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Operation of the Power Transformer

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Any opinions, findings, conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of UK Essays.

Published: Wed, 21 Feb 2018

CHAPTER 1

INTRODUCTION

1.1 Project Overview

A factor of main economic importance and safety in electrical utilities and industrial customers of electricity is dependent on the operation of the power transformer. In the current economic situation, most of the supply utilities and industries tighten their control on production spending of capital and make savings in maintenance as well as ensuring the reliability of electricity supply. A power failure can increase the electrical loads. These loads will defer purchasing additional plant capacity and can cause the stress on the transformer increases. Thus, monitoring should be conducted to ensure the reliability of the net effect of the thermal voltage, electrical and mechanical service requirements brought about by the increase. Regular sampling and testing of insulation oil taken from the transformer is a valuable technique in the preventative maintenance program. The transformer can be used longer if a proactive approach undertaken based on the transformer oil’s condition.

During an operation of a power transformer, transformer oil is subject to form electrical and mechanical stresses. Besides that, there are also contaminations caused by chemical interaction with windings and other solid insulations, catalysed by high operating temperature. Consequently, the original chemical properties of transformer oil changes gradually, cause it no longer function effectively after many years. Therefore, this oil should be tested periodically to ascertain its basic electrical properties, and make sure it is suitable for further use or necessary actions like filtration has to be done. The details of conducting these test is available in the standards issued by the IEC, ASTM, IS, BS.

1.2 Background Problem

The dielectric strength of insulating oil is the oils ability to withstand electrical stress without failure. This test is done by applying a controlled ac voltage to two electrodes which are immersed in the insulating oil. The gap between two electrodes placed in a specified distance. The voltage recorded when the current arc across this gap is the dielectric strength breakdown strength of the insulating liquid. Contaminants such as water, carbon, sediment and conducting particles can reduce the dielectric strength of insulating oil. Clean dry oil has an inherently high dielectric strength but this does not indicates the absence of all contaminates, it may indicate that the amount of contaminants present between the electrodes is not large enough to affect the average breakdown voltage of the liquid.

Power transformers are often operated under aged conditions. Thus the moisture content in oil increases, aging products become dissolved and particles are dispersed. Besides that, transformers are operated under novel environmental conditions, were low or high pressures exist. A safe service necessitates the thorough investigation of these influences.

1.3 Problem Statement

Monitoring system of transformer oil existing is usually done in periodically. Duration of each use of transformer oil has been established within a time period for the replacement of the new transformer oil. So, the used transformer oil cannot be fully ensured in accordance with the standards set and this could cause a disruption in the operation of transformer. In addition, the monitoring system of transformer oil existing is expensive as well as the impact of waste oil is hazardous and cannot be disposed of. Thus, a permanent monitoring system of transformer oil with minimal costs should be established to ensure the transformer oil is always good quality to use.

1.4 Objectives

The objectives of the project are important to ensure the research will fulfill the solution of the problem of the research. There are intentions conducting the research are shown below:-

  1. To study on the transformer oil and the maintenance procedures.
  2. To design a dielectric test device for transformer oil with using commercial off-the-shelf (COTS) equipment.

1.5 Scopes

The scopes of the project are important to ensure every step is followed in completing the research. The scopes also could be important reference to gain related data or information of the research. Those are the scopes of the project:-

  1. To study on the quality of transformer oil.
  2. To study on the dielectric strength of transformer oil.
  3. To study the maintenance of oil immersed distribution transformer.
  4. Literature research about the monitoring of Dielectric Breakdown of transformer oil.
  5. To design a Dielectric Strength testing circuit.
  6. To analyze the result of Dielectric Strength testing.

1.6 Thesis Outline

In preparing this project, the development of any information obtained should be gathered and described in each chapter are contained in the project report. Each chapter will discuss some important issues.

Through this project, Chapter 1 as an introduction to the project discuss on overview of the project, background problem and problem statement. The objectives and scopes of the project were also discussed in this chapter.

Then, Chapter 2 will explain in an inclusive literature review of transformers, transformer oil, the methods of monitoring and maintenance of transformer oil, equipments or tools required and software programming suitable for design the Dielectric Strength testing circuit.

Next, Chapter 3 will describe the methodology used in preparing this project. This chapter is important to ensure that methods and tools used systematically and effectively.

Chapter 4 will give an explanation and analysis of the circuit to be designed. This chapter also includes the methods and results of tests carried out by using the circuit designed. Problems occur in doing this project and steps to overcome the problems also discussed in this chapter.

