The localization of possible electrical faults in superconducting accelerator magnets may, in most cases, be a complex, expensive and time-consuming process. In particular, inter-turn short circuits and failures of the ground insulation are well detectable when the magnet is collared, but often disappear after disassembly for repair due to the release of the pre-stress in the coils. The fault localization method presented in this paper is based on the measurement and analysis of the magnetic field generated inside the magnet aperture by a high voltage pulse. The presence of the fault modifies the distribution of the current in the coils and produces a distortion of the magnetic field. The described method aims at locating both the longitudinal and azimuthal position of the fault-affected area. The test method, the transient case FEM models and the implemented experimental set-up are presented and discussed for the LHC dipole models

Komorowski, P A

Tommasini, D

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EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHEuropean Laboratory for Particle PhysicsLOCALIZATION OF ELECTRICAL INSULATION FAILURESIN SUPERCONDUCTING COLLARED COILS BY ANALYSISOF THE DISTORTION OF A PULSED MAGNETIC FIELDPiotr Komorowski1 and Davide Tommasini2The localization of possible electrical faults in superconducting accelerator magnets may, in most cases, bea complex, expensive and time-consuming process. In particular, inter-turn short circuits and failures of theground insulation are well detectable when the magnet is collared, but often disappear after disassembly forrepair due to the release of the pre-stress in the coils.The fault localization method presented in this paper is based on the measurement and analysis of themagnetic field generated inside the magnet aperture by a high voltage pulse. The presence of the faultmodifies the distribution of the current in the coils and produces a distortion of the magnetic field. Thedescribed method aims at locating both the longitudinal and azimuthal position of the fault-affected area.The test method, the transient case FEM models and the implemented experimental set-up are presentedand discussed for the LHC dipole models.1 Faculty of Physics, Warsaw University of Technology, Warsaw, Poland2 LHC Division, CERNPresented at the 16th International Conference on Magnetic Technology26 September-2 October 1999 - Ponte Vedra Beach, USAGeneva, Large Hadron Collider ProjectAdministrative SecretariatLHC DivisionCERNCH - 1211 Geneva 23SwitzerlandLHC Project Report 343Abstract1 December 19991Localization of Electrical Insulation Failures in Superconducting CollaredCoils by Analysis of the Distortion of a Pulsed Magnetic FieldPiotr Komorowski*, Davide TommasiniCERN, LHC Division, Geneva, Switzerland*Faculty of Physics, Warsaw University of Technology, Warsaw, PolandAbstractThe localization of possible electrical faults insuperconducting accelerator magnets may, in most cases, be acomplex, expensive and time-consuming process. In particular,inter-turn short circuits and failures of the ground insulationare well detectable when the magnet is collared, but oftendisappear after disassembly for repair due to the release of thepre-stress in the coils.The fault localization method presented in this paper isbased on the measurement and analysis of the magnetic fieldgenerated inside the magnet aperture by a high voltage pulse.The presence of the fault modifies the distribution of thecurrent in the coils and produces a distortion of the magneticfield. The described method aims at locating both thelongitudinal and azimuthal position of the fault-affected area.The test method, the transient case FEM models and theimplemented experimental set-up are presented and discussedfor the LHC dipole models.I. INTRODUCTIONThe pattern of the magnetic field in modernsuperconducting accelerator magnets is mainly determinedby the geometry of the coils. In an ideal case a current of thetype I(M) = I0 cos(M) distributed around the apertureproduces a pure dipole field. For practical reasons multi-layer coils graded with longitudinal wedge-shaped spacersinserted between the conductor blocks approximate thiscondition (Fig. 1). This paper is focussed on the study of thesingle-aperture superconducting short dipole models for theLHC in view of applying the results of the investigation tothe 15-m long double-aperture magnets.Since 1989 a test and evaluation program of shortsuperconducting dipole magnets for the LHC is under way atCERN. It was