TRANSFORMER CONSTRUCTION AND PRACTICAL CONSIDERATIONS

The type of construction adopted for transformers is intimately related to the purpose for which these are to be used; winding voltage, current rating and operating

frequencies. The construction has to ensure efficient removal of heat from the two seats of heat generation—

core and windings, so that the temperature rise is limited to that allowed for the class of insulation employed.

Further, to prevent insulation deterioration, moisture ingress to it must not be allowed. These two objectives are simultaneously achieved in power transformers, other than those in very small sizes, by immersing the built-up transformer in a closed tank filled with noninflammable insulating oil called transformer oil.

To facilitate natural oil circulation and to increase the cooling surface exposed to the ambient, tubes or fins are provided on the outside of tank walls. In large-size transformers tubes may be forced-cooled by air. For still larger installations the best cooling system appears to be that in which the oil is circulated by pump from the top of the transformer tank to a cooling plant, returning when cold to the bottom of the tank. In small sizes the transformers are directly placed in a protective housing or are encased in hard rubber moulding and are air-cooled. Figures 3.1. (a), (b) and (c) show the constructional details of practical transformers.

secondary. The reader should note that these are arbitrary terms and in no way affect the inherent properties of a transformer.

If the secondary voltage is greater than the primary value, the transformer is called a step-up transformer;

if it is less, it is known as a step-down transformer; if primary and secondary voltages arc equal, the transformer is said to have a one-to-one ratio. One-to-one transformers are used to electrically isolate two parts of a circuit. Any transformer may be used as a step-up or step-down depending on the way it is connected.

In order to ensure the largest and most effective magnetic linkage of the two windings, the core, which supports them mechanically and conducts their mutual flux, is normally made of highly permeable iron or steel alloy (cold-rolled, grain oriented sheet steel). Such a transformer is generally called an iron-core transformer. Transformers operated from 25–400 Hz are invariably of iron-core construction. However, in special cases, the magnetic circuit linking the windings may be made of nonmagnetic material, in which case the transformer is referred to as an air-core transformer. The air-core transformer is of interest mainly in radio devices and in certain types of measuring and testing instruments. An intermediate type, exemplified by a type of induction coils and by small transformers used in speech circuits of telephone systems, utilizes a straight core made of a bundle of iron wires on which the primary and secondary coils are wound in layers.

Fig. 3.1 (a) Single-phase transformer core and windings

Power transformers are provided with a conservative through which the transformer breathes into the atmosphere The conservative is a smaller-sized tank placed on top of the main tank. This arrangement ensures that surface area of transformer oil exposed to atmosphere is limited so as to prevent fast oxidization and consequent deterioration of insulating properties of the oil.

Fig. 3.1 (b) Three-phase transformer core and windings

The magnetic core of a transformer is made up of stacks of thin laminations (0.35 mm thickness) of cold-rolled grain-oriented silicon steel sheets lightly insulated with varnish. This material allows the use of high flux densities (1–1.5 T) and its low-loss properties together with laminated construction reduce the core-loss to fairly low values. The laminations are punched out of sheets and the core is then built of these punching.

Before building the core, the punched laminations are annealed to relieve the mechanical stresses set in at the edges by the punching process; stressed material has a higher core-loss. Pulse transformers and high-frequency electronic transformers often have cores made of soft ferrites.

The primary and secondary coils are wound on the core and are electrically insulated from each other and from the core. Two types of cores are commonly employed in practice—core-type and shell-type. In core-type construction shown in Fig. 3.2(a) the windings are wound around the two legs of a rectangular magnetic core, while in shell-type construction of Fig. 3.2(b), the windings are wound on the central leg of a three-legged core. Though most of the flux is confined to a high permeability core, some flux always leaks through the core and embraces paths which partially lie in the air surrounding the core legs on which the coils are wound. This flux which links one of the windings without linking the other, though small in magnitude,

has a significant effect on the transformer behaviour. Leakage is reduced by bringing the two coils closer.

In a core-type transformer this is achieved by winding half low-voltage (LV) and half high-voltage (HV) winding on each limb of the core as shown in Fig. 3.2(a). The LV winding is wound on the inside and HV on outside to reduce the amount of insulation needed. Insulation between the core and the inner winding is then stressed to low voltage. The two windings are arranged as concentric coils. In shell-type construction leakage is reduced by subdividing each winding into subsections (wound as pancake coils) and interleaving LV and HV windings as shown in Fig. 3.2(b).

