reading / British practice / Vol D - 1990 (ocr) ELECTRICAL SYSTEM & EQUIPMENT

Introduction

1 General design and construction

1.1 Types of transformer

1.1.1 Phase relationships — phasor groups

1.1.2 Star/star connected transformers 1.1.3 The interconnected star connection

1.2 Basic materials

1.2.1Dielectrics

1.2.2Basic materials — copper, iron and insulation

1.3 Transformer characteristics

1.3,1 Basic theory

1.3.2Leakage reactance 1,3.3 Core loss

1.3.4Load losses

1.4Transformer construction

1.4.1Core construction

1.4.2Transformer windings

1.4,3 Winding conductors

1.4.4Low voltage windings

1.4.5Transpositions

1.4.6Continuously-transposed strip

1.4.7High voltage windings

1.4.8Tapping windings

1.4.9Disposition of windings

1.4.10Impulse strength

1.4.11Thermal considerations

1.4.12Performance under short-circuit

1,5 Tappings and tapchangers

1.5.1Uses of tapchangers

1.5.2Impedance variation

1.5.3Tapchanger mechanisms

1.5.4Single compartment tapchangers

1 5.5 In-tank tapchangers 1.5.6 Off-circuit tapchangers

1.6 Tanks, connections and auxiliary plant

1.6.1Transformer tanks

1.6.2Oil preservation equipment — conservators

1.6.3Bushing connections

1.6.4SF6 connections

1.6.5Cable box connections

1.6.6Tank-mounted coolers

1.6.7Separate cooler banks

1.6.8Water cooling

1.6.9Cooler control

1.6.10Layout of transformer compounds

1.7Quality assurance and testing

1.7.1Quality assurance (QA)

1.7.2Tests during manufacture

1.7.3Processing and dry-out

1.7.4Final testing

1.7.5Power frequency overvoitage tests 1.7,6 Impulse tests

1.7,7 Switching-surge tests

1.7.8Load runs

1.7.9Short-circuit testing

1.8 Transport, installation and commissioning

1.8.1Transport

1.8.2Installation and site erection

1.8.3Site commissioning

2Special design features

2.1 Generator transformers 2.1.1 Required characteristics 2.1.2 General design features

2.1.3Single-phase generator transformers

2.1.4Performance and reliability

2.1.5Economics of operation

2.2Station transformers

2.2.1Station transformer characteristics

2.2.2General design features

2.3Unit transformers

2.3.1Unit transformer characteristics

2.3.2General design features

2.4Auxiliary transformers

2.4.1General design features

2.4.2Auxiliary transformer insulation systems

2.4.3Design features of dry-type transformers

2.4.4Special transformers

2.4.5Foil windings

2.5Neutral earthing

2.5.1Generator earthing transformers — basic principles

2.5.2Generator neutral earthing transformers — general design features

2.5.3Practical arrangement

2.5.4Loading resistor

2.5.5Generator busbar system earthing

2.5.6Harmonic suppressors

2.6 Series reactors

2.6.1General design features

2.6.2Testing of series reactors

2.7 Instrument transformers

2.7.1Voltage transformers

2.7.2Generator voltage transformers 2.7,3 Current transformers

2.7.4Current transformer construction

3 References

Introduction

The invention of the power transformer in the latter part of the nineteenth century made possible the development of the modern constant voltage AC supply

system, with power stations often located many miles from the centres of electrical load. Before that, in the early days of public electricity supplies, these were DC systems with the source of generation, of necessity, close to the point of loading.

The power transformer, not only permitted the development of large central power stations but, in addition, made a significant contribution to the development of the power station itself.

The amount of auxiliary plant needing an electrical supply in a power station is so great that it is necessary to provide an electrical system similar in magnitude and complexity to that of a small town. As a result, there is a need, in a 2000 MW station, for five or possibly six different voltage level systems, requiring 60 or more power transformers to provide the interconnections. These range from the largest, the generator transformer, which steps up the generator output voltage for connection to the transmission system, to the many very much smaller auxiliary transformers which provide supplies at several voltages down to 415 V. In addition, there is an almost countless number of transformers providing supplies at 110 V and lower, for control and instrumentation equipment.

