polarization index during utilizing time in HV electrical machines – A survey

Measurement 59 (2015) 21–29

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Review

Measurement variations of insulation resistance/polarization index during utilizing time in HV electrical machines – A survey H. Torkaman ⇑, F. Karimi Faculty of Electrical and Computer Engineering, Shahid Beheshti University, A.C., Tehran 1658953571, Iran

a r t i c l e

i n f o

Article history: Received 23 April 2014 Received in revised form 9 August 2014 Accepted 16 September 2014 Available online 28 September 2014 Keywords: Insulation quality measurement Insulation resistance Polarization index Insulation faults Fault analysis High voltage electrical machine

a b s t r a c t The concept of insulation system is chiefly concerned with the stator winding lifetime of the high voltage (HV) electrical machines. Along with the insulation testing, insulation resistance (IR) and polarization index (PI) techniques are suitable for different types of HV electrical machines and transformers. Since the power plant industries have to move toward higher reliability, the IR/PI accurate analysis needs to be optimized. This precise study is necessary, since the results of resistance measurement during a period of time are variable and depend on different factors. This point of view indicates the important role of the IR/PI variations assessment over the utilization time. Therefore, this paper is an attempt to evaluate the dependency of IR/PI variations on the main profiles as; current components, insulation conditions, and constant time. For this purpose, deviation of the resistance values in the mentioned tests are presented and discussed to achieve the optimum trend of IR/PI results interpreting. A comprehensive review of literature was done and industry experts were surveyed for their ideas and experience under a research project. The main results are summarized in this paper. Ó 2014 Elsevier Ltd. All rights reserved.

Contents 1. 2. 3.

4. 5. 6. 7.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Review on insulation resistance test theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Insulation current flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Capacitive current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Conduction current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Surface leakage current. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Dielectric absorption current or polarization current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Variation of IR/PI during utilization time according to currents behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Variation of IR/PI during utilization time with respect to the insulation condition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Variation of IR/PI during utilization time with respect to constant time response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

⇑ Corresponding author. E-mail address: [email protected] (H. Torkaman). http://dx.doi.org/10.1016/j.measurement.2014.09.034 0263-2241/Ó 2014 Elsevier Ltd. All rights reserved.

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1. Introduction The reliability of high voltage electric machines depends on the reliability and efficiency of their insulation system. So that the expected life time for stator and rotor windings is in association with their insulations [1–3]. Therefore calculating and monitoring the condition of the insulation system in the operating period through inexpensive and simple methods have been a fundamental issue in these kinds of studies [4,5]. On the other hand it is needless to say that the stator insulation system is one of the most important parts in HV electrical machines in terms of the production investment, maintenance and insulation lifetime [6,7]. The collapse of stator insulation may be the result of thermal, mechanical, electrical or environmental tensions or a combination of them which are produced during their operation and lead to fault increase in the machine [8–11]. Generally there are four reasons for testing and monitoring the machines and transformers [12–14]: a. Evaluating the insulation condition and the insulation remaining life time. b. Prioritizing the repairs and maintenance. c. Testing the guarantee and contraction of the manufacturer. d. Collapse diagnosis. Generally there are almost forty methods for testing and monitoring which are used to diagnose and detect the conditions of motor and generator windings [15–18]. An appropriate solution can be chosen on the basis of some criteria such as test price, the period of keeping the machine out of service for the test, reliability, availability, applying simplicity and high accuracy [19–21]. The results of these methods’ measurement depend on different factors such as; humidity [22], air pressure [23,24], mechanical pressure [25], type of fault [26], temperature [27], raw natural gas [28], power electronic arrangements [29], insulation class [30], high and low frequency pulses [31,32], partial discharges [33,34], and corona discharges [35]. HV insulation tests divide into two types individually [36]: a. Destructive tests. b. Nondestructive tests [37]. In destructive tests, the applied voltage and electrical field intensity are higher than their rated values. So deterioration is probable in these types of tests. In such tests the operator should check carefully that the high voltage equipment are under which kinds of tension during their operating time [38,39]. These tensions are the basis of the destructive insulation tests. In nondestructive tests the applied voltage is less than its rated value or equal. So the insulation of the healthy systems should not be destroyed in these kinds of tests. These tests include measuring IR, measuring capacity and dielectric loss factor, measuring polarization and depolarization currents, measuring partial discharge (PD) [40–42].

