slot discharge pattern of 10 kv induction motor stator coils under condition of insulation...

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IEEE Transactions on Dielectrics and Electrical Insulation Vol. 20, No. 6; December 2013 2091 1070-9878/13/$25.00 © 2013 IEEE Slot Discharge Pattern of 10 kV Induction Motor Stator Coils under Condition of Insulation Degradation Jiancheng Song, Chuanyang Li, Lingyan Lin, Zhipeng Lei, XiaoYu Bi Shanxi Key Laboratory of Coal Mining Equipment and Safety Control Taiyuan University of Technology, No.79 Yingze West Street, Taiyuan, 030024, China and Haoyuan Yang School of Electrical Engineering Xi’an Jiaotong University, Xi’an 710049, China ABSTRACT In the manufacturing process of the 10 kV induction motors, a semi-conductive layer with certain resistance covering the linear part of the stator coil is often used to suppress the partial discharges (PDs) happened in the slot. However, due to electromagnetic force while the motor is running, slot discharge is still one of the main causes of stator insulation degradation. In this paper, an adjustable width mockup core slot is designed using silicon steel sheet. Several 10 kV real machine stator coils subjected to varying aging degrees are adopted as test coils. The phase resolved partial discharge (PRPD) patterns of slot discharge are analyzed using a PRPD analyzer and the variation law of slot discharge patterns with voltage variation is illustrated. The investigation result has been applied to the online monitoring and pattern recognition of the insulation for a 10 kV induction motor in a coal mine and it has great application significance. Index Terms - Induction motors, partial discharge, discharge pattern, insulation degradation, online monitoring, pattern recognition. 1 INTRODUCTION THERE are several stresses which can affect the aging rate of stator winding insulation in 10 kV induction motor. Generally speaking, there are thermal, electrical, ambient, and mechanical stresses, as is the so-called TEAM stresses [1]. In the production process, the end-winding movement is restrained by insulating fiberglass bands to support the coils against centrifugal forces. In case that the stator coils are not manufactured properly in the production process, or not held tight enough inside the slot and in the overhang region, it will make the coils vibrate at twice the grid frequency with respect to the stator core under tremendous electromagnetic force [2, 3]. At this point, the core lamination which is extremely sharp could abrade the semi-conductive layer and further the ground wall insulation. In this case, a large air gap between the ground wall insulation and the stator core will be created. There will be displacement current flowing in the ground wall insulation and thus raise the potential in between [4]. When the potential exceeds a certain value, it can arouse gas ionization in the gap and finally result in discharge in the slot. This kind of discharge i.e. the slot discharge is quite strong and capacitive. In addition, the ionizing air can create an ozone area in the event of slot discharge. The ozone can react with nitrogen under the conditions of discharge and high temperature. This can produce nitrogen oxides which cause chemical erosion to the insulation, and this in turn can accelerate the aging of the insulation. According to statistics, it only takes less than a decade for some HV motors to be out of service due to slot discharge [5]. Figure 1 shows a stator coil of a 10 kV motor extracted from the stator core slot. It can be seen that the semi-conductive layer of the linear portion has been seriously abraded due to vibration. Therefore, it is necessary to monitor the occurrence and development process of slot discharge in order to provide early warning of faults. There are several steps in the online monitoring of slot discharge in the stator core slot. Firstly, the acquisition of the discharge signal is essential. This process is usually quite complicated. Stone has done the most excessive research in this field [6-9]. Secondly, the removal of background noise can ensure that the real PD signal is picked out from a most complicated waveform. It will determine the authenticity and the quality of the signal being collected. In this field, a great quantity of work has been done by earlier researchers [10-13]. Last but not least, the determination of the discharge type and the severity of insulation deterioration is also a crucial step. In order to recognize the discharge type and assess degradation severity, many experiments need to be performed and a large number of discharge patterns should be investigated. In this field, Hudon and his team have produced a variety of fault models in the stator winding insulation. Patterns of various PDs with respect to different outer stresses have been investigated Manuscript received on 30 January 2013, in final form 26 May 2013.

