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BASIC OIL TYPE TRANSFORMER DESIGN1. Overview

The design process starts with the design of the active part; this comprises the core and windings and is done according to customer specifications. Important aspects that must be satisfied include the impedance, load and no-load losses, test and service voltages withstand capability and cooling, to name but a few. Every transformer unit must be verified according to short circuit instances that may occur in the field.

The mechanical design ensures strength to withstand winding assembly pressures, short circuit forces, lifting, transport, pressure, vacuum, and earthquakes. The constraints due to manufacturing methods and transport limitations must be taken into consideration. Simultaneous fulfilment of all the requirements with economic optimization is achieved using an iterative design process and extensive design experience.

Figure 4. An illustration of the generation, transformation and distribution of electrical energy.

2. Basic Principles

A transformer is a static piece of apparatus which by electromagnetic induction, transforms alternating voltage and current between two or more windings, at the same frequency and usually at different values of voltage and current.

Figure 1. Transformer being prepared for testing

Figure 2. Stacking of core steel

Figure 3. Employee working on on the LV exits of transformer tank

In simpler terms, a transformer is used in various applications such as when the need arises to increase a voltage for transport of HVAC purposes. At higher voltages, electricity can be transported at a lower current, which allows for lower losses along the transmission line and hence the saving in material for a lower cross sectional area. Once the electricity has reached industrial or residential areas, it is again transformed several times, but this time to lower voltages depending on requirements.

There are many types of transformers available for different applications. Typical types of transformers manufactured include generator step-up transmission and distribution transformers.

2.1 Transformer Components

2.1.1 The Core

Cold rolled grain orientated steel has long been employed in transformer cores because of its unique characteristics, enabling the use of higher flux densities with consequent savings in material. Over the years, steels of continually lower loss have become available. Super oriented steel can operate continuously at high flux densities and provide significant reductions in no load loss, magnetising current and sound levels.

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Figure 6. Radiator cooled transformer.

Ia

Ib

Ic

Ia

Ib

Ic

Figure 5. Illustration of Star and Delta connections.

Transformer windings are designed and manufactured to withstand the electrical, mechanical and thermal stresses that will occur during service. In certain high-current applications, continuously transposed conductor coated with an epoxy coating improves mechanical strength against short circuit forces.

The two most common types of winding connections in three phase systems are delta and star connections as illustrated in Figure 5. In a star connection, the bottom tails of all three windings are connected, this point is called the star point or neutral. The top connections are connected to a bushing and then to an overhead line.

In a delta connection the top tail of the first winding is connected to the bottom tail of the second winding and the top tail of the third winding is connected to the bottom tail of the first winding. This is illustrated in Figure 5.

2.1.3 Cooling

Cooling requirements are customer specific and a range of different types of cooling are available. This includes ONAN (oil natural air natural), ONAF (oil natural air forced), ODAF (oil directed air forced), OFWF (oil forced water forced) or all combinations of the above.

The most basic method of cooling adopted within a transformer is by use of equipment commonly known as radiators that collect hot oil at the top of the transformer tank and return cooled oil lower down in the transformer tank.

It is well known that a magnetic field is found around any current carrying conductor and if the conductor carrying a current is situated in a magnetic field, then a force is imposed perpendicular to the field.

A magnetic field is normally illustrated with lines that form a closed loop; these lines are called magnetic flux lines and indicate the intensity of the magnetic field. This intensity is expressed as the flux density and the unit of measure is Tesla. An alternating current in the primary winding of a transformer will create a changing magnetic field in the core, which in turn will induce a current in the secondary winding.

The relationship is as follows:

NV

N

V

s

s

p

p=

The letters V and N represent voltage and number of turns respectively, and the subscripts S and P, represent secondary and primary respectively.

By specifying the primary voltage and varying the turn ratio between the primary and secondary sides, the desired voltage may be obtained on the secondary.

2.1.2 Windings and Connections

Thus far, reference has been made to the primary and secondary windings of a transformer; in industry these are referred to as the high voltage (HV) and low voltage (LV) windings. A transformer could possibly also have three windings per phase, for example a high voltage, medium and low voltage winding. The third set of windings would commonly be used as a stabilising winding. The purpose of which is to absorb unbalanced, short circuit or harmonic currents. Another function of the third set of windings could be auxiliary supply.

Rectangular electrolytic copper strip covered with paper is mainly used for winding conductors in the power transformers manufacturing. In recent years the usage of continuously transposed conductor (CTC) has increased and is now very common. The use of CTC has lead to a decrease in losses, overall dimensions and through-put time in manufacturing. In a highly competitive industry, this once again stresses the importance of optimization, reliability and cost effectiveness.

This cooling method acts very much like a circuit, where the inner circuit transfers the loss energy from the heat producing surfaces in the transformer to the surrounding oil. The outer circuit then transfers the loss energy from the oil to the ambient outside air temperature.

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Figure 7. AutoCAD model of a complete transformer tank.

Figure 8. Fleming’s left hand rule.

