Insulating Materials in Power Transformer.Pdf

Home , Grounding transformer, Isolation transformer

بسمميحرلا نمحرلا هللا

UNIVERSITY OF KHARTOUM

FACULTY OF ENGNEERING

DEPARTMENT OF ELECTRICAL AND ELECTRONIC ENGNEERING

INSULATING MATERIALS IN POWER TRANSFORMERS

HUSSAM MAGID AHMED ABDALLAH

INDEX NO.124041

Supervisor

Dr. Alfadel Zakariya

A REPORT SUBMITTED TO

University Of Khartoum

In partial fulfillment for the degree of

B.Sc. (HONS) Electrical and Electronics Engineering

(POWER SYSTEM ENGINEERING)

Faculty of Engineering

Department of Electrical and Electronics Engineering

October 2017

DECLARATION OF ORGINALITY

I declare this report entitled “(Insulating materials in power transformers)” is my own work except as cited in references. The report has been not accepted for any degree and it is not being submitted currently in candidature for any degree or other reward.

Signature: ______

Name: ______

Date: ______

ACKNOWLEDGEMENT

All the thanks, praises and glorifying is due to the Almighty GOD, without his uncountable blesses and uninterrupted gifts I wouldn„t be here a grown man about to graduate.

To my mother, who grew me up, fed me, guided me through the life, many thanks and thanks.

To my father who learnt me the patience and self-confidence.

For my supervisor Dr.alfadel Zakariya, for his endless guiding and support.

For my colleague, my partner in this project.

iii

Abstract

There are many types of transformers insulators, some of them are used with conductors and the others are used to insulate the metal slides from each other and the others used to insulate the windings form the iron core.

After collecting the parts of the transformer, it has to be dry enough because the insulators affected by the humidity which is make its isolation properties to be less than usual.

In case of high internal temperature, transformer oil is considered one of the most important part of the cooling system, preventing the chemical interactions and detecting the faults e.i (short circuit) in a transformer.

المستخلص

ػٕاسل انًحٕالث نٓا إَاع كثٍزِ, بؼضٓا ٌسخخذو يغ انًٕصالث ٔانبؼض االخز ٌسخخى نؼشل انشزائح انؼًذٍَت ػٍ بؼضٓا انبؼض ٔ يُٓا اٌضا يا ٌسخخذو فً ػشل انًهفاث ػٍ انقهب انحذٌذي.

ػُذيا ٌخى جًغ انًحٕالث ٌجب حجفٍفّ فً افزاٌ خاصّ بذنك الٌ انًٕاد انؼاسنّ فً انًحٕالث فً غانبٍخٓا حخأثز بانزطٕبّ يًا ٌؤدي انى َقصاٌ قابهٍّ انؼشل نذٌٓا.

انحزارة انذاخهٍت فً انًحٕالث حسبب يشاكم كبٍزة فً حال حذٔثٓا نذا ؼٌخبز سٌج انًحٕالث يٍ اْى انؼُاصز انًسخخذيّ فً يُظٕيّ انخبزٌذ, ٌٔسخخذو انشٌج اٌضا فً ػشل انًهفاث ػٍ بؼضٓا انبؼض, ٔ بٍٍ انًهفاث ٔ انقهب ٔانخشاٌ انخارجً ,كًا ٌسخخذو فً يُغ انخفاػالث انكًٍٍائٍّ ػٍ طزق حغطٍّ االجشاء انؼًذٍَّ, ٔاخٍزا ٌسخخذو انشٌج ككاشف نالػطال انخً قذ ححذد داخم انًحٕالث.

Table of Contents

CHAPTER 1: Introduction ...... 1

1.1 Overview ...... 1

1.2 Problem Statement ...... 1

1.3 Objectives ...... 1

1.4 Methodology ...... 1

1.5 Thesis Layout ...... 2

CHAPTER 2: Literature Review ...... 3

2.1 Power systems ...... 3

2.2 Transformers ...... 4

2.2.1 Introduction ...... 4

2.2.2 Basic Working of Transformers ...... 4

2.2.3 Equivalent circuit of transformer ...... 5

2.2.4 Classification of Transformers ...... 6

2.2.5 Types of Electrical Transformers ...... 6

2.2.6 Basic Parts of a Transformer ...... 8

2.2.7 Losses in Transformers ...... 10

2.2.8 Testing of Transformers ...... 12

2.3 Insulating materials used in transformers...... 16

CHAPTER 3: Methodology ...... 17

3.1 Overview ...... 17

3.2 Case study ...... 17

3.2.1 introduction ...... 17

3.2.2 transformer insulation life ...... 18

3.2.3 solid insulators ...... 18

3.2.4 transformer oil ...... 28

CHAPTER 4:Conclusion ...... 42

CHAPTER 5: Recommendation ...... 43

vii

Table of figures

FIGURE 2-1 MAGNETIC CORE ...... 5

FIGURE 2-2 EQUIVALENT CIRCUIT ...... 6

FIGURE 2-3CIRCUIT DIAGRAM FOR OPEN CIRCUIT TEST ...... 15

FIGURE 2-4 CIRCUIT DIAGRAM FOR SHORT CIRCUIT TEST ...... 17

FIGURE 3-1 THE SCHEMA ILLUSTRATING THE SAMPLE PREPARATION PROCEDURE ...... 21

FIGURE 3-2 THE RELATIVE PERMITTIVITY OF KRAFT PAPER AND POLYPROPYLENE IMPREGNATED ...... 23

FIGURE 3-3 THE LOSS TANGENT AT 1000 HZ AS A FUNCTION OF TEMPERATURE FOR ...... 23

FIGURE 3-4 THE BREAKDOWN STATISTICS OF PRISTINE PAPER. THE DATA ARE ANALYZED ...... 25

FIGURE 3-5 THE BREAKDOWN STATISTICS OF PP IMPREGNATED PAPER. THE DATA ARE ...... 25

FIGURE 3-6 FABRE-PICHON CURVES FOR MOISTURE EQUILIBRIUM OF THE AIR-OIL-PAPER COMPLEX AS A FUNCTION OF

THE AIR AND OIL SURROUNDING THE PAPER. FIGURE IS TAKEN FROM [Z] EXCEPT LABELS ARE TRANSLATED INTO

ENGLISH...... 27

FIGURE 3-8 OOMMEN‟S CURVES FOR MOISTURE EQUILIBRIUM FOR A PAPER-OIL SYSTEM ...... 28

FIGURE 3-9 MOISTURE IN WOOD PULP AS A FUNCTION OF RELATIVE HUMIDITY OF THE AMBIENT BELFRIES .PICTURE

SUPPLIED BY OOMMEN AT ABB-ET1 WHO REDREW JEFFRIES‟ DATA...... 29

FIGURE 3-10 MOISTURE IN PAPER VERSUS AMBIENT RELATIVE HUMIDITY CURVES COMPARING ...... 30

FIGURE 3-11GRIFFIN CURVES FOR WATER EQUILIBRIUM IN CELLULOSE MINERAL OIL SYSTEMS ...... 30

FIGURE 3-12 MIT-DEVELOPED CURVES FOR WATER EQUILIBRIUM IN CELLULOSE MINERAL OIL SYSTEMS FOR A WIDE

RANGE OF MOISTURE CONCENTRATIONS ...... 31

viii

CHAPTER 1: Introduction

1.1 Overview:

The life of a power transformer is limited to the life of its solid and liquid insulation, many diagnostic techniques and tests are used to prolong the service life of the power transformer and to insure that the transformer will operate in an efficient, effective and high quality functional life. These tests and techniques will be stated in detail in the next chapters

1.2 Problem statement:

Damaging of a power transformer due to external or internal faults and the part played by the insulation materials to prevent these damages

Understanding and perception the Tests and techniques performed on the soiled and liquid insulation materials

1.3 Objectives:

Better determination and perfect choice of a power transformer with high Specifications

Increase the ability of the transformer and its insulation materials to resist the faults and its impacts

1.4 Methodology:

Solid and liquid insulators and tests that had been made on them (paper and transformer oil)

1.5 Thesis layout:

Chapter 1: Introduction and problems and the objectives of this research

Chapter 2: Literature review and the concept of power system and its equipment

Chapter 3: Methodology and case study

Chapter 4: Conclusion

Chapter 5: recommendation

CHAPTER 2: Literature Review

2.1 Power systems

The world today is moving towards a state of complete transformation with the advancement in science and technology. The field of power systems is also not void of these changes. Today's generation of power systems consists of larger number of small-scale highly dispersed generation units that utilize diverse renewable energy sources. These are characterized by large scale networks with virtual interconnections and interactions apart from the conventional physical interconnections.