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Page 1: Slot discharge pattern of 10 kV induction motor stator coils under condition of insulation degradation

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 20, No. 6; December 2013 2091

1070-9878/13/$25.00 © 2013 IEEE

Slot Discharge Pattern of 10 kV Induction Motor Stator Coils under Condition of Insulation Degradation

Jiancheng Song, Chuanyang Li, Lingyan Lin, Zhipeng Lei, XiaoYu Bi Shanxi Key Laboratory of Coal Mining Equipment and Safety Control

Taiyuan University of Technology, No.79 Yingze West Street, Taiyuan, 030024, China

and Haoyuan Yang School of Electrical Engineering

Xi’an Jiaotong University, Xi’an 710049, China

ABSTRACTIn the manufacturing process of the 10 kV induction motors, a semi-conductive layer with certain resistance covering the linear part of the stator coil is often used to suppress the partial discharges (PDs) happened in the slot. However, due to electromagnetic force while the motor is running, slot discharge is still one of the main causes of stator insulation degradation. In this paper, an adjustable width mockup core slot is designed using silicon steel sheet. Several 10 kV real machine stator coils subjected to varying aging degrees are adopted as test coils. The phase resolved partial discharge (PRPD) patterns of slot discharge are analyzed using a PRPD analyzer and the variation law of slot discharge patterns with voltage variation is illustrated. The investigation result has been applied to the online monitoring and pattern recognition of the insulation for a 10 kV induction motor in a coal mine and it has great application significance.

Index Terms - Induction motors, partial discharge, discharge pattern, insulation degradation, online monitoring, pattern recognition.

1 INTRODUCTIONTHERE are several stresses which can affect the aging rate

of stator winding insulation in 10 kV induction motor. Generally speaking, there are thermal, electrical, ambient, and mechanical stresses, as is the so-called TEAM stresses [1]. In the production process, the end-winding movement is restrained by insulating fiberglass bands to support the coils against centrifugal forces. In case that the stator coils are not manufactured properly in the production process, or not held tight enough inside the slot and in the overhang region, it will make the coils vibrate at twice the grid frequency with respect to the stator core under tremendous electromagnetic force [2, 3]. At this point, the core lamination which is extremely sharp could abrade the semi-conductive layer and further the ground wall insulation. In this case, a large air gap between the ground wall insulation and the stator core will be created. There will be displacement current flowing in the ground wall insulation and thus raise the potential in between [4]. When the potential exceeds a certain value, it can arouse gas ionization in the gap and finally result in discharge in the slot. This kind of discharge i.e. the slot discharge is quite strong and capacitive. In addition, the ionizing air can create an ozone area in the event of slot discharge. The ozone can react with nitrogen under the conditions of discharge and high

temperature. This can produce nitrogen oxides which cause chemical erosion to the insulation, and this in turn can accelerate the aging of the insulation. According to statistics, it only takes less than a decade for some HV motors to be out of service due to slot discharge [5]. Figure 1 shows a stator coil of a 10 kV motor extracted from the stator core slot. It can be seen that the semi-conductive layer of the linear portion has been seriously abraded due to vibration. Therefore, it is necessary to monitor the occurrence and development process of slot discharge in order to provide early warning of faults.

There are several steps in the online monitoring of slot discharge in the stator core slot. Firstly, the acquisition of the discharge signal is essential. This process is usually quite complicated. Stone has done the most excessive research in this field [6-9]. Secondly, the removal of background noise can ensure that the real PD signal is picked out from a most complicated waveform. It will determine the authenticity and the quality of the signal being collected. In this field, a great quantity of work has been done by earlier researchers [10-13]. Last but not least, the determination of the discharge type and the severity of insulation deterioration is also a crucial step. In order to recognize the discharge type and assess degradation severity, many experiments need to be performed and a large number of discharge patterns should be investigated. In this field, Hudon and his team have produced a variety of fault models in the stator winding insulation. Patterns of various PDs with respect to different outer stresses have been investigated Manuscript received on 30 January 2013, in final form 26 May 2013.

