Life length and stress tests of electric machines for ...1141665/FULLTEXT01.pdf · Electrical...

KTH ROYAL INSTITUTE OF TECHNOLOGY

SCHOOL OF INDUSTRIALENGINEERING AND MANAGEMENT

DEGREE PROJECT IN MATERIAL SCIENCE AND ENGINEERING, SECOND CYCLE, 30 CREDITS STOCKHOLM, SWEDEN 2017

Life length and stress tests of electric machines for electric vehicles

RAÚL SANZ DESCO

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Abstract

Electrical machines have been widely used along the last decades with large life length under operating conditions. However, they will become more important in the upcoming years because of the emerging electric car industry. Thus, the maintenance cost of this technology can be reduced by extending the lifetime in the electrical machines. Despite the fact that existing numerous studies within the life length in these devices, only few study the effect of the thermomechanical stresses of insulation.

The core of this master thesis is to study the influence of these stresses in the insulation material of a winding. The tested electrical machines were subjected to different test conditions, allowing to analyse multiple aging effects in the winding. To achieve these effects, power cycling tests were carried out on stators, where the windings were tested in cycles with different ΔT and two cooling methods: air cooling and oil cooling.

The results showed large aging differences between the two cooling methods employed. The aging effect in the oil cooling method was higher than in the air cooling method for the same number of cycles. However, the aging effects regarding the same cooling process had not wide differences between the different test temperatures.

Keywords: Electrical machines, winding insulation, power cycling, thermomechanical stresses, winding cooling.

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Preface

This master thesis was conducted in Scania CV AB, at the Environmental Testing and Electromechanics group (RECT), in collaboration with Royal Institute of Technology (KTH). The study was developed under the supervision of Dr. Samer Shisha to whom I want to give my most honest gratitude for his guidance, continuous feedback and support, but especially for the opportunity of this project. My deep gratitude to the examiner, Prof. Mikael Hedenqvist who always had time to assist me. Special thanks to Dr. Maria Conde for her valuable help along the project, and to the entire RECT group for their support and help. I would additionally wish to thank Jan Hellgren tips every time I required it. Finally, huge gratitude to Claudia, my family and friends for the support received along this project.

Stockholm, September. 2017.

Raúl Sanz Desco

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Notations

𝐼 Current [A]

𝑇 Temperature [°C]

𝑡 Time [s]

P 𝑃 Power [W]

𝑉 Voltage [V]

𝑅 Resistance [Ώ]

𝑅0 Resistance for 20ºC[Ώ]

α Thermal coefficient

𝐿 Lifetime [h]

𝐸𝑎 Activation energy [𝑒𝑉]

𝑘 Boltzmann constant [𝑒𝑉/𝐾]

𝐴𝐹 Acceleration factor

𝑚 Coffin-Manson

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Abbreviations

EM Electrical Machine

TC Thermal Chamber

ΔT Difference of Temperatures [K]

PCT Power Cycling Test

ACPC Air cooling Power Cycling test

OCPC Oil Cooling Power Cycling test

FTIR Fourier-Transform Infrared Spectroscopy

ICE Internal Combustion Engine

SCPI Standard Commands for Programmable Instruments

CL Control Loop

ML Measurement Loop

RCL Relay Control Loop

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Contents

Abstract ................................................................................................................... i

Preface ................................................................................................................... iii

Notations ................................................................................................................ v

Abbreviations ........................................................................................................ vii

1 Introduction ................................................................................................... 1

1.1 Motivation .................................................................................................. 1

1.2 Social and ethical implications ..................................................................... 1

1.3 Aim and scope ............................................................................................. 2

1.4 Methodology ............................................................................................... 2

1.5 Assumptions and limitations ....................................................................... 3

1.6 Outline of the thesis .................................................................................... 3

2 Literature Review and State of the Art ............................................................ 5

2.1 Electrical machines ...................................................................................... 5

2.1.1 Stator .............................................................................................. 6

2.1.2 Electrical theory .............................................................................. 9

2.2 Stress aging types in electrical machines .................................................... 10

2.2.1 Thermal stresses ............................................................................ 10

2.2.2 Thermomechanical stress ............................................................... 11

2.2.3 Other stresses ................................................................................ 12

2.3 Accelerated aging tests .............................................................................. 12

2.3.1 Power cycling tests ........................................................................ 13

2.3.2 Endurance tests ............................................................................. 13

2.3.3 Electrical endurance tests .............................................................. 13

2.3.4 Multifactor stress tests .................................................................. 14

2.4 Diagnosis tests ........................................................................................... 14

2.4.1 Non-destructive tests ..................................................................... 15

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2.4.1.2 Insulation capacitance .................................................................. 15

2.4.2 Destructive tests ............................................................................ 16

3 Power cycling test methodology and setup ...................................................... 17

3.1 Introduction .............................................................................................. 17

3.2 Stator preparation ..................................................................................... 17

3.3 Test setup ................................................................................................. 19

3.3.1 Temperature test control ............................................................... 21

3.3.2 Power supply ................................................................................. 21

4 Air Cooling Power Cycle ................................................................................ 23

4.1 Temperature ranges .................................................................................. 25

4.1.1 Thermal chamber temperature ...................................................... 27

4.2 Fans .......................................................................................................... 28

5 Oil Cooling Power Cycle ................................................................................ 29

5.1 Temperature ranges .................................................................................. 32

6 LabVIEW Model ........................................................................................... 35

6.1 Introduction .............................................................................................. 35

6.2 Model Design ............................................................................................ 35

6.3 Safety measures ......................................................................................... 36

7 Results and discussion ................................................................................... 37

7.1 Introduction .............................................................................................. 37

7.2 Non-destructive tests ................................................................................. 37

7.2.1 Insulation resistance ...................................................................... 38

7.2.2 Insulation capacitance ................................................................... 40

7.3 Destructive tests ....................................................................................... 42

7.3.1 Water fault detection ..................................................................... 42

7.3.2 Camera inspection ......................................................................... 43

7.3.3 FTIR ............................................................................................. 45

8 Conclusions ................................................................................................... 49

8.1 Conclusions ............................................................................................... 49

8.2 Future work .............................................................................................. 50

References .............................................................................................................. 53

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1 Introduction

1.1 Motivation

Electrical Machines (EMs) convert mechanical energy into electrical energy working as generators. Alternatively, the opposite conversion is accomplished when working as motors. Nowadays, they are essential to our daily life, being present in nearly any application such as industrial, commercial and domestic ones. For this reason, achieving the maximum output of these machines has been the topic of numerous investigations. As a result, there is already extensive expertise on optimising these devices.

On the other hand, the profitable application of a product is the basic goal of its design. In that sense, the life cycle costs are decisive. When it comes to EMs, understanding how their real use affects their lifespan will not only reduce maintenance costs, but also production costs. This is because they are designed to be on the safe side and generally, this safety entails extra costs. Thus, determining the most relevant factors involved in the service life of the machine would avoid its oversizing.

In this context, the number of cycles EM can withstand during its lifespan together with the upper temperatures reached in service are considered crucial. A significant amount of publications addresses the topic of degradation of the materials subjected to specific conditions and temperatures. However, few of them focus on the degradation caused by the actual use of an EM. Indeed, it is remarkable the general absence in knowledge and research of life cycle optimization.

1.2 Social and ethical implications

Beyond the justification of the life length studies of engineering applications in EMs, many other benefits can be derived for the society and environment. For example, a longer life length in EMs will reduce the waste generation. In addition, a longer lifespan would reduce not only the energy demand and waste derived from EM, but also everything related with them.

Chapter

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On the other hand, the objective of the carried studies could help to reduce the price in all types of EM, since the time that they are used would be longer than before. All these benefits would help to reduce prices in many areas and the cost for the automotive sector would be especially reduced because they are on the path of electrification. In this way, a reduction of the cost and a longer life length in EM will increase significantly its commercial value. Because of this, a better technology would be available for the lower price.

1.3 Aim and scope

The overall aim of this thesis is to investigate the life length variation of EMs depending on the conditions to which they are subjected. The scope of this project is limited to the stress caused in the stator winding insulation of such EMs due to the temperature variation reached in the winding during its operation. More specifically, the objectives are to:

• Investigate the effect of different winding cooling methods in EM

• Determine the most relevant operation aspects affecting the condition of the insulation

• Ascertain the best approach to evaluate the insulation winding state in power cycling tests.

1.4 Methodology

In this master thesis, EMs were tested to analyse the impact of stress factors on the winding insulation.

Firstly, a complete literature review was carried out. Throughout this report, some of the most pertinent insulation test procedures and aging effects are presented, emphasising in those literature related to testing insulation methods and diagnosis insulation test for windings.

