Transformer Oil Or Insulating Oil Engineering Essay

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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.

http://www.electricityforum.com/images/electrical-transformer-design.jpg

Figure 2.1 General Structure of Transformer

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 in proportion 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:

(Equation 2.1)

By appropriate 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:

the natural circulation of air (AO) for 18000 kVA, or

forced-air cooling with fans (FA) for 24000 kVA, or

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.

TEMCo offers the largest selection of High Voltage Transformers.GE Ventilated Dry-type Transformer

Figure 2.2 Dry-Type Transformer Figure 2.3 Oil-Immersed Transformer

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 likened to be the blood within the transformer body. 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 stem 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 aid decisions in these fields 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. It should have the following properties:

High dielectric strength and good dielectric properties resulting in minimum power loss.

Low viscosity improves cooling.

Freedom from inorganic acids, alkali, and corrosive sulphur.

Resistant to emulsification.

Rapid settling of arc products.

Low pour point.

High flash point resulting in low evaporation losses due to high thermal stability.

High resistivity gives better insulation values between windings.

Excellent interfacial tension for quick water separation.

Proven resistance to electrical stresses.

High electrical strength.

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

Water Content

The standard for measuring water contain 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 may result in 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).

Breakdown Voltage

Dielectric strength is one of the important properties in insulation field. Breakdown voltage in insulating material is the maximum electric field strength that it can be withstand intrinsically without breaking down and without experiencing 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 is also very dependent 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).

Figure 2.4 Heating / Cooling Curve of Oil (Ahmad Norhakimi bin Ibrihim @ Ibrahim, 2010)

2.3.3 Types of Transformer Oil

Mineral Transformer Oil (Mineral Based Oil)

A mineral oil or liquid petroleum is a liquid by-product of the distillation of petroleum to produce gasoline and other petroleum based products from crude oil. A mineral oil in this sense is a transparent, colourless oil composed mainly of alkenes (typically 15 to 40 carbons) and cyclic paraffin, related to petroleum jelly (also known as "white petrolatum"). It has a density of around 0.8 g/cm3. Mineral oil is a substance of relatively low value, and it is produced in very large quantities. 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:

Paraffinic oils, based on n-alkenes.

Naphthenic oils, based on cycloalkanes.

Aromatic oils, based on aromatic hydrocarbons (not to be confused with essential oils).

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

°C

338

°F

Pour Point

-50

°C

-58

°F

Flash point

160

°C

320

°F

Auto ignition point

280

°C

536

°F

Specific heat capacity

1860

J/(kg*K)

0.444

BTU/(lb*°F)

Thermal conductivity at 20°C (68°F)

0.126

W/(m*K)

0.875

BTU*in/(hr*ft²*°F)

Thermal expansion at 20°C (68°F)

7.5*10-4

°Cˉ¹

4.2*10-4

in/(in* °F)

Breakdown strength

min.70

kV

min.70

kV

Dielectric dissipation factor at 90°C (194°F)

max.0.002

max.0.002

Permittivity at 20°C (68°F)

2.2

2.2

Silicon Transformer Oil (Polydimethylsiloxane based fluid)

Polydimethylsiloxane (PDMS) belongs to a group of polymeric organosilicon compounds that are commonly referred to as silicones. PDMS is the most widely used silicon-based organic polymer, and is particularly known for its unusual rheological (or flow) properties. PDMS is optically clear, and, in general, is considered to be inert, non-toxic and non-flammable. It is occasionally 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 (as dimethicone makes hair shiny and slippery), 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

°C

min.662

°F

Pour Point

max.-50

°C

max.-58

°F

Flash point

min.300

°C

min.572

°F

Auto ignition point

435

°C

815

°F

Specific heat capacity

1510

J/(kg*K)

0.360

BTU/(lb*°F)

Thermal conductivity at 20°C (68°F)

0.15

W/(m*K)

1.019

BTU*in/(hr*ft²*°F)

Thermal expansion at 20°C (68°F)

10.4*10-4

°Cˉ¹

5.8*10-4

in/(in* °F)

Breakdown strength

50

kV

50

kV

Dielectric dissipation factor at 90°C (194°F)

max.0.001

max.0.001

Permittivity at 20°C (68°F)