Electric power systems are comprised of components that produce electrical energy and transmit this energy to consumers.

A modern electric power system has mainly six main components:

1) Power plants which generate electric power

2) Transformers which raise or lower the voltages as needed

3) Transmission lines to carry power

4) Substations at which the voltage is stepped down for carrying power over the distribution lines

5) Distribution lines

6) Distribution transformers which lower the voltage to the level needed for the consumer equipment.

The production and transmission of electricity is relatively efficient and inexpensive, although unlike other forms of energy, electricity is not easily stored, and thus, must be produced based on the demand.

2.2 Transformers

2.2.1 Introduction:

Transformer definition:

A device used to transfer electrical energy from one circuit to another by electromagnetic induction (also called transformer action). With an alternating current, a transformer will either raise or lower the voltage as it makes the transfer.

Why are transformers used in the power system?

Transformers are used in the power system in order to step up or step down the voltages. In the transmission end the voltage is stepped up and in the distribution side the voltage is stepped down in order to reduce the power loss (i.e.) copper loss or I2R loss.

Principle of Operation:

Transformers work on the principle of Faraday‟s law of electromagnetic induction.

Faraday‟s law states that, “Rate of change of flux linkage with respect to time is directly proportional to the induced EMF in a conductor or coil”.

2.2.2 Basic Working of Transformers:

The basic transformer consists of three types of coils, namely:

1. Primary coil: The coil to which the supply is given.

2. Secondary coil: The coil from which the supply is taken.

3. Magnetic core: The flux produced in the primary passes through this magnetic core.

Figure 2-1 magnetic core

Based on the required output voltage the number of turns in the primary coil and the secondary coil are varied.

The processes occurring inside the transformer can be grouped into two:

1. Magnetic flux is produced in a coil whenever there is a change in current flowing through the coil.

2. Similarly change in magnetic flux linked with the coil induces EMF in the coil.

The voltage produced in the secondary coil depends mainly on the turns ratio of the transformer.

Their relationship between the number of turns and the voltage is given by the following equations.

N1/N2 = V1/V2 = I2/I1

Where,

N1= number of turns in the primary coil of the transformer.

N2= number of turns in the secondary coil of the transformer.

V1= voltage in the primary coil of the transformer.

V2= voltage in the secondary coil of the transformer.

I1= current through the primary coil of the transformer.

I2= current through the secondary coil of the transformer.

2.2.3 Equivalent circuit of transformer:

Figure 2‎ -2 Equivalent circuit 2.2.4 Classification of Transformers:

Parameter Types

Based on application Step up transformer

Step down transformer

Based on Construction Core type transformers

Shell type transformers

Based on the number of phases. Single phase

Three phase

Based on the method of cooling Self-air–cooled (Dry type)

Parameter Types

Air-blast–cooled (Dry type)

Oil-immersed, combination self-cooled

and air-blast

Oil-immersed, water-cooled

Oil-immersed, forced-oil–cooled

Oil-immersed, combination self-cooled

and water-cooled

2.2.5 Types of Electrical Transformers

1. Power transformer: You can find this type of transformer in distribution networks, substations, industries etc. It is used to step up and step down voltages based on the requirement. It is also used in the interconnection of two power systems. The main purpose of using power transformer is to reduce I2R loss causes in the transmission lines by increasing voltage level and reducing current flow.

2. Instrument transformer: devices cannot be directly connected to the line carrying high current or the line at extremely high voltages, since it causes damage to the equipment. Hence in order to facilitate the measurement of electrical parameters such as voltages, current, power, energy etc. instrument transformers are used.

There are two types of instrument transformers:

Potential / voltage transformer: Potential transformers are used for the measurement of high voltages and to provide protection against under and over voltages.

Current transformer: It is the current ratio transformer used to step down current levels and provide it to measuring instruments like an ammeter. It is also used along with protection relays. It is always connected in series to the circuit. The measure devices or protective relays are connected to the secondary winding of the current transformer.

3. Autotransformer: Auto transformer consists of a single winding that acts as both primary and the secondary. When the primary and the secondary are electrically connected so that a part of winding is common to both.

4. Audio frequency transformer: Audio frequency transformers are used in audio frequency electronic amplifiers for matching load to the output impedance of the power amplifiers. The operating range of audio frequency transformer is 20 Hz to 20

8 kHz. These transformers are used in the electronic circuits for stepping up the voltage or impedance.

5. Grounding transformer: When delta – delta transformers are used in the power system, neutral grounding is not possible. In such cases a special star- delta transformers are used. These transformers are called grounding transformers.

6. Welding transformer: To limit high short circuit currents during welding, welding transformers are used. These transformers have very high reactance both at the primary and the secondary.

7. Isolation transformers: Isolation transformers are used to isolate any equipment from an AC power source. The turns ratio of any isolation transformer is unity. Isolation transformer are used to maintain electrical safety. It is used to transfer power between two circuits that must not be connected. It provides protection against electric shock.

2.2.6 Basic Parts of a Transformer

1. Laminated core: The core is used to support the windings in the transformer. It also provides a low reluctance path to the flow of magnetic flux. It is made of laminated soft iron core in order to reduce eddy current loss and Hysteresis loss. The composition of a transformer core depends on such factors as voltage, current, and frequency

2. Windings: There are two windings wound over the transformer core that are insulated from each other. Windings consists of several turns of copper coils bundled together, and each bundle is connected in series to form a winding.

Windings can be classified in two different ways:

A. Based on the input and output supply : 1. Primary windings 2. Secondary windings B. Based on the voltage range : 1. High voltage winding 2. Low voltage winding

3. Insulating materials: Insulating paper and cardboard are used in transformers to isolate primary and secondary windings from each other and from the transformer core.

4. Transformer oil: Is another insulating material. Transformer oil can actually have two functions: in addition to insulating it can also work to cool the core and coil assembly. The transformer's core and windings must be completely immersed in the oil. Normally, hydrocarbon mineral oils are used as transformer oil.

Oil contamination is a serious problem because contamination robs the oil of its dielectric properties and renders it useless as an insulating medium.

5. Tap changer: The output voltage may vary according to the input voltage and the load. During loaded conditions, the voltage on the output terminal decreases, whereas during off- load conditions the output voltage increases. In order to balance the voltage variations, tap changers are used. Tap changers can be either on-load tap changers or off-load tap changers. Automatic tap changers are also available.

6. Conservator: The conservator conserves the transformer oil. It is an airtight, metallic, cylindrical drum that is fitted above the transformer. The conservator tank is vented to the atmosphere at the top, and the normal oil level is approximately in the middle of the conservator to allow the oil to expand and contract as the temperature varies. The

conservator is connected to the main tank inside the transformer, which is completely filled with transformer oil through a pipeline.