The second staged consisted in preparing and testing of EMs with various methods. The analyses were carried out with the stators of several EMs. Furthermore, different solutions were designed to handle the problems encountered during the testing and diagnosis process of the insulation.

Finally, the last stage of the work consisted in studying the influence of different aspects on the stator winding insulation of the tested EMs. After having analysed and compared the results obtained in the conducted tests, different conclusions about the insulation effects in windings were obtained.

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1.5 Assumptions and limitations

As it will be explained in the following chapters, the winding insulation tests were carried out in big detail. Nevertheless, the study relies on the following assumptions:

• The temperature was the same in the three phases of the winding

• The temperature was constant in each single phase

On the other hand, the testing methods required of a lot of time to reach the wished failure in the insulation. In addition, the preparation and setup for each test required a lengthy period of time, limiting the time available for the testing itself. For this reason, the project was focused only in the Power Cycling Test (PCT) for the stator insulation.

1.6 Outline of the thesis

The master thesis report is organized as follows: Chapter 2 provides some important knowledge about electrical machines and winding insulation tests necessary to understand the overall study, thus, it contains the research methodology. Chapter 3 includes the preparations required for the PCTs. Chapter 4 and Chapter 5 contain the main part of the work, which consists in the methodology followed to carry out the Air Cooling Power Cycling (ACPC) and the Oil Cooling Power Cycling (OCPC) respectively. Chapter 6 describes the LabVIEW [1] model designed for control and measurement. Chapter 7 presents the analysis and results achieved in the project. Finally, conclusions and recommendations for future work are provided in Chapter 8.

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2 Literature Review and State of the Art

In this section, theoretical research regarding EMs, stress tests and diagnosis tests in EM insulation windings was performed. In addition, there are defined all the necessary terms for a general understanding of the project.

2.1 Electrical machines

EMs are very important in each daily-life application because have an extensive working range and permit a high control of the delivered power. In addition, the EM offers many advantages against other forms of motors like the Internal Combustion Engine (ICE). In this way, the EM results the best possible option for delivering mechanical or electrical energy.

EMs have high efficiency when they work either as generators or motors. The standard motors operate in efficiencies between 83% and 92% of their maximum power [2]. However, there are energy-efficient motors with better performance. For example, electrical generators can achieve high efficiencies, reaching values close to 99% in high power applications [3].

An EM is composed of two main parts, stator and rotor. The stator is the fixed part. Working as a motor, the stator induces a rotary magnetic that turn the rotor. On the other handling, working as a generator, the rotor induces a rotary magnetic field, enabling the stator to generate energy. In this context, EM can deliver electrical or mechanical power, when they are working as generators or motors respectively [4].

It is important to remark that this project only contemplates the stator part of an EM. They are used in many different applications. Owing to the expensive price of propulsion EMs and their hand difficulties, alternators were chosen to be tested. More specifically, the EM studied were 24V alternators, which function in vehicles providing them with electrical power. Because of their small size, alternators are easier to handle in all aspects and they have similar structure as the other EMs. In addition, they are cheaper than other EMs. Figure 2.1 shows the alternator studied.

Chapter

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Figure 2.1: Tested alternator

2.1.1 Stator

As said before, the stator was the component investigated in this master thesis, and thereby the rotor was not tested. Figure 2.2 shows the stator and the rotor of the EM. The stator has got a star connection of the three phases, which details and testing procedures are explained in section 3.2.

Figure 2.2: Stator and rotor components. The left component is the stator used for the tests

The stator is composed of two main parts. The first of them is the winding, which is made of copper covered with insulation. The second part is the stator core, it holds the winding in the stator.

In Figure 2.3 and Figure 2.4, the detailed stator components can be seen.

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Figure 2.3: Cross section of the stator (sketch)

Figure 2.4: Cross section of the stator

2.1.1.1 Stator insulation

Regarding the insulation used in the stator winding, it was composed of two different materials:

• Aromatic polyamide varnish: Enamelled winding wire with outstanding thermal properties. The insulation is based on aromatic polyimide enamel for a temperature of 240ºC and it is used as a turn insulation [5]. Aromatic polyamides are considered high-performance materials due to their strong thermal and mechanical properties. They can be cast into varnishes with cut-resistant and high tensile strength properties, and they provide a fantastic electrical insulation. Thus, aromatic polyamides are used in a wide number of applications [6].

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• ISONOM NHN 0885: class H rated for a maximum temperature of 180ºC. It is Nomex-Kapton-Nomex laminate. The NHN 0885 is used as a ground wall insulation in low or medium voltage electrical motors. It separates and protects the copper conductors from the stator core. In addition, the ground wall insulation holds the copper conductors and it prevents the conductors from vibrating in response to the magnetic forces [7]. The ground wall insulation is composed by three insulation layers. The materials used are Nomex-Kapton-Nomex. The layout of these materials is performed in order to achieve electrical insulation between the winding and the stator core and preventing damages in the turn insulation. The electrical insulation is provided by the Kapton material. Kapton is used in a large number of applications as a thermal and electrical insulation [8]. In addition, it has low dielectric constant (∈𝑟 ≈ 3,4) with high dielectric strength. In this way, this material can withstand high electric fields without experience a breakdown [9]. On the other hand, Nomex is a material used as a fire-resistant material in high temperature applications. It can withstand high temperatures and even fire flames. Besides, Nomex combines high mechanical and chemical resistance [10]. Thus, with the sandwich configuration, the two layers of Nomex protect the Kapton material layer from high operating temperatures and external agents, guaranteeing an electrical insulation between conductors and ground in any conditions.

The winding insulation materials can be understood in Figure 2.5.

Figure 2.5: Winding insulation materials. Materials identified in Table 2.1

Table 2.1: Material and element identification of Figure 2.5

Element number Description

1 Aromatic polyamide varnish (Turn insulation)

3 1

2

4

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2 ISONOM NHN 0885 (Ground wall insulation) 3 Stator core 4 Copper (Conductors)

2.1.2 Electrical theory

Basic electrical knowledge will be necessary to understand some sections in the following chapters. This section summarizes the electrical principles used to measure and to raise temperatures in the stator winding of EM.

2.1.2.1 Ohms law

The ohms law relates current (𝐼), resistance (𝑅) and voltage (𝑉) in as equation (2.1) [11].

𝑉 = 𝑅 ∗ 𝐼 (2.1)

2.1.2.2 Temperature dependence

The electrical resistivity of most materials changes with temperature. In some materials, a linear approximation can be used. Hence, the temperature of a material and its electrical resistance can be related with equation (2.2) [12].

𝑅𝑇 = 𝑅0 ∗ (1 + 𝛼 ∗ (𝑇 − 𝑇0)) (2.2)

Where 𝑅T relates the resistance for a certain temperature, 𝑅0 relates the resistance for 20°C, 𝛼 is the thermal coefficient, 𝑇 is the real temperature and 𝑇0 is 20°C. For copper, 𝛼 is 0,00393 at 𝑇0 = 20°C.

2.1.2.3 Heat electrical generation

Heat generation is an effect produced by load losses when the current run through a conductor. It can be expressed as equation (2.3).

𝑃 = 𝑅 ∗ 𝐼2 (2.3)

Where 𝑃 is the power generation, 𝑅 express resistance and 𝐼 represents current. That heat generation effect was used during the tests to raise the temperature in the stator winding with DC current. No other heat sources were needed to warm the conductors.

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2.2 Stress aging types in electrical machines

During the testing process, some stresses were induced to test the windings in order to produce aging in the insulation. In this section, the main stress types affecting the winding insulation in EM are described. These stresses are mainly caused by the temperatures reached in operating conditions.

The stresses in the insulation can be divided into two types: constant and transient. In constant stresses, the time to get the failure depends on the operating time of the EM. On the other hand, when the failure is originated by transient stresses, the failure time depends on the number of transients experienced by the EM. However, the stresses presented in this section are only constant because effects reached in the project were constant stresses [4].

2.2.1 Thermal stresses

Thermal stresses are produced owing to the high temperatures reached in the insulation material. The high temperatures cause oxidation in the insulation of EMs when they operate over the threshold material temperature. In this way, the oxidation produces brittle insulation and causes delamination in form-wound coil ground walls. Nevertheless, high temperatures can also be favourable for the insulation. For example, these high operating temperatures can swell the insulation reducing thereby the dimensions of any air pockets in the material and they decrease the partial discharge activity. In addition, in some cases, these high temperatures can also prevent moisture from settling on windings and it can reduce the risk of electrical tracking failures [4].