2.7

2.7

Synthetic Transformer Oil (Organic Esters Based Fluid)

Synthetic oil is a lubricant consisting of chemical compounds which are artificially made (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

°C

612

°F

Pour Point

-60

°C

-76

°F

Flash point

275

°C

527

°F

Autoignition point

438

°C

820

°F

Specific heat capacity

1880

J/(kg*K)

0.448

BTU/(lb*°F)

Thermal conductivity at 20°C (68°F)

0.144

W/(m*K)

0.98

BTU*in/(hr*ft²*°F)

Thermal expansion at 20°C (68°F)

7.5*10-4

°Cˉ¹

4.2*10-4

in/(in* °F)

Breakdown strength

min.75

kV

min.75

kV

Dielectric dissipation factor at 90°C (194°F)

max.0.006

max.0.006

Permitivity at 20°C (68°F)

3.2

3.2

2.3.4 Transformer Oil Testing

Regular sampling and testing of insulation oil taken from transformers is a valuable technique in a preventative maintenance program. If a proactive approach is adopted based on the condition of the transformer oil, the life of the transformer can be extended. Hence, transformer oil must be periodically tested to ensure its basic electrical properties. These tests can be divided into:

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. The new oil has very low power factor less than 0.1% at 25 ° C and 1.0% at 90 ° C. As the oil ages and moisture accumulates, or if the unit is contaminated, the liquid power factor tends to increase. Many owners make the mistake of having this transformer testing at only one temperature. While the test is more sensitive to 90 ° C, both the temperature should be used. The relationship between 25 ° and 90° values can assist in making the diagnosis as to whether the problem of moisture, oxidation, or contamination. (A Guide to Transformer Oil Analysis, by I.A.R. GRAY)

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)

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 transformer at 20 ° C that containing 20 ppm moisture in oil is considerably wetter than a same unit, with a similar 20 ppm moisture, but it is operating at 40 ° C. The new transformer must be less than 0.5% moisture by dry weight. Anything more than 3.0% (or 30% saturation) is considered very wet. Many owners dehydrate transformer when the moisture level exceed 1.5 to 2.0% moisture by dry weight. (A Guide to Transformer Oil Analysis, by I.A.R. GRAY)

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. Acceptable values for acid number are 0.20 and lower. Unacceptable values are over 0.20.These are the values that are used by TNB. (A Guide to Transformer Oil Analysis, by I.A.R. GRAY)

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. Brand new oil is often 40-50 mN/m. A value that is acceptable for the in-service oil is greater than 25 mN/m or greater; unacceptable results are below 28 mN/m. (A Guide to Transformer Oil Analysis, by I.A.R. GRAY)

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)

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)

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)

Dissolved Gas Analysis

In contrast to the tests and the methods discussed to this point, the dissolved gas analysis (DGA) did not measure the gradual changes in the quality of oil. DGA has a very limited utility in determining the continued suitability of the transformer oil. The purpose and functions of the DGA is to provide an indication as to whether there may be an active or incipient transformer fault affecting the operation and continued health of the equipment. DGA is used to detect and measure nine of dissolved gases which are Hydrogen, Oxygen, Nitrogen, Methane, Carbon Monoxide, Carbon Dioxide, Ethan, Ethylene, and Acetylene. (A Guide To Transformer Oil Analysis, by I.A.R. GRAY)

Dissolved Metals Analysis

Analysis of dissolved metals (in particular, for the three metals: iron, copper, and aluminium) can be used in further identifying the location of transformer faults discovered by dissolved gas analysis. For example, the dissolved metal analysis indicating the presences of conductor metals may indicate a fault is occurring in the winding or at a connection while the presence of iron indicates involvement of the core steel. (A Guide To Transformer Oil Analysis, by I.A.R. GRAY)

