On-site Non-destructive Dielectric Response Diagnosis of Rotating Machines

Transcription

1 IEEE Transactions on Dielectrics and Electrical Insulation Vol. 17, No. 5; October On-site Non-destructive Dielectric Response Diagnosis of Rotating Machines Supatra A. Bhumiwat Independent High Voltage Diagnostics Consultant P.O. Box Glen Innes Auckland 1743, New Zealand ABSTRACT An on-site off-line non-destructive diagnostic tool for insulation assessment of rotating machines by means of the dielectric spectroscopy is introduced. The technique is based on time-domain measurement of both Polarization and Depolarization Current (PDC) in addition to frequency-domain processing of the measurement results in terms of the Dielectric Dissipation Factor () and Capacitance (C). The article emphasizes the importance of depolarization current, or equivalently the absorption current, which shall be measured in conjunction with polarization current for equal length of time, in order to identify the aging type of insulating materials. Factors affecting the measurement are discussed. The proposed interpretation of the dielectric response results demonstrates a need for using both time-domain and frequency-domain analyses as well as provides a decisive measure for safe operation of rotating machine. Index Terms Dielectric, absorption, conduction, polarization, depolarization, capacitance, dissipation factor. 1 INTRODUCTION MAINTAINING the life of machine insulation requires a nondestructive diagnosis which can detect and identify problems. Online partial discharge measurement has been popular for the detection of local defects especially in ground-wall insulation, but gaining more information on overall health (or global problem) is also necessary. Insulation Resistance measurement has been applied for decades in the detection of surface contaminants especially water and carbon dust. Several methods were introduced to determine absorption current from the total charging current during the insulation resistance measurement, in order to identify the aging type, but none is simple. Since 2002 the author has applied the dielectric response technique based on [1] in the diagnosis of insulation problems in both generators and motors. The key advantage of the analyzer is its ability to measure both polarization and depolarization currents (the latter is the absorption current) in the range of A which allows conduction and polarization, two basic properties of the dielectric, to be identified non-destructively. For a clear insight into how the technique works, the article starts with classifying problems in machine insulation based on the basic dielectric properties, conduction and polarization, before introducing the technique, all factors involved and a methodology for interpretation. Although details on diagnosis of problems such as thermal aging, moisture, contaminants, corrosive dust, products of spilled lubricating oil, etc. are in [2 4], different case studies are presented with more details included for the benefit of readers. Manuscript received on 20 January 2010, in final form 19 May BACKGROUND OF PROBLEMS IN MACHINE INSULATION Every kind of insulating material in rotating machines possesses two basic electrical properties. One is an ability to persist in an electrostatic field for a long time and the other is an ability to be polarized. The first property, that any electrical insulation shall have low conduction (or high resistance), is well-known but here are some explanations of polarization: Polarization is the result of a relative shift of positive and negative charges in a material. During all of these processes, the electric field is not able to force the charges to escape from the material, which would cause inherent electrical conduction [5] and When the effect of the voltage applied to a dielectric is discontinued, the displaced charges may tend to return to their initial positions, which never happens in the phenomenon of electrical conduction [6]. More explanations are in [5-7]. There are different types of polarization. The accumulation of electric charges at the interfaces of different dielectric materials, e.g. at mica-resin interface, etc. is called Interfacial Polarization. Polarization takes place in all the molecules of a dielectric material and causes chemical change or deterioration. At the same time, conduction in a dielectric is often determined by the presence of impurities or contaminants but is not attributed to its basic substance. Problems in every dielectric are produced by the mechanisms of one of these two. A classification of problems in machine insulation based on its basic properties is shown in Table /10/$ IEEE