2.2.1.1 Arrhenius rate law

Arrhenius law describes the oxidation rate with a first-order chemical reaction. This effect defines the impact of temperatures over equilibrium constants in chemical reactions. For aging tests, this relation can be calculated by equation (2.4).

𝐿 = 𝐴 ∗ 𝑒−𝐸𝑎/(𝑘∗𝑇) (2.4)

Where L is the lifetime of the insulation in hours, A is a constant determined by test, 𝐸𝑎 is activation energy (eV), T is the temperature in Kelvin and 𝑘 is the Boltzmann constant (8.61 × 10−5 𝑒𝑉/𝐾). The equation is useful to correlating time at an operating temperature with time at a different testing temperature. According to equation (2.4), the lifetime in the insulation is reduced around 50% for each 10ºC increase. However, each insulation material has got a threshold operating temperature and the insulation degradation below the threshold is not very significant. These temperature limits are indicated by insulation thermal classifications [13].

2.2.1.2 Insulation classification

The insulation material is a substance used to provide electric isolation in conductors. Thus, the electrical conductivity is quite low in such materials. Depending on the

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application requirements, the material used must stand a specific limit temperature. As said in 2.2.1.1, a faster degradation of the material starts over this temperature.

In order to provide a clear temperature limit, IEC 60085 classify the insulation materials in different groups, as seen in Table 2.2.

Table 2.2: Thermal classification of rotating machine insulation materials [14]

Letter Classification

Numerical Classification

Temperature (°C)

A 105 105

B 130 130

F 155 155

H 180 180

N 200 200

According to IEC 60085, the group indices indicate the materials serviceability for 20,000 hours (approximately 3 years) operating in the classified temperatures. For instance, a class F insulation should be able to hold 20,000 h at 155°C.

2.2.2 Thermomechanical stress

Thermomechanical stresses are produced by temperature differences (ΔT) in windings. The insulation has lower coefficient of thermal expansion than the copper. Thus, when the winding temperature raises fast to high temperatures, the copper conductor expands axially. It results in shear stresses between insulation and copper. After many cycles, it may occur that the link between them delaminates. Thus, a higher ΔT in operation cycles will produce a faster insulation break.

2.2.2.1 Coffin-Manson

The aging effect in thermomechanical stresses resulted from PCTs is given by an Acceleration Factor (AF). This factor, defined in equation (2.5), relates de life service at normal operating conditions and tested conditions.

𝐴𝐹 = (∆𝑇𝑡𝑒𝑠𝑡/∆𝑇𝑢𝑠𝑒)𝑚 (2.5)

Where, ∆𝑇𝑡𝑒𝑠𝑡 is the temperature difference in the carried tests, ∆𝑇𝑢𝑠𝑒 is the temperature difference in operating conditions and 𝑚 is the Coffin-Manson exponent that depends on the material. Slight alterations of such exponent yield large cycles variations and therefore, it should be carefully examined.

The number of operating cycles can be obtained from equation (2.6).

𝑡𝑒𝑠𝑡 𝑐𝑜𝑛𝑑𝑖𝑡𝑖𝑜𝑛𝑠 𝑐𝑦𝑐𝑙𝑒𝑠 ∗ 𝐴𝐹 = 𝑜𝑝𝑒𝑟𝑎𝑡𝑖𝑛𝑔 𝑐𝑜𝑛𝑑𝑖𝑡𝑖𝑜𝑛𝑠 𝑐𝑦𝑐𝑙𝑒𝑠 (2.6)

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Note that the previous equations are only applicable to cases where the threshold temperature of the insulation material is never exceeded. In the event that the aforementioned condition is not satisfied, it results necessary to consider other aging effects to predict the aging of the material [15].

2.2.3 Other stresses

While this project focuses on thermal and thermomechanical stresses, other types of stresses may also affect the degradation of the winding insulation. This subsection describes some of the relevant ones, which are: electrical, ambient and mechanical stresses [4]. Due to the tested conditions, those stresses were neglected in all tests. The aging tests of the project where completed under laboratory conditions with DC currents, without ambient stress factors and with non-movable parts. For these reasons, the following stresses where not considered:

• Electrical stresses: They are affected by power frequencies. However, the aging effects of these stresses are very low in stator windings when they work with less than 1000V of voltage operation. Accordingly, the insulation thickness is firstly determined by mechanical aspects.

• Ambient stresses: Originated from environmental factors present in the location of the EM, such as moisture, oil, aggressive chemicals, abrasive particles and radiation. In some cases, the existing aging process may be caused by some of them together.

• Mechanical stresses: The forces caused by the power frequencies that are acting in the stator winding can produce vibrations in the coils. Subsequently, the ground wall insulation is eroded.

2.3 Accelerated aging tests

The objective of the accelerating tests is to reach stresses in the materials faster than in normal service conditions. They allow evaluating material failures in a short period of time. They are particularly useful for components of which life length is excessively long for being tested during the entire time. In those cases, the accelerated tests can give much information about the behaviour in real service. Moreover, such tests show the weak parts of the components in their future real conditions.

There are two approaches available for evaluating the life length of a material drawing on these accelerated tests:

1. Accelerated tests and equations: it is based in the comparison between the results obtained from the accelerated tests and those calculated by equations. The real behaviour of the insulation in real operation can thereby be determined.

2. Comparing testing results: a tested new material can be compared with a reference material that has been proven to offer satisfactory service in real

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processes. When both materials have been tested with the same method, the results can show the improvements or vulnerabilities of the new material.

In this section, some accelerated aging tests applicable to an EM are explained in detail. The methods described belong to the group previously denoted as 1, which provide a great deal of information concerning insulation failures.

2.3.1 Power cycling tests

PCTs try to simulate the effect of large load changes produced in the real operation of EM. Power variations produce fast temperature changes in the winding and it causes thermomechanical stresses in the insulation. This effect is described in section 2.2.2 and it causes gradually the cracking of the insulation material [4].

The standardised power cycling procedure is described in IEEE 1310. Where the lowest temperature is 40ºC and the highest ones is given by the insulation classification (see Table 2.2), to avoid thermal aging.

As said in section 2.2.1, temperatures exceeding the insulation classification limit of a material accelerates the aging effects. This may be attributed to different stresses originated by thermomechanical faults. Thus, in order to prevent such stresses from appearing, the threshold temperature shall be respected in all the cases.

According to [4], the preparation and completing of PCTs require a significant amount of resources in terms of energy and time. Obviously, that implies also important costs. Furthermore, these tests are to be complemented with diagnosis tests in order to analyse the state of the material after certain number of cycles. The explanation rests in the excessive length of time that some materials need for reaching its failure.

As a final remark, and due to the generalised lack of literature about thermomechanical stresses in EM insulation, this project only contemplates the PCT for evaluating the aging in the insulation material.

2.3.2 Endurance tests

Endurance tests are based in exposing the insulation material to high temperatures during a long period of time, simulating thereby the EM in high power operating conditions. The performance behaviour of a material under high temperatures can be estimated by equation (2.4). Consequently, the life length of the material decreases by increasing the tested temperatures in endurance tests [4]. In addition, this method is used to determine the insulation classification in Table 2.2.

2.3.3 Electrical endurance tests

The function of the electrical insulation is the prevention of short-circuits between conductors or conductor and core. Thus, the capacity to withstand voltage should be measured in tests. To predict the effect of the high voltage over the stator insulation,

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there are some standardized methods. These methods are used in windings prepared to work up to 1000V. They help to determine the best insulation system against voltage stresses. Nevertheless, electrical endurance tests are good to see the effect of voltage in the slot section of a coil, but they are not able to harm in the end-winding [4]. Regarding the standard procedures of these methods, they can be consulted in IEEE 1043-1996.

2.3.4 Multifactor stress tests

As said in section 2.2.3, some failures can be produced by some stress factors simultaneously. In order to duplicate these failures, several stresses must be applied. These tests are the multifactor accelerated aging tests. To complete one of the multifactor tests, many stresses are simultaneously or sequentially applied over the material.

There are many documents concerning general procedures that should be employed in multifactor aging tests. However, the exact test measures have not been standardised, but some of the principles according to them are collected in the standards IEEE 1776 and IEC 60034-18-33.

2.4 Diagnosis tests

The purpose of diagnosis methods is to evaluate the insulation condition without the creation of any damage in the insulation material. These tests are necessaries to undertake assessments and comparisons between the insulation winding state and the conducted aging methods. In this way, they present the aging effects over winding insulation for the different tests. In order to achieve this, diagnosis tests must be completed before and after each aging test.