Furanic Compounds

When paper breaks down, the cellulose chains are broken and glucose molecules (which serve as the "building blocks" of the cellulose) are chemically changed. Each of the glucose monomer molecules that are removed from the polymer chain becomes one of a series of related compounds called "furans" or "furanic compounds". Because these furanic compounds are partially soluble in oil, they are present in both the oil and the paper. Measuring the concentration of the oil can tell us a little more about the paper. The standard method typically tests for five compounds that are normally only present in the oil as a result of the paper breaking down. Those five compounds, and their probable causes, are 5-hydroxymethyl-2-furaldehyde, 5H2F (typically formed by oxidation of paper), 2-furyl alcohol, 2FOL (typically formed in connection with a high moisture content), 2-furaldehyde, 2FAL (very common, formed by all overheating and aging conditions), 2-acetyl furan, 2ACF (very rare, may be related to electrical stress), and 5-methyl-2furaldehyde, 5M2F (typically formed as a result of overheating). (A Guide To Transformer Oil Analysis, by I.A.R. GRAY)

2.3.4 Instrument / Device for Transformer Oil Testing

Oil Test Set (Megger OTS 60 PB)

The OTS 60PB is a 0 - 60 kV, battery powered portable dielectric strength oil test set. Its size and weight make it suitable for on-site assessment of insulating oil quality. The dielectric strength test it performs is an important deciding factor in knowing whether to retain or replace the oil. Breakdown voltage is measured, averaged and displayed under the control of built-in programmed sequences. Go/no-go testing is available.

Figure 2.4 Oil Test Set (Megger OTS 60 PB)

OTS 60PB follows the oil testing sequences described in many national and other specifications among which are: British BS 148, BS 5730a (automatic proof testing), BS 5874; International IEC 156, American ASTM D877 ASTM D1816, German VDE 0370, French NFC 27, Spanish UNE 21, Italian CEI 10-1, Russian GOCT 6581, South African SABS 555, Australian AS 1767 and Institute of Petroleum IP 295. Two types of withstand (proof) testing of an oil sample are available. The principle with these tests is to subject the oil sample to a specified voltage for a defined length of time (1 minute) to see if it will withstand that voltage. In one of the tests the voltage is removed after a minute, in the other test, the voltage continues to rise after the minute until breakdown or the maximum value is reached. Withstand (proof) tests can be set up to the user's own requirements, and then repeatedly called up to quickly test oil under known fixed conditions.

The OTS 60PB is used for determining the dielectric strength of liquid insulants such as insulating oils used in transformers, switchgear, cables and other electrical apparatus. It is portable and suitable for testing on site as well as in the laboratory. The test set is fully automatic. The operator has only to prepare the test vessel, load it with sample oil, place it in the test chamber, select the appropriate specification for the tests and then start the test sequence. The test set carries out automatically (and if necessary unattended) the sequence of tests as defined by the pre-selected national specification. Oil testing specifications, for which the set is pre-programmed, are as follows:-

Figure 2.5 Oil testing specifications

A 5 minute test sequence is also provided so that the operator may quickly obtain an idea of the breakdown value of an oil sample. Two types of semi automatic withstand (proof) testing of an oil sample are available. The principle with these tests is to subject the oil sample to a specified voltage for a defined length of time (1 minute) to see if it will withstand that voltage. In one of the tests the voltage is removed after a minute, in the other test the voltage continues to rise after passing for one minute until breakdown or the maximum value is reached. Withstand (proof) tests can be set up to the user's own requirements, and then repeatedly called up to quickly test oil under known fixed conditions. The test results can be reviewed on the LCD or printed via the RS232 interface. An optional, battery operated printer is available to obtain a hard copy of the results. The safety features incorporated in the test set's design include two forced break switches used as described in BS 5304. These are interlocked with the oil vessel loading door.

Volumetric titration system Metrohm Titrino SM 702

An automatic potentiometric titration system "Titrino SM 702 with Exchange Unit 806" made by Metrohm measured the acidity of the oils. Here the Total Acid Number (TAN) was determined by a volumetric titration with potash to neutralize the carboxylic acids. The titration took place as follows: At first 10 g of the oil were dissolved in 40 ml of solvent toluene / ethanol in a ratio of 5 to 4. Potash (KOH, 0,1 mol/l) was added as titre with volume increments of 0.001 ml or 0.005 ml depending on the expected acidity. The system detects, when the acid-base-equivalence-point EP is reached by a voltage measurement in the solution. From the volume of potash at the EP equation below calculates the acidity as TAN:

TAN - total acid number

EP1 - equivalent point

C31 - blind value of the solvent toluene/ethanol

CO1 - 0.1 mol/L, concentration of titre

CO2 - 1

CO3 - 56106 g/mol, molar mass of titre

CO0 - weight of the oil sample

Figure 2.6 Volumetric itration systems Metrohm Titrino SM 702

Kelman TRANSPORT X Portable DGA Unit And Moisture In Oil

Dissolved Gas Analysis (DGA) is an established technique and is recognised as the most important test in monitoring power transformers. It is now being successfully extended to other oil filled equipment such as tap changers and circuit breakers. The TRANSPORT X unit has been designed to be very rugged and user friendly with an emphasis placed on field operation. The unit is used by over 200 companies and utilities and has sold in excess of 600 units worldwide.

Figure 2.7 Kelman TRANSPORT X Portable DGA Unit And Moisture In Oil

The TRANSPORT X test uses state of the art infrared measurement technology to give accurate, reliable results in a matter of minutes. The TRANSPORT X product represents an invaluable tool for Asset Management and will increase the power of any DGA program. Extensive field and laboratory use worldwide has proven that the TRANSPORT X test gives highly reliable results and that it is genuinely suitable for field conditions. The TRANSPORT X equipment minimizes the risk of carryover between tests. With the ability to go from high gassed samples (such as tap changers) to subsequent low gassed samples (such as main tanks) with no contamination of results the user can confidently test all types of oil filled equipment.

Internal diagnostic software helps to translate ppm data into valuable information by employing standard DGA interpretation rules e.g. Duval's triangle, key gas analysis etc. These established algorithms assist the user to analyse the condition of the transformer. The accompanying TransportPro PC software allows the user to download records to a PC database for export to Kelman PERCEPTION software or Excel.

2.4 Monitoring Method

Basically, there are two method of monitoring transformer oil which is on-line monitoring and off-line monitoring.

2.4.1 On-Line Monitoring

On-line monitoring and predictive technologies that have been used can reduce the inherent deficiencies in many current maintenance practices. Many of these technologies have become more intelligent so that it requires less expertise in interpreting the results. It is not always valid, and difficult for companies to hire enough qualified workers in each location to understand the various kinds of data from various types of equipment. Many monitors provide more information than the normal end user can understand, but is available to members for additional diagnostics and prognostics. A new acronym has emerged to Intelligent Electronic Devices (IED).

Unfortunately there is no technology that is the "Holy Grail" for the assessment of electrical equipment. In many cases, several technologies must be used to perform a complete diagnosis. Most of the monitoring system designed to warn the user or unusual problems and provide additional diagnostic data. Most of these technologies have been built in communications capability that allows you to forecast long-distance monitoring through Ethernet, serial communications such as Modbus, DNP 3.0 or customized data streams and wireless modems, and analog. Remote monitoring allows companies to streamline forecasting expertise in centralized locations or outsourcing to the right experts. This can be done continuously or periodically or event driven. Event driven systems send out reminders via, pager, phone, e-mail or fax to the appropriate person, and then communicate back to the monitor for further analysis and recommendations.

Most industrial facilities do very little monitoring of their Large Power Transformers. Modern day systems now control and monitor all aspects of a transformer including temperatures, loads, cooling systems, pressures, bushings and windings. Four great examples include:

The monitoring of the loads on the cooling fans and pump circuits to indicate abnormal conditions such as locked rotor or loss of cooling capacity.

Monitoring the temperature differential across the connection board between the main tank and the load tap changer compartment

Continuous monitoring of the power factor and capacitance of High Voltage Bushing

Central data concentrator and communication RTU for all third parties monitors such as Partial Discharge (PD) and DGA.