7. Breather: The transformer's breather is a cylindrical container that is filled with silica gel. When the atmospheric air passes through the silica gel of the breather, the air's moisture is absorbed by the silica crystals. The breather acts like an air filter for the transformer and controls the moisture level inside a transformer. Moisture can arise when temperature variations cause expansion and contraction of the insulating oil, which then causes the pressure to change inside the conservator. If the insulating oil encounters moisture, it can affect the paper insulation or may even lead to internal faults. Therefore, it is necessary that the air entering the tank is moisture-free.

8. Cooling tubes: Are used to cool the transformer oil. The transformer oil is circulated through the cooling tubes. The circulation of the oil may either be natural or forced (an external pump is used).

9. Buchholz Relay: Is a protective device container housed over the connecting pipe from the main tank to the conservator tank. It is used to sense the faults occurring inside the transformer. It is a simple relay that operates by the gases emitted due to the decomposition of transformer oil during internal faults.

10. Explosion vent: is used to expel boiling oil in the transformer during heavy internal faults in order to avoid the explosion of the transformer. During heavy faults, the oil rushes out of the vent. The level of the explosion vent is normally maintained above the level of the conservatory tank.

Of the above, the first four are found in almost all the transformers, whereas the rest are found only in transformers that are more than 50 KVA.

2.2.7 Losses in Transformers:

1. Core loss: When the core of the transformer undergoes cyclic magnetization power losses occur in it. There losses are together called as core loss. There are two kinds of core losses namely:

A. Hysteresis loss:

This phenomenon of lagging of magnetic induction behind the magnetizing field. This is because, during a cycle of magnetization, the molecular magnets in the specimen are oriented and reoriented a number of times. This molecular motion results in the production of heat. The shape and size of the hysteresis loop is characteristic of each material because of the differences in their retentivity, coercivity, permeability, susceptibility and energy losses etc.

Steinmetz gave an empirical formula to simplify the computation of the hysteresis loss based on his experimental studies. The formula given by him is as follows:

n Ph=khfB m W

Where kh is a characteristic constant of the core material, Bm is the maximum flux density and n is caller steinmetz constant

B. Eddy current loss:

When the magnetic core flux varies in a magnetic core with respect to time, voltage is induced in all possible paths enclosing the flux. This will result in the production of circulating currents in the transformer core. These currents are known as eddy currents. These eddy currents leads to power loss called Eddy current loss. This loss depends upon two major factors. The factors affecting the eddy currents are:

Resistivity of the core and Length of the path of the circulating currents for a given cross section.

The eddy currents can be expressed as

2 2 Pe = kef B W/m3 2 ke = ke'd /p

Where, d is the thickness of the lamination p is the resistivity of material of the core

2 2 2 Pe = ke'd f B /p W/m3

Hence from the above equations it is evident that Eddy current loss is directly proportional to the square of the thickness of the lamination and that of the frequency of supply voltage.

Reduction of eddy current loss can be achieved by using core with high resistivity and increasing the path of circulating currents.

Total core loss = Hysteresis loss + Eddy current loss.

2. Copper loss: Copper loss occurs in the winding of the transformer due to the resistance of the coil. When the winding carries current, power loss occurs due to its internal resistance. The copper loss can be expressed as below

2 Pcu = I R

Where I is the current through the winding and R is the resistance of the winding.

Copper loss is proportional to the square of current flowing through the winding.

3. Load (stray) loss: Stray loss results from leakage fields including Eddy currents in the tank wall and conductors. The winding of the transformers should be designed such that the stray loss is small. This can be achieved by the splitting of conductors in to small strips to reduce Eddy

13 currents in the conductors. The radial width of the strips should be small and they should be transposed.

4. Dielectric loss: This loss occurs in the transformer oil and other solid insulating materials in the transformer.

The major losses occurring in the transformer are Core loss and copper loss. Rest of the losses are very small compare to these two. All the losses occurring in transformer are dissipated in the form of heat in the winding, core, insulating oil and walls of the transformer. Efficiency of the transformer increases with decrease in the losses.

2.2.8 Testing of Transformers

Tests done on transformers are:

1. Open circuit (OCC) test

2. Short circuit (SCC) test

3. Sumner‟s or Back to back test

1. Open circuit (OCC) test:

Open circuit test is conducted on the transformer for the following purposes

1. I. To determine the shunt parameters in the equivalent circuit

2. II. To determine core loss

3. III. To determine the magnetizing current (Im)

During this test, the rated voltage is supplied to one of the winding while the other winding is kept open. Normally LV side is provided with the rated voltage and the LV side is kept open. If the transformer is used at voltages other than the rated voltage, then the test should be conducted at that voltage. The meters are connected to the transformer as shown in the circuit diagram.

After applying the voltage the meter readings are noted. The ammeter reading corresponds to the no load current Io and the watt meter reading corresponds to the core loss or iron loss Pi.

Pi = Po (Iron loss)

Shunt parameters in the equivalent circuit can be calculated from the following formula.

Yo = Gi - jBm

Yo = Io / Vi

2 Vi Gi = Po

Hence,

2 The conductance Gi = Po / Vi

The suscepta

nce Bm = 2 √ (Yo - 2 Gi )

Figure 2‎ -3Circuit diagram for Open circuit test

Short circuit (SC) test: Short circuit test is conducted to determine the following

I. The full load cu- loss (Copper loss). II. Leakage reactance and equivalent resistance.

In short circuit test supply arrangements are made at the HV side and the LV side is short circuited. The voltage needed for the short circuit test is 5 - 8% of rated voltage of the transformer.

Since the test on the HV side requires less current than that on the LV side supply is provided on the HV side. The supply voltage is gradually raised from zero till the transformer draws its full load current.

Voltage = Vs; current = Isc; Power input = Psc

The iron loss during the short circuit test is negligible due to very low excitation voltage. Therefore power drawn will be sufficient to satisfy the copper loss.

Hence the watt meter reading corresponds to the full toad copper loss.

Psc = Pcu (Copper loss)

Z = V sc / I sc

= √ (R2 + X2)

2 Equivalent resistance R = Pcu/ I sc

Equivalent reactance X = √ (Z2 - R2)

Since the iron loss is neglected the shunt branch in the equivalent circuit can also be neglected

Figure 2‎ -4 circuit diagram for short circuit test

Sumpner‟s test or back to back test: Sumpner's test is conducted by connecting to transformers in parallel across a single rated voltage supply. The two secondary winding are connected in series with opposing polarity. Since the transformers used are identical secondary voltages cancel each other. The two transformers acts like open circuited as their secondary are in phase opposition and no current flow through them. The current drawn from the primary source is twice the no load current and the power is twice the core loss. The secondary voltage source is adjusted to circulate full load current and the power fed is twice the full load copper loss. Thus by doing so full load core losses and full load copper losses are achieved in both the transformers without connecting actual load and the energy requires to fulfill the losses alone is drained from the sources.

Circuit diagram for sumpner's test

Figure 2-5 Circuit diagram for sumpner's test

2.3 Insulating materials used in transformers

In transformers mainly three insulating materials are used.

1. Transformer oil: It is normally obtained by fractional distillation and subsequent treatment of crude petroleum. That is why this oil is also known as mineral insulating oil. Transformer oil serves mainly two purposes one it is liquid insulation in electrical power transformer and two it dissipates heat of the transformer e.i. acts as coolant. In addition to these, this oil serves other two purposes, it helps to preserve the core and winding as these are fully immersed inside oil and another important purpose of this oil is, it prevents direct contact of atmospheric oxygen with cellulose made paper insulation of windings, which is susceptible to oxidation.

2. Insulating paper: Is made from the vegetable fibers mainly consist of cellulose. It must has these properties: Grammage , Density (0.6 to 1.3 gm/c.c.), Air Permeability ( 0.2 - 0.5.), Tension (78 - 85 N-mt/gm in wrapping direction)

3. Press board: Also made up of vegetable fibers and contains cellulose. The important parameters considered are density, tensile strength, elongation, conductivity, oil absorption, moisture content, compressibility etc.