There are different diagnosis tests methods available to analyse the condition variations in the winding insulation. They can be classified in non-destructive and destructive tests. In this way, non-destructive tests do not affect the insulation material and PCT can be carried out afterwards. On the other hand, destructive tests damage the insulation material and PCT cannot be done after the tests. Each group comprises the following tests:

• Non-destructive tests: Insulation resistance and insulation capacitance.

• Destructive tests: Water fault detection, Fourier-Transform Infrared Spectroscopy (FTIR) and microscopy analysis.

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2.4.1 Non-destructive tests

2.4.1.1 Insulation resistance

The goal of the insulation is to avoid electrical connection between the winding conductors. Accordingly, the insulation resistance must be measured to detect some variations in the material conditions and it should be tested with a megger. Such megger can measure large resistances between its contacts applying high voltages, allowing the insulation resistance measurements.

In this project, two different measurement alternatives were completed: phase/phase and phase/core. The insulation resistances were evaluated between the following stator elements indicated in Figure 2.6.

Figure 2.6: Stator parts involved in the measurements

• Phase - Phase: Stator resistances were measured between the three stator phases in order to detect variations in the insulation.

• Phase - Core: Stator resistances were also measured between each phase and stator core.

On the other hand, the applied voltage in the test can break the insulation material if it exceeds the limit. To prevent this from happening, the voltage used for the measurements must be below a certain number depending on the operation voltage in the EM. For this project, the EM operated in normal conditions with 24 V.

According to [4], the voltage applied in the insulation resistance tests must be 500 V for EM with operation voltages below 100 V, in order to prevent damages in the material.

2.4.1.2 Insulation capacitance

This test is used to measure the variations of capacitance because of changes in the winding insulation. As in the previous case, the measurements can be accomplished in two different alternatives, phase - phase and phase - core. Thus, the test was conducted with the same 6 measurements as the insulation resistance (see Figure 2.6).

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2.4.2 Destructive tests

The insulation material can show some visual changes or other effects, such as colour change or roughness surface variations. These differences in the material can show aging effect, but they cannot display the performance drop in the insulation.

Due to this fact, different material analysis must be used to evaluate the real aging of the insulation. Some useful methods in material analysis are provided in the following sections.

2.4.2.1 Water fault detection

Water immersion test can provide very useful information about cracks in the insulation material. After having detected a crack, it can be located in order to analyse the material in this area. Water immersion test is one method that makes possible the detection of cracks in the insulation material. It should be used when the insulation failure is detected to find the insulation break in the winding because the method destructive to the objects.

This diagnosis test only needs simple steps to be completed. First, a salt-water solution is obtained by mixing water, salt and soap. Then, the stator must be placed in the solution while a voltage difference is applied between the phases where the break is detected. It produces bubbles in the insulation crack area. This happens because of the short circuit produced in the winding. In addition, a corrosion takes place in the stator core.

2.4.2.2 FTIR

FTIR is a usual technique used to characterize organic materials. FTIR uses the infrared radiation for recognising chemicals in the aged material, collecting high spectral resolution data. Nevertheless, FTIR have some limitations when it analyses complex multi component materials. In these cases, the absorption bands from other chemicals can interfere in the results [16]. The FTIR is very useful for recognizing differences in the winding insulation material. It enables comparison between the insulation in the new and tested stators, providing valuable information.

2.4.2.3 Microscopy analysis

Microscopy allows the analysis in detail of the insulation material, because the image size can be increased considerably. Comparing the insulation microscopy images of the tested windings with new windings, it enables to observe differences between both materials that were impossible previously. For example, it can be used to detect delamination in the material. In that sense, the microscopy analysis provides large amount of details regarding aging effects [17].

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3 Power cycling test methodology and setup

3.1 Introduction

This master thesis was focused in the PCT. More precisely, two different tests were conducted depending on the cooling method considered:

1. Air cooling 2. Oil cooling

After each aging test, diagnosis tests were conducted to obtain the insulation conditions. Comparing both cooling methods allows evaluating the effect of each cooling method over the insulation material. Moreover, testing different ΔT in cycles with the same cooling method provided information about the thermomechanical stress aging effect.

3.2 Stator preparation

EMs can have different phase distributions in the winding. The machine used in this study was a claw-pole EM as shown in Figure 2.2. It is the rotor that shows it is a claw-pole machine. The stator is no different from other stators of conventional machines (see Figure 3.1). This specific EM had a three phases star connection, which means that the three phases of the winding were connected to a neutral point (see Figure 3.2) [4].

Chapter

18

Figure 3.1: Stator before splitting phases Figure 3.2: Star phases configuration

According to that phase configuration, it was impossible to warm the three phases of a stator with current if the neutral point was not split. It can be seen in Figure 3.3. In this case, the neutral point was a copper bar joining the three phases.

Figure 3.3: Stator neutral bar

To conduct the test, the neutral point of the stator was split to make the three phases independent. Then, the phases were connected in series layout to let the same current run through them during the tests. This configuration enables to increase the temperature simultaneously in the whole winding, because the same current was running through the phases. Consequently, the same heat power generation was produced in each single phase.

The result of split the neutral bar and the series configuration sketch of phases can be seen in Figure 3.4 and Figure 3.5 respectively. The neutral bar in detail after its division is shown in Figure 3.6.

Neutral bar

ΔV

19

Figure 3.4: Stator with split phases

Figure 3.5: Split phases sketch

Figure 3.6: Neutral bar division

3.3 Test setup

PCT are long and require a considerable amount of time for the preparation, the calibration and the diagnosis between a certain number of cycles. Besides, the life length for the power cycling of an EM is hard to determine without testing. For this reason, the PCT were completed with four stators simultaneously. In this way, it was possible to save time and energy.

In order to run the four stators at the same time, they were connected in series. Thus, the 12 phases had the same current through them while the test was running. The phases layout can be seen in Figure 3.7.

Neutral bar

removed

20

Figure 3.7: 4 stator phases layout

Figure 3.8 show the 4 stators series configuration during the ACPC.

Figure 3.8: 4 stator air cooling configurations

As can be easily observed in Figure 3.8, this setup of the alternators allowed to test faster than a single stator, but it required a longer preparation, diagnosis and calibration time. In addition, the 4-stator series configuration required a more complex control of the test compared to 1-stator test. In this manner, the tests were running as fast as possible with the available resources. Note that all the EM tested belonged to the same production batch and therefore they were identical, allowing an accurate comparison of the results.

21

3.3.1 Temperature test control

As mentioned before in the limitations of the project, the temperature readings from the stators phases were considered constant in each conductor. Moreover, the temperature in the winding was considered as an average of the three stator measurements.

Since temperatures in conductors were not easy to extract, several procedures were considered to achieve these readings successfully. Some of them, such the use of thermocouples and thermal cameras, were rapidly discarded. That was because the thermal inertia could affect the measurements. For this reason, the only method contemplated for the temperature readings was the phase winding resistance.

This method is based on the phase voltage drop in each stator phase. As previously explained in section 2.1.2.1, knowing the voltage drop in a conductor and the current through it, the resistance can be obtained using Ohms law. Thereupon, the phase temperature was obtained from the phase resistance according to equation (2.2). To obtain all the necessary parameters involved in this equation, two different stator calibrations methods were considered for each PCT.

• 1st calibration: consisted in setting the temperature of the Thermal Chamber [TC] in four different temperatures and measuring the resistance of each phase in the stators. Afterward, the thermal coefficient (α) of each phase was obtained from the resistances and the temperatures used in this calibration. The thermal coefficient resulted from equation (3.1) using the measured resistances in each temperature.

𝑅𝑇 = 𝑅0 ∗ (1 + 𝛼 ∗ (𝑇 − 𝑇0)) (3.1)

The value of α considered for each alternator was the average of all its three phases. All of them were around 0,0039 as expected at the beginning of the calibration, ensuring that no differences were detected between the four stators tested.

• 2nd calibration: this calibration was done before each test sequence. It used current through the phases to measure the resistance in a temperature of 20ºC. This process set the 𝑅0 for the equation (2.2) of each phase. Thus, the real temperature (𝑇) during the tests was obtained using all the other variables included in equation (2.2). This calibration was especially important because small resistance variations modified on a large scale the temperature measurements. For this reason, real-time calibration and the implemented filters were necessary in the LabVIEW model to correct wrong data. The significance of these features will be highlighted and explained in section 6.2.

3.3.2 Power supply

During the PCTs, four different power supplies were used. Via LabVIEW [1], one of the power supplies was real time controlled with Standard Commands for

22

Programmable Instruments (SCPI). These commands set the power supply settings twice per cycle to produce either heating or cooling, but providing continuous current through all the phases as the temperature measurement method required. Therefore, the power supply changed to high current (120 A) for the heating and later it changed to low current (10 A) for the cooling of the stator winding. Besides, these currents were constant in all completed cycles throughout this project in order to compare results in the same test conditions. The currents and the conductor warming can be understood in Figure 3.9.