1 General design and construction

The transformer interconnects and transfers power between systems at different voltages. It does this with very high efficiency, usually 99% or better, and even the imperfection which results from the incomplete magnetic coupling of primary and secondary, i.e., leakage reactance, is a feature which can be used to advantage by the system designer to reduce system fault levels and the consequent rupturing capability of the system switchgear.

1.1 Types of transformer

Transformers in power stations are generally threephase and almost invariably double-wound, i.e., they have electrically separate primary and secondary windings. Auto transformers are not used.

Since in a power station each three-phase system requires an earth, it is convenient if one of these windings can be star connected and thereby provide a neutral for connecting to earth either solidly or via a fault current limiting resistor. it is also desirable that a three-phase system should have a delta to provide a low impedance path for third harmonic currents in order to eliminate or reduce third harmonic voltages in the waveform. This requirement is most easily met by connecting the other winding in delta.

1.1.1 Phase relationships — phasor groups

If a two winding three-phase transformer has one winding star connected and the other in delta, there will be a phase shift produced by the transformer, as can be seen by reference to Fig 3.1. In the example shown in the diagram, this phase shift is 30 ° before twelve o'clock (assuming clockwise rotation) which is referred to as the 'eleven o'clock' position. The

(a)

A

2

A

1

(b)

(c)

Fio. 3.1 Winding connections, phasor and polarity diagram

COILS

(a)

(b)

FIG. 3.3 Interconnected star winding arrangement

thick, which make up the core or magnetic circuit, and copper or, more precisely, hard-drawn high-conductivity copper, from which the windings are formed. In mot transformers, winding turns are insulated by paper supplemented in some cases by enamel, whilst the major insulation, insulating winding from winding and windings from core, is almost entirely paper or cotton-fibre based board or laminated board, with small amounts of wood or wood laminates used where high mechanical strength is demanded. The properties of these basic materials will be dealt with in further detail in the sections dealing with core and windings.

1.3 Transformer characteristics

For a detailed treatment of basic transformer theory, the reader is referred to a standard text book [2]. However, it is necessary to carry out a brief review of the basic theory in order to obtain an understanding of how the characteristics demanded in power stations affect the design, how these interact with each other and what performance might reasonably be expected from a given transformer design.

1.3.1 Basic theory

1.2 Basic materials

1.2.1 Dielectrics

The majority of transformers installed at power station sites are oil-filled, using a mineral oil which in the UK complies with BS148. This serves the dual purpose of providing insulation and as a cooling medium to conduct away the losses which are produced in the transformer in the form of heat.

Mineral oil is, of course, combustible — it has a fire point of 170 ° C — and transformer fires do someti mes occur. It is usual, therefore, to locate these out of doors where a fire is more easily dealt with and

consequentially the risks are less. It is necessary to consider the need for segregation from other plant and

incorporate measures to restrict the spread of fire. Because of the fire hazard associated with mineral

oil, it has been the practice to use designs for smaller auxiliary transformers which do not contain any oil. These may be entirely dry, air insulated; or they may contain non-flammable or reduced flammable liquid; they have the advantage that they may be located inside buildings in close proximity to the associated switchgear. More will be said about this type of transformer in Section 2.4 of this chapter.

1.2.2 Basic materials — copper, iron and insulation

The other basic materials which go to make a transformer are iron, nowadays almost exclusively cold-rolled grain-oriented in the form of laminations 0.28-0.30 mm

A transformer usually consists of two coils linking an iron core. An alternating voltage applied to one of these coils produces an alternating flux within the core. This, in turn, induces an alternating voltage within each turn linking the flux, which, in accordance with Lenz's law, has such a polarity as to oppose that flux if current is allowed to flow.

This is normally expressed in the form

E = - N(dq5/dt)

but, for the practical transformer, it can be shown that the voltage induced per turn is

where is the total flux linking that turn and K is a constant which depends on the supply frequency and the units in which cl) is measured.

For a 50 Hz supply, RMS voltage, and total flux measured in Webers, K is equal to 4.44.

This expression is a measure of the voltage induced per turn regardless of which winding a particular turn might be associated with. If the transformer had only one winding this induced voltage, or back EMF, would balance the supply voltage and an equilibrium would be established with the 'transformer' taking a very small current, the magnetising current, sufficient to establish flux within the core. Any other winding would thus develop a voltage proportional to the number of turns and, for a two winding transformer, the familiar relationship exists that

where Ei and Nt are the voltage and turns, respecril,ely, of the primary winding and E2 and N2 those

of the secondary winding.

