (FPA)

dUmag (unfiltered nQPS symmetric QD)


dUmag max during opening of switches at higher currents

50150

250350

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012345678

dUmag max at 6kA EE, s34

dUmag max [mV]

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tions

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dUmag max [mV]

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dUmag max [mV]

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tions

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0 50 100 150 200 250 300 350 400 450

dUmag max (mean) vs. current, s34

dUmag max (mean) [mV]

Curr

ent [

A]

SymQ threshold (200mV)

Plots courtesy J. Steckert TE-MPE

The adaptive filter

Trigger condition fulfilled-1.6V

Fast power abort

Standard threshold: 200mV

Elevated value: 1300mV

Duration: 1300ms

∆Umag

Elevate threshold for some time after fast power abort to ignore the spike !After 1300ms threshold goes back to standard valueFilter has to be rearmed for further action in the next powering cycleCopes with one transient per powering cycleWorked very well (tested up to 6kA)

Trigger condition: Umag < -1.6V

Wait for rearm conditionElevated thres.armedFilter states

Input:Magnet voltage

Detection:dUmag + threshold

Standard threshold: 200mV

Plots courtesy J. Steckert TE-MPE

PC oscillation along the magnet string

Plots courtesy J. Steckert and B. Auchmann

Power converterAnd Switch

Switch

L4R3

…

…

↓ amplitude maximum

amplitude minimum ↑

Amplitude of Umag vs. position along magnet string

Power converter oscillation measured for whole arc Amplitude varies with position, PC voltage at switch-off, current and other parameters Simulation (red) agrees with measurements Pspice model reproduces these results

Simulations: E. Ravaioli TE-MPE

Power converter oscillations

• If a power converter is switched off during ramping (160V 0V), its output filter capacitor will discharge over the magnets

• This effect can be measured as a damped oscillation over each magnet in the arc

• The amplitude of oscillation is mainly dependent on the voltage of the output capacitor and the current in the magnets


“Side effects” of power converter oscillations

• Oscillations influence the proper functionality of the Symmetric quench detection cards adaptive filter Quench heaters of 50 magnets were fired at 3.5kA on Feb. 24

• Oscillations on top of a Fast Power Abort (EE switch opening) can cause strong perturbations and can trigger the old QPS as well Quench heaters of 11 magnets fired at 6kA


50 quench event of feb-24 mass firing after FPA from 3.5kA

Ramping with 10A/s

Simulated Powering failure at ~3500A

Oscillation in Umag triggers adaptive filter

Std Threshold, filter armed

Elevated threshold

Std. Threshold, filter blocked

I magnet

Umag

~1V~-50mV

Coasting (several minutes)

FPA

-6V

FPA without adaptive filter enabled

No rearm condition

• Power converter switch-off caused oscillation <-2V on Umag• Adaptive filter is activated • Filter switches back to std threshold after 1.3s• During coasting Umag stays negative (no rearm cond.)• At Fast Power Abort filter is not available, threshold stays

low

SymQ triggers (…in 50 cases)

Superposition of both oscillations

Umag osc = -4V

Energy extraction (opening of switches)

Power converter switch-off oscillation


Measures taken to reach 6kA-3.5TeV

• Increased thresholds of nQPS SymQ quench detectors– Detection threshold set above dUmag caused by Fast Power Abort ringing– Calculations for 6kA showed acceptable thresholds up to 1010mV– SymQ is immune to FPA ringing even without adaptive filter

• Temporarily reduced current ramp rate to reduce the amplitude of the PC oscillations

• Delay extraction switch with respect to PC fault in order to decouple both oscillations (ongoing)– Nominal ramp rate can be restored

500ms are sufficient and feasible, old QPS heater firing will be mitigated

With this measures safe operation at 6kA will be ensured


New symmetric quench detection threshold

1010 mV

MIITsmax

36.6 MA2s @ B = 3.5 T

max. delta

MIITs 6.8 MA2s @ B = 3.5 T

MIITsafter detec.

29.8 MA2s

detect

heater fired

heater effective

I0

time

currentTemp

MIITs

350 K

302010 40

• MIITs calculations for max. temp. of 350 K.

