transformer engineering design and practice 1_phần 3 docx

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Transformer Fundamentals

1.1 Perspective

A transformer is a static device that transfers electrical energy from one circuit to

another by electromagnetic induction without the change in frequency. The

transformer, which can link circuits with different voltages, has been instrumental

in enabling universal use of the alternating current system for transmission and

distribution of electrical energy. Various components of power system, viz.

generators, transmission lines, distribution networks and finally the loads, can be

operated at their most suited voltage levels. As the transmission voltages are

increased to higher levels in some part of the power system, transformers again

play a key role in interconnection of systems at different voltage levels.

Transformers occupy prominent positions in the power system, being the vital

links between generating stations and points of utilization.

The transformer is an electromagnetic conversion device in which electrical

energy received by primary winding is first converted into magnetic energy which

is reconverted back into a useful electrical energy in other circuits (secondary

winding, tertiary winding, etc.). Thus, the primary and secondary windings are not

connected electrically, but coupled magnetically. A transformer is termed as either

a step-up or step-down transformer depending upon whether the secondary

voltage is higher or lower than the primary voltage, respectively. Transformers can

be used to either step-up or step-down voltage depending upon the need and

application; hence their windings are referred as high-voltage/low-voltage or

high-tension/low-tension windings in place of primary/secondary windings.

Magnetic circuit: Electrical energy transfer between two circuits takes place

through a transformer without the use of moving parts; the transformer therefore

has higher efficiency and low maintenance cost as compared to rotating electrical

Copyright © 2004 by Marcel Dekker, Inc.

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machines. There are continuous developments and introductions of better grades

of core material. The important stages of core material development can be

summarized as: non-oriented silicon steel, hot rolled grain oriented silicon steel,

cold rolled grain oriented (CRGO) silicon steel, Hi-B, laser scribed and

mechanically scribed. The last three materials are improved versions of CRGO.

Saturation flux density has remained more or less constant around 2.0 Tesla for

CRGO; but there is a continuous improvement in watts/kg and volt-amperes/kg

characteristics in the rolling direction. The core material developments are

spearheaded by big steel manufacturers, and the transformer designers can

optimize the performance of core by using efficient design and manufacturing

technologies. The core building technology has improved from the non-mitred to

mitred and then to the step-lap construction. A trend of reduction of transformer

core losses in the last few years is the result of a considerable increase in energy

costs. The better grades of core steel not only reduce the core loss but they also

help in reducing the noise level by few decibels. Use of amorphous steel for

transformer cores results in substantial core loss reduction (loss is about one-third

that of CRGO silicon steel). Since the manufacturing technology of handling this

brittle material is difficult, its use in transformers is not widespread.

Windings: The rectangular paper-covered copper conductor is the most

commonly used conductor for the windings of medium and large power

transformers. These conductors can be individual strip conductors, bunched

conductors or continuously transposed cable (CTC) conductors. In low voltage

side of a distribution transformer, where much fewer turns are involved, the use of

copper or aluminum foils may find preference. To enhance the short circuit

withstand capability, the work hardened copper is commonly used instead of soft

annealed copper, particularly for higher rating transformers. In the case of a

generator transformer having high current rating, the CTC conductor is mostly

used which gives better space factor and reduced eddy losses in windings. When

the CTC conductor is used in transformers, it is usually of epoxy bonded type to

enhance its short circuit strength. Another variety of copper conductor or

aluminum conductor is with the thermally upgraded insulating paper, which is

suitable for hot-spot temperature of about 110°C. It is possible to meet the special

overloading conditions with the help of this insulating paper. Moreover, the aging

of winding insulation material will be slowed down comparatively. For better

mechanical properties, the epoxy diamond dot paper can be used as an interlayer

insulation for a multi-layer winding. High temperature superconductors may find

their application in power transformers which are expected to be available

commercially within next few years. Their success shall depend on economic

viability, ease of manufacture and reliability considerations.

Insulation and cooling: Pre-compressed pressboard is used in windings as

opposed to the softer materials used in earlier days. The major insulation (between

windings, between winding and yoke, etc.) consists of a number of oil ducts

Copyright © 2004 by Marcel Dekker, Inc.

Transformer Fundamentals 3

formed by suitably spaced insulating cylinders/barriers. Well profiled angle rings,

angle caps and other special insulation components are also used.

Mineral oil has traditionally been the most commonly used electrical insulating

medium and coolant in transformers. Studies have proved that oil-barrier

insulation system can be used at the rated voltages greater than 1000 kV. A high

dielectric strength of oil-impregnated paper and pressboard is the main reason for

using oil as the most important constituent of the transformer insulation system.

Manufacturers have used silicon-based liquid for insulation and cooling. Due to

non-toxic dielectric and self-extinguishing properties, it is selected as a

replacement of Askarel. High cost of silicon is an inhibiting factor for its

widespread use. Super-biodegradable vegetable seed based oils are also available

for use in environmentally sensitive locations.

There is considerable advancement in the technology of gas immersed

transformers in recent years. SF6 gas has excellent dielectric strength and is non￾flammable. Hence, SF6 transformers find their application in the areas where fire￾hazard prevention is of paramount importance. Due to lower specific gravity of

SF6 gas, the gas insulated transformer is usually lighter than the oil insulated

transformer. The dielectric strength of SF6 gas is a function of the operating

pressure; the higher the pressure, the higher the dielectric strength. However, the

heat capacity and thermal time constant of SF6 gas are smaller than that of oil,

resulting in reduced overload capacity of SF6 transformers as compared to oil￾immersed transformers. Environmental concerns, sealing problems, lower

cooling capability and present high cost of manufacture are the challenges which

have to be overcome for the widespread use of SF6 cooled transformers.

Dry-type resin cast and resin impregnated transformers use class F or C

insulation. High cost of resins and lower heat dissipation capability limit the use of

these transformers to small ratings. The dry-type transformers are primarily used

for the indoor application in order to minimize fire hazards. Nomex paper

insulation, which has temperature withstand capacity of 220°C, is widely used for

dry-type transformers. The initial cost of a dry-type transformer may be 60 to 70%

higher than that of an oil-cooled transformer at current prices, but its overall cost

at the present level of energy rate can be very much comparable to that of the oil￾cooled transformer.

Design: With the rapid development of digital computers, the designers are freed

from the drudgery of routine calculations. Computers are widely used for

optimization of transformer design. Within a matter of a few minutes, today’s

computers can work out a number of designs (by varying flux density, core

diameter, current density, etc.) and come up with an optimum design. The real

benefit due to computers is in the area of analysis. Using commercial 2-D/3-D

field computation software, any kind of engineering analysis (electrostatic,

electromagnetic, structural, thermal, etc.) can be performed for optimization and

reliability enhancement of transformers.

Copyright © 2004 by Marcel Dekker, Inc.