Figure 3.9: Currents used in PCT

As Figure 3.9 shows, the current was run through windings during the heating and cooling process of the conductors. It was necessary to measure the phase resistances and as such, the temperatures. In addition, the power supply contributed the safety measures of the cycle, but it will be described in the section 6.3.

23

4 Air Cooling Power Cycle

ACPC was the longest test done during this master thesis. It consisted of a large amount of cycles between two different temperatures. This test was carried out with four stators at the same time. Figure 4.1 shows the main components of the ACPC system.

Figure 4.1: ACPC layout. The elementas are explained in Table 4.1

1

2

3

7

6 5

4

Chapter

24

Table 4.1: Elements description of Figure 4.1


1 Cooling fans 2 Stators 3 Data acquisition cable 4 Thermal chamber 5 Power supply 6 Computer 7 Data acquisition LabVIEW modules

The setup with four stators was very useful to save time, energy and Scania’s resources. One of the benefits of the configuration was that two stators were running with different ΔT than the other two. That was possible owing to the fans installation in the Thermal Chamber (TC). In Figure 4.2, the whole ACPC setup can be seen, with plastic separation between both sides of the TC. Thus, both tests were running in a completely independent way. It is worth highlighting that the plastic was working as a protection for the operator from the tested warm windings as well.

Figure 4.2: Air cooling layout. Elements described in Table 4.2



1 Data acquisition cable 2 High current cable joining split phases. 3 Resistance measurement points 4 Plastic separation between cycles

4

3

2

1

25

4.1 Temperature ranges

As explained in section 2.3.1, the purpose of PCT was to reach the failure in the insulation due to a single stress type: thermomechanical stress. For this reason, the temperature ranges in cycles were selected seeking to achieve large ΔT. To achieve that, the temperatures were chosen according to the following criteria:

• High cycle temperature: to avoid thermal stresses in the insulation, all cycles were run under the threshold temperature of the insulation material. In this case, the insulation material was class H, and the threshold temperature was 180°C according to Table 2.2. As discussed in the section 2.2.1, when the material gets temperatures above the limit, the degradation of the material starts to accelerate. Since, the aim of the project was only to study the effects caused by thermomechanical stresses. The maximum temperature achieved during the power cycles was 180°C.

• Low cycle temperature: to achieve a large ΔT in the cycles with a short time cycle, the low cycle temperatures chosen were 30°C and 40°C. These low temperatures were similar because the stators needed close temperatures to work properly with the fans control cooling.

Figure 4.3 shows the temperatures of a complete cycle in ACPC, with a low temperature of 30°C and 40°C and a high temperature of 180ºC in each pair of stators, with -12°C in the TC. Accordingly, the ΔT in cycles were 140ºC and 150ºC, corresponding to a large ΔT with 465s cycle time. For larger ΔT in cycles, the cycle time became very long because of the small ΔT between cooling air and windings.

Figure 4.3: Air cooling temperatures cycle 30/40ºC to 180ºC

40ºC

180ºC

30ºC

26

The lower limit was originally supposed to be varied after the stators failure. However, as no failure was reached, these low temperatures were changed to -20ºC in order to achieve higher degradation in the insulation.

At the beginning of the ACPC, other low temperatures were considered, but the cycle time has extremely large for the ΔT reached in the cycle (see Figure 4.4)

After 4500 cycles between 30/40ºC and 180ºC, the diagnosis test detected some degradation in the insulation, but the completed cycles were not enough to get a failure in the stators. For this reason, the lower temperature limit was changed to -20ºC in order to obtain different results, and other 200 cycles were conducted between -20°C and 180ºC. Moreover, the temperature in the TC was changed to -40ºC. Figure 4.4 shows the cycles with ΔT of 20, 150 and 200.

Figure 4.4: ACPC with ΔT of 20ºC, 150ºC and 200ºC

In addition, the high temperature reached in the winding of a stator is shown in Figure 4.5. It belonged to the ACPC. It can be observed that the temperature in the copper was 180ºC, after 205 seconds operating at 120 A. However, the insulation surface temperature was 153ºC. Accordingly, it was proved that the temperature in the copper cannot be measured in the surface of the insulation because of the thermal inertia in the material and the stator core.

27

Figure 4.5: ACPC stator thermal picture with 180ºC in the conductors

4.1.1 Thermal chamber temperature

An improper selection of the TC temperature would yield wrong data. 4500 cycles in air cooling were ran with a TC temperature of -12 °C. The reason was to prevent problems in the temperature measurements. As previously explained (see section 3.3.1.), the method employed for reading temperatures is sensitive to variations in the resistances. In that sense, small variations can lead to large differences in the results. Owing to that fact, temperatures far below zero in the TC produced problems in the measurements arising from the thermal dilatation in the screws. This was evidenced in the last test corresponding to a temperature of -40ºC in the TC, in which the measurements of some of phases had wrong data. The problem was solved with data filters (see section 6.2).

Figure 4.6 shows the TC layout with the air-cooling system.

Figure 4.6: Components air cooling layout in TC. Elements are described in Table 4.3

1

2

3

4

5

28



1 Fans 2 Stators with ΔT of 150ºC 3 Plastic separation 4 Stators with ΔT of 140ºC 5 TC

4.2 Fans

Fans were placed in both sides of the TC. Their position can be seen in Figure 4.6. The fans were crucial to accelerate the cooling time of the cycle with air.

Each fan was controlled to run at different speed, producing a different cooling rate between the two ΔT levels. When the low temperature was reached in the conductors, the respective fan stopped until the next cycle started.

The fan with ΔT of 140ºC was working at the beginning of the heating cycle stage to reach the same temperature in the four stator windings. In this way, the four conductors reached 180 ºC at the same time.

As can be gathered from Figure 4.6, fans were protected with a metallic grid in order to protect the operator.

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5 Oil Cooling Power Cycle

The second test conducted was the OCPC. While in this test the stators were refrigerated with oil, the testing principles were identical to the air cooling test. Figure 5.1 shows the main components in the OCPC system.

Figure 5.1: OCPC layout. The elementas are described in Table 5.1

5

7

8

4

3

2 1

Chapter

6

30



1 Oil hoses 2 Oil heat exchanger 3 Fans 4 High pressure oil pumps 5 Oil showers 6 High current cables 7 Oil tank 8 Low pressure oil pump

As can it be observed in Figure 5.1 the setup was composed of two plastic boxes containing the stators, two high current power supplies, an oil tank, three oil pumps and the data acquisition system. Each stator was completing cycles subjected to different ΔT with respect to the other one (see Figure 5.2). That is, these cycles temperatures were controlled separately by two different pumps.

Despite oil refrigerates faster than air, its test preparation requires a long period of time. Furthermore, for safety reasons this test is to be run only during work hours. As a consequence, the number of completed cycles significantly lower than in the air cooling test.

Figure 5.2: Oil cooling stators setup. Elements described in Table 5.2

2

3

1

4

5

31



1 Data acquisition cable 2 Oil protection box 3 Plastic oil houses 4 Oil shower 5 Stator winding

Figure 5.3 shows in detail the stator layout inside the box during the cooling process.

Figure 5.3: Stator layout in oil box

As it can be seen in Figure 5.3, the picture was taken during the cooling phase of the cycle. The oil was sprayed over the insulation surface in order to decrease its temperature.

On the other hand, for a better understanding of the components location inside the TC see Figure 5.4. A radiator was placed in the top tray of the TC to reduce the oil tank temperature. In the middle tray, the two fans were refrigerating the high temperature of the pumps. Finally, the two pumps were in the bottom tray.

32

Figure 5.4: Oil cooling components in the TC. Elements described in Table 5.3



1 Oil cooling radiator 2 Cooling fans for the pumps 3 Plastic oil houses 4 Oil high pressure pumps

5.1 Temperature ranges

The OCPC completed 650 cycles in four different temperature ranges. These temperatures were 50/160°C, 70/160°C, 60/150°C and 80/150°C, where the first number is the low temperature and the second is the high temperature of the completed cycles. In this way, the four stators were operated in four different temperature ranges. As it can be observed, these temperature ranges differ from those used in the air cooling test. Owing to the small amount of space available into the TC, the oil cooling setup was set outside of the TC. Thus, only the pumps, the radiator and the fans were installed inside the TC. In addition, the oil tank and stators were placed in the surroundings of the TC, at a temperature of 27ºC.