intensified in 1995 when a production of anew series of 1-m long dipole magnets was launched [1].The design is based on a dismountable structure allowingeasy implementation of variants (Fig. 2). The coils areconfined in a fixed volume by the collars with a pre-compression sufficient to counteract the electromagneticforces during magnet excitation. The outer layer of the coilsis separated from the collars by 0.5 mm of ground insulationand by a 0.7-mm thick austenitic steel protection sheet called‘collaring shoe’. A ferromagnetic yoke is clamped aroundthe collared assembly by an external bolted stainless steelcylinder. The yoke enhances the magnitude of the magneticfield induction inside the aperture and prevents the collarsfrom expanding when submitted to the Lorentz forcesexerted on the coils.The superconducting cable is insulated by two layers ofpolyimide film followed by a third adhesive one, whichbonds adjacent turns of the coil. The effective thickness ofthe cable insulation is about 125 Pm; i.e. two adjacent turnsare separated by about 250 Pm of insulation. This dielectricbarrier can be damaged during one of the magnet assemblyphases, causing thus an inter-turn short circuit. The changein the distribution of the current in the coils of the magnetdue to such a failure produces a distortion of the magneticfield. This effect is used here for the localization of the shortcircuits by means of transient magnetic field analysis.II. INTER-TURN SHORT-CIRCUITSInter-turn short circuits are well detectable when themagnet is collared. The presence of the insulation failure inthe magnet can be diagnosed by means of pulse propagationmethods at room temperature. A voltage impulse is producedbetween the terminals of the magnet by a dischargegenerator. The coil response in form of voltage and currentoscillations is then recorded and analyzed in terms ofpseudo-period of the oscillations, inductance and effectiveresistance of the coils. Fig. 3 shows the discharge curvesacquired on the collared assembly during a test at 1 kV.Fig. 1. Ideal current shell vs. actual LHC dipole geometryFig. 2. Cross section of a single-aperture dipole magnet53612 mm197collarsiron yokebolted stainless steel cylinder56 m m  aperturecoilsManuscript received September 27, 1999.2 ƒf ¸¸¹·¨¨©§ 1101 cossin),(nnnnr nanbrrBrB TTT ƒf ¸¸¹·¨¨©§ 1101 sincos),(nnnnnanbrrBrB TTTT(1)In case of an inter-turn short circuit, the current isredirected to the neighboring cable turn through the short-circuit path (Fig. 4). As a consequence of this redistributiona high current is induced in the passive winding deprived ofthe initially imposed transport current. This induced currentgenerates a transient magnetic field opposing the fluxchange in the area spawn by the fault-affected loop(schematically marked with a thin line in Fig. 4.) Energy isdissipated additionally at the point of fault. The value of themagnet inductance decreases, whereas the damping factorand the frequency of the oscillations increase. The dischargecurves already give a valuable clue concerning the azimuthalposition of the defect, i.e. the bigger the frequency change,and the greater the self-inductance drop, the closer thefailure to the median plane of the magnet (see Fig. 3 and 4).The existence of the electrical failure can be thus verified,however its longitudinal position cannot be established. Theanalysis of the transient magnetic field during the dischargeprovides a solution to this problem, as demonstrated in thefollowing sections.III. MAGNET MODELINGA. Multipole Expansion of the Magnetic FieldThe radial and azimuthal components of the magneticfield can be expressed as:Coefficients bn and an indicate respectively the normal andthe skew field multipoles measured at a reference radius r0and normalized to the main dipole field B1. The main normalcoefficient of the multipole expansion (b1) is thus alwaysnormalized to 1. Higher harmonic coefficients representmagnetic field errors relative to the dipole B1 and are givenin dimensionless units of 10-4.B. Reference Magnet ModelsBoth static and transient models of the 5-block collareddipole magnet (2 conductor blocks in the outer, and 3 in theinner layer of the coil quadrant) without iron yoke weredeveloped and analyzed using two different CAD programs.In the static model the conductors are excited with a 20 Adirect current. This model was created with The Routine forthe Optimization of Magnet X-Sections, Inverse ProblemSolving and End Region Design (ROXIE) program packagedeveloped at CERN [2], and used for the