If a current were then allowed to flow in a secondary winding, by connecting some external load, this current would itself produce a flux, the sense of

hich woulc be the same as that of the back EMF an d it would thus neutralise the back EMF developed

in the primary winding, thereby allowing current to be drawn from the supply applied to the primary ‘vincling. The flux produced by this primary current is such as to balance the neutralising flux created by the secondary current and equilibrium is established once more. Primary and secondary currents are in inverse proportion to the turns ratio, since IiN1 =I2N2.

The transformer has thus accomplished the necessary transformation of voltage levels and 'reflects' into the primary circuit those events which occur in the

secondary.

The transformation is, however, not perfect. Not all the flux produced by the primary winding links with the secondary for example, so that events as seen from the primary are not a total reflection of those occurring on the secondary, i.e., the transformer has leakage reactance. As already mentioned, establishing flux in the core involves the drawing of a magnetising current. Associated with this flux there are hysteresis losses and eddy current losses in the core. When load current flows there are resistive losses and eddy current losses in the windings. There are also eddy current losses in the tank and the core frame.

1.3.2 Leakage reactance

•.s explained above, the leakage reactance of a transformer arises from the fact that all the flux produced by one winding does not link with the other winding. As would be expected, therefore, the magnitude of this leakage flux is a function of the geometry and construction of the transformer. Although there are other forms of construction, the majority of transformers and certainly all power station transformers produced in the UK are of the core-type construction, as shown in Figs 3.4 (a) and 3.4 (b), consisting of a central core surrounded by two or more concentric windings. This central wound section is known as the leg, or limb, and the iron circuit is completed by a return section or yoke. Figure 3.4 (c) shows a partsection of a core-type transformer taken axially through the centre of the wound limb and cutting the primary and secondary windings. The principal dimensions are marked in the figure, as follows:

is axial length of windings (assumed the same for primary and secondary)

a is the radial spacing between windings

for a hypothetical winding occupying the space between inner and outer windings, then the leakage reactance in percent is given by the expression

070){ = KF(3amit a + brratb + cmIt c )/01 (3.3)

where K is a constant of value dependent on the system of units used

increase as the transformer rating increases. This is of little consequence in most transformers, as almost any required reactance can normally be obtained by appropriate adjustment of the physical dimensions, but it does become very significant for large generator transformers, as permissible limits of dimensions are reached.

F is equal to the ampere-turns of primary or secondary winding, i.e., MMF per limb

0 is the total flux in the core

Equations (3.1), (3.2) and (3.3) determine the basic parameters which fix the design of the transformer. The MMF is related to the MVA rating of the transformer and the total flux, 0, is the product of flux density and core cross-sectional area. Flux density is determined by the choice of core material and the duty of the transformer. The transformer designer can, therefore, select a combination of ig5 and 1 to provide the value of reactance required by the customer. A larger core cross-section, usually referred to as the frame size, and a longer 1 will reduce reactance and, conversely, reducing frame size and winding length will increase reactance. Unfortunately, the designer's task is not quite as simple as that since variation of any of the principal parameters affects the others which will then also affect the reactance. For example, increasing 0 not only reduces reactance, because of its appearance in the denominator of Equation (3.3), but it also reduces the number of turns, as can be seen by reference to Equation (3.1), which will thus reduce reactance still further. The value of I can be used to adjust the reactance since it mainly affects the denominator of Equation (3.3). Nevertheless, if I is reduced, say, to increase reactance, this shortening of the winding length results in an increase in the radial depth (b and c) of each winding, in order that the same number of turns can be accommodated in the shorter axial length of winding. This tends to increase the reactance further. Another means of fine tuning of the reactance is by variation of the winding radial separation, the value 'a' in Equation (3.3). This is more sensitive than changes in b and c since it is multiplied by the factor three, and the designer has more scope to effect changes since the dimension 'a' is purely the dimension of a 'space'. Changes in the value of 'a' also have less knock-on effect although they will, of course, affect `mlt,'. For a given transformer 'a' will have a minimum value, determined by the voltage class of the windings and the insulation necessary between them. In addition, the designer will not wish to artificially increase 'a' by more than a small amount since this is wasteful of space within the core window.