2 1454 S. A. Bhumiwat: On-site Non-destructive Dielectric Response Diagnosis of Rotating Machines Table 1. Classification of problems in machine insulation. Polarization By-products of Partial Discharges Aging molecules at interfaces Arcing by-products Oxidation by-products Products of spilled lubricating oil & dust Chemical dust including salts Corrosive products Thermal aging products Moisture in adsorbed or molecular state Conduction Surface humidity Free water or droplet Surface contaminants Tracking Carbon dust Metal dust Debris from fault Leakage path While free water or surface contaminants causes conduction, water in the insulation e.g. in molecular state or adsorbed state causes polarization, especially interfacial polarization. In addition, the oxygen in water, HOH, has a negative dipole similar to the oxygen in the ester bond of adhesives (for bonding mica together) such as epoxy resin, etc. Water will thus compete with the ester for association with the potassium ions in mica [8]. A similar case is shown in Figure 1. This new machine had been exposed to free water during transport and some off-white solids were observed. The infrared spectra of off-white solids exhibited absorption peaks indicative of a polyester-based material, which is polyethylene terephthalate or PET used as an adhesive. PDC measurement results of phase-to-phase insulation, even after refurbishment, shows both currents have the pattern of reversal charges such as case 4 in [3], which refers to absorption current due to polarisation phenomena (see 3.1 in section 3). Figure 1. The white PET deposits were found in a new machine having been exposed to water during transport. 3 DIELECTRIC RESPONSE DIAGNOSIS All the dielectric response results in this article are from the commercially available PDC-Analyser-1MOD 1 which measures the time-domain polarization and depolarization currents (PDC) and evaluates (by the PDC software) the frequency-domain capacitance and tan delta (or dielectric dissipation factor, ). The evaluation results also include insulation resistance (IR) & polarization index (PI) and recovery voltage polarization spectrum. The latter is sensitive only when the insulation is in very bad condition, see [4]. It will not be used as a diagnostic parameter in the interpretation of this article. 3.1 HOW PDC IDENTIFIES CONDUCTION AND POLARIZATION IN MACHINE INSULATION Figure 2 shows the principle of test arrangement for PDC measurement on phase-to-phase and phase-to-ground Figure 2. Principle of test arrangement for PDC measurement of phase-tophase insulation (top) and ground insulation (bottom). insulation. Under the action of constant direct voltage, the current during charging (or polarization) consists of absorption current due to polarization phenomena and conduction current caused by conduction phenomena while the current during discharging (or depolarization) consists of absorption current only (as conduction current exists only when there is power supply). Capacitive current will not be mentioned, as it occurs during switching and will disappear before 1 s, the starting time of PDC measurement. When the insulation has very low conduction, the polarization current and the depolarization current will be nearly equal for about one-tenth of the charging time. This is how PDC identifies conduction and polarization in machine insulation (or problems which are classified in Table 1). The measurement of depolarization current is very important, as it is the measurement of absorption current. Absorption current increases due to aging of materials at any interfaces, aging of the adhesive which bonds mica together (e.g. asphalt, resin, varnish, etc.), oxidation aging, thermal aging, by-products of partial discharges, etc. Absorption current can decrease due to formation of voids or gap (e.g. from delamination). The other advantage of depolarization current measurement is an ability to monitor the remaining charges in the insulation. See more details in 4.1 of section EVALUATION RESULTS OF C & As mentioned, the frequency-domain C & are from the evaluation of PDC measurement results, not from direct measurement. The analytical transition from time to frequency domain is described in [5]. Only the frequency range corresponding to the measurement time will be considered, not others from the extrapolation. In addition, is used instead of C, which varies from one machine to another. is the ratio of capacitance at corresponding frequency to capacitance

3 IEEE Transactions on Dielectrics and Electrical Insulation Vol. 17, No. 5; October at 50 Hz. When the insulation system contains negligible amount of deterioration products (which cause polarization phenomena), the capacitance value is constant from 50 Hz, 1 Hz towards low and very low frequencies. In another words, =1.00. Deterioration products especially water in the adsorbed state or molecular state, increase capacitance at low and very low frequencies which means higher. This is due to the slow process of interfacial (or migrational) polarization. The frequency scan of and are both used as diagnostic parameters in the interpretation. While provides a decisive measure of insulation condition, it cannot identify the cause of problems. Both conduction and polarization phenomena increase. In addition, the dependency of on frequency is different from one type of insulating material to another. 