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2092 J. Song et al.: Slot Discharge Pattern of 10 kV Induction Motor Stator Coils under Condition of Insulation Degradation

under specific environments [14-17]. Song and Cheng have studied the development of the PDs under multi-factor aging of the stator bars [18, 19]. However, reports on slot discharge patterns of the stator coils for 10 kV HV motor with different abrasion levels under different voltages are rarely to be seen.

The aim of this paper is to reproduce the slot discharge occurring in the 10 kV motor through a mockup stator core slot which can automatically adjust its slot width. In order to reduce background noise, the experiment is conducted in an electromagnetic shielding room with background noise below 2 pC. The real new stator coils of the 10 kV motor are adopted in order to simulate the real situation as closely as possible. The air gap between the ground wall insulation of the experiment coils and the mockup stator core slot is sized by laser beams to insure accuracy. The coils, with different abrasion levels, are embedded in the mockup stator core slot and a certain range of voltage is applied with a no PD step-up transformer. The acquisition of the discharge signal is finished by DDX9101 PD analyzer.

2 DISCHARGE MECHANISMBeing one of the strong discharge types taking place in HV

motors, the slot discharge is a typical type of dielectric barrier discharge (DBD). The schematic equivalent of slot discharge can be found in Figure 2. The DBD is the electrical discharge between two electrodes separated by an insulating dielectric barrier. According to the position of the organic insulating medium, the DBD is sorted out into two types which are: (a) the organic insulating medium suspended in the discharge space and (b) covering the electrode surface. In this case, the slot discharge taking place in HV motors is of the latter type.

When the gap is created, the potential will rise on the inner surface of the ground wall insulation. Then the electric field will exist between the ground wall insulation and the stator core which is equivalent to the ground electrode. The spatial electrons in the gap gain energy under the action of external electric field and transfer the energy to the surrounding atoms or molecules through collisions. The excitation and ionization will result in avalanche. The air in the gap will breakdown when the voltage is over the discharge inception voltage. During the breakdown process, the current flowing through the discharge passage is actually the displacement current through the organic insulation, rather than the short circuit current. Therefore, in the process of slot discharge, the current is blocked by the medium insulation (i.e. the ground wall insulation) and cannot grow freely. So it will not form a spark or arc, only showing several stable discharge filaments across the air gap. During the discharge process, because of the cumulative effect of the dielectric surface charge, the discharge filaments will have certain location rules and will be renewed and extinguished in certain positions as the voltage increases and decreases continuously. The visual effect is of several discharge filaments forming across the insulation and the stator core. Lévesque has taken local pictures of slot discharge as shown in Figure 3 to investigate the slot discharge under effect of different relative humidity [20]. However, DBD analysis was not involved. Of course, if the electrode system was changed to silicon steel sheet, the DBD phenomenon could not be visible because the silicon steel sheet is not transparent.

3 EXPERIMENTAL DESCRIPTION3.1 TEST COILS

The insulation material and the production methods of the stator coils are determined mainly by the voltage level. It has no direct relationship with the motor’s capacity and body size. In this paper, several new stator coils of the 10 kV motors are adopted as the test coils which can be seen from Figure 4. All the coils are manufactured by Shanxi Changsheng Wire Co., Ltd. The ground wall insulation material of the coil is epoxy mica with a thickness of 2.03 mm. It can well reflect the characteristics of the ground wall insulation of the 10 kV motors in domestic use and has great representative significance. The manufacturing process of the coils is shown in Figure 5:

Figure 3. Picture of the slot discharge taken at 8 kV with UV camera.

Figure 2. The schematic equivalent of the environment of slotdischarge.

Figure 1. A stator coil damaged by vibration abrasion.

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IEEE Transactions on Dielectrics and Electrical Insulation Vol. 20, No. 6; December 2013 2093

3.2 MOCKUP STATOR CORE SLOT As is shown in Figure 6, the mockup stator core slot is stacked

by industrial silicon steel sheet, with two slots and a slot tooth which can slide in between. First, several epoxy glass panels are placed in the bottom of the slot to adjust the depth. Before embedding the coil into the slot, one piece of anti-corona epoxy glass board is stacked onto the slot bottom to simulate the real situation on site. After embedding the coil into the slot, the left side of the core slot is adjusted through inserting a certain number of the silicon steel laminations on the left side. This step is to make sure that the inner wall of the slot has close contact with the coil. The wedge made of magnetic glass epoxy is knocked into the mockup slot gently with a rubber hammer. At last, to adjust the gap between the coil and the wedge, one or two pieces of epoxy anti-corona glass plate is knocked in between the wedge and the upper surface of the coil to make sure that the coil cannot move in the slot. These steps are to follow the industrial requirement strictly.