Of the three, the major insulating material used is transformer oil. 18

CHAPTER 3: Methodology

3.1 Overview

Generally transformers have to be protected from overheating due to external and internal faults by two or more insulating materials like mineral insulating oil, press board or insulating paper, and for other purposes e.i; preserve the winding and the core, prevent direct contact between windings and prevent direct contact of atmospheric oxygen and the insulating paper that is made of cellulose which is liable to oxidation.

Materials quality and Insulating testing are important things to be considered in insulating transformers.

Due to the earth faults, zero sequence current flows into the substation network and it should be blocked (cleared) by the auxiliary (earthing) transformer, but after several faults the auxiliary cannot handle these faults and it may be damaged, so the zero sequence current flows to the power transformer.

Cellulose paper and the press board is the most commonly used solid insulation in oil-field power transformer

The major insulating material used is transformer oil.

3.2 Case study

3.2.1 Introduction

Oils that combine a high flashpoint with high dielectric strength have long been used as an insulating medium in transformers, switchgear and other electrical apparatus. To ensure that the dielectric strength of the oil does not deteriorate however, proper maintenance is essential, and the basis of proper maintenance is testing.

3.2.2 Transformer insulation life

For a given temperature of the transformer insulation, the total time between the initial state for which the insulation is considered new and the final state foe witch di electric stress, short circuit stress, or mechanical movement, which could occur in normal service, and would cause an electrical failure.

There are materials critical to functional life of a transformer which are: Conductor insulation, duct spacers and lead insulation.

3.2.3 Solid insulators

Paper is widely used in various engineering applications due to its physical properties and ease of manufacture. As a result paper has been selected or designed as an electrical insulation material for parts and components in high voltage technology.

The thermal contraction of paper is low enough such that it is near that of metals, and elastic elongation at low temperatures is reasonable without leading to mechanical problems.

In the current research we present the dielectric properties of Kraft paper and present how one can improve the properties by modifying the paper.

3.2.3.1 Sample preparation procedure:

Figure 3‎ -1 the schema illustrating the sample preparation procedure

Conventional Kraft paper was used without further modification of its chemistry. Low molecular weight polypropylene (PP) was used to impregnate the paper to fill the void space. The PP was isotactic and had typical molecular weight around 12000 with Brookfield Thermoset viscosity of 6000 poise at 190 C. A sheet of paper was cut in 152mm squares, and a few PP pellets were placed under and over the sheet. Hot plates were heated to 175 C and applied until the

PP pellets melted. Later a hydraulic press was used to promote better impregnation of the PP; the pressure was set to 2 kPa, After the impregnation, the samples were tested with the dielectric spectroscopy method at fixed frequency between 15K and 295K; dielectric breakdown measurements were conducted in a liquid nitrogen open-bath, at 77K, using an alternating current voltage source. The remaining voids present in samples, if any, were impregnated with liquid nitrogen. In the tests silver electrodes were evaporated on the samples; the procedure is presented elsewhere. The sample thicknesses before and after the impregnation were 65 m and 69 m for pristine and PP impregnated paper, respectively. The PP impregnated paper had the appearance of a wax paper. Dielectric permittivity measurement would determine the impregnation level and also the electrical insulation properties. The relative permittivity‟s of pristine and PP impregnated papers are shown in Fig. 2 as a function of temperature for a fixed frequency at 1 kHz. The relative permittivity of the paper was increased with PP impregnation since the void space in the paper were substituted with the PP. The measurements were performed in a low pressure chamber using a crycooler. Simply using a geometrical addition rule

(layered dielectric mixture) for the porous cellulose and PP-filled cellulose using their concentrations q and (1 - q), respectively,

q"void + (1 - q)"paper = 4:716 (pristine) q"PP + (1 - q)"paper = 5:629 (PP impregnated) (1)

First solving for q above, one can estimate the relative permittivity of paper. The relative permittivity of void space and PP were taken to be 1 and 2.2 in the calculations. The measurement data on the right-hand side of Eq. (1) were the permittivity values at the lowest temperatures, where most of the polarizations were frozen permittivity can be assumed constant. The concentration of pore space was estimated to be 0.761, which is comparable to data available on paper porosities. The relative permittivity of the paper was unanticipated high, "paper = 15:537. To our knowledge no one has reported the permittivity of 100% paper. The values estimated above would be different if one considers the shape of voids and their spatial distribution; however, expressions similar to Eq. (1) are complex to compute with simple algorithms. The dielectric loss tangent data for the samples are presented in Fig. 3 at 1 kHz as a function of temperature. The dielectric losses had a crossover region around 160K, where the losses in PP impregnated paper became lower than the pristine paper when the temperature was decreased. Above 130K losses were similar in two materials.

Figure 3‎ -2 the relative permittivity of Kraft paper and polypropylene impregnated

Paper as a function of temperature at 1 kHz.

Figure 3‎ -3 the loss tangent at 1000 Hz as a function of temperature for

Kraft paper and polypropylene impregnated paper.

3.2.3.2 Dielectric breakdown:

Dielectric breakdown information on the puncture strength is one of the design parameters taken into consideration determining the thickness of electrical insulation for the desired voltage level. The dielectric breakdown data of pristine and PP impregnated paper were analyzed using two different distribution functions, Weibull and log-normal, to better estimate a design value. The data were analyzed using the Data plot platform of the National Institute of Standards and Technology .The pristine paper had localization and scale parameters and equal to 27:96kVmm- 1 and 7.73 for the Weibull distribution. The probability of breakdown at 0.1% was 11:44kVmm- 1. Similarly the log-normal distribution yields localization and standard deviation parameters _ and _ equal to 26:19kVmm-1 and 0.135, respectively. The probability of having a breakdown at 0.1% using the log-normal distribution is 17:25kVmm-1. Repeating the same analysis on the PP impregnated paper, we obtained improved values for the breakdown characteristics. The most important parameters would be the 0.1% breakdown strength estimations, which are 17:82kVmm-1 and 27:98kVmm-1 for the Weibull and lognormal analysis, respectively. These values indicate that PP impregnation improves the dielectric breakdown design values by 55%- 62%. The significance of the impregnation is the potential to increase the voltage level for the same insulation thickness, or decrease the insulation thickness and reduce the size of equipment. In summary, a novel, easy to implement method was employed to improve the dielectric properties of Kraft paper.

Figure 3-4 the breakdown statistics of pristine paper. The data are analyzed

With Weibull (red) and log-normal (blue) distributions. Confidence levels at

10% are also shown with dashed lines.

Figure 3-5 the breakdown statistics of PP impregnated paper. The data are

Analyzed with Weibull (red) and log-normal (blue) distributions. Confidence

Levels at 10% are also shown with dashed lines.

3.2.3.3 Water in Paper:

Water in paper may be found in four states: It may be adsorbed to surfaces, as vapor, as free water in capillaries, and as imbibed free water. The paper can contain much more moisture than oil. For example, a 150 MVA, 400 kV transformer with about seven tons of paper can contain as much as 223 kg of water. The oil volume in a typical power transformer is about 80,000 liters. Assuming a 20 PPM moisture concentration in oil, the total mass of moisture is about 2 kg, much less than in the paper. The unit for moisture concentration in paper is typically expressed in %, which is the weight of the moisture divided by the weight of the dry oil-free pressboard.

Water vapor pressure is the partial pressure exerted by water vapor. When the system is in equilibrium with the liquid or solid form, or both, of the water, it reaches the saturation water vapor pressure. Saturation vapor pressure is a measure of the tendency of a material to change into the gaseous or vapor state, and it increases with temperature. At the boiling point of the water, the saturation water vapor pressure at the surface of water becomes equal to the atmospheric pressure.