• ROXIE quench simulations verified by SM18 measurements give max. MIITs after detection.

• QP3 program calculates MIITs up to detection for given threshold.

A threshold voltage below 1 V is safe for operation up to 6 kA B. Auchmann, A. Verweij

Symmetric quench detection - threshold change

• Old version of field-bus controller limited threshold to 200mV

• New firmware version sets threshold to 800mV with the option to change threshold up to 1000mV if required– In the same way the adaptive filter settings were modified

• The firmware update was required only for the field bus controllers storing the device parameters of the associated symmetric quench detection boards– 54 x 8 + 4 = 436 controllers were re-programmed

R. Denz TE-MPE-CPMAC – 26th April 2010 Andrzej Siemko, TE-MPE 18

19/04/2023

Decoupling of the oscillations

S56, 10A/s, 6kA, no quench, 8/4/10, 21:30, data from first 28 magnets

• Delay of the EE switches opening was tested and validated in sector 56• up to 6kA and with ramp rate of 10A/s no single quench observed but there is no margin


(n)QPS Outlook• Transient oscillation are now well understood– New Pspice model was developed

• Solutions were developed to allow operation at 3.5TeV

• Operation above 3.5TeV will require:– Reduction of the oscillation amplitudes

• Installation of the snubber capacitors for the energy extraction switches (in preparation)

• Implementation of the additional resistances in the PC passive filter (in preparation)

• Modification of the passive filter position in PC (2012)

– Improvement of the nQPS• Development and implementation of the new adaptive filter (2012)


Andrzej Siemko, TE-MPE

nQPS as powerful diagnostic tool


Measurements of the sc splices


nQPS as powerful diagnostic tool


Measurements of the voltages across the sc circuits

Will help to study the noise spectrum in the tunnel

Possible hump diagnostics ???


Overall performance of the (n)QPS systems


• Since the end of March the (n)QPS systems are stable and in their present configuration are working remarkably well

• In practice only minor problems were encountered:• replacement of 3 QH power supplies• replacement of several noisy QD cards• several trips of RQD/RQF circuits (due to the noisy cards and/or bad contacts in the

quench loop)

• trips of RQTD/RQTF circuits due to the tune fit-back loop requirements exceeding the original specifications

• … • but all together these problems are “normal” for the complex system like the (n)QPS

RRR measurements - motivation• Measuring the RRR of the copper stabilizer of the busbars of the LHC has

been proven difficult and we do not have yet the accurate and complete measurements.

• This has led us to take the conservative approach to assume a RRR of 100 for the whole machine.

• The value of the RRR has a direct impact at the energy that the LHC can safely be operated on.– If lowest RRR is 160 instead of 100 the safe energy increases by 0.3TeV per beam

• Measuring the RRR is one of the easiest ways to increase our knowledge of the LHC as far as splices are concerned.

• A method has been proposed to measure the RRR with a precision of a few % using the nQPS system by injecting a low current (20-30A) to the three main circuits of a sector.

• A type test was performed (21-31 January 2010)– 53 bus bar segments (33 RB, 20 RQ)– Two temperature transitions (up and down)


Definition of RRR

5 10 15 20 25 300

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Typical bus bar segment with resistance of 2mOhms

at warm

RRR = 100RRR=200

Temperature (K)

Resis

tance o

f bus b

ar

segm

ent

(uO

hm

s)

Mike Koratzinos TE/MPE

RRR is the Residual Resistivity Ratio and is defined by the resistivity formula for copper:

ρCu = C0/RRR + ƒ(T) + ƒ(B)

Where

RRR = ρCu (290K)/ ρCu(4.2K)

∴ RRR is related to an offset

RRR is easiest to measure at the transition temperature

RRR - a real data example


[BA23.L1<->BB22.L1]

•Temperature in a segment needs to rise to about 15K•Temperature rise needs to be homogeneous•An equivalent measurement can be taken when cooling down•RRR is easily calculated as the ratio of Rwarm/Rtransition

Problems seen during the type test

• The calculation of resistances (and therefore RRR) is not trivial due to the low input impedance of the QPS system [the QPS system was not designed for measurements at non-superconducting temperatures].