Top core clamp

L.V. winding

H.V. winding

Oil ducts L.V. SIDE L.V. insulating

cylinder H.V. insulating

cylinder

Bottom core clamps

Coil stack end insulation

Tapping leads to switches

H.V. SIDE Conservator

Fig. 3.1 (c) Transformer showing constructional details

The core-type construction has a longer mean length of core and a shorter mean length of coil turn. This type is better suited for EHV (extra high voltage) requirement since there is better scope for insulation. The shell-type construction has better mechanical support and good provision for bracing the windings. The shell-type transformer requires more specialized fabrication facilities than core-type, while the latter offers the additional advantage of permitting visual inspection of coils in the case of a fault and ease of repair at substation site. For these reasons, the present

practice is to use the core-type transformers in large high-voltage installations.

Transformer windings are made of solid or stranded copper or aluminium strip conductors. For electronic transformers, “magnet wire” is normally used as conductor. Magnet wire is classified by an insulation class symbol, A, B, C, F and H, which is indication of the safe operating temperature at which the conductor can be used. Typical figures are the lowest 105 °C for class-A and highest 180 °C for class-H.

The windings of huge power transformers use conductors with heavier insulation (cloth, paper, etc.) and are assembled with greater mechanical support and the winding layers are insulated from each other—this is known as minor insulation for which pressed board or varnished cloth is used.

Major insulation, insulating cylinders made of specially selected pressed board or synthetic resin bounded cylinders, is used between LV and core and LV and HV. Insulating barriers are inserted between adjacent limbs when necessary and between coils and core yokes.

Transformer Cooling (Large Units)

Some idea of transformer losses, heating and cooling has been presented above. Details of transformer losses will be presented in Section 3.6. Basically, there are two seats of losses in a transformer namely:

(1) Core, where eddy current and hysteresis losses occur (caused by alternating flux density).

(2) Windings (primary and secondary) where I²R or copper loss occurs because of the current flowing in these.

Heat due to losses must be removed efficiently from these two main parts of the transformer so that steady temperature rise is limited to an allowable figure imposed by the class of insulation used. The problem of cooling in transformers (and in fact for all electric machinery) is rendered increasingly difficult with increasing size of the transformer. This is argued as below:

The same specific loss (loss/unit volume) is maintained by keeping constant core flux density and current density in the conductor as the transformer rating is increased. Imagine that the linear dimensions

1/2 LV 1/2 HV

Windings

(a) Core-type transformer

Core yoke Core Core yoke 1/2 LV

1/2 HV

Windings

Sandwiched LV HV windings

Core

(b) Shell-type transformer f/2

f/2

Fig. 3.2

of transformer are increased k times. Its core flux and conductor current would then increase by k² times and so its rating becomes k⁴ times. The losses increase by a factor of k³ (same as volume), while the surface area (which helps dissipate heat) increases only by a factor of k². So the loss per unit area to be dissipated is increased k times. Larger units therefore become increasingly more difficult to cool compared to the smaller ones. This can lead to formation of hot spots deep inside the conductors and core which can damage the insulation and core properties. More effective means of heat removal must therefore be adopted with ducts inside the core and windings to remove the heat right from the seats of its generation.

Natural Cooling

Smaller size transformers are immersed in a tank containing transformer oil. The oil surrounding the core and windings gets heated, expands and moves upwards. It then flows downwards by the inside of tank walls which cause it to cool and oil goes down to the bottom of the tank from where it rises once again completing the circulation cycle.

The heat is removed from the walls of the tank by radiation but mostly by air convection. Natural circulation is quite effective as the transformer oil has large coefficient of expansion. Still for large sizes, because of the arguments presented earlier, the cooling area of the tank must be increased by providing cooling fins or tubes (circular or elliptical) as shown in Fig. 3.3. This arrangement is used for all medium size transformers.

Forced Cooling

For transformer sizes beyond 5 MVA additional cooling would be needed which is achieved by supplementing the tank surface by a separate radiator in which oil is circulated by means of a pump. For better cooling oil-to-air heat exchanger unit is provided as shown in Fig. 3.4(a). For very large size transformers cooling is further strengthened by means of oil-to-water heat exchanger as shown in Fig. 3.4(b).

Conservator

Water inlet

Oil/air heat

exchanger Oil/water

heat-exchanger

Oil

pump Fan

(a) (b)

Fig. 3.4 Forced cooling in transformers

Fig. 3.3 Natural cooling in transformers

As already pointed out, ducts are provided in core and windings for effective heat removal by oil. Vertical flow is more effective compared to horizontal flow but for pancake coils some of the ducts will have to be horizontal.