3.3 EVALUATION RESULTS OF INSULATION RESISTANCE AND POLARISATION INDEX (P.I.) PDC shape itself identifies the type of insulation trouble. The PDC measurement results depend not only on insulation condition but also geometry or capacitance of the insulation system. Insulation resistance is the result of constant applied voltage divided by the measurement results of polarization currents at a defined time of electrification. So the insulation resistance is also influenced by both insulation condition and capacitance. The comparison of insulation resistance can only be made among similar insulation having similar design. The resistance of phase-to-phase stator insulation cannot be compared or judged with the resistance of ground insulation, since their capacitance values are much different. Considering P.I. from the polarization current between 60 and 600 s of all results in section 5, the actual meaning of P.I. will be fully understood. 4 FACTORS AFFECTING THE MEASUREMENT This section describes all factors which influence the PDC measurement and how to overcome. 4.1 REMAINING CHARGES IN THE INSULATION When a machine is switched off from operation, all line terminals are usually brought to earth for discharging. The time taken for winding temperature to be stabilized is normally longer than the time required for discharging. This means by the time all line and neutral terminals are disconnected and isolated from any connected devices, remaining charges in the insulation system will be normally low enough to start the first measurement. In case the ground resistance is not low or the ground system is poor, the remaining current due to residual charges will not be well discharged and the constant value will be obtained. This background level will be used later for making correction. The remaining current is the measurement of depolarization current without any voltage application. 4.2 INFLUENCE OF TEMPERATURE PDC measurement is more or less temperature dependent, so it shall be carried out when stator winding temperature is as stabilized as possible. Normally when a machine is shutdown, heaters inside will be automatically switched on, in order to prevent moisture ingress. The winding temperature will be controlled and stabilized by means of a thermostat. Waiting time before PDC measurement depends on the winding temperature at shutdown and the time taken for the winding temperature to decrease and stabilize. Temperature-controlled heaters shall be switched on throughout the measurement. 4.3 INFLUENCE OF SURFACE LEAKAGE AND AMBIENT HUMIDITY Both line and neutral terminals of a stator are normally indoors or at least inside an enclosure which has a built-in heater. This means air humidity and contaminants are minimized. So many tests on both phase-to-phase and ground insulation were successfully carried out overnight. Although the internal-guard design of PDC-Analyzer always allows the test in the rain of insulation between windings of a power transformer, this is not the case for a stator. When all stator terminals are wet or heavily polluted, no test instrument will be able to assess its internal condition. 4.4 TEST CONNECTION & GROUND RESISTANCE As PDC analysis is a dc test, the test can be applied to either line or neutral terminal of the stator. The line and neutral terminal of each phase under test do not need bonding like AC diagnostic tests. Both line and neutral terminals of the stator shall be isolated from bus, instrument transformers, surge arresters and capacitors. The neutral of all three phases shall be also isolated and disconnected from earth. In case the isolation of neutral is not possible, only one test of three phases to ground can be done. All three line terminals of the stator are bonded during the measurement. For PDC test on phase-to-phase insulation, the voltage is applied to one phase while the current is sensed from another phase. The non-tested phase is earthed. The ground reference of the PDC shall be connected to the same point where the stator frame is earthed, in order to prevent any potential difference which may cause unstable measurement. Care shall be taken in case of machine after refurbishment, as new paint may prevent good ground contact. For PDC test on phase-to-ground insulation (or ground insulation ), an accessory of the PDC-Analyser called Phantom is used. The voltage is then applied from the Phantom to the phase under test, phase by phase, while the other two non-tested phases are floated from earth. Again, the ground of phantom shall be connected to the same point where the stator frame is earthed. 5 INTERPRETATION OF RESULTS Three diagnostic parameters are proposed for interpretation. These are PDC shape, and. The methodology for interpretation is demonstrated through case studies. All figures used in this section are from the actual results of either new or in-service machines where aging may have more than one type but the dominant one is described.