Curves Methods for Water vapor pressure:

1. Fabre-Pichon Curves

Figure 3-6 Fabre-Pichon curves for moisture equilibrium of the air-oil-paper complex as a function of the air and oil surrounding the paper. Figure is taken from [Z] except labels are translated into English.

2. Oommen Curves

Oommen‟s method is based on the principle that the equilibrium curves represent the same relative saturation for the oil and for the paper at the same temperature The Moisture in Oil versus Relative Humidity curves are straight lines with the relationship

x,=x„,xR.H. (1)

Where x, is the moisture in oil in PPM, x“, is the water solubility in oil in PPM, and R.H. is the relative humidity of oil. Oommen used the oil equilibrium curves along with the Moisture in Wood Pulp versus Relative Humidity Curves and generated the moisture equilibrium curves for a paper-oil system shown in Figure ‎3-7. The dashed lines indicate desorption curves (diffusion of moisture out of cellulose), whereas the solid lines indicate the adsorption curves (diffusion of moisture into cellulose). For the same relative 27

humidity, the moisture content of the desorption curves is slightly higher than that of the adsorption curves.

The advantage of Oommen's method is that it is much easier to determine the water equilibrium between the gas space and paper without the presence of the liquid insulation, as the moisture diffusion coefficients of oil-impregnated pressboard are about two orders of magnitude smaller than those of oil-free pressboard.

Figure 3-8 Oommen‟s curves for moisture equilibrium for a paper-oil system

Figure 3-9 Moisture in wood pulp as a function of relative humidity of the ambient belfries .Picture supplied by Oommen at ABB-ET1 who redrew Jeffries‟ data.

3. equilibrium curves for water vapor pressure and moisture Content:

From both Fabre and Oommen's statements, we see that the equilibrium curves for water vapor pressure and moisture content in paper can be used to derive the partition curves between oil and paper.

C = 2.173 x 10-7 x Pv 0.6685 e (4725.61\T) (4)

Pv = 9.2683 x 1O 9 x c (1.4595) x e (9-7 069\T) (5)

Figure 3-10 Moisture in paper versus ambient relative humidity curves comparing

4. Grien's Curves

Figure 3-11Griffin curves for water equilibrium in cellulose mineral oil systems

5. MIT Curves

Figure 3-12 MIT-developed curves for water equilibrium in cellulose mineral oil systems for a wide range of moisture concentrations

Additional tests on mechanical properties would be significant to determine the full potential of the presented modified paper. Using low temperature dielectric measurements, in which most of the polarization effects were frozen at the lowest temperature, the porosity and the relative permittivity of paper were estimated. The dielectric breakdown characteristics of the PP impregnated paper were better than those of pristine paper.

3.2.4 Transformer oil

3.2.4.1 Parameters of Transformer Oil

The parameters of transformer oil are categorized as,

1. Electrical parameters: – Dielectric strength, specific resistance, dielectric dissipation factor.

Dielectric strength of transformer oil:

Is also known as breakdown voltage of transformer oil or BDV of transformer oil. Break down voltage is measured by observing at what voltage, sparking strands between two electrodes immerged in the oil, separated by specific gap. Low value of BDV indicates presence of moisture content and conducting substances in the oil. For measuring BDV of transformer oil, portable BDV measuring kit is generally available at site. In this kit, oil is kept in a pot in which one pair of electrodes are fixed with a gap of 2.5 mm (in some kit it 4mm) between them. Now slowly rising voltage is applied between the electrodes. Rate of rise of voltage is generally controlled at 2 KV/s and observe the voltage at which sparking starts between the electrodes. That means at which voltage dielectric strength of transformer oil between the electrodes has been broken down. Generally this measurement is taken 3 to 6 times in same sample of oil and the average value of these reading is taken. BDV is important and popular test of transformer oil, as it is primary indication of health of oil and it can be easily carried out at site.

Dry and clean oil gives BDV results, better than the oil with moisture content and other conducting impurities. Minimum breakdown voltage of transformer oil or dielectric strength of transformer oil at which this oil can safely be used in transformer, is considered as 30 KV.

Specific Resistance of Transformer Oil:

This is another important property of transformer oil. This is measure of DC resistance between two opposite sides of one cm3 block of oil. Its unit is taken as ohm-cm at specific temperature. With increase in temperature the resistivity of oil decreases rapidly. Just after charging a transformer after long shut down, the temperature of the oil will be at ambient temperature and during full load the temperature will be very high and may go up to 90oC at over load condition. So resistivity of the insulating oil must be high at room temperature and also it should have good value at high temperature as well. That is why specific resistance or resistivity of transformer oil should be measured at 27oC as well as 90oC. Minimum standard specific resistance of transformer oil at 90oC is 35 × 1012 ohm–cm and at 27oC it is 1500 × 1012 ohm–cm.

Dielectric Dissipation Factor of Tan Delta of Transformer Oil:

Dielectric dissipation factor is also known as loss factor or tan delta of transformer oil. When insulating materials is placed between live part and grounded part of an electrical equipment, leakage current will flow. As insulating material is dielectric in nature the current through the insulation ideally leads the voltage by 90o. Here voltage means the instantaneous voltage between live part and ground of the equipment. But in reality no insulating materials are perfect dielectric in nature. Hence current through the insulator will lead the voltage with an angle little bit shorter than 90o.

2. Chemical parameter: - Water content, acidity, sludge content.

Water Content in Transformer Oil:

Moisture or water content in transformer oil is highly undesirable as it affects adversely the dielectric properties of oil. The water content in oil also affects the paper insulation of the core and winding of transformer. Paper is highly hygroscopic in nature. Paper absorbs maximum amount of water from oil which affects paper insulation property as well as reduced its life. But in loaded transformer, oil becomes hotter, hence the solubility of water in oil increases as a result the paper releases water and increase the water content in transformer oil. Thus the temperature of the oil at the time of taking sample for test is very important. During oxidation acid are formed in the oil the acids give rise the solubility of water in the oil. Acid coupled with water

33 further decompose the oil forming more acid and water. This rate of degradation of oil increases. The water content in oil is measured as pm (parts per million unit).

Acidity of Transformer Oil:

Acidity of transformer oil, is harmful property. If oil becomes acidic, water content in the oil becomes more soluble to the oil. Acidity of oil deteriorates the insulation property of paper insulation of winding. Acidity accelerates thee oxidation process in the oil. Acid also includes rusting of iron in presence of moisture. The acidity of transformer oil is measure of its acidic constituents of contaminants. Acidity of oil is express in mg of KOH required to neutralize the acid present in a gram of oil. This is also known as neutralization number.

3. Physical parameters: - Inter facial tension, viscosity, flash point, pour point.

Inter Facial Tension of Transformer Oil:

Inter facial tension between the water and oil interface is the way to measure molecular attractive force between water and oil. It is measured in Dyne/cm or mili-Newton/meter. Inter facial tension is exactly useful for determining the presence of polar contaminants and oil decay products. Good new oil generally exhibits high inter facial tension. Oil oxidation contaminants lower the IFT.

Flash Point of Transformer Oil:

Flash point of transformer oil is the temperature at which oil gives enough vapors to produce a flammable mixture with air. This mixture gives momentary flash on application of flame under standard condition. Flash point is important because it specifies the chances of fire hazard in the transformer. So it is desirable to have very high flash point of transformer oil. In general it is more than 140o (>10o).

Pour Point of Transformer Oil:

It is the minimum temperature at which oil just start to flow under standard test condition. Pour point of transformer oil is an important property mainly at the places where climate is extremely cold. If the oil temperature falls below the pour point, transformer oil stops convection flowing and obstruct cooling in transformer. Paraffin based oil has higher value of pour point, compared to Naphtha based oil, Pour Point of transformer oil mainly depends upon wax content in the oil. As Paraffin based oil has more wax content, it has higher pour point.