It should be noted that since the kVA or MVA factor appears in the numerator of the expression for percent reactance, the value of reactance tends to

1.3.3 Core loss

The purpose of a transformer core is to provide a low reluctance path for the magnetic flux linking primary and secondary windings.

In doing so, the core experiences iron losses due to hysteresis and eddy currents flowing within it which, in turn, show themselves as heating of the core material.

Hysteresis loss can be reduced by increasing silicon content but, since this also makes the material brittle and hard, there is a practical limit if the material is to remain sufficiently workable to permit reasonably straightforward core manufacture. This limit is about 41/2% silicon for steel produced by the hot rolling process.

Orienting the grain structure by cold rolling so that the magnetic domains are uniformly aligned rather than random also reduces hysteresis and, in fact, can produce such an improvement that the silicon content can be reduced whilst still permitting the use of higher flux densities than the non-oriented steel.

Reducing the silicon content (a figure of about 3 07o is used for cold rolled steel) also reduces resistivity, so there is a tendency for eddy current loss to be increased. This is countered by a reduction in plate thickness.

This is the basis of the development of cold-rolled grain-oriented steel, 0.28 mm thick, which has been current in the UK since the 1950s.

Specific loss for cold rolled steel is very dependent OP internal stress and increases sharply for any com-

pressive stress in the material. It is therefore necessary to anneal frequently during the rolling process. In order to prevent fusion between adjacent layers of the coiled strip, it is given a coating of magnesium oxide in the rolling mill. This is usually supplemented by an additional phosphate coating during the final anneal. These coatings also serve as the insulation between laminations to restrict eddy loss in the completed core.

As cores get larger, and core plates wider, higher voltages are induced and the duty of the interlaminar insulation becomes more onerous, so that in practice, for cores greater than about 640 mm in diameter, phosphate/magnesia coated plates are usually given an additional coat of insulation, china clay or varnish, by the core manufacturer.

This additional insulation also makes good any damage to the phosphate coating brought about by the

grinding off of cutting burrs produced when the strip, initially about one metre wide by several hundred metres

long, is cut into individual plates perhaps only 300 mm wide by 2-3 m long.

For a given grade of material, hysteresis loss (Wh) is proportional to the area of the hysteresis loop, the maximum flux density and the frequency:

where n varies from about 1.6 to 2.5 depending on

the material.

Eddy cu. rent loss (W e ) depends on square of frequency arid on the thickness of the steel, so that

k i , k2 = constants

=frequency, Hz

=thickness of material, mm

1.5

0.5

Beff = flux density corresponding to the RMS value of the applied voltage.

In the UK, the supply frequency is 50 Hz and the thickness of the material is determined by the steel manufacturer, so the only variables are B,„,, and Bell'. The steel manufacturer normally quotes the specific loss, in W/kg, at a stated working flux density, usually 1.5 tesla, and provides a curve giving specific loss at other flux densities. A typical curve is reproduced in Fig 3.5. Specific loss is obtained by cutting samples of the material 25 mm wide by 250 mm in length along the rolling direction and building these into a square 'core' with overlapped corner joints, called an Epstein Square, and measuring the loss on this.

The total loss of a built up core is then theoretically equal to this specific loss value multiplied by the total weight. In practice, the measured loss ex-

ceeds this figure by 15-25 07o and this is known as the building factor of the core. Its precise value depends on the type of core and the form of construction. More will be said on this subject in Section 1.4.1 of this chapter which deals with core construction.

1.3.4 Load losses

The load loss of a transformer is that proportion of the losses generated by the flow of load current and which varies as the square of the load. This falls into three categories:

•Resistive loss within the winding copper and leads.

•Eddy current loss in the winding copper.

•Eddy current loss in the tanks and structural steelwork.

Resistive loss can be reduced by dropping the number of winding turns, by increasing the cross-sectional area of the turn conductor, or by a combination of both. Reducing the number of turns requires an increase in 0, i.e., an increase in core cross-section (frame size), which increases iron weight and iron loss. So load loss can be traded against iron loss and vice versa. Increased frame size requires reduced winding length to compensate in Equation (3.3) and thus retain the same impedance, although as already explained there will be a reduction in number of turns (which was the object of the exercise) by way of partial compensation. Reduction of the winding axial length means that the core leg length is reduced, which also offsets the increase in core weight resulting from the increased frame size to some extent. There is thus a band of one or two frame sizes for which the loss variation is not too great, so that optimum frame size can be chosen to satisfy other factors, such as ratio of fixed to load losses or transport height.