4 1456 The identification of aging type is mostly based on PDC shape, both I pol and ) and. Suggested criteria for safe operation, as proposed in [3] and are shown here in figure 3, are based on and. The criteria for phaseto-phase insulation () are different from ground insulation (L-G). Additional information can be gained from insulation resistance and P.I., but with care (see item 3.3 of section 3) In this section, the word moisture will be used to represent water in the dielectric which causes polarization phenomena. Free water will be used to represent water or humidity at insulation surface which cause conduction. Case studies chosen for this article are as follows. More case studies are in [2-4]. - New and good insulation condition - Moisture - Overheating or thermal aging - Moisture and conductive contaminants - Combination of aging (an example for end-of-life) 5.1 NEW AND GOOD INSULATION CONDITION Figure 4 presents dielectric response results, or three diagnostic parameters: PDC and, of a new generator stator (not yet in service). It is considered as good insulation because of the following characteristics: PDC: PDC shape is the first to be considered. I pol and I depol. are both straight in log-log scale, following inverse power law. (In many cases n of inverse power law is about but n as low as 0.5 is found in some old machines which are still in acceptable condition. A few cases of phaseto-phase insulation of a new machine has n= The value of n will not be mentioned any more in this article.) In addition, I pol and I depol are very close for about onetenth of charging time before I depol decays. Amplitude of PDC in numerical value is not used in interpretation. But moisture can increase PDC amplitude Max. limit for Max. limit for L-G 1.E E-0 3 Max. limit for Max. limit for L-G 1.E E-0 3 Figure 3. Suggested in-service criteria (at about o C) of and for phase-to-phase insulation () and ground insulation (L-G). S. A. Bhumiwat: On-site Non-destructive Dielectric Response Diagnosis of Rotating Machines substantially in spite of good PDC shape. This occurs in case of wet machine after drying such as G-01A of [3]. : When is 1.00 for the whole frequency range observed, the machine insulation has no problems with polarization phenomena. Free water and conductive contaminants which cause conduction phenomena do not increase. : is used as a decisive measure only. It does not identify the aging type or problems in machine insulation. As long as the whole shape of is far below the suggested 1.E-0 3 PDC at 100V, 20 o C ,000 Suggested limit for L-G, L-G 1.E-0 3 Suggested limits for L-G at 20 o C 1.E-0 3 Figure 4. Dielectric response results of ground insulation for case study 5.1: New and good insulation condition PDC and limit, the insulation system is in good condition. 5.2 MOISTURE Figure 5 shows PDC, and of another new generator stator (not yet in service) but became wet during storage before installation. The drying was done on site and could obtain good results for ground insulation but not phaseto-phase insulation. PDC: PDC shape is straight in log-log scale, following inverse power law. I pol and I depol are very close, similar to case 5.1. But if there is a new sister stator with good condition to compare, it will be found that the current amplitude of this stator is substantially higher. In case there is nothing to compare, then moisture will be identified by.

5 IEEE Transactions on Dielectrics and Electrical Insulation Vol. 17, No. 5; October E-11 PDC at 100V, 19 o C ,000 Suggested limit for, 1.E ,000 10,000 PDC at 50V, 18 o C Suggested limit for L-G, L-G 1.E-03 1.E-02 1.E-01 1.E+00 at 19 o C Suggested limit for 1.E-0 3 Figure 5. Dielectric response results of phase-to-phase insulation for case study 5.2: Moisture PDC and : is higher than 1.00 from 1 Hz and above the suggested limit in some frequency range. This is unacceptable. Interfacial polarization in dielectric with water inclusions increases more than other deterioration products. This is due to the very high permittivity of water. : is not acceptable because it is above the suggested limit for phase-to-phase insulation () in some frequency range. is very high from 1 Hz ( at 1 Hz should be less than 10-2 for new machine) and increase exponentially to 0.1 Hz, then it is quite constant towards low and low frequencies. The constant shape of at low and very low frequencies is also found in other new and good machines. 5.3 OVERHEATING OR THERMAL AGING This is a case of overheating due to high load and insufficient cooling. Figure 6 shows the results of PDC, C ratio and of ground insulation. PDC: The PDC shape, I pol as well as I depol, has one prominent crook. The initial current is straight but not certain for how long (see the results of other machines having thermal trouble in [3]. I pol and I depol are very close and have the same shape. This refers to absorption current caused by polarization phenomena and it means conductive contaminants in this machine are very low. 1.E-0 3 Suggested limits for L-G at 18 o C Figure 6. Dielectric response results of ground insulation for case study 5.3: Overheating or thermal aging PDC and Because the crook or bending shape causes a decrease in current at longer time, thus the current at 10 minutes is much lower than the current at 1 minute, which means P.I. is higher. In many cases P.I. > 7 is found in ground insulation. : As this is the case of polarization phenomena, C ratio is higher than unity. The lower the frequency, the higher the. In case of thermal aging, is rarely higher than the suggested limit such as in the case of moisture. It can also be seen that at 0.1 Hz is slightly above unity so the frequency of 0.1 Hz is not sensitive enough to detect thermal aging. The higher sensitivity is at lower frequencies. : increases from 1 Hz towards low and very low frequencies and can have different shapes. Since in this case is lower than the suggested limit for the whole frequency observed, the insulation is considered acceptable though improvement of cooling or ventilation was suggested. 5.4 MOISTURE & CONDUCTIVE CONTAMINANTS The dielectric response results of an old machine before and after refurbishment are presented, with phase-to-phase insulation in Figure 7 and ground insulation in figure 8. The contaminants especially carbon dust in the stator before refurbishment is clearly seen in Figure 9, which also includes the picture of machine after refurbishment.