Viscosity of Transformer Oil:

In few wards, viscosity of transformer oil can be said that viscosity is the resistance of flow, at normal condition. Obviously resistance to flow of transformer oil means obstruction of convection circulation of oil inside the transformer. A good oil should have low viscosity so that it offers less resistance to the convectional flow of oil thereby not affecting the cooling of transformer. Low viscosity of transformer oil is essential, but it is equally important that, the viscosity of oil should increase as less as possible with decrease in temperature. Every liquid becomes more viscous if temperature decreases.

3.2.4.2 Causes of Insulating Oil Deterioration

In normal service life of an oil filled equipment, the insulating oil inside the equipment is subjected to deterioration, due to various causes. In normal service span of the equipment there

are many maintenance occasions, when due to opening of blanking or any other reason the oil comes in contact with air. The air can also be entered in the oil due to unattended leakages in the body of the equipment.

Contact of atmospheric air with the oil causes unwanted oxidation reaction in the oil. These oxidation reactions are further accelerated due to temperature and presence of catalysts like solid copper, iron and other dissolved metallic materials in the oil. Temperature of oil rises due to loading of the equipment and any flash or arc formed in oil filled space of the equipment. As a result of these oxidation reactions the color of the oil becomes darker and resistivity of the insulating oil decreases. At the same time acidity of the oil increases. Also the tan delta or dielectric dissipation factor of the oil increases. That means over all insulating properties of the oil deteriorated. Deteriorated oil also affects the other insulating parts (mainly paper insulation) of the equipment. Deteriorated oil decreases the normal life span of the oil immersed equipment and may increase the no load (fixed) losses of the equipment.

3.2.4.3 Collecting Oil Sample from Oil Immersed Electrical Equipment

Process of Collecting Oil from Oil Immersed Electrical Equipment Like Transformer. We collect oil sample for performing different tests on the oil to determine different physical, chemical and electrical characteristics of the oil. The sample is the representative of the oil inside the equipment. That is why, we take special care during collecting oil sample from the equipment. Otherwise surrounding atmosphere may affect the characteristics of the oil which may differ the outcome of the tests form the actual results.

The normal procedures of collecting oil from a transformer or other oil immersed electrical equipment are given in following points

1. We should always take at least 1 to 2 liter of oil from bottom valve or oil sample valve (if available) depending upon the tests to be performed on that sample.

2. We should ensure that sampling is done by well experienced technical personnel.

3. We should not take sample of the oil in rainy, stormy, foggy, and snowy weather since it can affect the water contaminant of the oil. We should also avoid oil sampling in high atmospheric humidity for same reason.

4. We should use dry and clean containers preferably made of amber colored glass or clear glass or seamless stainless steel bottle. We can also use plastic bottle provided its suitability has been approved.

5. First we should run of some of the oil from oil sampling valve of the equipment to ensure the removal of any contaminants in the orifice of the valve.

6. Before toping up the bottle, we should rinse the sampling bottle with some oil

7. We should fill the bottle slowly and gently. We should allow the oil to flow against the wall of the bottle to avoid any formation of air bubble in oil so that no air can be trapped in the sample.

8. We should fill the bottle up to 95% of its capacity

9. Now we carefully close the bottle.

10. We must avoid any additive tape like thing at the cover.

11. Now, the bottle must be labeled with:

a. Name of the equipment from where sample is collected b. Rating of the equipment c. Serial No of the equipment d. Installed location of the equipment e. Other identifications of the equipment if required. f. Temperature of the oil at the time of collection g. Date of collection h. Identification of the sampling point (whether it is from top or bottom)

3.2.4.4 Acidity Test of Transformer Insulating Oil

Causes of Acidity in Insulating Oil

In insulating oil in the transformer may occasionally come into contact with air. It may be during opening any blanking or due to leakage in the oil tanks or in associated pipe lines. Because of that the oxidation reaction in the transformer oil takes place, which further be accelerated due to temperature and presence of catalysts like iron, copper and dissolved metallic compounds in the transformer oil.

Effects of Acidity in Insulating Oil

Increased acidity of the oil, causes decrease in resistivity of the oil. It also increases the dissipation factor of the oil. Excessive oxidation accelerates the slug formation rates in the oil. It may also causes abnormal deterioration of paper used for insulation in the transformer windings.

Acidity Test Kid

We can determine the acidity of transformer insulating oil, by a simple portable acidity test kid. It consists of one polythene bottle of rectified spirit (ethyl alcohol), one polythene bottle of sodium carbonate solution and one bottle of universal indicator (liquid). It also consists of clear and transparent test tubes and volumetrically scaled

How do we measure Acidity of Insulating Oil?

The acidity of insulating oil is generally measured by the required quantity in milligram of KOH to entirely neutralize the acidity of a specific quantity in gram of the oil. Acidity of an insulating oil is 0.3 mg KOH / g means 0.3 milligram of KOH is required to neutralize 1 gram of that insulating oil.

Principle of Acidity Test of Insulating Oil

When a specific quantity of alkali is added to a specific quantity of oil, the oil will either become acidic, neutral or alkaline depending upon the quantity of acid present in the sample. If the fixed quantity of added alkali is just same as the quantity of alkali required to neutralize the acid presents in the oil sample, the oil will have pH value of 7. If the fixed quantity of added alkali is

38 more than that required to neutralize entire acid in the oil sample, the oil becomes alkaline and it will have any pH value from 8 to 14 depending upon the quantity of acid was present in the oil. If the fixed quantity of added alkali is less than that required to neutralize entire acid in the oil sample, the oil becomes acidic and it will have any pH value from 0 to 6 depending upon the quantity of acid was present in the oil. The universal indicator is a chemical solution which gives different colors for different pH values of the oil. So, we can visually determine the pH value of the sample, hence the acidic nature of the oil by viewing its colors.

Procedure of Acidity Test for Insulating Oil

For that we have first to take exactly 1 gram of insulating oil. We normally do this by taking 1.1 milliliter of the oil to be tested by provided volumetric syringe. Actually, 1.1 milliliter oil is taken as 1 gram of oil.

Before test we have to extract the dissolved acid in the oil. That we do by adding exactly 1 ml of rectified spirit (ethyl alcohol) in the test sample. This is because the acid produced in the mineral oil is highly soluble in the alcohol.

After shaking the test sample well we add 1 ml of sodium carbonate in the sample. Sodium carbonate is the most suitable alkali for that purpose because it does not change its characteristics much when comes in contact with atmosphere during use.

At last after re-shaking the sample mixture we have to add 5 drops of universal indicator in the sample.

3.2.4.5 Observation

The color of the sample oil after mixing rectified spirit, sodium carbonate and universal indicator, describes the value of acidity of the oil sample in mg KOH / g as follows.

3.2.4.6 DGA or Dissolved Gas Analysis of Transformer Oil | Furfural or Furfur aldehyde Analysis

DGA or Dissolved Gas Analysis of Transformer Oil

Whenever electrical power transformer goes under abnormal thermal and electrical stresses, certain gases are produced due to decomposition of transformer insulating oil, when the fault is major, the production of decomposed gases are more and they get collected in Buchholz relay.

But when abnormal thermal and electrical stresses are not significantly high the gasses due to decomposition of transformer insulating oil will get enough time to dissolve in the oil. Hence by only monitoring the Buchholz relay it is not possible to predict the condition of the total internal healthiness of electrical power transformer. That is why it becomes necessary to analyses the quantity of different gasses dissolved in transformer oil in service. From dissolved gas analysis of transformer Oil or DGA of transformer oil, one can predict the actual condition of internal health of a transformer.

It is preferable to conduct the DGA test of transformer oil in routine manner to get prior information about the trend of deterioration of transformer health and life. Actually in dissolved gas analysis of transformer oil or DGA of transformer oil test, the gases in oil are extracted from oil and analyze the quantity of gasses in a specific amount of oil. By observing percentages of different gasses present in the oil, one can predict the internal condition of transformer.