3.3 EXPERIMENTAL STEPS Before the experiment, fix all the test coils in the mockup

core slot one by one to guarantee whether they can satisfy the test standards. Elevate the voltage to 15 kV gradually and observe the distribution of the discharge pattern. If there are only internal discharge characteristics, i.e. the discharge number of the positive and negative half cycle are basically the same, and the maximum discharge amplitude is less than 200 pC, then the coil is ready for the experiment. Figure 7 shows the internal discharge pattern of an intact coil under 6 kV. This step is to exclude defective coils that can bring interference to the test results. Figure 8 shows the surface tracking phenomenon occurring at 14.5 kV during this test stage. Through attentive examination, it is found that there is a small region of epoxy paint which is not yet completely dry in the end arm, leaving some dust attached to its surface. The pattern of surface tracking disappears after removing the dust. However, as is shown in Figure 9 that a slight slot discharge phenomenon still exists. Therefore, this coil cannot be used.

The coils which meet the experimental requirements are divided into three groups, namely A, B and C. Each group contains five coils with labels attached to them as described in Table 1. The coils of the three groups are abraded with the lamination stack equipment which could be found in the schematic diagram of Figure 10. The abrasion level is divided into three grades: thickness of 0.2mm, 0.4mm and 0.8mm. It is determined from careful observation and measuring of the abrasion areas located in the aged coils withdrawn from

Figure 7. Internal discharge pattern of the intact bar under 6 kV, with the full scales of PD reaching 160 pC.

Figure 6. A photo of the test coils in the slot.

Figure 5. Manufacturing process flow chart of the stator coil.

Figure 4. Test coils.

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2094 J. Song et al.: Slot Discharge Pattern of 10 kV Induction Motor Stator Coils under Condition of Insulation Degradation

several 10 kV motors which have suffered from severe slot discharge. The adjustment of the gap size is made by using laser beams. The purpose of this step is to ensure that the artificial abrasion is the same as the real abrasion existing in HV motors as closely as possible. The artificial abrasion thickness is made to simulate the real abrasion at three levels: 1. the semi-conductive layer being completely worn out to expose the ground wall insulation; 2. the ground wall insulation being a little abraded; 3. the ground wall insulation being seriously abraded. Since there are many uncertainties such as where the abrasion area is located and how large the abrasion size is in the real situation, in this paper, a 29 mm × 29mm abrasion section in the middle of the linear section of the coil is used.

The coils with each abrasion levels are embedded separately into five mockup core slots as is shown in Figure 11. Accordingto IEEE 1434 or IEC 60034-27, when a coil is first energized, the PD is initially high and then decreases over the next 30 minutes [21, 22]. Therefore, in this paper, 6kV is applied to the coils first for 1 hour. Then the acquisition which lasts for 120s in turn is performed under 3, 4, 5, 6, 7, 8, 9 and 10 kV separately in order to get the corresponding PD patterns.

3.4 EXPERIMENT SYSTEM The experiment was carried out in the electromagnetic

shielding room with background noise no larger than 2 pC. The test system schematic diagram is shown in Figure 12. Wherein, Cx is for the test coils; R is for the protecting resistor; BPF is for the band-pass filter used to filter out the interference signal and let the power frequency (50 or 60 Hz) pass; LPF is for the no PD isolation transformer low-pass filter which is used to filter the high-frequency interference mixed from the former grade equipment; T is for a no PD inflatable test transformer which is to ensure the reliability and uniqueness of the partial discharge signal source; Cc is for an 1000 pF coupling capacitance which produces a high-frequency path for partial discharge signal; A is for the HAEFELY DDX9101 PD analyzer which can present the two dimensional p-q graph, as well as three-dimensional p-q-n graph and the maximum discharge amplitude. The control is performed by a remote computer to show the information of

Figure 11. Test diagram.

