Testing of the Inter-Turn Insulation of High Voltage Induction Motor Coils
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TESTING OF THE INTER-TURN INSULATION OF HIGH VOLTAGE INDUCTION MOTOR COILS Michael John Hopkins A Dissertation Submitted to the Faculty of Engineering Uni v a r s ity o f "the Wi tw a te rs ra n d , Johannesburg f o r th e Degree o f Master o f Science Johannesburg 1976 ABSTRACT It is recognised that a need exists for a reliable set of tests of induction motor coil inter-turn insulation, due to the dangers presented to such insulation by fast rising surges. The occurrence and form of such surges are discussed. The behaviour of surges on coils is ana lysed both in theory and in practice to derive the maxi mum s tr e s s l i k e ly to be imposed on a c o il by a fa s t r is in g surge. Based on t h is a c r it e r io n is proposed fo r i n te r- tu rn te s ts . Two tests which do not require the coil to be cut, are proposed. The firs t consists of discharging a charged c a p a c ito r th ro u g h the c o il and a llo w in g a damped o s c i l l ation to occur which effectively stresses the inter-turn insulation. The second test consists of inducing a high frequency voltage wave into the coil using an electronic oscillator. Refinements are proposed for both tests to a llo w f o r a p ro d u c tio n te s t programme as well as t o pe rm it an unambiguous detection i_T breakdown between turns. A number of test circuits are proved in practice. Tests are derived to assess the validity and safe6\ o-' +he c a p a c ito r discharge te s t . A number o f m a te ria ls and samp les are tested to assess their behaviour in the proposed inter-turn tests. The results of such tests are compared with those obtained with the standard impulse test. The breakdown process between the turns of a coil is examined and analysed to reveal a weakness In in d u c tio n m otor c o ils which can lead to a reduced resistance to i nter-turn breakdown. I/ Michael John Hopkins do state that this dissertation is my own unaided work and has not been presented in whole or in part for any other degree or at any other university. ACKNOWLEDGEMENT The author wishes to thank GEC Machines (Pty) Ltd. and Marth i nusen Ltd. for assistance end materials supplied. CONTENTS PAGE 1 INTRODUCTION • 1 2 AN ANALYSIS OF IMPULSES LIKELY TO STRESS THE INTER TURN INSULATION OF MOTORS .. 3 3 THEORETICAL BEHAVIOUR OF SURGES ON MOTOR COILS .. 6 4 TRANSIENT MEASUREMENTS ON COILS USING LOW VOLTAGE SURGES .. .. .. • .. .. 17 4.1 Mode I Coils - Results .. .. 19 4« 2 Commepci a l l y Produced M otor C o ils . - •• 26 5 THE CAPACITIVE DISCHARGE TEST . 32 5.1 Introduction .. .. .. .. 32 5.2 A Practical Capacitive Discharge Test .. 38 5-3 Detection of Breakdown in tl.e Capacitive Discharge Test .. ., .. .. 47 5.4 An Improved Test Suitable for Factory Use ,. 55 5.5 Statistical Tests on the Capacitor Discharge Method.. .. .. .. 58 5.6 Various Effccts Observed Concerning the Capacitive Discharge Test .. ., 65 5.6.1 The Discharge between Turns .. .. 65 5 .6 .2 Noise on the Discharge Waveform 68 6 RADIO FREQUENCY COIL TESTING .. 70 7 PRACTICAL BREAKDOWN TESTS PERFORMED ON A NUMBER . OF INSULATING MATERIALS .. 81 7.1 Introduction ., .. 81 7 .2 Tests on Rea I Co i I Samp I os : , , .. 81 7.3 Tests using Sheet Samples of Insulating Materia Is • .. ., ,. .. , 86 7.4 Analysis of the Test Results .. •., 90 ' 7«4.1 Impulse Tests .. .. .. 90 7.4.2 Radio Frequency Tests .. .. 96 CONCLUSION APPENDIX A 101 APPENDIX B 104 APPENDIX C 106 APPENDIX D 109 APPENDIX E 111 REFERENCES 129 1 ' INTRODUCTION Insulation breakdown is thf most feared failure occurring in large high voltage induction motors. Damage to the ground insulation is often extensive, and it is the breakdown of this insulation which is most often cited os the initial cause o f the damage. In some cases, however, i t is suspected that the initial failure occurs between turns; heat gener a tio n a t t h is p o in t weakens the ground in s u la tio n which fails catastrophically destroying all evidence of the i n i t i a l f a :Iu re . Cl] I t has been n o tic e d by local m an ufa ctu re rs and r e p a ir ers that the incidence of insulation failure is much higher du rin g summer, [2] As the South A fric a n in d u s tria l areas are subjected to a large amount of lightning during this season it is likely that lightning surges are the root cause of this trouble, by the above process. Lightning surges can travel into the windings, which because of their distributed parameters, behave in a similar fashion to a tra n s m is s io n lin e . The steep surge fr o n t is imposed across a coil (or even a few turns) causing a high transient stress of the !