Another subdivision is in terms of applying insulation tests which consists of online and offline types [43,44]. If a system like a generator is tested while working and connected to the load, this type of test is called online test. Offline test is done when the generator is disconnected from the load or grid. The main privilege of offline tests is that they can be run under various voltage and temperature ranges. Also with respect to the curve of voltage and temperature variations, more comprehensive evaluation of the insulation can be obtained [45]. Further destructive tests which will detect general and major faults in insulation can only be applied in offline mode. According to the given information, it can be concluded that monitoring the insulation status, continuously, is a necessary issue in order to prevent destructive faults and to remove minor faults. Thus users have to apply different types of tests according to the conditions aforementioned. In this regard, one nondestructive offline test namely as IR/PI test and its variations during utilizing time are assessed. This study helps to have precise interpretation of the resulted test values and detect the insulation status correctly. The organization of the paper is as follow: part II will have a review on IR test theory. In part III current components in insulation will be analyzed. part IV deals with analyzing IR test according to current responses then this issue will be investigated with respect to insulation condition in part V and it is going to be analyzed in part VI from the sight of time constant behavior. Finally the conclusions will be presented in part VII. 2. Review on insulation resistance test theory Insulation resistance technique [46–48] is one of the most functional testing techniques for evaluating rotor and stator windings in motors and generators. Insulation resistance shows the resistance value of the insulation between copper conductor and the zero point or the core of the electrical machine. This type of test can be applied to majority of motors and various types of windings of course except squirrel cage induction motors whose motor winding does not have any insulation to be tested. This test is successful in fault detections which are caused by pollution and contamination of winding insulation. In systems with old insulation, this test is able to detect thermal deteriorations. The ideal value for this resistance is as much as it prevents current flow from winding to the core i.e. ideally this resistance value is infinite because the insulation is meant to protect the current from flowing between the copper and the core. Practically and according to insulation characteristics, the value of this parameter is not infinite and always a limited current passes through the insulation. Often when the value of insulation resistance is low, there probably has to be a fault or a problem in the insulation. The insulation resistance amount can be resulted from applying a DC current and measuring the current with respect to V = R  I. This resistance is often measured by a metering instrument namely as Megger. Measuring the insulation resistance is often run in four techniques:

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a. Measuring insulation resistance in short time:1 min. b. Measuring dielectric absorption rate (DAR): in rate of 300 /600 . c. Measuring polarization index (PI): in 1000 /600 . d. Measuring stepped voltage [49]. Each of these measuring types can be used for insulation resistance analysis or in other word the insulation condition. Reminding that each of these test types shows different values in its own range which has to be analyzed according to the test type in order to observe and analyze the type of fault, fault occurrence and fault severity. The minimum value for insulation resistance is a relative factor, however, most of HV synchronous machine users have adapted themselves for IEEE std. 43 suggestions in which the minimum DC insulation resistance for the whole winding (in MX) can be obtained through equation below;

Rmin ½M X ¼ kV þ 1

ð1Þ

where kV is the line nominal voltage in machine. The value which has to be compared with the above number is the value which is read after applying voltage for one minute and after correction for 40 °C of temperature. Resistance in 40 °C of temperature is resulted from;

Rc ¼ K t  Rt

ð2Þ

where Rc is the corrected resistance for 40 °C, Rt is the measured resistance in ambient temperature, Kt is the correction factor. For different temperatures Kt can be obtained from the standard which is based on a fact that for every 10 °C increase of temperature insulation resistance is halved. This can be written as;