Generally the gasses found in the oil in service are hydrogen (H2), methane (CH4), Ethane

(C2H6), ethylene (C2H4), acetylene (C2H3), carbon monoxide (CO), carbon dioxide (CO2), nitrogen (N2) and oxygen (O2).

Most commonly used method of determining the content of these gases in oil, is using a Vacuum Gas Extraction Apparatus and Gas Chronographs. By this apparatus first gasses are extracted from oil by stirring it under vacuum. These extracted gasses are then introduced in gas Chronographs for measurement of each component.

Generally it is found that hydrogen and methane are produced in large quantity if internal temperature of power transformer rises up to 150oC to 300oC due to abnormal thermal stresses. If o temperature goes above 300 C, ethylene (C2H4) are produced in large quantity. At the o temperature is higher than 700 C large amount of hydrogen (H2) and ethylene (C2H4) are produced.

Ethylene (C2H4) is indication of very high temperature hot spot inside electrical transformer. If

41 during DGA test of transformer oil, CO and CO2 are found in large quantity it is predicted that there is decomposition of proper insulation.

Furfural Test or Furfur aldehyde Analysis of Transformer Oil

Transformer core and winding have mainly paper insulation. Base of paper is cellulose. The Cellulose has a structure of long chain of molecules.

As the paper becomes aged, these long chains are broken into number of shorter parts. This phenomenon we often observe in our home. The pages of very old books become very much brittle.

In transformer, the aging effect of paper insulation is accelerated due to oxidation occurs in oil. When insulating paper becomes mechanically weak, it cannot withstand the mechanical stresses applied during electrical short circuit and leads to electrical breakdown. It is therefore necessary to monitor the condition of paper insulation inside a power transformer. It is not possible to bring out a piece of paper insulation from a transformer in service for testing purpose. But we are lucky enough, that there is a testing technique developed, where we can examine the condition of paper insulation without touching it. The method is called Furfur aldehyde analysis of in short Furfural test. Although by dissolved gas analysis one can predict the condition of the paper insulation primarily, but it is not very sensitive method. There is a guide line in IEC-599, where it is told that if the ratio of CO2 and CO in DGA results is more than 11, it is predicted that the condition of paper insulation inside the transformer is not good. A healthy cellulose insulation gives that ratio in a range of 4 to 11. But still it is not a very sensitive way of monitoring the condition of paper insulation. Because CO2 and CO gases also produced during oil breakdown and sometimes the ratio may misleads the prediction. When oil is soaked into paper, it is damaged by heat and some unique oil soluble compounds are realized and dissolved in the oil along with CO2 and CO. These compounds belong to the Furfur aldehyde group. These are sometimes called Furfural in short. Among all Furfurals compounds

Furfural is the most predominant. These Furfural family compound can only be released from destructive heating of cellulose or paper. Furfur aldehyde analysis is very sensitive as because damage of few grams of paper is noticeable in the oil even of a very large size transformer. It is a very significant diagnostic test. It is best test for assessing life of transformer. The rate of rise of percentage of Furfurals products in oil, with respect to time, is used for assessing the condition and remaining life of paper insulation in power transformer.

3.2.4.7 Principle of Water Content Test of Insulating Oil

The measure water content in an insulating oil we use Karl Fisher Titration as basic technique. In

Karl Fisher Titration, water (H2O) chemically reacts with iodine (I2), sulfur dioxide (SO2), an organic base (C5H5C) and alcohol (CH3OH) in an organic solvent.

Here, the sample is mixed with sulfur dioxide, iodide ions and organic base / alcohol. Here, iodide ions are produced by electrolysis and take part in above reactions in the solution. So there would not be any trace individual iodide ion in the solution as long as the reaction continues. The iodide ions produced by electrolysis are totally consumed in reaction as long as the water molecules present in the solution. As soon as there will be no more water molecule to react, the Karl Fisher Reactions stop.

There are two platinum electrodes immersed in the solution from beginning. Just after the Karl Fisher reaction is over the existence of iodide ions in the solution depolarizes the platinum electrodes. As a result the voltage current ratio of the electrode circuit changes. So, this change indicates the end point of the Karl Fisher Reaction in the solution.

According to the Faraday law of electrolysis, the quantity of iodine participated in the reaction is proportional to the electricity consumed for electrolysis during Karl Fisher Reactions. From, the consumption of electricity till the reaction ends, one can easily calculate the actual mass of

43 iodine participated in reaction. Again, from the first equation of reactions, it is found that one mole iodine reacts with one mole water. That means 127 grams iodine will react with 18 grams of water. So from calculated weight of iodine we can determine the exact quantity of water presented in the sample of insulating oil.

Which types of insulating oil can be tested?

While the generic term „oil‟ is almost universally used to describe insulating fluids, there are currently five different types of insulating fluid in common use. These are:

Mineral oil

High molecular weight hydrocarbon (HMWH) fluids

Silicone fluids

Synthetic ester fluids

Natural ester (vegetable oil) fluids

Why, when and how often is oil testing performed?

The dielectric breakdown voltage test is a relatively quick and easy way of determining the amount of contamination

In insulating oil. Usually the contaminant is water, but it can also be conductive particles, dirt, debris, insulating particles and the by-products of oxidation and aging of the oil.

For in-service equipment, the dielectric breakdown voltage test offers a useful and convenient way to detect Moisture and other contamination in the oil before it leads to a catastrophic failure. The information gained from the test can also be used as an aid to:

a. Predicting the remaining life of a transformer

b. Enhancing operational safety

c. Preventing equipment fires

d. Maintaining reliability

CHAPTER 4: Conclusion

The transformer is a very essential apparatus in an electric power system and its reliability is of utmost importance as a transformer failure results is a very costly and difficult to predict interruption of energy delivering .In turn, transformer‟s performance depends heavily on its insulation system; there for the insulation is perhaps the most critical transformer part.

Solid insulators as paper has been selected or designed as an electrical insulation material for parts and components in high voltage technology.

Several sets of classic moisture equilibrium curves were studied and a comparison is given for each method. Caution should be taken when using such curves because they differ from measurement techniques, data sources, and generating methods. An experimental case study shows that Oommen's curves match the experimental data best. When the system is not in equilibrium, these equilibrium curves cannot be used to find the moisture in paper. A three- wavelength interdigital dielectrometry sensor developed at the MIT High Voltage Research Laboratory is able to measure the spatial profile of the moisture distribution in the pressboard.

Insulating oil primary functions are to insulate, cool, prevent the chemical interactions and detecting the faults e.i (short circuit) in a transformer, there for it must have high dielectric strength, thermal conductivity and chemical stability.

CHAPTER 5: Recommendations

Power Transformers are one of the major components in power system that have to be chosen very carefully and efficiently and with aid of professional experiences to insure an effective and efficient functional life. Insulation material quality in a power transformer is extremely important that the operational lifetime of transformer depends on it , insulation materials quality is varied from one company to another and the higher the quality the higher the price , maybe the price is reasonable comparing to its service lifetime and its ability to resist the impacts of faults but it‟s a part of an engineer job to choose a high quality equipment‟s within low prices, as an example German transformers is extremely good but its price is high, and Chinese transformer have a low price but it may not have a satisfying specifications like the “XD TRANSFORMERS” in Garri that after its damage got replaced with Turkish transformer “BEST TRANSORMERS” that have satisfying specifications and a reasonable price. When power transformer is chosen several tests have to be performed as it stated in this report, pressed board and solid insulators tests are performed in the factories of the transformer but the oil tests are performed in the installation location and should be performed in a very professional way to overcome the goal of a high effective, efficient and satisfied functional lifetime.