Finally, Chapter 5 which is the last chapter in this project as the conclusion of the project and some suggestions for further research on this project.

CHAPTER 2

LITERATURE REVIEW

2.1 Introduction

Transformer is one of the most useful appliances ever invented. Transformer can raise or lower the voltage or current in alternating current (AC) network, the circuit can be isolated from one another, and to increase or decrease the apparent value of a capacitor, inductor, or resistor. Furthermore, the transformer allows us to transmit electricity long distances and to circulate safely in factories and homes. (Electrical Machines, Drives, and Power Systems, 6th Edition).

The cost of a transformer is high. The failure of one transformer resulted in a loss in terms of the price of one transformer or in terms of energy supply disruptions to consumers. Therefore, to monitor the transformer oil is one the right way and good for detecting the causes of damage to transformers.

2.2 Transformer

Transformer is one of the most important electrical devices. Transformer is widely used in power systems and electronic devices. Transformer can also raise and lower voltage levels and the alternating current to suit application. Transformer can transfer power from one section to another on the same frequency but different voltage levels and currents. Transformer basically consists of two coils of a conductor which acts as an inductor electrically separate but magnetically attached.

Transformer consists of two loops wrapped around the core base, core and coil which are a part of the transformer structures. Figure 2.1 shows the general structure of a transformer.

When alternating current connected to the transformer primary windings, current will flow through the primary winding. Alternating current flows will create an alternating magnetic flux in the transformer core. The magnetic flux can flow to the secondary winding of the transformer through the transformer core.

According to the Faraday law, the electromotive force or voltage is induced in the coil-winding transformer when the flux is changes in value. Because of the magnetic flux in the transformer core is an alternating flux whose value is constantly changing over time, the electromotive force or voltage is always induced in the coil-winding transformer.

Electromotive force in the primary winding is known as the self-induced electromotive force is due to the flux generated by the coil itself. While the electromotive force induced in the secondary winding is known as mutual induction electromotive force due to the induced electromotive force is caused by magnetic flux generated from the primary winding.

In an ideal transformer, the induced voltage in the secondary winding (Vs) is comparable to the primary voltage (Vp), and is given by the ratio of the number of turns in the secondary (Ns) to the number of turns in the primary (Np) as follows:

VsVp= NsNp
(2.1)

By the selection of the ratio of turns, a transformer thus allows an AC voltage to be “stepped up” by making Ns greater than Np, or “stepped down” by making Ns less than Np.

There are many types of transformer are designed to meet the specific industrial applications. These include autotransformer, control, current, distribution, general-purpose, instrument, isolation, potential (voltage), power, step-up, and step-down.

To avoid rapid damage of the insulating materials inside a transformer, sufficient cooling of the windings and the core must be provided.

Indoor transformers below 200 kVA can be directly cooled by the natural flow of the surrounding air. The metallic housing is equipped with ventilating louvres so that the convection currents that can flow over the windings and around the core. Large transformers can be constructed in the same way, but the forced circulation of fresh air must be provided. Such as a dry-type transformers are used inside the building, away from the hostile atmosphere.

Distribution transformers below 200 kVA are usually immersed in mineral oil and sealed in a steel tank. Oil carries the heat away to the tank, which it is lost by radiation and convection to the outside air. Insulating oil is much better than air, consequently, it is often used in high voltage transformers.

As the power rating increased, external radiators are added to increase cooling surface of the tank contains oil. Oil circulates around the transformer windings and moving through the radiator, where heat released into the surrounding air. For still higher levels, cooling fans blow air over the radiators.

For transformers in the megawatt range, cooling can be effected by the oil-water heat exchanger. Hot oil drawn from the transformer tank is pumped into the heat exchanger where it flowing through the pipes that are in contact with cold water. Such as heat exchanger are very effective, but also very expensive, because water itself must continuously cool and recirculated.

Some large transformers are designed to have multiple ratings, depending on the cooling method used. Thus, the transformer may have triple ratings depending on whether it is cooled by:

  1. the natural circulation of air (AO) for 18000 kVA, or
  2. forced-air cooling with fans (FA) for 24000 kVA, or
  3. the forced circulation of oil accompanied by forced-air cooling (FOA) for 32000 kVA.

These elaborate cooling systems are nevertheless economical because they enable a much greater output from the transformer of a given size and weight. The type of transformer cooling is designated by the following symbols:

AA – dry-type, self-cooled

AFA – dry-type, forced-air cooled

OA – oil-immersed, self-cooled

OA/FA – oil-immersed, self-cooled/forced-air cooled

AO/FA/FOA – oil-immersed, self-cooled/forced-air cooled/forced-air, forced-oil cooled

The temperature rise by the resistance of oil-immersed transformers is either 55°C or 65°C. The temperature must be kept low to preserve the oil quality. By contrast, the temperature rise of dry-type transformer may be as high as 180°C, depending on the type of insulation used.