integrated designof superconducting accelerator magnets. The geometry ofthe coils defined in ROXIE served as a reference for thetransient model implemented in OPERA-2D [3]. Table Icontains a geometry transfer cross-check between the twoCAD environments.TABLE ICOMPARISON OF STATIC MODELS AT R0 = 10 MMStatic Case B1 [ 10-4 T] b3 b5 b7ROXIE -122.2 -0.52 0.017 0.013OPERA-2D -119.3 -0.55 0.020 0.013The transient model uses a FEM approach to simulate thebehavior of the magnet during a 1 kV pulsed discharge inwhich the peak current reaches a value of 20 A. In this casethe voltage-current decay, the skin, eddy currents andproximity effects in the conductors, copper wedges,collaring shoe, and in the collars are taken into account. Theeffective conductivity of the collars was adjusted in themodel in order to fine-tune the computational results to theexperimental data acquired by measuring the peak fieldinduction inside the aperture and in several locations aroundthe collars. For setting the correct value of the conductivityof the cables an additional test was carried out to evaluatethe magnitude of the eddy current effects. For this purposeseparate layers of the coils were excited one at a time toanalyze the screening effects. The parameters of the modelhave been optimized until the simulated magnetic field mapinside and outside the magnet matched the experimentalresults.Fig. 3. Discharge curves at 1 kV-0.6-0.4-0.200.20.40.60.811.20 50 100 150 200 250 300 350 400Time [ms]Voltage [kV]ReferenceShort Circuit AShort Circuit BShort Circuit CReference         f = 3800 Hz      L = 1.50 mH     Reff = 29.4 WShort Circuit A    f = 4850 Hz         L = 0.88 mH        Reff = 25.3 WShort Circuit B     f = 4550 Hz          L = 1.00 mH         Reff = 26.7 WShort Circuit C  f = 4070 Hz        L = 1.26 mH      Reff = 29.9 WABCRefFig. 4. Schematic representation of an inter-turn short-circuit30153154 313031153154X-section 1X-section 2ABCA3Fig. 5. Multi-layer printed circuit pick-up coilsFig. 7. Magnet coils equipped withmicro-relaysIV. TRANSIENT MAGNETIC FIELD IN PRESENCE OF A SHORT-CIRCUITTABLE IISHORT-CIRCUIT ANALYSIS AT R0 = 10 MMNormal Reference X-section 1 X-section 2B1 -82.210-4 T -60.910-4 T -61.410-4 Tb2 0.0 113.2 0.0b3 -156.4 -768.4 -697.0b4 0 31.6 0.0b5 9.8 -9.9 2.8b6 0.0 5.2 0.0b7 0.3 1.4 3.1Skew Reference X-section1 X-section 2a1 0.0 271.5 0.0a2 0.0 1320.9 1455.7a3 0.0 45.8 0.0a4 0.0 195.0 204.5a5 0.0 1.0 0.0a6 0.0 23.5 22.7a7 0.0 -0.7 0.0In case of short-circuit the transport current follows a pathdifferent from the nominal one (Fig. 4). On one side of theinter-turn short-circuit the current distribution in the coilwindings is asymmetric (cross-section 1). On the other sideit becomes symmetric again (cross-section 2). In terms of themagnetic field quality this corresponds to the vanishing ofeven-order harmonics of the normal field and of odd ordersof the skew multipoles in the area where the currentdistribution is symmetric. Table II lists the computedmultipoles values for the short-circuit in position denoted asA (Fig. 4) in the two sections before and after the defect incomparison to the pulsed magnetic field generated in aflawless magnet.V. EXPERIMENTAL SET-UPA. Detection MethodThe development of thefield harmonics during apulsed discharge isdetected with a system ofpick-up coils (Fig. 5)aligned on the medianplane of the magnet. Theschematic position of theindividual pick-up coilsinside the aperture isshown in Fig. 6. In orderto visualize and explainthe detection method thepure normal and skewquadrupoles in Fig. 6 were chosen as the representatives ofthe even-order field harmonics, and the pure normal andskew sextupoles as the representatives of the odd-ordermultipoles. If the even orders of the normal field harmonicsare present in the magnetic field a difference between thevoltage induced in coils C3 and C5 will be observed. Thecontribution  of  the  even-order   multipoles  cancels  out  inthe central coil. Since higher order odd multipoles arenegligible close to the center of the aperture, coil C4 picksup mainly the dipole field. Altogether coils C3, C4 and C5are transparent to the flux resulting from the skewharmonics, which is detected by the coils C1 and C2. Thesum of the voltage induction on these two coils willcorrespond to twice the total of the odd-order skewmultipoles.B. Test Set-upOne of the short dipolemagnets was equipped withmicro-relays soldered onthe internal layers in twoclusters; one centeredaround the cross section inthe middle of the magnet(Fig. 7), and the other