Figure 10. The schematic diagram of the coil under abrasion.

Table 1. Groups of the coils under test.

Group Coil Abrasion Conditions

A 1# - 5# Abrasion thickness of 0.2 mm B 1# - 5# Abrasion thickness of 0.4 mm C 1# - 5# Abrasion thickness of 0.8 mm

Figure 9. Discharge pattern after removing the contamination, with the full scales of PD reaching 2500 pC.

Figure 8. Discharge pattern of surface tracking, with the full scales of PD reaching 2500 pC.

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IEEE Transactions on Dielectrics and Electrical Insulation Vol. 20, No. 6; December 2013 2095

the PRPD patterns. Since the slot discharge is very wide band and of high-frequency, in this paper, the frequency bandwidth is controlled between 80-500 kHz.

4 DISCHARGE PATTERN ANALYSISFigure 13 shows the discharge pattern distribution of the

coils with the abrasion thickness of 0.2mm under 4, 6, 8 and 10 kV respectively. It can be drawn from Figure 13 that the maximum amplitude of the discharge during the negative voltage half cycle (Qmax+, i.e. pulses with positive amplitude) is almost twice as large as during the positive voltage half cycle (Qmax-, i.e. pulses with negative amplitude) under 4kV. Moreover, the discharge pattern has a sharp increase at the onset of the pattern during the negative voltage half cycle,

which is the characteristic of the slot discharge [23]. In addition, once the slot discharge takes place, the amplitude can be greater than 1000 pC, which is larger than the maximum discharge amplitude of the internal discharge. It may be observed that there is a group of discharges in the positive cycle, with rather low magnitude but with high number of occurrences. This is due to the discharge taking place in the voids between the copper and the ground wall insulation. The experiment coils must go through the compression process, during this period the different characteristics of copper and epoxy resin when heated may create voids between the copper and the ground wall insulation. However, it is of little consequence to the normal operation of the machine for the reason that the modern epoxy resin can withstand internal PD for several decades.

The discharge pulse during the negative voltage half cycle is a positive discharge (q+) with a sharp increase at the onset of the pattern all the time, with the slope remaining constant almost.

The pattern of q+ forms a certain geometric shape which is approximately like a right triangle. With higher voltage, its geometric shape changes gradually: the hypotenuse bends in the middle and the corresponding angle gradually decreases from 180 to close to 90 , but always larger than 90 °. This change makes its geometric shape vary from the former triangle and tends to form a quadrangular shape which could be mistaken for the pattern of corona discharge that occurs at the junction of the semi-conductive layer and the corona

Figure 12. The schematic diagram of partial discharge detection system.

(b) Abrasion thickness of 0.2 mm, with the full scales of PD reaching 8000pC.

(a) Abrasion thickness of 0.2 mm, with the full scales of PD reaching 6000pC.

(c) Abrasion thickness of 0.2 mm, with the full scales of PD reaching 7000pC.

10kV

(d) Abrasion thickness of 0.2 mm, with the full scales of PD reaching 5000pC.

Figure 13. Distribution of the slot discharge for stator bars with abrasion thickness of 0.2 mm under different voltage.

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2096 J. Song et al.: Slot Discharge Pattern of 10 kV Induction Motor Stator Coils under Condition of Insulation Degradation

control layer. The change is assumed to be caused by the follow reason: with higher voltage, the electric field in the air gap changes and the position of the dominant discharge shifts from between the bare ground wall insulation and the stator core slot to between the bare ground wall insulation and the edge of the semi-conductive layer. However, the mechanism of the phenomenon is to be further studied.

Meanwhile, the bottom side of the slot discharge pattern becomes longer with the elevation of the voltage. It can be illustrated from the reason that with the higher voltage, the DIV is reached earlier before that under the former voltage.

Then the discharge occurred earlier and maintains a longer phase segment.