2.3 Transformer Oil

Transformer oil or insulating oil is usually a highly refined mineral oil that is stable at high temperatures and has excellent electrical insulating properties. It is used in oil-filled transformers. Transformer oil is like the blood in the body of transformer. It must be periodically tested to monitor condition of the transformer.

Transformer oil serves three basic functions which are to insulate, to cool and maintain the transformer functions at all times. To keep these functions the industry has agreed on certain standards. The two leading transformer oil specifications in the world are IEC 60296 and ASTM D 3487. In these standards there are many specific requirement and limits based on physical and chemical properties.

Many of these properties and their limitations derived from the chemistry of refined mineral oils in combination with application specific requirements of electrical insulation. In an age when alternative to mineral oil being developed, it is important both to know what is desirable and what is likely to achieved in technical terms. Whereas some brands of transformer oil could only meet the specifications, the others excel.

In the end, transformer oil consumers should decide which properties are most important to their intended use. Technical specifications also have an impact on issues such as asset management, maintenance planning and investment budget. To help make decisions in these areas it is helpful to have a basic understanding of the science underlying specifications and limitations.

In Malaysia, mostly used transformer oil is mineral crude oils (uninhibited mineral oils) which contains Paraffic, Naphteric or mixed. It is supplied by Hyrax Oil Sdn. Bhd.

2.3.1 Transformer Oil Properties

The main function of transformer oil is insulating and cooling of the transformer. Thus, it should have the following properties:

  1. High dielectric strength and good dielectric properties resulting in minimum power loss.
  2. Low viscosity improves cooling.
  3. Freedom from inorganic acids, alkali, and corrosive sulphur.
  4. Resistant to emulsification.
  5. Rapid settling of arc products.
  6. Low pour point.
  7. High flash point resulting in low evaporation losses due to high thermal stability.
  8. High resistivity gives better insulation values between windings.
  9. Excellent interfacial tension for quick water separation.
  10. Proven resistance to electrical stresses.
  11. High electrical strength.
  12. Remarkably low sludge and acidity formation in both ageing and oxidation tests gives longer life to oil and equipment during storage and service.

2.3.2 Theory of Transformer Oil Parameters

a) Water Content

The standard for measuring water content in oil is IEC 60814. (Marcel Dekker, 1990). The important function in transformer oil is to provide electrical insulation. When oil has higher moisture content, it can reduce the insulating properties of the oil, which can cause dielectric breakdown. This is the particular importance with fluctuating temperatures because, transformer will cools down if any dissolved water will become free and this oil become poor insulating power and fluid degradation. (Azliza binti Mohd Jelan,2009).

b) Breakdown Voltage

Dielectric strength is one of the important characteristic in insulation field. Breakdown voltage of the insulating material is the maximum electric field strength that it can be withstand intrinsically without breaking down and without failure of its insulating properties, dielectric strength also means that a certain configuration and electrode dielectric material that produces minimal damage to the electric field. (Rohaina bt Jaafar, 2003).

Breakdown strength in liquid according to various factors influenced in the experiment which is electrode material and surface state, geometry electrode, the presence of chemical pollutants, the presence of physical pollutants, oil molecular structure, temperature and pressure. There also various factors in the theory of voltage breakdown which is like electronic theory, suspended particle theory, cavitations theory and bubble theory were postulated. (Olive Oil from the Tree to the Table).

Dielectric strength also depends on the time and method of tension, purity materials, the type of tension as well as experimental and environmental parameters, until set of dielectric strength unique to the specific material is difficult, a range of values can be found and used for application purposes. (Noraniza binti Toriman, 2003).

2.3.3 Types of Transformer Oil

a) Mineral Transformer Oil (Mineral Based Oil)

A mineral oil is a liquid by product of the petroleum refineries to produce gasoline and other petroleum based products from crude oil. A mineral oil in this sense is transparent and colourless oil composed mainly of alkenes and cyclic paraffin, related to. Mineral oil is a substance of relatively low value, and it is produced in very large amounts. Mineral oil is available in light and heavy grades, and can often be found in drug stores. There are three basic classes of refined mineral oils:

  1. Paraffinic oils, based on n-alkenes.
  2. Naphthenic oils, based on cycloalkanes.
  3. Aromatic oils, based on aromatic hydrocarbons.