The problem of cooling in transformers is more acute than in electric machines because the rotating member in a machine causes forced air draft which can be suitably directed to flow over the machine part for efficient heat removal. This will be discussed in Section 5.10.

Buchholz Relay

Buchholz relay is used in transformers for protection against all kinds of faults. It is a gas-actuated relay and installed in oil-immersed transformers. It will give an alarm in case of incipient faults in the transformer.

This relay also disconnects the transformer in case of severe internal faults. A Buchholz relay looks like a domed vessel and it is placed between main tank of transformer and the conservator.

The upper part of the relay consists of a mercury-type switch attached to a float. The lower part contains mercury switch mounted in a hinged-type flat located in the direct path of the flow of oil from the transformer to the conservator. The upper mercury-type switch closes an alarm circuit during incipient fault, whereas the lower mercury switch is used to trip the circuit breaker in the case of sever faults. The Buchholz relay is shown in Fig. 3.4 (c).

Figure 3.5 shows the schematic diagram of a two-winding transformer on no-load, i.e. the secondary terminals are open while the primary is connected to a source of constant sinusoidal voltage of frequency f Hz. The simplifying assumption that the resistances of the windings are negligible, will be made.

+

i₀

e₁ N₁ N₂ e₂ v₂

Primary

Core (magnetic material) –

Secondary terminals

open +

–

+ +

– –

Secondary f

Fig. 3.5 Transformer on no-load

Conservator

Buchholz relay 9.5°

Transformer main tank

Fig. 3.4 (c) Buchholz relay set up

The primary winding draws a small amount of alternating current of instantaneous value i₀, called the exciting current, from the voltage source with positive direction as indicated on the diagram. The exciting current establishes flux f in the core (positive direction marked on diagram) all of which is assumed confined to the core i.e., there is no leakage of flux. Consequently the primary winding has flux linkages,

l1 = N₁f which induces emf in it is given by

e₁ = d dt

l1 = N₁d dt

f (3.1)

As per Lenz’s law, the positive direction of this emf opposes the positive current direction and is shown by + and – polarity marks on the diagram. According to Kirchhoff’s law,

v₁ = e₁ (winding has zero resistance) (3.2) and thus e₁ and therefore f must be sinusoidal of frequency f Hz, the same as that of the voltage source. Let

f = fmax sin wt (3.3)

where fmax = maximum value of core flux

w = 2pf rad/s ( f = frequency of voltage source) The emf induced in the primary winding is

e₁ = N₁d dt

f = w N1fmax cos wt (3.4)

From Eqs (3.3) and (3.4) it is found that the induced emf leads the flux by 90°*. This is indicated by the phasor diagram of Fig. 3.6. The rms value of the induced emf is

E₁ = 2 pf N1fmax = 4.44 f N₁fmax (3.5) Since E₁ = V₁ as per Eq. (3.2),

fmax = E V f N

1 1

4 44 1

( )

.

= (3.6)

Even if the resistance of the primary winding is taken into account,

E₁ª V1 (3.7)

as the winding resistances in a transformer are of extremely small order. It is, therefore, seen from Eq. (3.6) that maximum flux in a transformer is determined by V₁/f (voltage/frequency) ratio at which it is excited.

According to Eq. (3.6) the flux is fully determined by the applied voltage, its frequency and the number of winding turns. This equation is true not only for a transformer but also for any other electromagnetic device operated with sinusoidally varying ac and where the assumption of negligible winding resistance holds.

All the core flux f also links the secondary coil (no leakage flux) causing in it an induced emf of e₂ = N₂ d

dt

f (3.8)

The polarity of e₂ is marked + and – on Fig. 3.5 according to Lenz’s law (e₂ tends to cause a current flow whose flux opposes the mutual flux f). Further, it is easily seen from Eqs. (3.1) and (3.8) that e1 and e₂ are in

* cos wt leads sin w t by 90°

phase. This is so indicated by phasor diagram of Fig. 3.6 where E₁, and E₂ are the corresponding phasors in terms of the rms values. As the secondary is open-circuited, its terminal voltage is given as

v₂ = e₂

From Eqs (3.1) and (3.8) we have the induced emf ratio of the transformer windings as e

e¹2

= N N¹2

= a E

E

1 2

= N N

1 2

= a ratio of transformation (3.9) This indeed is the transformation action of the transformer. Its current transformation which is in inverse

ratio of turns will be discussed in Section 3.4.