The paths of eddy currents in winding conductors are complex. The effect of leakage flux within the transformer windings results in the presence of radial and axial flux changes at any given point in space and any moment in time. These induce voltages which cause currents to flow at right angles to the changing fluxes. The magnitude of these currents can be reduced by increasing the resistance of the path through which they flow, and this can be effected by reducing the total cross-sectional area of the winding conductor or by subdividing this conductor into a large number of strands insulated from each other. The former alternative increases the overall winding resistance and thereby the resistive losses. Conversely, if the overall conductor cross-section is increased with the object of reducing resistive losses, one of the results is to increase eddy current losses. This can only be offset by a reduction of strand cross-section and an increase in the total number of strands. It is costly to wind a large number of conductors, in parallel and so a

manufacturer will wish to limit the total number of strands in parallel. Also, the extra insulation resulting on the increased number of strands results in a poorer winding space-factor.

As explained above, eddy currents in winding conductors are the result of leakage flux, so a reduction in leakage flux results in smaller eddy currents. It can be seen (Fig 3.6) that a tall slim winding produces less leakage flux than a short squat winding. This can be proved by flux plots and is also evident from Equation (3.3) in that the greater the value of 1, the less the leakage reactance, so there is also a minimum acceptable value of 1 if the eddy current losses are to be restricted to a reasonable level. In practice, manufacturers aim to limit eddy current loss to a value about 25 07o of that of the resistive loss.

In terms of the total load losses, the stray losses in the transformer tanks and other structural steelwork, such as core frames only constitute a small proportion. However, they can produce significant amounts of heating in areas of the tank surface and, particularly, in heavy-section flanges which, as well as attracting large amounts of leakage flux, tend to have

(a) Leakage flux paths in single phase core

(e) Leakage flux in squat core

FIG. 3.6 Leakage flux paths in tall and squat windings

large cross-sectional areas and hence low resistance to the circulation of eddy currents. Local overheating of flanges can cause rapid deterioration of gaskets and consequent serious leakage of oil. The oil itself suffers rapid degradation if it remains in contact with metal at temperatures much above 130 ° C. Even if the overheating is not severe enough to give rise to either of these problems, there is still the hazard faced by operating staff making accidental contact with the overheated tnk surface. For this reason, it is usual to specify that the tank temperature should not exceed

° C up to a height of 2.8 m above plinth level. A

80 ° C is usually permitted above temperature of up to I00

this height. The transformer designer may comply with these requirements by careful design of the core and by routing of heavy-current leads within the tank well clear of the sides and of large-section flanges in particular. If these measures are insufficient, then it might be necessary to provide packets of core steel to act as Flux shunts between the source MMF and the tank side. A typical arrangement of flux shunts is shown

3.7.

1.4 Transformer construction

1.4.1 Core construction

Laminations are built up to form a limb or leg having as near as possible a circular cross-section (Fig 3.8) in order to obtain optimum use of space within the cylindrical windings. The stepped cross-section approximates to a circular shape depending only on how many different widths of strip a manufacturer is prepared to cut and build. For the smaller cores of power station auxiliary transformers, this can be as few as seven. In the larger station and generator transformers, the number is eleven or more. Theoretically, these fill from just over 9307o to over 95 070, respectively, of the available core circle. In reality, the actual uti-

FOR PROTECTION AGAINST FLUX FROM WINDINGS

FOR PROTECTION FROM FLUX

PRODUCED BY HV LINE LEAD

FIG. 3.7 Arrangement of shunts for leakage flux

lisation is probably slightly less than this since the manufacturer aims to standardise on a range of plate widths to cover all sizes of cores, and will therefore be unlikely to have available widths which would give the ideal cross-section for every size of core. Manufacturers normally denote frame size by quoting the width of the widest plate, starting at about 200 mm for small auxiliary transformers and increasing in 25 mm steps up to about one metre for the largest generator transformers.

The cylindrical wound limb forms the common feature for all power station transformer cores. The form of the complete core will be one of the arrangements

shown in Fig 3.9; of these, by far the most common is the three-phase, three-limb core. Since, at all times the phasor sum of the three fluxes produced by a