Rt ¼ ð0:5Þð40TÞk

ð3Þ

So the correction factor for each specific temperature can be calculated. The accuracy of this relation is only accepted for partial differences between the measured temperature and 40 °C. When the phases are tested one by one and other phases are grounded, the value should be divided by two. Also in case of using guard circuits in other phases the resulted values for each phase should be divided by three. 3. Insulation current flow Four types of current pass through the insulation under the electric field. Each of them represent a specific condition of the insulation and their quantities in test data analysis can be an appropriate guide for the fault type recognition. 3.1. Capacitive current The current which passes to charge the capacitance formed by the insulation between copper conductor of winding and the stator or rotor core is called capacitive charging current [50,51]. This current decreases exponentially and is reduced to zero in approximately first 10–20 s of external DC field

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applying. The capacitance of these capacitors in large generators is almost 10 nF but in large hydro generators it increases to 1 lF. As an example 3-phase to ground capacitance of the hydro generator installed in Karun4 Dam [52] (located in Iran) is equal to 3.85 lF. This quantity for 250 MW hydro generator which is installed in Gotvand-e-Olya Dam [53], in single phase to ground is equal to 0.96 lF. The single phase to ground capacitance of Ansaldo Co. [54] designed 160 MW turbo generator manufactured by ParsGen. Co. [55] is equal to 0.427 lF. 3.2. Conduction current Conduction current in insulation material [56,57] is a current that is caused by the transmission of free electrons and ions between two electrodes. This current in new insulation materials (Polyester/Epoxy-mica) is zero except when the insulation is wet. When this type of current is high in amount (i.e. the level of the insulation resistance is very low) there has to be a fault in insulation. Generally this current rate is constant as the time passes, which is ideally zero. It increases as the humidity and temperature increase. This current also flows if there is a crack, cut or a small cavity or some kinds of pollution in insulation. 3.3. Surface leakage current This kind of DC current flows on the surface of insulation or in end-winding [58,59]. The amount of this current is constant but increases when temperature and humidity increase. This kind of current is caused by the following reasons: Pollution in part of the conductor (oil or moisture combined with dust, dirt, ash, chemical substances) of winding. If there is high amount of such a current the pollution is probably caused by electrical gap. In addition, if the ambient temperature is as much as the dew point the surface resistance decreases as a layer of moisture covers the surface of bus. 3.4. Dielectric absorption current or polarization current Dielectric absorption current is caused when the insulation molecules are polarized [60–63]. When an external electric field is applied to insulation, molecules in insulation which are dispersed get aligned in the field direction. This change causes a current which reduces to zero as the time passes, with respect to the following equation:

Ipol ¼ Ktn

ð4Þ

where Ipol is polarization current and t is time. The range of this current in dry insulations in appropriate conditions may not reduce to zero for hours. But in old insulations this current in first 10–15 min and for new insulations like Polyester/Epoxy-mica for almost first 4 min reduces to zero. In high voltage laminated insulations, despite the molecules alignment, because of the trapped electrons in the gaps, absorption current may increase. Unfortunately neither of these currents can be measured directly.

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4. Variation of IR/PI during utilization time according to currents behavior As it was mentioned, applying a DC voltage to a winding insulation causes an insulation current which includes four components. Capacitive charging current, which has transient and unstable natures and during few seconds reduces to zero, absorption current, which has a damping nature but takes more time in comparison with capacitive charging current and in some cases minutes to hours are needed until it reduces to a negligible amount. The main factor of insulation current is leakage current which passes through the insulation volume and through faults and also the surface. This current has a constant behavior during utilization. According to the nature of the insulation current in which the leakage current is the most effective component, measuring insulation resistance is sensitive to humidity and surface pollution of coils (surface current) and also slots and perpendicular cracks in insulation which may be filled by pollution. This test is also sensitive to temperature and should be applied when the winding has adapted its temperature with the environment and reached the stable conditions in terms of the temperature and humidity. The results will be corrected for 40 °C. Through insulation resistance variation or total current variation during utilization, useful evaluation can be done to the insulation so that better information of cleanness and dryness can be obtained. As shown in Fig. 1, if the winding is wet or exposed to pollution total current (IT) will be constant during utilization because leakage current (IL) and conducting current (IG) are much larger than absorption current (Ia). As it is illustrated in Fig. 2, in case the winding is dry and clean, total current will normally decrease during operation because total current is affected by absorption current. In other words it can be said that four types of currents; capacitive, conducting, surface leakage and absorption form the total current, and if the applying voltage is divided by this current the resistance insulation will be