The final selection of the temperatures was conditioned by:

• The oil tank was outside the TC. Consequently, the oil tank temperature was around 12ºC.

4

3 1

2

33

• Power losses in the pumps heated the oil up to 35ºC, impeding temperatures below 50ºC due to the large cycle time for lower temperatures.

• The oil type used was ATF, which started producing too much fume after 160ºC. For safety reasons, the upper limit was settled at this temperature.

On the other hand, the TC temperature was settled to 1°C because of the ATF oil viscosity, which became too high subjected to negative temperatures and the pump was overloaded.

Figure 5.5 shows the temperatures of the cycles in OCPC.

Figure 5.5: OCPC with ΔT of 110ºC, 90ºC and 70ºC cycles

Figure 5.6 shows the temperatures in OCPC cycles. It shows four stators tested in the temperature ranges of 70/160°C, 50/160°C, 60/150°C and 80/150°C with 1°C in the TC.

34

Figure 5.6: Oil temperatures cycle 50/70ºC to 160ºC

In Figure 5.6, the area identified as (1) indicates a temperature reduction the stator 1 during the heating part of the cycle. In order to reach the same high temperature in the windings, the cooling method was activated to balance the temperatures in the heating process. In this way, when the temperature difference between the stators was higher than 3ºC, the temperature control started the oil pump for some seconds.

In conclusion, the cooling method activation during the heating process was essential to reach the high temperatures in ACPC and OCPC as stators operated in different low temperatures.

(1) 70ºC

50ºC

160ºC

35

6 LabVIEW Model

6.1 Introduction

LabVIEW [1] is a development and design software for visual programming language. It is commonly used in data acquisition, automation and instrument control in a wide number of operating systems.

Throughout the project, LabVIEW data acquisition modules and LabVIEW software were used to design and control all PCTs. After several weeks of work, an extensive study of the software was completed to use in the two control test systems. One for ACPC and the other one for OCPC.

As said in previous sections, both tests had many similarities between them. The control systems were using almost the same number of signals although in a different way.

The signals acquired from the test were:

• 12 phase voltages measurements (inputs)

• 4 current measurements (input)

• 4 thermocouple measurements (input)

• 2 relay controls (output)

• Power supply connection (input/output)

As it can be gathered from the previous list, LabVIEW permits the acquisition of many different signal types. The great amount of data in real-time reduced the required winding testing time and also it allowed for redundancy in the measurements, providing correct measurements and high performance in the test.

6.2 Model Design

The LabVIEW [1] models for ACPC and OCPC were designed to work with a large number of signals. This data was very useful to accurately control the tests. These

Chapter

36

signals were summarized before being introduced in the control. In order to do that, the models had different filters and loops, where the main functions were:

• The filters combined measurements and removed wrong signal inputs in the system. This function was extremely important to delete wrong measurements caused by thermal expansion in the screws.

• The loops were in charge of reading signals and control the outputs in the system. In this way, they were classified in three main loops: Control Loop (CL), Measurement Loop (ML) and Relay Control Loop (RCL). Where the CL was in charge of the power supply control. It sent the signal to the power supply to change between high and low current when it was required. ML saved the inputs signal in the system, all the measurements were saved by ML each second in order to trace signs of aging in the insulation and possible variations in the tests conditions. Finally, RCL controlled the relays switching, turning the cooling on/off.

Moreover, both tests had a real-time calibration. It enabled the recalibration of the resistance changes caused by the thermal expansion in the screws. In this way, the interruption of the test was not needed to have correct measurements.

6.3 Safety measures

As said in previous sections, PCTs are time consuming and these tests should be able to run without human supervision, reducing the testing time. This function allowed the tests to operate non-stop all day, accelerating the aging process.

To run tests without supervision, some security measures were taken in advance. Moreover, the PCT operated in high currents, and severe effects could appear when operating in these conditions. For this reason, all possible problems were considered and safety measures applied. The main safety measures were:

• Overvoltage protection: The power supplied was switched off automatically when a voltage limit was reached. During the cycles, the current remained constant and the voltage was increasing according to the heating of the copper. If the connection with the computer was lost due to abnormal reasons, the voltage increased until the predefined limit.

• LabVIEW stop detections: The system stopped in case the measurements indicated that something strange was happening during the tests.

• Physical protections: They were composed of fuses to the low current circuits.

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7 Results and discussion

7.1 Introduction

This chapter covers the results achieved throughout this project and its subsequent discussions. The chapter first presents the results and discussion of non-destructive tests. It then proceeds with those extracted from destructive tests.

The PCT carried out during the project are summarised in Table 7.1. It includes number of cycles, cooling method, higher temperature, lower temperature and ΔT for each PCT completed.

In addition, the larger insulation resistance results were 1000 GΩ, but the actual insulation resistance value was beyond the measurement capability of the Megger used.

Table 7.1: PCT completed

#/n. Cycles PCT High Temp. Low Temp. ΔT

4500 Air Cooling 180°C 30°C 150°C

4500 Air Cooling 180°C 40°C 140°C

200 Air Cooling 180°C -20°C 200°C

650 Oil Cooling 160°C 50°C 110°C




7.2 Non-destructive tests

The non-destructive tests were conducted after a certain number of PCT. They were used to evaluate the state of the insulation material in order to detect signs of aging.

Chapter

38

The 2 non-destructive tests were insulation resistance and insulation capacitance as explained in section 2.4. In addition, 6 different elements were measured in each diagnosis test. Nevertheless, as it can be observed from these results, not all the measurements are presented, only the most relevant values are displayed.

7.2.1 Insulation resistance

The results of the insulation resistance tests are shown in this section. They are divided in two parts: ACPC and OCPC.

Air Cooling Power Cycle

The ACPC resistance results from the first 4500 cycles are gathered in Table 7.2 and Table 7.3.

Table 7.2: ACPC Resistance 180/40°C (ACPC140)

Cycles Phase/Phase Phase/Core

0 1000 GΩ 1000 GΩ

2100 1000 GΩ 1000 GΩ

2800 1000 GΩ 30 GΩ

3300 1000 GΩ 17 GΩ

4500 1000 GΩ 10 GΩ

Table 7.3: ACPC Resistance 180/30°C (ACPC150)

Cycles Phase/Phase Phase/Core

0 1000 GΩ 1000 GΩ

2100 1000 GΩ 1000 GΩ

2800 1000 GΩ 20 GΩ

3300 1000 GΩ 20 GΩ

4500 1000 GΩ 18 GΩ

The resistance results from the additional 200 cycles with ΔT of 200ºC are presented in Table 7.4.

Table 7.4: ACPC Resistance 180/-20°C (ACPC200)

Cycles Phase/Phase Phase/Core Voltage test

4500 + 0 1000 GΩ 10 GΩ 500 V

4500 + 200 20 GΩ 10 GΩ 500 V

4500 + 200 0 GΩ 0 GΩ 1000 V

39

As it can be observed in Table 7.2 and Table 7.3, the resistance measurements between ACPC140 and ACPC150 had not significant differences after 4500 cycles. When it comes to phase/phase resistances, the measurements did not suffer any change, they remained constant in 1000GΩ. On the other hand, the phase/core resistances had small variations, reaching values of 10 GΩ and 18 GΩ for ACPC140 and ACPC150 respectively.

The same stators of ACPC 140 and ACPC 150 were run for 200 cycles and large resistance variations were achieved. These results showed a strong impact of large ΔT in ACPC. However, for ΔT lower than 150ºC, the ACPC had not fast aging effect over the insulation material, and a larger number of cycles would be needed to achieve these changes. After these cycles, a failure between one of the winding’s phases was observed. It was only achieved in one of the stators. However, this only took place after running 200 cycles in ΔT of 200C and by increasing the test voltage to 1000V.

Oil Cooling Power Cycle

The OCPC resistance results from the 650 cycles are collected in Table 7.5.

Table 7.5: OCPC resistance results

OCPC Cycles Phase/Phase Phase/Core Voltage test

0 1000 GΩ 1000 GΩ 500 V

160/50°C 650 10 GΩ 8 GΩ 500 V

OCPC110 650 10 GΩ 8 GΩ 1000 V

0 1000 GΩ 1000 GΩ 500 V

160/70°C 650 17 GΩ 12 GΩ 500 V

OCPC90 650 17 GΩ 12 GΩ 1000 V

0 1000 GΩ 1000 GΩ 500 V

150/60°C 650 15 GΩ 12 GΩ 500 V

OCPC90-2

650 15 GΩ 12 GΩ 1000 V

0 1000 GΩ 1000 GΩ 500 V

150/80°C 650 18 GΩ 15 GΩ 500 V

OCPC70 650 18 GΩ 15 GΩ 1000 V

As it can be gathered from Table 7.5, the resistance measurements dropped significantly after 650 cycles in OCPC. The resistance reduction was not only in the phase/core measurements, but also in the phase/phase measurements, reaching lower values than what was noted after 4500 cycles in ACPC.