Table 2.1 Properties of Mineral Transformer Oil (http://www.substech.com)

Property

Value in metric unit

Value in US unit

Density at 60°F (15.6°C)

0.880 *10³

kg/m³

54.9

lb/ft³

Kinematic viscosity at 68°F (20°C)

22

cSt

22

cSt

Kinematic viscosity at 212°F (100°C)

2.6

cSt

2.6

cSt

Fire point

170

ºC

338

ºF

Pour Point

-50

ºC

-58

ºF

Flash point

160

ºC

320

ºF

Auto ignition point

280

ºC

536

ºF

Specific heat capacity

1860

J/(kg*K)

0.444

BTU/(lb*ºF)

Thermal conductivity at 20ºC (68ºF)

0.126

W/(m*K)

0.875

BTU*in/(hr*ft²*ºF)

Thermal expansion at 20ºC (68ºF)

7.5*10-4

ºCˉ¹

4.2*10-4

in/(in* ºF)

Breakdown strength

min.70

kV

min.70

kV

Dielectric dissipation factor at 90ºC (194ºF)

max.0.002

 

max.0.002

 

Permittivity at 20ºC (68ºF)

2.2

 

2.2

 

b) Silicon Transformer Oil (Polydimethylsiloxane based fluid)

Polydimethylsiloxane (PDMS) belongs to a group of polymeric organosilicon compounds that is often referred to as silicones. PDMS is the most widely used silicon-based organic polymer, and is known for its unusual rheological properties. PDMS is optically clear, and, in general, is considered to be inert, non-toxic and non-flammable. It is called dimethicone and is one of several types of silicone oil (polymerized siloxane). Its applications range from contact lenses and medical devices to elastomers; it is present, also, in shampoos, caulking, lubricating oils, and heat-resistant tiles.

Table 2.2 Properties of Silicon Transformer Oil (http://www.substech.com)

Property

Value in metric unit

Value in US unit

Density at 60°F (15.6°C)

0.960 *10³

kg/m³

59.9

lb/ft³

Kinematic viscosity at 68°F (20°C)

55

cSt

55

cSt

Kinematic viscosity at 212°F (100°C)

15

cSt

15

cSt

Fire point

min.350

ºC

min.662

ºF

Pour Point

max.-50

ºC

max.-58

ºF

Flash point

min.300

ºC

min.572

ºF

Auto ignition point

435

ºC

815

ºF

Specific heat capacity

1510

J/(kg*K)

0.360

BTU/(lb*ºF)

Thermal conductivity at 20ºC (68ºF)

0.15

W/(m*K)

1.019

BTU*in/(hr*ft²*ºF)

Thermal expansion at 20ºC (68ºF)

10.4*10-4

ºCˉ¹

5.8*10-4

in/(in* ºF)

Breakdown strength

50

kV

50

kV

Dielectric dissipation factor at 90ºC (194ºF)

max.0.001

 

max.0.001

 

Permittivity at 20ºC (68ºF)

2.7

 

2.7

 

c) Synthetic Transformer Oil (Organic Esters Based Fluid)

Synthetic oil is a lubricant consisting of chemical compounds which are synthesized using chemically modified petroleum components rather than whole crude oil. Synthetic oil is used as a substitute for lubricant refined from petroleum when operating in extremes of temperature, because it generally provides superior mechanical and chemical properties than those found in traditional mineral oils.

Table 2.3 Properties of Synthetic Transformer Oil (http://www.substech.com)

Property

Value in metric unit

Value in US unit

Density at 60°F (15.6°C)

0.970 *10³

kg/m³

60.6

lb/ft³

Kinematic viscosity at 68°F (20°C)

70

cSt

70

cSt

Kinematic viscosity at 212°F (100°C)

5.3

cSt

5.3

cSt

Fire point

322

ºC

612

ºF

Pour Point

-60

ºC

-76

ºF

Flash point

275

ºC

527

ºF

Autoignition point

438

ºC

820

ºF

Specific heat capacity

1880

J/(kg*K)

0.448

BTU/(lb*ºF)

Thermal conductivity at 20ºC (68ºF)

0.144

W/(m*K)

0.98

BTU*in/(hr*ft²*ºF)

Thermal expansion at 20ºC (68ºF)

7.5*10-4

ºCˉ¹

4.2*10-4

in/(in* ºF)

Breakdown strength

min.75

kV

min.75

kV

Dielectric dissipation factor at 90ºC (194ºF)

max.0.006

 

max.0.006

 