To analyze the PD pattern with regard to the Qmax, the Qmax+is always dominant, with its amplitude 1.5 to 2 times larger than that of the Qmax-. Additionally, with the voltage increases, Qmax- tends to catch up with Qmax+. It also can be seen that Qmax is changeable with different abrasion levels under the same voltage. Qmax+ decreases with the exacerbation of the abrasion level when the voltage is more than 6 kV, which is not the case when it comes to Qmax-. When the voltage is below 6 kV, Qmax+ increases with the increasing of the voltage and reaches the top under 6-7 kV (when the abrasion level is 0.2 mm, Qmax+ is nearly 8000 pC). When the voltage is above 7 kV, Qmax+ has decreasing trend, and the decreasing rate of the Qmax+ is faster than that of the Qmax- . Therefore, it is unreliable to make the judgment only through the maximum discharge amplitude. Because the Qmax is only the max discharge which takes place in the largest void in the insulation, it cannot reflect the overall activity of the insulation system.

Figure 14 shows the pattern distribution of the coils with different abrasion levels under 10 kV. Wherein a, b and c represent the abrasion thickness of 0.2 mm, 0.4mm and 0.8 mm respectively. It can be seen from Figure 14 that the PD patterns are to shift left and the shifting distance tends to become narrower with the exacerbating of the abrasion level. The reason for this phenomenon is as follows: when the electric field in the air-gap reaches a certain value, the air in the gap will start ionizing under this field. The electric field between the mockup core slot and the ground wall insulation is created by both the outer grid voltage and the residual charge left by the former discharge. So, even if the voltage was at the zero crossing, there still exists a potential electric field in the air space. And this field, together with the rising of the voltage during the next voltage half cycle, can excite the air and finally result in slot discharge. As to the PD activity with respect to the AC cycle, more details can be found in contribution [24].

In addition to the phase shifting of the slot discharge, the amplitude of the discharge and the corresponding discharge number also follow certain laws. Figure 15 and Figure 16 show the two-dimensional and three-dimensional distribution of the discharge pattern extracted from the coils with different abrasion levels under 6 and 10 kV:

From Figures 15 and 16, the conclusion could be drawn that there is a rapid increase in the PD number of q+ once the discharge amplitude exceeds nearly 900 pC as is marked by an ellipse. It should be taken as a warning that more severe degradation has occurred in the slot. Note that the patterns ofcorona PD activity which occurs at the junction of the semi-conductive coating and stress control coating could sometimes be located in the same phase section where the rapid increase in the PD number of q+ occurred by slot discharge. Therefore, caution is required to observe the discharge during the online monitoring process. When the discharge amplitude is over 1800 pC, the number of q+ begin to decline; the discharge number of q+ is relatively low when the discharge amplitude is between 400-600 pC. It is unavoidable that voids will exist

(a) Abrasion thickness of 0.2 mm, with the full scales of PD reaching 5000pC.

(b) Abrasion thickness of 0.4 mm, with the full scales of PD reaching 4500pC.

(c) Abrasion thickness of 0.6 mm, with the full scales of PD reaching 4500pC.Figure 14. Phase shifting distribution of slot discharge of stator coils with different abrasion level under 10 kV.

Page 7: Slot discharge pattern of 10 kV induction motor stator coils under condition of insulation degradation

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 20, No. 6; December 2013 2097

in the ground wall insulation during the manufacturing process of the stator coils. The size of the voids can determine the frequency and the max amplitude of the discharge in some way. However, these kinds of discharges taking place in the voids have not enough energy to reach 400 pC according to the test of the new coils of 10 kV (phase to phase) before the experiment. Discharge amplitude less than 400 pC are largely due to the air ionization taking place in the voids within the ground wall insulation which is called internal PDs. It is caused by the high electric field density in the internal voids in the ground wall insulation.

5 CONCLUSIONA mockup stator core slot of a 10 kV induction motor was

produced in this paper. The slot discharge patterns were obtained by using the real stator coils at three abrasion levels under different test voltages. The experiment conclusions are as follows:

(1) Once slot discharge takes place, the amplitude will reach 1000 pC, and the pattern has significant characteristics which are easy to recognize.

(2) The slot discharge patterns of the stator coils with the same abrasion level changes obviously with the increasing of the voltage: the original triangle can turn to a quadrangle gradually, and the Qmax shows an increasing trend and then decrease with respect to the increasing of the voltage.