The results showed a significant difference between OCPC110 and OCPC90. The OCPC110 had more aging effect in the insulation material than the OCPC90,

40

demonstrating the high impact of the ΔT in OCPC. In this way, larger ΔT led to faster insulation degradations.

On the other hand, no substantial differences were observed between OCPC90 and OCPC-2. These stators were operating in different temperature ranges, but they completed cycles with the same ΔT=90°C.

Comparing the 2 PCT, it is clear that the OCPC produces a faster resistance reduction in the material than the ACPC. In addition, the different ΔT used in oil cooling methods had visible resistance impact on the insulation, and small increments of ΔT in OCPC lead to a reduction of the measured resistances.

Insulation resistance is a good parameter to evaluate the state of the material. This test is a non-destructive test that permits to examine the insulation without detrimental effects on it.

7.2.2 Insulation capacitance


The ACPC capacitance measurement results are shown in Figure 7.1 and Figure 7.2.

Figure 7.1: Insulation capacitance of the ACPC phase – phase

Figure 7.2: Insulation capacitance of the ACPC phase – core

As it can be seen in Figure 7.1 and Figure 7.2, the capacitance measurements varied widely during the ACPC. These changes were not due to aging in the material but because of the room temperature changes during the measurements. In this test, the temperature in the winding affected more than the capacitance variations due to the material aging. The capacitance measurements remained almost constant during the first 4500 cycles in ACPC, where the insulation material did not suffer strong aging effects operating with ΔT around 150ºC.

41

However, in the last 200 cycles with ΔT of 200ºC, a considerable drop of the capacitance was detected in the phase/phase measurements as well as in the phase/core measurements. This indicated changes in the material state, but these changes were also detected during the insulation resistance diagnosis test without temperature disturbances. For this reason, the insulation capacitance cannot be considered as a good aging indicator of the material for ACPC.


The results of the OCPC capacitance tests are summarised in Table 7.6.

Table 7.6: OCPC capacitance results

OCPC Cycles Phase/Phase Phase/Core

160/50°C 0 887 pF 1634 nF

OCPC110 650 1035 nF 1895 nF

160/70°C 0 882 pF 1636 nF

OCPC90 650 1016 nF 1879 nF

150/60°C 0 882pF 1620nF

OCPC90-2 650 1036nF

1889nF

150/80°C 0 837pF 1657nF

OCPC70 650 1002nF 1858nF

As it can be gathered from Table 7.6, the capacitance measurements in OCPC presented a high capacitance variation after 650 cycles. The capacitances presented an increment in the phase/phase as well as the phase/core measurements.

It is necessary to remark that larger changes in the capacitance and resistance measurements were observed in the OCPC than in the ACPC, even when the number of completed cycles in OCPC was 650 cycles instead of 4700 cycles, as in ACPC.

One of the reasons for these capacitance phase/core changes was the oil absorption by the insulation material. The oil penetrated in the insulation material by filling the air gaps. The relative permittivity (𝜀𝑟) of the air (𝜀𝑟 ≈ 1) is lower than in the oil (𝜀𝑟 ≈2.8). Thus, when the air was replaced by oil, the capacitance increased. It can be related from equation (7.1).

𝜀𝑟 ∗ 𝐶0 = 𝐶 (7.1)

Where 𝜀𝑟 is relative permittivity, 𝐶0 is the capacitance measured with vacuum between its plates and 𝐶 is the measured capacitance [18].

The second reason of these changes was the produced by the aging effect in the insulation material. The phase/phase capacitance results were much higher than the

42

measurements taken before the tests. In this way, these increment variations were caused by the oil between the phases as well as by the insulation aging effect.

Finally, it is important to mention that the capacitance was not affected by the temperature in the oil cooling method. In this way, this diagnosis test resulted more convenient in OCPC than in ACPC.

7.3 Destructive tests

Destructive tests were performed either when a failure was reached or when no more PCT were necessary. Similar, to the non-destructive tests, destructive tests are used for evaluating the state of the insulation. However, such destructive tests produce irreversible effects in the material and therefore they may only be tested once. As said in section 2.4, the three destructive tests completed in the project were water fault detection, camera inspection and FTIR.

7.3.1 Water fault detection

Water fault detection was the first destructive tests carried out after a failure in a winding. As explained in section 2.4.2.1, this test produced bubbles and corrosion in the stator core, revealing thereby the location of the insulation damage. In this way, the locations of the failures were detected in the material for its subsequent analysis.

The bubbles formed throughout the diagnosis test are shown in Figure 7.3. The red square indicates their location and therefore, it also indicates the location of the insulation crack.

Figure 7.3: Water fault detection test

43

In Figure 7.4, the detection procedure and the crack location of the winding can be understood in further detail.

Figure 7.4: Bubbles produced by insulation fault

Finally, there is another useful parameter on this test. The current through the phases is an indicator to reveal insulation cracks. Comparing data from different test conditions, insulation degradation can be related to the operating conditions when the voltage applied between phases was the same.

7.3.2 Camera inspection

The camera inspection of the insulation was useful to detect changes in the appearance. In this section, a visual analysis of the conductor and turn insulation was completed. After the fault detection, the inspection of the insulation was completed in the areas where faults were detected. The elements presented in the section are: copper conductor and turn insulation. Figure 7.5 shows the visual aspect of the insulation material after the PCT.

Figure 7.5: Reference, ACPC and OCPC insulation material

Reference Turn Insulation Material

OCPC Turn Insulation Material

ACPC Turn Insulation Material

44

As it can be gathered from Figure 7.5, reference and ACPC material had the same appearance. On the other hand, OCPC material was covered of ATF oil. It explained the oil absorption of the material and the capacitance changes.

However, when the ACPC insulation material was analysed in further detail, a thin brown line appeared in the insulation (see Figure 7.6).

Figure 7.6: Colour change in ACPC

The material from Figure 7.6 completed 4700 cycles in ACPC. All conductors from this test presented the same colour change regardless of the ΔT used in the cycles. These changes could be due to a higher temperature in this area of the conductor.

On the other hand, the material from OCPC did not present any colour change after the 650 cycles, but the material was completely covered by ATF oil (see Figure 7.7).

Figure 7.7: Insulation material tested in OCPC

Figure 7.7 shows oil in the surface of the insulation. It was present in all samples taken from the OCPC. It explained the oil absorption by the material in these tests. Finally, no other visual changes were detected in this stage, but these images could help to explain later outcomes in the FTIR analysis.

45

7.3.3 FTIR

FTIR was a non-destructive analysis completed. The purpose of this test was analysing the material composition for recognizing material changes in the winding insulation material (see Figure 7.8). The tests have been done on two winding insulation materials, where Aromatic Polyamide Varnish 240ºC was used as a turn insulation and ISONOM NHN 0885 was used as a ground wall insulation.

Analysis aromatic polyamide varnish

The FTIR analysis comparing the ATF oil, the turn insulation reference material, to the aged materials are shown in Figure 7.8. The black line relates de the material tested in ACPC with ΔT=140ºC whereas the blue one corresponds to the material tested in OCPC with ΔT=110ºC.

Figure 7.8: FTIR results of ATF oil, reference aromatic polyamide, aromatic polyamide tested in OCPC110 and aromatic polyamide tested in ACPC140

As it can be gathered from Figure 7.8, there are three important regions to analyse between the reference material and the aged materials. The first change (1) can be found in frequencies around 1600 − 1680 𝑐𝑚−1. This region corresponds to the carbon-carbon double bond (𝐶 = 𝐶). As it can be seen, all 𝐶 = 𝐶 from the reference material have disappeared in the aged materials.

Other important region (1) in the FTIR is situated in frequencies around 1680 −1750 𝑐𝑚−1. Carbon-oxygen double bond (𝐶 = 𝑂) can be found in this region. However, there was not reduction of the 𝐶 = 𝑂 when the material was tested.

(2) (1)

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The last important change (2) in the results appeared in frequencies around 2850 −3000 𝑐𝑚−1. It corresponds to the carbon-hydrogen bond (𝐶 − 𝐻), where the hydrogen is linked to a carbon which is single bonded to everything else. Regarding to the ACPC materials, it is observed that the 𝐶 − 𝐻 disappeared in the aged materials. It can be due to pyrolysis effects or hydrogen substitution. A possible hypothesis could be an aromatic electrophilic substitution, where the hydrogen is replaced by an electrophilic group. In addition, in the ACPC line there was an increasing of the bonds corresponding to 3500 𝑐𝑚−1, it can suggest that the oxygen from the broken 𝐶 = 𝑂 bonds reacted to O groups at these frequencies [19].