2.4.2 Off-Line Monitoring

Off-line tests are "go/no go" tests. Most of the techniques whether chemical or electrical methods, or destructive or non-destructive methods, only provide partial information about the state of the insulation condition of power transformers. More advanced condition monitoring or condition assessment techniques have been developed and are now starting to come into more general use. They have been developed in response to the need for new materials assessment methods. However, in some advanced diagnostics tools are still in the developmental stage, either in the technical development or, more likely, in the methods of analysis and interpretation of the test data. Examples of Off-Line Monitoring:

Recovery Voltage Measurement (RVM)

Polarization and Depolarization Current Measurement (PDC)

Frequency Domain Dielectric Spectroscopy (FDS)

Frequency Response Analysis (FRA)

PD Measurement

RVM, PDC & FDS are based on the use of the dielectric response of insulating materials to the application of electric fields - Conductivity, Polarization & Dielectric Response.

2.4.3 Monitoring Method of Dielectric Breakdown

There are several existing methods for measuring the dielectric strength of such interference, ASTM D877 ASTM D1816 and IEC 60156 method. . While these methods can often be performed on-site with portable equipment and are valuable laboratory tests, they suffer from poor repeatability, and due to their destructive nature, cannot be used on-line. Progress has been made in controlling the destructive energy released by the test device (American Society for Testing and Materials (ASTM), West Conshohocken, PA, 1999), but not to the level or the payment to be suitable for use on-line.

Like the arcs produced in contacting equipment, the arcs produced in the test instruments degrade the breakdown strength of the oil. Since the dielectric breakdown strength is the quantity under measurement, this limits the number of successive tests that can be run on a given sample. In the ASTM method, five-shot test performed on the samples provided before should be discarded. With only five samples, it may be difficult to obtain statistically valid representation of oil and, in fact, the ASTM standard allows the range of 92% of means a five-shot test will be valid. (ASTM International, West Conshohocken, PA, 2005)

With some kind of chemistry and physical particulate contaminants are generally present, the oil can be a homogeneous media. Temperature variations can locally influence the relative saturation of moisture; turbulence of flow and proximity to sources of pollution can influence the type and concentration of particulate contaminants. So, it may be difficult to obtain a representative sample of the actual state of the oil with a small sample size used in the existing instruments and the limited number of test specimens to be drawn from the size of the oil compartment.

The inhomogeneity of oil, combine with the fact that the number of test shots that can be restricted more to reduce the ability of existing test methods. It is not surprising that while the dielectric breakdown strength are important, lack of faith is placed on the ability of standard test methods to measure accurately.

The key to both improving the accuracy of laboratory test methods and enabling on-line testing is the reduction of energy dissipated during the breakdown of the oil. If the shot did not damage the oil test, more test sample shots can be done and more accurate statistics can be developed.

Traditionally, the test method is to use a big scheme, but simply to generate the high voltage necessary to break down the oil. This device comprises essentially a variable autotransformer which is used to increase tension in the cell test until damage occurs, at which point the relay to close off the current transformers. With all the energy stored in magnetic and capacitance of the transformer, the energy released into the cell into the test after the relay shut off (assuming that the relay works in real time) to several tens of joules.

Many cheaper dielectric breakdown test set available today is still depending on the approach of the variable autotransformer. Recently, efforts have been made to reduce the energy lost during the damage even by using a resonant test set. These test sets limit the stored energy available during a breakdown event and can very quickly detect a breakdown and de-energize the test set. Such sets are capable of limiting the energy dissipated during a breakdown to a mere 20 mJ (American Society for Testing and Materials, West Conshohocken, PA, 1999). Unfortunately, this advanced capability comes at a price of complexity and cost, making this device a laboratory test device which is very good, but not suitable for use on-line.

2.5 National Instrument Company and Products

National Instruments or in the other word is NI, is an American company with approximately 5,000 employees and does direct operations in 41 countries all over the world (National Instrument). The company's headquarter is in Austin, Texas and it is a producer of virtual instrumentation software and automated test equipment. The software products include LabVIEW, a graphical development environment, LabWindows/CVI, which provides VI, tools for C, TestStand, a test sequencing and management environment, and Multisim, which is formerly Electronics Workbench is an electrical circuit analysis program. Hardware products is including the VXI, VMEbus, and PXI frames and modules, as well as interfaces for GPIB, I²C, and other industrial automation standards. The company also sell real-time embedded controllers, including CompactRIO and Compact FieldPoint. Applications which commonly used is data acquisition, instrument control and machine vision.In 2006, the company had sold products to more than 25,000 companies in 90 countries with revenues of $660 million. National Instrument also is in the list of 100 best companies in the world (National Instruments (press release), 2009).