Short circuit currents give rise to mechanical forces and increases in temperatures. These mechanical forces are developed using the relationship:

cosF IBL= a

It is well known that when a current carrying conductor lies within a magnetic field, a force is originated and applied to the current carrying conductor according to Fleming’s left hand rule.

Figure 8 illustrates the electromagnetic forces created by the interaction of winding currents and magnetic leakage flux is. Where B is the flux density, I the current intensity, L the current carrying conductor length, a the angle between B and I vector, and F is the resultant force. The direction and orientation of the force is determined according to Fleming’s left hand rule.

The through fault currents increase the volume of flux between the winding blocks and exert inward forces on inner windings and outward forces on outer windings. By accurate calculations and finite element method (FEM) modelling of the leakage flux and flux distributions in the windings, the short circuit currents can be correctly modelled and predicted.

2.1.4 Mechanical Parts

There are commonly two types of transformers:

· dry type; and· oil type.

A transformer that is designed to operate under oil must be placed in a tank. Oil acts both as a coolant and insulation medium for the transformer. A transformer can easily hold up to 100,000 litres of oil depending on the size and specifications of the unit.

A conservator is a reservoir of spare oil that is placed on top of a transformer via a sloped pipe. Expansion and contraction of oil is associated with temperature variances. The purpose of a conservator is to compensate for this changing transformer oil volume. The transformer tank (illustrated in Figure 7) is also built equipped with stiffeners, these are metal columns welded to the sides of the transformer tank. Stiffeners serve the purpose of stabilising the tank walls, by compensating for both inward and outward pressures exerted on the tank walls under normal operating conditions.

3. Important Design Optimisation Criteria

3.1 Short Circuit

The short circuit integrity of a transformer is verified by its ability to withstand the dynamic effects of a short circuit occurrence. This is verified according to the International Electrotechnical Commission (IEC) by the implementation of tests, calculations and well developed design considerations.

Short circuit currents arise during system disturbances. These currents can lead to consequential damages if the design is not well prepared for such events. There are different types of short circuit events that may occur; these are single-phase-to-earth, double-phase with or without simultaneous earth fault and three-phase (“bolted”) short circuits with or without simultaneous earth fault.

The occurring forces during a short circuit can be separated into radial and axial components. Failure modes due to the radial forces may be buckling of the inner winding or a diameter increase of an outer winding. An inner winding will experience a compressive or inward force, whereas an outer winding will experience a tensile or outward force. The failure modes due to the presence of axial forces may arise from damage to the insulation system, or to the windings that may tilt or collapse.

3.2 Thermal Stability

An ideal transformer would be that all flux generated by the primary winding links all turns of the remaining windings and itself. This means that all the flux lines will lie between the high and low voltage windings, within the winding block. However this is not the case and thus this is termed leakage flux.

Finite Element Method (FEM) allows for the investigation into the stray flux within the winding block of a transformer. Leakage flux from the winding block will lead to eddy current losses in the

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Figure 11. Equipotential lines around transformer windings.

Figure 10. Stress criterion analysis.

The geometry of the area of interest is drawn, and appropriate boundary conditions are defined. This method is used to analyse the top edges of transformer windings where the electric field stresses are highest.

Creep occurs on a surface between two materials with different permittivities. This surface creates an easy path for free electrons to flow which may then create a flash-over. Due to the high volt-age difference between several windings and between the windings and core, creep becomes a bigger problem with increasing system voltages.

The probability of creep occurrence is also analysed using FEM software. It is evident that the more equipotential lines cut a sur-face, the higher the probability of creep along that surface. Figure 11 is a cross sectional view which shows the equipotential lines inside the transformer.

Figure 9. FEM model showing flux distribution in the winding block.

conductors as well is in the core clamps and tank walls. Using this analysis, the temperatures in these critical areas can be determined with great accuracy.

Losses in the clamp structures and tank walls are not directly related to the load currents in the windings, but in fact are directly related to the leakage flux from the windings. Therefore FEM modelling proves to be an effective method of interpreting these leakage flux lines. It is important for the designer that leakage flux be reduced as much as possible, as this adds to the load loss cost of the transformer as well as overheating in the tank structure.

High temperatures in the active part and tank structures can lead to deterioration in the insulating materials of the transformer, which can in time, reduce the lifetime of a transformer. The temperature rise limits in these critical areas are specified in the IEC standards.

The flux line distribution between the core and the winding block is illustrated in Figure 9. The leakage flux lines can be seen headed toward the tank side of the model.

3.3 Dielectrics

Due to the fact that power transformers operate at various high voltage levels it is crucial for the device to be dielectrically sound. To analyse the dielectric integrity of the transformer FEM software is once again used to calculate the electric field vectors inside the transformer.

There are two main phenomena which need to be analysed. The first is the probability of an electron avalanche forming between any two areas within the transformer, and the other is the probability of creep occurring on certain surfaces.

An electron avalanche forms when there is a localised area densely populated with free electrons, situated in a space charged region capable of sustaining electron acceleration. Figure 10 illustrates an example of such an analysis.