On the other hand, the peaks in the OCPC line showed an increment of 𝐶 − 𝐻 bonds. It could be explained because of the absorption of ATF oil by the insulation material.


The FTIR analysis comparing the turn insulation reference material, the materials tested in ACPC with ΔT=140ºC and ΔT=150ºC are shown in Figure 7.9.

Figure 7.9: FTIR results of reference aromatic polyamide, aromatic polyamide tested in ACPC150 and aromatic polyamide tested in ACPC140

In it can be seen in Figure 7.9, all aged materials showed a decreasing of the 𝐶 − 𝐻 and

𝐶 = 𝐶 bonds in the frecuencies around 3000 𝑐𝑚−1 𝑎𝑛𝑑 1650 𝑐𝑚−1 respectively. The results of the four materials show the aging effects explained in Figure 7.8. Nevertheless, the ACPC S3 presented a lower aging effect than the rest of the tests, and it should present similar results as ACPC S4 presented. A possible reason for this result may be that the material analysed in ACPC S3 was operated in lower temperatures than the rest of the samples. It could be caused because it was located in a different section of the windings.

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The FTIR analysis comparing the ATF oil, the turn insulation reference material and the OCPC aged material are shown in Figure 7.10.

Figure 7.10: FTIR results of ATF oil, reference aromatic polyamide, aromatic polyamide tested in OCPC110 and aromatic polyamide tested in OCPC90

As it is presented in Figure 7.10, the results showed a higher presence of 𝐶 − 𝐻 bonds

in the insulation material at frequencies around 3000 𝑐𝑚−1. One explanation could be that the oil was absorbed by the insulation material. The OCPC with ΔT=110ºC had got higher oil presence in the material than the OCPC with ΔT=90ºC. The main reason of that is the longer cooling time needed in the ΔT=110ºC.

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ISONOM NHN 0885

The FTIR analysis comparing the ground wall insulation reference material, the material tested in ACPC with ΔT=150ºC, the material tested in ACPC with ΔT=140ºC and the material tested in OCPC with ΔT=110ºC are shown in Figure 7.11.

Figure 7.11: FTIR results of reference ISONOM NHN 0885 material, ISONOM NHN 0885 tested in ACPC150, ISONOM NHN 0885 tested in ACPC140 and ISONOM NHN 0885 tested

in OCPC110

In Figure 7.11, there were not aging differences in the aged materials. OCPC insulation material showed a high presence of 𝐶 − 𝐻 at frequencies around 3000 𝑐𝑚−1. It was caused by the oil as in the OCPC turn insulation. On the other hand, no other composition differences were observed from this analysis.

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8 Conclusions

8.1 Conclusions

The main purposes defined in this master thesis have been accomplished. That included the analysis and comparison between different cooling methods. Moreover, the effect of different operating conditions in EM was also investigated.

A significant number of winding samples were tested according 2 cooling methods: oil cooling and air cooling. As a result of this testing process, a failure of the winding insulation was reached. Such failure demonstrated that the considered testing method permitted to develop cracking in the insulation material due to thermomechanical stresses. Thus, all this data from the tests provided valuable information regarding the insulation winding behaviour in PCT when only thermomechanical stresses are involved.

In this way, the conclusions resultant from this master’s thesis are summarized as follows:

• Larger ΔT in PCT produced a significant decreasing of the life length in the windings. This was demonstrated after all cycles completed in ACPC and OCPC. While it was observed that ACPC needed higher ΔT in cycles to produce alterations in the insulation than the OCPC, larger ΔT have more impact in the aging process in OCPC than in ACPC. Therefore, ΔT differences have more impact in the aging process in OCPC than in ACPC. This fact became evident when after 4500 cycles, there were no difference in the diagnosis tests between the two ΔT (of 140°C and 150°C) contemplated in ACPC whereas after 200 cycles in ΔT=200°C a failure was achieved. On the other hand, distinct result was extracted in OCPC, in which larger ΔT produced a fast deterioration of the insulation winding. In addition, these variations were observed for ΔT of 90°C and 110°C which results presented significant differences between the two ΔT.

Chapter

50

• The ACPC and OCPC had huge differences in the results when they were compared. ACPC needed 2800 cycles to show the first aging sign, and it was only a small reduction of the phase/core resistance. On the other hand, OCPC presented large insulation resistance differences after 650 cycles with ΔT of 110°C. Regarding the FTIR analysis, the windings tested in OCPC showed an oil adsorption by the material, changing the original attributes of the insulation. Finally, oil cooling method produces faster decreasing of the winding temperatures than air cooling, and it produces large thermomechanical stresses in the material. In conclusion, the aging degradation was faster in oil cooling methods than in air cooling methods, and the results from the project support them. Could be both due to the chemical reaction between the oil and the insulation as well as the faster thermal changes.

• Diagnosis methods evaluated the insulation condition after aging tests, but not all these results were equally effective. After the project, the most valuable diagnosis test to determine the insulation state was the insulation resistance test. Such non-destructive test was crucial to detect failures after they occurred. It could not alert of future failures, but in comparison to the other alternatives, it resulted by far the best evaluating the winding state.

• Testing and preparation time was the biggest issue of this project, mainly due to the lengthy time that PCT requires for reaching cracks in the insulation material. This challenge together with the test setup preparation reduced significantly the time available to conduct cycles. In this way, PCT is time consuming.

Conversely, PCT can provide valuable information in case the following conditions are fulfilled: it is performed in large scale with fast preparation, short diagnosis time and configured for testing several windings in different operation conditions simultaneously.

8.2 Future work

Although the main objectives have been fulfilled, this project could be improved and extended in different ways. Some proposed lines of work for this project are collected in this section. The proposals for future work are:

• Accelerate the testing process by pre-aging the windings. This would reduce considerably the sources, especially in terms of time, for reaching the failure in the insulation winding. In other words, the power cycles required for the appearance of cracks would decrease significantly. Subsequently, a larger number of samples with different operating conditions could be analysed and compared.

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• Provide better temperature control by applying an independent cooling method in each tested stator. The cooling stage of PCT spent most of the testing time. Operating the tests with independent cooling approaches would allow complete cycles with different ΔT. It would also decrease the cooling stage time, which results the longest stage in the cycles.

• Continue the study and broaden the analysis for different ΔT in the cycles. This permits the comparison of a larger number of operating conditions. A good work line would be the comparison between short and large ΔT in PCT. Other approach would be the analysis and comparison of the same ΔT for different temperature ranges. These analyses would show further information about the effect of these power load variations in EM.

• Increase the ΔT of the OCPC. The results showed a higher aging effect of the oil cooling method, however, the cycles could not be compared for the same ΔT in cycles as in the air cooling method. The solution would be a compact OCPC setup inside of the thermal chamber and better ventilation to extract the oil fumes.

• Investigate delamination in different locations of the windings. Not all insulation areas are subjected to the same temperatures. The delamination analysis would show the weakest parts of the insulation, providing valuable information for the life length extension of the EM

.

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[12] P. Wang, X. Wang, J. Du, F. Ren, Y. Zhang, X. Zhang and E. Fu, “The temperature and size effect on the electrical resistivity of Cu/V multilayer films,” Acta Materialia, 2016.

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[17] P. Quennehen, I. Royaud, G. Seytre, O. Gain, P. Rain, T. Espilit and S. François, “Determination of the aging mechanism of single core cables with PVC insulation,” Polymer Degradation and Stability, 2015.

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[20] M. Popescu, D. Staton, A. Boglietti, A. Cavagnino, D. Hawkins and J. Goss, “Modern Heat Extraction Systems for Electrical,” IEEE, 2009.

[21] IEEE, “Recommended Practice for Thermal Cycle Testing of Form-Wound Stator Bars and Coils for Large Rotating Machines,” 2012.

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[23] IEC, “60034-18-33 Functional evaluation of insulation systems – Test procedures for form-wound windings – Multifactor evaluation by endurance under simultaneous thermal and electrical stresses,” 2010.

[24] IEEE, “1776-2008 - Recommended Practice for Thermal Evaluation of Unsealed or Sealed Insulation Systems for AC Electric Machinery Employing Form-Wound Pre-Insulated Stator Coils for Machines Rated 15 000 V and Below,” 2008.

[25] MATLAB, MathsWorks, 2015.

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