Large shell type transformers aspects of design [extracted 139 141]

6 Large-Shell-Type Transformers: Aspects of Design Xose M. L贸pez-Fern谩ndez CONTENTS 6.1 Introduction......................................................................................................................... 140 6.2 Core Form and Shell Form Transformers........................................................................ 140 6.3 Transformer Basic Construction....................................................................................... 141 6.3.1 Different Magnetic Circuit Construction (Core Structures)............................. 142 6.3.2 Core........................................................................................................................... 144 6.3.2.1 Core Construction.................................................................................... 144 6.3.3 Winding.................................................................................................................... 147 6.4 Shell-Form-Type Transformer Construction................................................................... 148 6.4.1 Design Concept and Requirements..................................................................... 150 6.4.2 Pancake Windings.................................................................................................. 154 6.5 Performance Aspects of Shell-Type Transformers......................................................... 155 6.5.1 Leakage Magnetic Field......................................................................................... 155 6.5.2 Flux Distribution around a Coil........................................................................... 156 6.5.3 Flux Distribution across a Coil............................................................................. 157 6.5.4 Magnetic Shielding................................................................................................. 158 6.5.4.1 T-Beam Shielding..................................................................................... 159 6.5.5 Third Component of Flux...................................................................................... 159 6.5.6 Stray Losses............................................................................................................. 160 6.5.6.1 Calculation of Eddy-Current Losses..................................................... 161 6.5.6.2 Circulating Currents................................................................................ 161 6.5.6.3 Eddy Losses due to the Horizontal Flux.............................................. 162 6.5.6.4 Eddy Losses due to the Vertical Flux.................................................... 164 6.5.6.5 Circulating Current Losses..................................................................... 164 6.5.6.6 Effect of Transposition............................................................................ 165 6.5.7 Forces between Windings..................................................................................... 165 6.5.7.1 Short-Circuit Performance of Shell Form Transformers.................... 165 6.5.8 3D Shell Form Transformer Performance............................................................ 167 6.5.9 Electric Insulation................................................................................................... 168 6.5.10 Cooling..................................................................................................................... 169 6.5.11 Remarks on Shell Form Power Transformer Construction.............................. 171 6.6 Conclusions.......................................................................................................................... 173 Acknowledgments....................................................................................................................... 173

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Transformers: Analysis, Design, and Measurement

6.1 Introduction A transformer is a device for stepping up, isolating, or stepping down the voltage of an alternating electric signal, and it is widely used for transferring energy from an alternating current in the primary winding to that in one or more secondary windings.

The importance of transformers is due to the vital role that they play in the electrical power systems, which has made possible the power generated at low voltages (LVs) to be stepped up to extra-high voltages (EHVs) for transmission over long distances and then transformed to LVs for utilization at proper load centers. So power transformers represent the largest portion of capital investment in transmission and distribution substations. In addition, power transformer outages have a considerable economic impact on the operation of an electrical network. More than that, it has its own manufacturing problems associated with insulation, dimensions, and weights because of demands for ever rising voltages and capacities.

There are two main different types of power transformer construction: the core form type and the shell form type. The core form unit is adaptable to a wide range of design parameters and economical to manufacture. However, shell form transformers have a high kVA-to-weight ratio and find favor on EHVs and high MVA applications. Shell forms have better short-circuit strength characteristics and are more immune to transit damage but have a more laborious intensive manufacturing process.

While core form transformer predominates all over the world, shell form large power transformers are widely used in North America. Therefore, this chapter tries to show the main different aspects of design and their influence in the performance of shell form transformers. A picture of 675 MVA 352/22 kV one-phase GSU shell-type transformer is shown in Figure 6.1.

6.2 Core Form and Shell Form Transformers As an example of the former type of classification, one has generator step-up (GSU) transformers, which are connected directly to the generator and raise the voltage up to the line transmission level or power distribution transformers (DTs), which are the final step in

FIGURE 6.1 Shipping of 675 MVA 352/22 kV one-phase GSU shell-type transformer.

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Large-Shell-Type Transformers: Aspects of Design

Core

Windings

Core (a)

(b)

FIGURE 6.2 Single-phase transformer: (a) core form and (b) shell form.

a power system, transferring single-phase power directly to the household or customer.

One late type of classification, perhaps the most important is the distinction between the following: • Core-form-type transformers • Shell-form-type transformers Conventionally, in the core form construction, the two or more vertically arranged laminated steel core limbs have two or more windings concentrically arranged around each core limb. In its simplest form, the windings are commonly separated into LV and highvoltage (HV) winding sections. In an alternative construction, the LV and HV coils are interleaved vertically for shell form construction. In any case, the coils are separated from each other by a dielectric (insulating) material.

The basic difference between a core form and shell form transformer is illustrated in Figure 6.2. Both of them refer to the arrangement of the steel core with reference to the windings. In the core-type transformer, the winding surrounds the laminated steel core.

In the shell-type transformer, the steel core surrounds the windings. The choice depends on the economics, both labor and material involved in construction and installation.

Each of these types of construction has its own advantages and disadvantages. Perhaps the ultimate determination between the two comes down to a question of cost.

6.3  Transformer Basic Construction Although transformers are primarily classified according to their function in a power system, they also have subsidiary classifications according to how they are constructed.

The basic design of a transformer consists of two or more electrical circuits comprising primary and secondary windings, each made of multi-turn coils of conductors with one or more magnetic cores coupling the coils by transferring magnetic flux between them.

Therefore, in its simplest form, a transformer consists of a laminated iron core around which are wound two or more sets of windings. Voltage is applied to one set of windings of the primary, and the load is connected to the secondary.

The core can be constructed in different ways with reference to the windings.