(3) Discharge inception phase shows variation laws with different abrasion levels: with increasing voltage, the discharge phase tends to shift left. The distance of the shifting will be farther with the increase of the voltage, and the shifting distance is decreased when the aging level becomes more serious under the same voltage.

(4) When the voltage is constant, the number of the discharges in a specific region of the pattern can change differently. It shows that the number of the discharge in a certain region will increase with respect to the abrasion level.

ACKNOWLEDGMENTThe authors wish to express their deepest gratitude to the

financial support of the State Major Project and the Special Program for International Cooperation Project of the Ministry of Science and Technology, which ensured the successful completion of the experiment. In addition, Michaeljohn Clement’s proofreading is highly appreciated by the authors.

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(c) Abrasion thickness of 0.8 mm, with the full scales of PD being 4500 pC.

Figure 16. Discharge patterns distribution with different abrasion level under 10 kV.

(b) Abrasion thickness of 0.4 mm, with the full scales of PD being 4500 pC.

(a) Abrasion thickness of 0.2 mm, with the full scales of PD being 5000 pC.

(c) Abrasion thickness of 0.8 mm, with the full scales of PD reaching 7000pC.

Figure 15. Discharge distribution with different abrasion level under 6 kV.

(b) Abrasion thickness of 0.4 mm, with the full scales of PD reaching 7000pC.

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2098 J. Song et al.: Slot Discharge Pattern of 10 kV Induction Motor Stator Coils under Condition of Insulation Degradation

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[22] IEC 60034-27:2006, “Rotating Electrical Machines – Part 27- PD Measurements of Rotating Machinery.

[23] C. Hudon and M. Bélec, “Study of Slot Partial Discharges in Air-cooled Generators”, IEEE Trans. Dielectr. Electr. Insul., Vol. 15, pp.1775-1690, 2008.

[24] Y. J. Kim and J. K. Nelson, “Assessment of Deterioration in Epoxy/Mica Machine Insulation”, IEEE Trans. Dielectr. Electr. Insul., Vol. 27, pp.1026-1039, 1992.

Jiancheng Song received the B.Sc. degree from Taiyuan University of Technology, China, in 1982, the M.Sc. degree from Newcastle University, England, in 1987, respectively and the Ph.D. degree from Xian Jiaotong University, China, in 1999. Currently, he is a professor of the College of Electrical and Power Engineering at Taiyuan University of Technology. He has experience in the field of condition assessment, remaining life assessment and intellectual automation technology. He has performed a number of electrical

failure investigations about coal mine. He has presented a number of technical and scientific papers at international conferences and seminars.

Chuanyang Li was born in Shandong, China, on February 6th, 1987. He received the B.S. degree from Taiyuan University of Technology, China, in 2011. He has been studying in Taiyuan University of Technology for the M.S. degree since 2011. His main research interest is condition monitoring and PD pattern recognition for HV motors and generators.

LingYan Lin was born in Shanxi, China, on 16 August 1969. She received the B.S. degree from Taiyuan University of Technology, China, in 1991, the M.S. degree from Taiyuan University of Technology, China, in 1994. Currently, she is an Associate Professor of the College of Electrical and Power Engineering at Taiyuan University of Technology.

Zhipeng Lei was born in Shanxi Province, China, on 14 April 1983. He received the B.S. degree in electrical engineering from East China Jiaotong University in 2005 and the M.S. degree in electrical machines and electrical apparatus from Taiyuan University of Technology in 2010. He is now pursuing the Ph.D. degree in Taiyuan University of Technology. His main research interest is condition assessment high voltage cable failure and associated partial discharges characteristics.

Hao Yuanyang was born in An hui, China, on 22 June 1992. He has been studying in Xi’an Jiaotong University for the B.S. degree since 2010. His main research interest is the high voltage insulation.

Xiaoyu Bi was born in Hebei, China,on 24 March 1986. She received the B.S. degree from Taiyuan University of Technology, China, in 2010. She has been studying in Taiyuan University of Technology for the M.S. degree since 2010. Her main research interest is insulation condition monitoring and fault diagnosing method of HV motors and generators.