> uI ati on between turns. If this insulation has o n ly been designed to w ithsta nd the normal working potential between turns oP at the most a few hundred volts, or if the coil is faulty* then i nterfcurn faljuue is I j k<? I y. To ensure a reliable motor all manufacturers test the insulation during and after manufacture. This is usually only done for strength of ground insulation, as testing of interturn- Insulation is difficult, especially before the c o ils have boon in s ta lle d in the s tn to r . At present the usual method of impulse testing inter turn insulation of motor coils is to cut a coil yielding a number of parallel conductors which are then impulsed using the normal type o f impulse g e ne rato r. Such a te s t is s p e c ifie d by both The Rand Water Board and ESCOM. (ESCOM spec. DCS 1551) The major defect of this best is th a t i t de stroys the c o il, whether sound or unsound. Con sidering the high cost of manufacturing such coils, most firms are I oath to use it. It is suitable for locating design defects, or for locating systematic defects in batrhes of coils. As it cannot be applied to every coil it cannot locate the isolated coil with a latent defect. Furthermore, it is a tedious test requiring a skilled oper ator. 1 he process of cutting tne coil s-.ay also damage the insulation, making the test invalid, A test is therefore required which is non-destrucci ve to a ll bu t unsound c o ils . I t should be both safe and easy to apply, and be suited to routine testing* The prime limiting fact 'r in inter-turn testing is the very low impedance o f m otor c o ils . A t normal freq ue ncie s very high currents would be required to produce a s u ita ble p o te n tia l drop across the c o il. The normal type o f impulse generator is unsuitable due to its relatively high-internaI impedance. To increase th e impedance o f a c o i l , high f r e quencies are necessary. T h is can he done in two ways. The first consists of discharging a high voltage capa citor into the inductance of the coil. The resulting 1C com bination executes damped o i l la t i o n s which s tre s s the coiI insuIation. The second method uses an e le c tr o n ic valve c i r c u i t to generate high voltage radio frequency oscillations which are induced into the coil to stress the insulation. There are decided advantages and disadvantages to both methods and an analysis of this is attempted. It is a I so proposed to analyse the nature of impulses encountered in practice as well as impulse behaviour on coils in an attempt to set a criterion for such tests and their ability to ensure a motor resistant to natural impulses. 3 2 m ANALYSIS Of IMPULSES LIKELY TO STRESS THE 1HTEK-TURH INSULATION OF MOTORS Impulses travelling on transmission systems can be caused by a number o f events [3] . These are b a s ic a lly e ith e r switching surges, or lightning induced impulses. The classic type of switching surges found on transmission lines are usually too slow to cause trouble [4,53 ■ However,vacuum quenched circu it breakers are be I no used increasingly for motor switching and can cause extremely fast transients to occur at the motor terminals due primarily to re-ignition phenomena (6,7) L ig h tn in g surges can have very steep fro n ts and it is worthwhile considering studies undertaken on light ning in an effort to estimate the type of surge likely to cause distress to a motor's inter-turn insulation. The shape o f impulses used to te s t tra n sfo rm e rs have been s p e c ifie d by va rio u s bodies [%, 9, Id] .