2.5.1 NI USB - TC01 Thermocouple Measurement Device

The NI USB-TC01 thermocouple measurement device features NI InstantDAQ technology so that can instantly take temperature measurements with the PC. Just plug it in and built-in software for viewing and logging data automatically loads. No driver installation is necessary. Connect to any USB port to use the PC as a display and monitor data in real time. The USB-TC01, which is compatible with J, K, R, S, T, N, and B thermocouples, uses a standard miniplug for easy thermocouple connection. Additional applications for alarming, triggering, and scheduled data logging are available as free downloads. For even further customization, it can build the applications with NI LabVIEW graphical programming and NI-DAQmx driver software. (http://sine.ni.com/ds/app/doc/p/id/ds-215/lang/en)

Figure 2.8 NI USB-TC01 Thermocouple Measurement Devices

2.5.1.1 NI InstantDAQ Technology

The USB-TC01 features NI InstantDAQ technology that automatically loads software for viewing and logging data after connecting the device to the computer. No prior driver installation is necessary. Simply plug the device into the USB port, and the USB-TC01 launch screen is loaded from built-in memory on the device. 

The launch screen displays the current thermocouple reading so that can configure the thermocouple type and temperature units. It can log data with the temperature logger application, open and customize the temperature logger source code in LabVIEW. (http://sine.ni.com/ds/app/doc/p/id/ds-215/lang/en).

Figure 2.9 USB-TC01 Launch Screen

2.5.1.2 Built-In Temperature Logger

Directly from the USB-TC01 launch screen, it can load the USB-TC01 temperature logger. With the temperature logger, it can graph live measurements from the USB-TC01 and log data with timestamps to a text file. (http://sine.ni.com/ds/app/doc/p/id/ds-215/lang/en).

Figure 2.10 USB-TC01 Temperature Logger

Figure 2.11 USB-TC01 Temperature Logger Block Diagram

2.5.1.3 Taking Measurements with Software

Logging Temperature

From the NI USB-TC01 Launch Screen, click "Temperature Logger". In the NI USB-TC01 Temperature Logger window that opens, select the "Thermocouple Type" and "Temperature Units". If want to capture, or log, the temperature readings, select "Log Data". Click "Start" to acquire NI USB-TC01 and graphs the temperature until click "Stop". Click "View Log" to open the log file. (USER GUIDE AND SPECIFICATIONS; NI USB-TC01 Single Channel Thermocouple Input Module).

Downloading Additional Application

Additional ready-to-run applications that provide added functionality for the NI USB-TC01 is available as free download. It can access these applications by selecting "Do More with your NI USB-TC01" from the NI USB-TC01 Launch Screen. (USER GUIDE AND SPECIFICATIONS; NI USB-TC01 Single Channel Thermocouple Input Module).

Creating Custom Software

In addition to taking measurements with the NI USB-TC01 Launch Screen, it can also build custom software for the NI USB-TC01 with LabVIEW and NI-DAQmx driver software. LabVIEW uses graphical icons and wires that resemble a flowchart, so it can graphically wire together function blocks to create own applications for logging data, alarming, triggering, reporting, and performing real-time data analysis. To learn more, select "Do More with your NI USB-TC01" from the NI USB-TC01 Launch Screen. (USER GUIDE AND SPECIFICATIONS; NI USB-TC01 Single Channel Thermocouple Input Module).

2.5.1.4 Connecting Input

The NI USB-TC01 provides connections for one thermocouple. Thermocouple types J, K, R, S, T, N, E, and B are supported. The NI USB-TC01 has a two-prong uncompensated thermocouple input that accepts a standard two-prong male mini thermocouple connector.

Figure 2.12 NI USB-TC01 Terminal Assignments

Connect the positive lead of the thermocouple connector to the TC+ terminal, and the negative lead of the thermocouple connector to the TC- terminal. Figure 2.13 shows the NI USB-TC01 terminal assignments. If it is unsure which of the thermocouple leads is positive and which is negative, check the thermocouple documentation or the thermocouple wire spool.