6 1458 S. A. Bhumiwat: On-site Non-destructive Dielectric Response Diagnosis of Rotating Machines 1.E ,000 10,000 Suggested limit Suggested limits for Figure 7. Dielectric response results of phase-to-phase insulation for case 5.4: Moisture and conductive contaminants PDC and (d) L-G ,000 10,000 (e) Suggested limit L-G 1.E E-0 3 (f) Suggested limits for L-G 1.E E-0 3 Figure 8. Dielectric response results of ground insulation of the same stator as figure 7 (d) PDC (e) and (f) PDC: All results of I depol in Figures 7 and 8 have straight shape, no prominent crook. This means the only cause of polarisation phenomena, if exists, is moisture. The deviation of I pol from I depol and the deviation of I pol from a straight line with the appearance of flat dc component (Figure 7) are the pattern of conductive contaminants. The earlier the deviation, the higher the risk that the insulation will fail due to contaminants. An improvement is seen after refurbishment as both deviations mentioned above are later and much less. As mentioned in item 5.2 that moisture increases absorption current (or increases both I pol and I depol) substantially. The difference of PDC amplitude before and after refurbishment in figure 7 and 8 refers to moisture. The drying through shortcircuit method did remove moisture. : The unacceptably high is seen from this machine especially before refurbishment. This is mostly caused by moisture, as free water or carbon dust does not increase C ratio. is in acceptable level after refurbishment. : is very high and exceeds the suggested limits of both phase-to-phase and ground insulation. The refurbishment could bring the acceptable results to ground insulation, but not the phase-to-phase insulation. Figure 9. The condition of the old stator in case study 5.4 before refurbishment and after refurbishment.

7 IEEE Transactions on Dielectrics and Electrical Insulation Vol. 17, No. 5; October PDC at 500V, 30 o C ,000 Suggested limit for, at 30 o C Suggested limit for 1.E E-0 3 Figure 10. Dielectric response results of phase-to-phase insulation of a motor stator in case study 5.5: Combination of discharges, thermal and mechanical stresses PDC and 5.5 COMBINATION OF DISCHARGES, THERMAL AND MECHANICAL STRESSES Figure 10 shows the dielectric response results, PDC, and, of phase-to-phase insulation of a motor stator (V-W of case II in [4]. Because of the bad results, the machine was stripped to verify this dielectric response technique. Figure 11 shows the worst area. The phase-to-phase slot packer was very deteriorated, with abrasion of insulation due to vibration, heat and discharges. The machine was rewound. Other results of this stator and its sister are in [4]. PDC: The shape of I pol and I depol at the initial time of about 6 seconds are very similar and they are steep, or higher absorption current. This refers to by-products (such as discharge by-products, thermal aging by products, etc.) closer to the conductor. that I depol changes slope to normal straight line while I pol has the pattern of conductive contaminants. The change of slope of I depol is due to interfacial polarization (e.g. at the interface of by-products and the insulation, etc.). Conductive contaminants are likely those particles from the abrasion of conductive paint. : is very high and exceed the suggested inservice limit for phase-to-phase insulation. The deviation of C ratio from 1.00 is due to the problems related to polarization phenomena. The shape of is unusual with the change of slope which means more than one process of polarization (e.g. thermal aging by products and discharges by-products). : is very high (exceeds suggested limit excessively) especially between Hz, which corresponds to the initial time of I pol or So it is likely caused by the deterioration products or interfacial polarization closer to the conductor, as described above in the paragraph of PDC. 6 CONCLUSION Three diagnostic parameters, PDC, and, are proposed in this article for the interpretation of dielectric response