Fig. 1. Types of currents for Asphaltic-mica insulation [48].

Fig. 2. Types of currents for an Epoxy-mica insulation with a relatively low structure leakage current and no conductance current [48].

Fig. 3. Typical insulation resistance measurements for three different systems [48].

resulted. Therefore according to current behaviors and taking into account that applying voltage is constant, insulation resistance varies as the time passes. But so much attention should be paid to this point that after 1 min with complete elimination of IC the resistance value is only resulted from IG, IA and Ipol. So during operation voltage is applied first then increases slowly until it reaches a constant point, the measured insulation resistance usually increases fast (Fig. 3). This incremental trend may continue for hours with a fixed applying voltage in dry winding in appropriate conditions. For older type of insulations, it reaches a stable reasonable value during 10–15 min. For new insulations in stator winding, such as Film-coated, Epoxy-mica or Polyester-mica it reaches a constant resistance in less than four minutes. If the winding is wet and dirty a stable point will be resulted within one or two minutes after applying the test voltage. As mentioned before polarization index is defined normally as the division of insulation resistance in 10 to 1 min;

PI ¼ IR10 =IR1

ð5Þ

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Fig. 4. Variation of insulation resistance versus time of applied voltage [65].

PI indicates the slope of the characteristic curve (Fig. 3) and is able to indicate the insulation condition independent of time. In order to have accuracy in one minute, and evaluating in logarithmic coordinate, it is common to sample other ranges such as 15–30–45 s and 1, 1.5, 2, 3 to 10 min [48]. When PI is less than 1 the insulation is destructed and needs immediate repairs. This type of test is used for dry insulation systems like dry transformers, cables and electric machines, etc. 5. Variation of IR/PI during utilization time with respect to the insulation condition The study of structural and chemical changes to which the insulation has been exposed during aging is not enough and the insulation cannot be assessed accurately by this information. Therefore it is essential to evaluate the mechanism of insulation degradation during utilization [64]. Deviation in IR value during a measurement in a period of time is more reliable and applicable than its value taken instantly or separately. This is indicated in the slope of the curve which is drawn as a proportion of resistance and time. This curve is called dielectric absorption curve. In order to draw it, standard test voltage is applied for 10 min and IR values are read every minute or less. Fig. 4 shows some curves of these types which belong to some insulation systems of different conditions. If this curve reduces to a low and constant value less than three minutes, this indicates the superiority of leakage current versus absorption current which demonstrates the pollution and humidity on the surface of the insulation. In this case the winding should be cleaned completely and tested again. In case of severely wet insulations, dielectric

Fig. 5. Variation of insulation resistance versus time of insulation drying [66].

absorption curve may rise first and then fall to a lower value than the initial one in starting the test. Measured IR values are functions of insulation dryness in spite of being the function of time. Fig. 5 shows the IR variations in one minute and 10 min as a function of time in which the insulation is under the drying process. In the first period of insulation drying, the insulation temperature does not reach the vaporization temperature of the moisture inside, and therefore drying process is not started yet. So IR values in this limitation should be disregarded until the temperature reaches the drying temperature. Generally the changes in measured resistance in first few minutes depend on the insulation condition and exter-