The value of exciting current i₀ has to be such that the required mmf is established so as to create the flux demanded by the applied voltage (Eq. (3.6)). If a linear B-H relationship is assumed (devoid of hysteresis and saturation), the exciting current is only magnetizing in nature and is proportional to the sinusoidal flux and in phase with it. This is represented by the phasor Im, in Fig. 3.6, lagging the induced emf by 90°. However, the presence of hysteresis and the phenomenon of eddy-currents, though of a different physical nature, both demand the flow of active power into the system and as a consequence the exciting current I0 has another component Ii in phase with E₁. Thus, the exciting current lags the induced emf by an angle q0 slightly less than 90° as shown in the phasor diagram of Fig. 3.6. Indeed it is the hysteresis which causes the current component Ii leading Im by 90° and eddy-currents add more of this component. The effect of saturation nonlinearity is to create a family of odd-harmonic components in the exciting current, the predominant being the third harmonic; this may constitute as large as 35–40% of the exciting current. While these effects will be elaborated in Sec. 3.10, it will be assumed here that the current Io and its magnetizing component Im

and its core-loss component Ii are sinusoidal on equivalent rms basis. In other words, Im is the magnetizing current and is responsible for the production of flux, while Ii is the core-loss current responsible for the active power* being drawn from the source to provide the hysteresis and eddy-current loss.

To account for the harmonics, the exciting current Io is taken as the rms sine wave equivalent of the actual non-sinusoidal current drawn by the transformer on no-load. Since the excitation current in a typical transformer is only about 5% of the full-load current, the net current drawn by the transformer under loaded condition is almost sinusoidal.

From the phasor diagram of Fig. 3.6, the core-loss is given by

P_i = E₁I₀ cos q0 (3.10)

In a practical transformer, the magnetizing current (I_m) is kept low and the core-loss is restrained to an acceptable value by use of high permeability silicon-steel in laminated form.

From the no-load phasor diagram of Fig. 3.6, the parallel circuit model** of exciting current as shown in Fig. 3.7 can be easily imagined wherein conductance G_i accounts for core-loss current Ii and inductive susceptance B_m for magnetizing current I_m. Both these currents are drawn at induced emf E1 = V1 for resistance-less, no-leakage primary coil; even otherwise E₁ ª V1.

* Suffix i is used as this current provides the core-loss which occurs in the iron core and is also referred as iron-loss.

** Series circuit is equally possible but not convenient for physical understanding.

E₂

I_m l_i

a₀

q₀ E₁= V₁

l₀

f

l_i G_i V₁

l₀

l_m B_m +

–

E₁ +

–

Fig. 3.6 Phasor relationship of induced emf, Fig. 3.7 Circuit model of transformer on

EXAMPLE 3.1 A transformer on no-load has a core-loss of 50 W, draws a current of 2 A (rms) and has an induced emf of 230 V (rms). Determine the no-load power factor, core-loss current and magnetizing current. Also calculate the no-load circuit parameters of the transformer. Neglect winding resistance and leakage flux.

SOLUTION

Power factor, cos q0 = 50

2 230¥ = 0.108 lagging;

q0 = 83.76°

Magnetizing current, I_m = I₀ sin q0 = 2 sin (cos^–1 0.108) = 1.988 A

Since q0ª 90°, there is hardly any difference between the magnitudes of the exciting current and its magnetizing component.

Core-loss current, I_i = I₀ cos q0

= 2 ¥ 0.108 = 0.216 A

In the no-load circuit model of Fig. 3.7 core loss is given by G_i V₁² = P_i

or G_i = P

V

i

12 = 50 230 ²

( ) = 0.945 ¥ 10^–3

Also I_m = B_m V₁

or B_m = I_m

V1

= 1 988

230

. = 8.64 ¥ 10^–3

EXAMPLE 3.2 The BH curve data for the core of the transformer shown in Fig. 3.8 is given in Problem 2.10.

Calculate the no-load current with the primary excited at 200 V, 50 Hz. Assume the iron loss in the core to be 3 W/kg. What is the pf of the no-load current and the magnitude of the no-load power drawn from the mains?

Density of core material = 7.9 g/cc.

200 V

10 cm

20 cm 150 turns

5 cm thick

75 turns

25 cm

Fig. 3.8

SOLUTION

Substituting values in Eq. (3.5)

200 = 4.44 ¥ 50 ¥ 150 ¥ fmax

or fmax = 6.06 mWb

B_max = 6 06 10 10 5 10

In document [Kothari] Electric Machines(BookZZ.org) (Page 67-76)