closeto the end region of the coil.The relays, excitedexternally with DC one at atime, create short circuitsbetween adjacent cableturns in well-definedpositions. A pulsed currentis supplied from a dischargegenerator by releasing theenergy   accumulated   in   a1 PF  capacitor,  charged  at1 kV, into the magnet. Thevoltage induced in the pick-up coils is measured during thedischarge and integrated thereafter to obtain the timeevolution of the magnetic field induction. The pick-up coilplatform slides longitudinally on two vetronite rods alignedinside the aperture of the magnet.C1C3 C4 C5C260b2C5C460C1C3C2b3Fig. 6. Schematic representation of the detection methodC1C3 C4 C5C260a2C1C3 C4 C5C260a34Fig. 8. Test results for short circuit A. In reality  the signals picked upby the coils C1 and C2 are of opposite polarity.Cross-section No. 1Absolute values of the integrated peak field01020304050607080Coil C1 Coil C2 Coil C3 Coil C4 Coil C5Peak Field [ T ]ExperimentModelx 10-4Cross-section No. 2Absolute values of the integrated peak field01020304050607080Coil C1 Coil C2 Coil C3 Coil C4 Coil C5Peak Field [ T ]ExperimentModelx 10-4Fig. 10. Indications of the coil C1 as a function of the azimuthalposition of short-circuit05101520253035Peak Field [ T ]Short-Circuit A Short-Circuit B1 Short-Cirucit B2 Short-Circuit CExperimental Data, Coil No. 1, Cross-section No.2Coil C1x 10-4C. ResultsThe test results confirm the computational analysisdemonstrated in previous sections. The signals registered bythe five pick-up coils of Fig. 5 allowed to identify thelongitudinal position of all triggered faults, and also gave anindication of the azimuthal position of the short-circuits. Asan example, typical results of the measurements arepresented in Fig. 8 for a short-circuit in position A. Asexpected, the signals of the absolute peak field induction onthe coils C1 and C2, as well as on the coils C3 and C5became unbalanced while passing the point of fault fromcross-section No. 2 towards cross-section No. 1. The highmagnitude of the signal on the coils C1, C2 is due to thehigh current induced in the passive winding loop deprived ofthe imposed transport current. For the short circuit underanalysis the peak intensity of this current amounts toapproximately 510 A.  In case of the flawless magnet withno short-circuits engaged, virtually no signal is induced inthe coils C1 and C2 (Fig. 9).Also, as predicted by the model, the amplitude of thesignal picked up by the coils C1 and C2 is very sensitive tothe azimuthal position of the short-circuit (Fig. 10). Thedifference in the coil indications is visible even for thedefects located close to each other as shown for the short-circuits B1 and B2 between two pairs of adjacent cable turnsin the proximity of the position B in Fig. 4.VI. CONCLUSIONSTransient magnetic field distortions created by electricalinsulation failures in superconducting collared coils wereanalyzed in view of the localization of possible inter-turnshort circuits. The detection method was based on the studyof the magnetic field harmonics induced by the currentredistribution in the magnet coils. The experimentalprocedure has been simulated with OPERA-2D and testedon a dedicated dipole magnet equipped with micro-relayscapable of activating artificial short circuits in the internallayer of the coils. The longitudinal position of the short-circuits was successfully established for all of the installedmicro-switches with the precision of a few millimeters. Themethod gave also the indication of the azimuthal position ofthe defects.ACKNOWLEDGMENTThe authors would like to thank N. Siegel and C. Wyss fortheir constant support and H. Kummer for his contribution tothe assembly of the test magnet.REFERENCES[1] N.Andreev et al. " State of the short dipole model program for theLHC", presented at the 6th EPAC, Stockholm, June 22-26, 1998[2] S. Russenschuck et al., "Integrated design of superconductingaccelerator magnets. A case of study of the main quadrupole", TheEuropean Physical Journal 1998, AP 1, 93-102[3] OPERA-2D, Software Environment for Electromagnetic Design,Workstation ver. 7.0, 1998, Vector Fields Ltd., Oxford, EnglandFig. 9. Pick-up coil indications in a flawless magnet0102030405060708090Peak Field [ T ]Coil C1 Coil C2 Coil C3 Coil C4 Coil C5Absolute values of the integrated peak fieldExperimentModelx 10-4

Localization of Electrical Insulation Failures in Superconducting Collared Coils by Analysis of the Distortion of a Pulsed Magnetic Field

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