Permitivity at 20ºC (68ºF)

3.2

 

3.2

 

2.3.4 Transformer Oil Testing

Regular sampling and testing of insulation oil taken from the transformer is a valuable technique in the preventative maintenance program. The transformer can be used longer if a proactive approach undertaken based on the transformer oil’s condition. Hence, transformer oil must be periodically tested to ensure its basic electrical properties. These tests can be divided into:

a) Liquid Power Factor

The IEC standard method for this test is IEC 247. This involves measuring the power loss through a thin film of liquid test. Water, contamination, and the decay products of oil oxidation tend to increase the power factor of oil. (A Guide to Transformer Oil Analysis, by I.A.R. GRAY)

b) Dielectric Breakdown Strength

The dielectric breakdown voltage is a measure of the ability of the oil to withstand electric stress. Dry and clean oil showed the inherent high breakdown voltage. Free water and solid particles, especially the latter in combination with high levels of dissolved water, tend to migrate to areas of high electric stress and dramatically reduce the breakdown voltage. The measurement of breakdown voltage, therefore, serves primarily to indicate the presence of contaminants such as water or conducting particles. A low breakdown voltage can be indicating that one or more of these are present. However, a high breakdown voltage does not necessarily indicate the absence of all contaminants. This test was conducted in accordance with IEC 156. (A Guide to Transformer Oil Analysis, by I.A.R. GRAY)

c) Moisture

The purpose of dielectric tests are conducted is to ensure the monitoring moisture can be done directly. IEC 733 is a well established and can measure the moisture down to the low part of the million levels. While the acceptable values have been set by the voltage class for moisture, these are somewhat misleading. A truer picture of moisture in the transformer must be taken into account so that percentage saturation of the oil by moisture and percentage moisture by dry weight of the solid insulation can be calculated. (A Guide to Transformer Oil Analysis, by I.A.R. GRAY)

d) Neutralization Number (Acidity)

This value, measured by IEC standard method IEC 1125A reported as mg KOH / g sample, reports the relative amount of oil oxidation products, especially acids, alcohol and soap. As oil continues to oxidize, the acid increased gradually, generally over the years. Running the acid number regularly provides guidance as to how far oxidation of the oil has proceeded. The acceptable limit by the test is usually used as general guidelines to determine when the oil should be replaced or reclaimed. (A Guide to Transformer Oil Analysis, by I.A.R. GRAY)

e) Interfacial Tension

The test methods for interfacial tension (IFT), IEC 6295, measuring the strength in mN/m from the interface that will form between service aged oil and distilled water. Because the decay products of oil oxidation are oil and water soluble, their presence would tend to weaken the interface and reduce the interfacial tension value. (A Guide to Transformer Oil Analysis, by I.A.R. GRAY)

f) Colour/Visual

Field inspection of liquid insulation (IEC 296) includes examination for the presence of cloudy or sediment and the general appearance as well as a colour inspection. As oil ages, it will be darken gradually. Very dark oil or oil that changes drastically over a short period of time may indicate a problem. Any cloudiness or sediment indicates the presence of free water or particles that may be harmful to continued the equipment operation. Taken alone, without considering the past history or other test parameters, the colour is not very important to diagnose transformer problems. If the oil has an acrid or unusual odor, consideration should be given to carrying out further tests. (A Guide To Transformer Oil Analysis, by I.A.R. GRAY)

g) Sludge/Sediment

The IEC 296 test distinguishes between the sediment and sludge. Sediment is an insoluble substance present in the oil. Sediment may consist of insoluble oxidation or degradation products of solid or liquid materials, solid products such as carbon or metallic oxide and fibres or other foreign matter. Sludge is polymerized oxidation products of solid and liquid insulating material. Sludge is soluble in oil up to a certain limit. At sludge levels above this, the sludge comes out of the solution contributing an additional component to the sediment. The presence of sludge and sediment can change the electrical properties of the oil and prevent the exchange of heat, so encouraging damage to the insulating material. (A Guide to Transformer Oil Analysis, by I.A.R. GRAY)

h) Inhibitor Content

Inhibited oil deteriorates more slowly than uninhibited oil so long as active oxidation inhibitor is present. However, after the oxidation inhibitor is consumed, the oil can be oxidized at a higher level. Determination of oxidation inhibitor remaining in the in-service transformer oil is based on IEC 666. (A Guide to Transformer Oil Analysis, by I.A.R. GRAY)

i) Dissolved Gas Analysis

The purpose and functions of the DGA is to provide an indication as to whether there may be an active or incipient tran


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