• A correction of about 40% of the voltage signal needed to be applied on RB and about 5% on the RQ.

• Cross checks were made by measuring voltages down in the tunnel and comparing them with the calculated voltages


0 0.2 0.4 0.6 0.8 1-0.400

-0.200

0.000

0.200

0.400

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1.000

f(x) = 1.05447092886866 x − 0.0679202492323864R² = 0.982626778336283

Voltage drop measured and calculated

voltage drop measured RBLinear (voltage drop measured RB)voltage drop measured RQ

Voltage drop calculated

Vonta

ge d

rop m

easure

dCorrelation for RB (where the correction is important) is very good

Results of the type test

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RRR

Segment number

RR

R


•RRR for RQ is between 210 and 280 (250±20)•RRR for RB is between 150 and 210 (180±20)•There is a marked difference between RB and RQ – this needs to be understood or verified.•The worst RRR seen is 140±10

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RRR RBRRR RQ

RRR

RRR Measurements Outlook

• Measuring the RRR is one of the easiest methods to increase our knowledge of the machine from the splices point of view.

• A type test was performed, demonstrating the feasibility of the method. However, another test would be needed to gain confidence on the (large) corrections that need to be applied (5 days in the shadow of other tests).

• The minimum RRR found (140) was only modestly higher than the (conservative) value used in simulation (100).

• Ideally, measuring the RRR for all busbar segments in the machine would be advantageous – failing that, a sample from every sector would be advisable

• If for any reason a sector is emptied of Helium, the measurement can come nearly for free (low current needed; the sector needs to only reach 15K for a good measurement)


(n)QPS Conclusions• Two effects causing the EM transients on the magnets have affected

the behaviour of the (n)QPS systems– Both are now well investigated and understood

• Two “side effects” of transients were observed:– Symmetric quench detector can fire heaters due to multiple transients – Old QPS can fire heaters due to superposition of transients

• Symmetric quench detector vulnerability can be treated by raising the thresholds – Firmware upgrade of field bus controllers was required

• Old QPS triggering can be mitigated by delaying the extraction switches

• Solutions for operation above 6kA needs to be further developed

• The overall performance of the (n)QPS systems in their present configuration is more than satisfactory



Reserve slides


Tune feedback and RQTD / RQTF trips

• Protection elements for the superconducting part of the RQT circuits:– 2 x detection systems for the leads– 1 x detection system for the magnets and bus-bars– Parallel extraction resistors + external energy extraction

system

Plot courtesy R. Denz

Tune feedback and RQTD / RQTF trips• The tune feedback applies only small changes in current but

creates a voltage signal, which QPS cannot distinguish from a real quench

Plot courtesy H. Thiesen


• Voltage signal exceeds detection settings for |I| > 50 A– Power converter: DV ≈ 50 to 100 mV, ts = 0.1 s

– Quench detection: VTH = 100 mV, tr = 20 ms, (18 ms evaluation time + 2 ms discrimination time)

• Can detection settings be changed?– 600 A protection scheme covers magnets and bus-bars including

splices– Critical current range between and 50 A and 200 A as bus-bar

quench may propagate very slowly– In consequence an increase of threshold is not recommended– Reaction time is the by far less critical parameter at low currents– A. Verweij: tr = 200 ms ok for |I| ≤ 200 A (sufficient for 3.5 TeV run)

• Requires for the time being only a change of a parameter• Code can be modified to change detection settings dynamically at a later stage


• Proposed solution– Change discrimination time only, keep threshold

• tr = 190 ms, (18 ms evaluation time + 172 ms discrimination time) • Time discriminator is retriggerable• Can reject signals (ts = 100 ms) with a duty cycle of about 80%

– Requires local reprogramming of 64 circuit boards in the even points (about 1 hour per point)

• Roadmap– Code recompilation and lab test successfully done– Approval– Implementation in one point and test– Full deployment– Extension to other circuits (e.g. RCBX ...)?

Documents

(n)QPS performance and RRR measurements