Figure 2.13 Connecting a Thermocouple Input Signal to the NI USB-TC01

For best results, NI recommends the use of insulated or ungrounded thermocouples when possible. If need to increase the length of the thermocouple, use the same type of thermocouple wires to minimize the error introduced by thermal EMFs. Temperature measurement errors depend in part on the thermocouple type, the temperature being measured, the accuracy of the thermocouple, and the cold-junction temperature. Error graphs for each thermocouple type connected to the NI USB-TC01 are shown in the specifications section. (USER GUIDE AND SPECIFICATIONS; NI USB-TC01 Single Channel Thermocouple Input Module).

2.5.1.5 NI USB-TC01 Circuitry

The NI USB-TC01 device's thermocouple channel passes through a differential filter and is sampled by a 20-bit analog-to-digital converter (ADC), as shown in Figure 2.13. (USER GUIDE AND SPECIFICATIONS; NI USB-TC01 Single Channel Thermocouple Input Module).

Figure 2.13 NI USB-TC01 Input Circuitry

2.6 Discharge Circuit

2.6.1 High Voltage Zappers

Circuit shown in figure 2.14 is use to generates a high voltage by discharging the energy stored in a large-value capacitor through the primary winding of a high-turns-ratio step-up transformer is known a Capacitor-Discharge (CD) system. It is the same concept used by many of the high-performance auto-ignition systems to produce a super-hot spark. It's also the same kind of system used by some of the top of the line electric fence chargers. Electronic device such as Stun-Gun which selling now in market is also generates its zap with capacitor-discharge circuit.

Figure 2.14 High Voltage Zapper circuit

To achieve a maximum spark CD ignition coil should be selected, and use a 440uF, 75-100V DC electrolytic capacitor for C1. Using a DC voltmeter, monitor the voltage across C1. Adjust R4 so that the Q3 fires when the charging voltage across C1 reaches between 50-55 volts. That setting should produce a spark 1.25 to 1.5 inches long every second or so. To obtain a faster pulse rate, with some reduction in the output, change C1 to a 10uF, 220VAC motor capacitor. Experiment with different component values to obtain the desired results.

2.6.2 Vacuum Discharge Driven by a Magnetic Pulse Compression Circuit

Low pressure discharges in a compact coaxial geometry were produced by applying either positive or negative high voltage pulses delivered by a three-stage magnetic pulse compression circuit. The driver provided repetitive pulses of up to 15 kV and 20 J maximum transferred energy per pulse. The inner electrode was a rod having either a pointed or flat end with sharp edges. When using the negative flat-end electrode, the breakdown occurred down to lower pressures (about 2.5mPa).The discharge was continued inside the inter electrode gap, and the discharge current had higher values (around 4 kA), the discharge characteristics being very reproducible. The measurements suggested that the field effect was responsible for the discharge onset in this configuration.

Figure 2.15 Schematic diagram of Vacuum discharge circuit

On the other hand, low pressure discharges are interesting from the point of view of plasma radiation sources (e.g. lasers, electron sources, flash X-ray sources). For such radiation sources, it is important to have very reproducible breakdown, plasma development and emission parameters. A high-power low-pressure discharge can be also used as fast switch or as triggering means for more powerful discharge configurations. This circuit concern on the breakdown and discharge characteristics for a large range of working power.

2.6.4 Charge/Discharge Equalization Management Circuit

Figure 2.18 shows a circuit is composed by one switch pipe Q, one diode D and one inductance L. The connection mode is that after Q and D is parallel connected, they are connected with L in series, and then respectively connected with the anode and the cathode of the battery, where, the cathode of D connects the anode of the battery and L connects the cathode of the battery. In the automatic equalization equipment of series-wound storage battery pile, various equalization circuits are series-wound.

Figure 2.18 Principle of Charge/Discharge circuit

`This circuit can be used with charge management and discharge management at the same time, and they are independent each other, and the equalization manager can be started in any stage of charge/discharge. The equalization voltage management of charge/discharge enhances the coherence of the single battery, reduces the accumulated influences of disequilibrium factors, and better solves the problem of a great lot of battery discarding induced by hybrid series-wound batteries with differences in the electrical cars.

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