REFRENCES

1. F. M. Clark, “Factors Affecting the Mechanical Deterioration of Cellulose Insulation,” Transactions of Electrical Engineering, Vol. 61, pp. 742-749, October 1942. 2. J. Fabre and A. Pichon, “Deteriorating Processes and Products of Paper in Oil. Application to Transformers,” 1960 International Conference on Large High Voltage Electric System (CIGRE), Paris, France, Paper 137, 1960. 3. H. I? Moser, Transformer board, Special print of Scientia Electrica, Translated by EHV-Weidmann Lim., St., Johnsbury, Vermont, USA, Section C, 1979. 4. A. J. Morin, M.Zahn, and J.R. Melcher, “Fluid Electrification Measurements of Transformer Pressboard/Oil Insulation in a Couette Charger,” IEEE Transactions on Electricallnsulation, Vol. 26, No. 5, pp. 870-901, October 1991. 5. A. I! Washabaugh, I? A. von Guggenberg, M. Zahn, and J. R. Melcher, “Temperature and Moisture Transient Flow Electrification Measurements of Transformer Pressboard/Oil Insulation Using a Couette Facility,” Proceedings of The 37d International Conference on Properties and Applications of Dielectric Materials, Vol. 2, Tokyo, Japan, 6. W A. Fessler, W J. McNutt, and T. 0. Rouse, “Bubble Formation in Transformers,” EPRI Report EL-5384, EPRI, Palo Alto, CA, August 1987. 7. l? J. Griffin and J. Christie, “Effects of Water and Benzotriazole on 49 Electrostatic Charge Generation in Mineral Oil/Cellulose Systems,” Proceedings: Static Electrification in Power Transformers, EPRI TR-102480, Project 1499-99, June 1993. 8. T. 0. Rouse, “Mineral Insulating Oil in Transformers,” Electrical Insulation Magazine, Vol. 14, No. 3, pp. 6-16, May/June 1998. 9. “Relative Humidity,” Britannica Online, 1994-1997 Encyclopedia Britannica, Inc, URL: http://www.britannica.com/. pp. 867-870, July 8-12, 1991. IO. G. Beer, G. Gasparani, F. Osimo, and F. Ross, “Experimental Data on The Drying-out of Insulation Samples and Test Coil for Transformers,” CIGRE Paper No. 135, 1966. 11. B. Fallou, “Summary of Work Done at L.C.I.E. on the Paper-Oil Complex,” internal report of Laboraroire Centre des Industries electriques, France. 12. T. V Oommen, “Moisture Equilibrium in Paper-Oil Systems,” Proceedings of the Electrical I Electronics Insulation Conference, Chicago, IL, pp. 162-166, October 3-6, 1983. 13. R. B. Kaufman, E. J. Shimanski, and K. W McFaydynen, “Gas and Moisture Equilibrium in Transformer Oil,” Communication and

Electronics, No. 19, pp. 312-318, July 1955. 14. D. N. Ewart, “Laboratory and Factory Measurements of Moisture Equilibrium in Power Transformer Insulation,” GE TIS Report 60PT44, August 1960. 15. E. T. Norris, “High Voltage Power Transformer Insulation,” Proceedings 50 I.E.E., Vol. 110, No.2, pp. 433, February 1963. 16. J. K. Nelson, “Dielectric Fluids in Motion,” IEEE Electrical Insulation Magazine, Vol. 10, No. 3, pp.16-28, MayIJune 1994. 17. A. G. Schlag, “The Recovery Voltage Method for Transformer Diagnosis,” Tettex Instruments booklet. 18. R. Jeffries, “The Sorption of Water by Cellulose and Eight Other Textile Polymers,” Jonrnal of the Textile Institute Transactions, Vol. 51, No. 9, pp. 339-374, 1960. 19. J. Reason, “Cost-effective Transformer Maintenance,” Electrical World, pp. 17-30, October 1997. 20. Handbook of Chemistry and Physics, D-94,46th Edition, the Chemical Rubber Co., Cleveland, Ohio, 1966. 21. J. D. Piper, „„Moisture Equilibrium between Gas Space and Fibrous Materials in Enclosed Electric Equipment,” AIEE Transactions, Vol. 65, pp. 791-797, December 1946. 22. A. R. Urquhart and A. M. Williams, “The Moisture Relations of Cotton, The Effect of Temperature on the Absorption of Water by Soda-Boiled Cotton,” Journal, Textile Institute (Manchester, England), Vol. 15, pp. T559,1924. 23. S. M. Neale and W A. Stringfellow, “The Primary Sorption of Water by Cotton,” Transactions, Faraday Society (London, England), Vol. 37, pp. 525, 1941. 24. C. C. Houtz and D. A. McLean, „Adsorption of Water by Papers at Elevated Temperatures,” Journal of Physical Chemistry, Baltimore, 51 25. L. M. Pidgeon and 0. Maass, “The Adsorption of Water by Wood,” Journal, American Chemical Society, Washington, D.C., Vol. 52, pp. 1053, 1930. 26. W W Guidi and H. P. Fullerton, “Mathematical Methods for Prediction of Moisture Take-up and Removal in Large Power Transformers,” Proceedings of IEEE Winter Power Meeting, C-74 Md., Vol. 43, pp. 309-321, 1939. 242-244, 1974. 27. T. V Oommen, “Moisture Equilibrium Charts for Transformer Insulation Drying Practice,” IEEE Transaction on Power Apparatus and Systems, Vol. PAS-103, No. 10, pp. 3063-3067, October 1984. 28. S. Glasstone, Textbook of Physical Chemistry, Second Edition, D. Van Nostrand Co., 1946, Chapter XIV 29. T. V Oommen, E. M. Petrie and S. R. Lindgren, “Bubble Generation in Transformer Windings Under Overload Conditions,” Minutes of the

Sixty-Two Annual International Conference of Doble Clients, Sec. 8-5.1, March 1995. 30. PJ. Griffin, C. M. Bruce, and J. D. Christie, “Comparison of Water Equilibrium in Silicone and Mineral Oil Transformers,” Minutes of the Fifty-Fifth Annual International Conference of Doble Clients, Sec. 3 1. S. D. Foss, “Power Transformer Drying Model,” Report prepared for General Electric Company, Large Transformer Operation, Pittsfield, MA, and Consolidated Edison Corp., New York, Ny, by Dynamic Systems, Pittsfield, MA, 1987. 52 32. I? F. Ast, “Movement of Moisture through A50P281 Kraft Paper (Dry and Oil-Impregnated,” test report HV-ER-66-41, General Electric, June 1966. 33. E. I<. Steele, “Moisture Redistribution in Simulated Transformer Paper-Oil Systems,” memo report MATL 70-37, General Electric, November 1970. 10-9.1, 1988. 34. Shell Diala Oils, Shell Oil Company, One Shell Plaza, 900 Louisiana Street, Houston, Texas 77002, (800)23 1-6950, Shell Lubricants Technical Bulletin SOC: 39-92. 35. Y Du, M. Zahn, A. VT Mamishev, and D. E. Schliclter, “ Moisture Dynamics Measurements of Transformer Board Using a Three-Wavelength Dielectrometry Sensor,” IEEE International Symposium on Electrical Insulation, Montreal, Quebec, Canada, pp. 53-56, June 1996. 36. Y Du, M. Zahn, and B. C. Lesieutre, “Dielectrometry Measurements of Effects of Moisture and Anti-Static Additive on Transformer Board,” IEEE Conference on Electrical Insulation and Dielectric Phenomena, Minneapolis, MN, USA, pp. 226-229, October 1997. 37. Moisture Equilibrium in Transformer Paper-Oil Systems, Y. Du, M. Zahn, B.C. Lesieutre, and A.V. Mamishev Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology and S. R. Lindgren Electric Power Research Institute