results. An increase in indicates there is a problem but does not tell what the problem is. An increase in at lower frequencies indicates the deterioration of insulation caused by polarization phenomena. Any problem due to conduction does not increase. PDC shape can identify not only polarization and conduction but more specific to e.g. thermal aging, conductive contaminants, etc., although some problems require the PDC shape together with for diagnosis. Nevertheless, the acceptable health condition is judged by and, not the PDC. While conduction and polarization of machine insulation are identified by the shape of both polarization current and depolarization currents, it is the shape of depolarisation current (or absorption current) which characterizes the aging type of electrical insulation. Finally, the overall health of machine insulation can be assessed non-destructively by the dielectric response technique. The local problem such as partial discharges can also be detected indirectly through the discharge by-products which increase absorption current or depolarisation current at initial time, increase and. ACKNOWLEDGMENT Many thanks to my clients from whose machines the data presented here were collected. Figure 11. The deteriorated phase-to-phase slot packer and the coil. REFERENCES [1] J. Alff, V. Der Houhanessian, W. S. Zaengl and A.J. Kachler, A novel, compact instrument for the measurement and evaluation of relaxation currents conceived for on-site diagnosis of electrical power apparatus, IEEE Intern. Sympos. Electr. Insul., Anaheim, California, USA, pp , 2000.

8 1460 S. A. Bhumiwat: On-site Non-destructive Dielectric Response Diagnosis of Rotating Machines [2] S. Bhumiwat, Application of polarisation depolarisation current (PDC) technique on fault and trouble analysis of stator insulation, CIGRE SC A1 & D1 Joint Colloquium, Gyeongju, Korea, pp , 2007 {can be downloaded from [3] S. Bhumiwat, Practical experiences on condition assessment of Stator insulation using Polarisation / Depolarisation Current technique, CIGRE 2008 Session, Paris, France, pp. D1-210, [4] S. Bhumiwat, Field experience in insulation diagnosis of industrial high voltage motors using dielectric response technique, IEEE Electr. Insul. Conf., Montreal, Canada, pp. 21-2, [5] W. S. Zaengl, Dielectric spectroscopy in time and frequency domain for HV power equipment, Part I: Theoretical considerations, IEEE Electr. Insul. Mag., Vol. 19, No.5, pp. 5-19, [6] B. Tareev, Physics of Dielectric Materials, Mir Publishers, Moscow, Ch.2, [7] T. W. Dakin, Conduction and polarization mechanisms and trends in dielectrics, IEEE Electr. Insul. Mag., Vol. 22, No.5, pp , [8] D. M. Hepburn, I. J. Kemp and A. J. Shields, Mica, IEEE Electr. Insul. Mag., Vol. 16, No.5, pp , PDC-Analyser-1MOD available from ALFF Engineering (Switzerland). Supatra A. Bhumiwat (M 81) was born in Bangkok, Thailand in She graduated with the B.Eng. degree from Chulalongkorn University, Bangkok, Thailand, in March She started her career with Italian-Thai Development Company for electrical installation of Bangkok main water treatment plant. In January 1980 she joined Electricity Generating Authority of Thailand (EGAT) and has become a test engineer at the end of In 1983, while working for EGAT, she received practical training on high voltage tests from BBC in Switzerland and Germany in addition to ETH- Zurich and ITR-Rapperswil in Switzerland. From 1984 she worked in EGAT- Extra-High Voltage Laboratory until February Her main field of interest was dielectric diagnoses. Her research works included insulation aging of instrument transformers and power transformers. She was a member of CIGRE WG12.15 and CIGRE WG In 1997, she emigrated to New Zealand and has been an independent high voltage diagnostics consultant. Her services at present include Transformer Oil Analysis and PDC Analysis of high voltage equipment, mostly transformers, stators and power cables. She was a member of CIGRE WG D1.11 and has been a member of CIGRE WG D1.17.