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nal factors like pollution and humidity. This response of the insulation leads to introduce polarization index. PI is used to evaluate the cleanness and dryness and in some cases the severe collapse of the winding insulation. This parameter also depends on winding components and the insulation class. B class insulation of windings usually shows larger PI values than A class insulations. Of course on the condition that the end-windings are covered by anti-corona semi-conductor, these values will be lower a little. Industrial motor users regard the value of almost 1.5–2 as PI. The value of 1–1.5 for dirty/wet windings and the value of less than 1 belongs to a severely polluted area with humidity and the value of 2–5 often indicates dry and clean windings. High PI amounts especially in Epoxy systems are portent of the laminated insulations. Studies have shown that PI decreases as the machine ages. This may be related to the pollution penetration into the windings. So that in first years of the machine operation, cleaning the insulation has a significant effect on the PI increase. Whereas, when the machine ages and more pollution penetrates, even winding cleaning does not show its former efficiency and small changes will be found in PI. Therefore, high values of IR and PI confirm the dryness of the winding and crack exclusion in the insulation but do not necessarily confirm the health of the winding insulation. For consecutive measuring if the PI has a decrease of 25% a cleaning process is necessary. In an appropriate insulation system the IR shows increase when the voltage is applied during a period of time. On the other hand an insulation which is polluted with humidity, dirt, etc. shows low IR value in response. In a perfect insulation system, the effects of absorption current will be decreased during the time. It is significant to say that a raw IR value alone may not include so much information to judge the machine insulation system status. But ‘‘Trend Analysis’’, which is resulted from the periodic measuring in equal conditions, is more authenticated to be the basis of the assessments. Figs. 6 and 7 show the IR and PI values of 3 and 6 kV machines as a function of their operating time. Black points indicate the measured accepted values and white points

Fig. 7. Time effects on polarization index of complete windings during utilizing time [67].

Fig. 8. Variation of insulation resistance with increasing number of aging cycles during utilizing time [68].

show the values after cleaning and drying. Despite high dispersion in data, both cases show the decremental behavior. Cleaning and drying process have improved the value to some extent. As shown in Figs. 6 and 7 after 20 years of age, the insulation characteristics cannot be returned i.e. after almost 20 years, cleaning does not result a positive effect [67]. Test results on high voltage electric machines with F thermal class (155 °C) are shown in Fig. 8 [68]. As it is observed in figure, IR value tends to low amounts with increasing aging cycle. But it has to be mentioned that IR amount is still too much after 3090 h and in 215 °C of temperature (almost 100 GX) that Epoxy decomposition may be a reason for these changes. 6. Variation of IR/PI during utilization time with respect to constant time response Fig. 6. Time effects on insulation resistance of complete windings during utilizing time [67].

Analyzing different results of IR and PI for several machines shows a lot of information. Insulation resistance

H. Torkaman, F. Karimi / Measurement 59 (2015) 21–29

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Fig. 9. Insulation time constant versus winding age [69,70].

of different machines cannot be compared with each other directly. It is due to the fact that each winding has its own capacitance. That is why instead of the insulation resistance; insulation time constant is used. This metering parameter is the product of insulation resistance multiplied by winding capacitance. Fig. 9 shows insulation time constant as a function of winding age. This curve is drawn from the analyzed information of 50 electrical machines. In these machines insulation resistances are measured under the voltage of 2 KV and in 25 °C of temperature after 30 min. The marked numbers in this figure are as followed: (1) Shellac mica-folium insulation. (2) Discontinuous insulation-mica wool folium on a paper backing pre-impregnated with epoxy resin. (3) Continuous insulation-mica and glass tape vacuum impregnated with solventless thermosetting epoxy resin. As it is observed from Fig. 9, time constant of the modern and old insulation systems are classified at different levels. Windings with shellac-folium insulation are com-

pletely in dry and clean condition and their insulation time constant tends to increase as they age. First rapid rise can be related to the rest of drying and curing of insulation during the first years. Most time constants are in a range of 800–2000 after 12 years of utilization. Further analyses have shown that time constants lower than 400 belong to polluted or wet insulations. It is more common to use one minute IR to calculate time constant. Regarding PI as 3.6 for shellac system which is in proper condition, insulation time constant is between 220 s and 560 s. The amounts less than 200 s (PI almost 2) indicate polluted or humid insulations. In these calculations IR in 10 min and 30 min are assumed to be equal. The Epoxy–resin curve is not drawn completely since a short time has passed from the usage of Epoxy–resin based systems, although the measures have shown that the time constant of such systems is significantly higher than the highest amount of the same factor in shellac insulation systems. In [69] time constant behavior as a function of winding age for 11 kV machines with non-synthetic insulation system is studied by ERA Company in which 1 min IR test is used. The achieved results in Table 1 shows the

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Table 1 Relationship between insulation’s constant time with contamination level [69].

[11]

Contamination level

Insulation’s constant time (XF)

[12]

Low Normal High Severe

>400 180–400 40–180 <40

[13]

[14]

ranges which can be used as a guide line for contamination degree evaluation. It is difficult to relate part of the time constant to the pollution range. 7. Conclusion In this paper firstly in order to evaluate the insulation system condition, IR test solution was presented and it was observed that in order to analyze the insulation condition it is better to analyze the insulation response during the utilization time so that reliable and applicable results will be achieved. In this case important parameters which lead to significant variations in insulation during the operation were investigated. Some of these most important parameters such as currents flowing in insulation, insulation condition and the insulation time constant, their variation process and reasons, were analyzed. It was concluded that offline DC IR/PD test is better to be run with analyzing its variation trend during the operation of the insulation. Since the result numbers are functions of different conditions such as ambient, test and the machine specific conditions which should be taken into consideration, this study helps researchers and engineers to achieve more reliable data in HV machines insulation testing and accurate interpreting of them. References [1] S. Huang, H. Yu, Intelligent fault monitoring and diagnosis in electrical machines, Measurement 46 (9) (2013) 3640–3646. [2] N. Lahoud, J. Faucher, D. Malec, P. Maussion, Electrical aging of the insulation of low-voltage machines: model definition and test with the design of experiments, IEEE Trans. Ind. Electron. 60 (9) (2013) 4147–4155. [3] S. Grubic, J. Restrepo, T.G. Habetler, Online surge testing applied to an induction machine with emulated insulation breakdown, IEEE Trans. Ind. Appl. 49 (3) (2013) 1358–1366. [4] C. Sumereder, M. Muhr, M. Marketz, C. Rupp, M. Krüger, Unconventional diagnostic methods for testing generator stator windings, IEEE Electr. Insul. Mag. 25 (5) (2009) 18–24. [5] J. Ramirez-Niño, A. Pascacio, J. Carrillo, O. de la Torre, Monitoring network for online diagnosis of power generators, Measurement 42 (8) (2009) 1203–1213. [6] I.A. Andreev, V.V. Amosov, Y.Z. Lyakhovskii, Evaluation of the state of the insulation system of a stator winding of high-voltage electric machines based on measurements of statistical characteristics of partial discharges, Russ. Electr. Eng. 82 (4) (2011) 184–188. [7] J.J. Park, Properties of EMNC and EMNSC for insulation new material as apply to high voltage heavy electric machine, Trans. Korean Inst. Electr Eng. 61 (10) (2012) 1454–1460. [8] B. Fruth, J. Fuhr, Partial discharge pattern recognition – a tool for diagnosis and monitoring of aging, CIGRE (1990). [9] M. Farahani, H. Borsi, E. Gockenbach, M. Kaufhold, Partial discharge and dissipation factor behavior of model insulating systems for high voltage rotating machines under different stresses, IEEE Electr. Insul. Mag. 21 (5) (2005) 5–19. [10] M. Kaufhold, K. Schaefer, K. Bauer, A. Bethge, J. Risse, Interface phenomena in stator winding insulation-challenges in design,

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