Analysis Of Transformer Losses

ABSTRACT

In any electrical machine, ‘loss’ can be defined as the difference between input power and output power. An electrical transformer is an static device, hence mechanical losses are absent in it. A transformer only consists of electrical losses. Transformer losses are similar to losses in a DC machine, except that transformers do not have mechanical losses. The aim of this work is to analyze these transformer losses.

 CHAPTER ONE

INTRODUCTION

BACKGROUND OF THE STUDY

A transformer is described as a static electrical device that transfers electrical energy between two or more circuits. A varying current in one coil of the transformer produces a varying magnetic flux, which, in turn, induces a varying electromotive force across a second coil wound around the same core. Electrical energy can be transferred between the two coils, without a metallic connection between the two circuits. Faraday’s law of induction discovered in 1831 described the induced voltage effect in any coil due to changing magnetic flux encircled by the coil.

Transformers are used for increasing or decreasing the alternating voltages in electric power applications, and for coupling the stages of signal processing circuits.

Since the invention of the first constant-potential transformer in 1885, transformers have become essential for the transmission, distribution, and utilization of alternating current electric power. A wide range of transformer designs is encountered in electronic and electric power applications. Transformers range in size from RF transformers less than a cubic centimeter in volume, to units weighing hundreds of tons used to interconnect the power grid.

An ideal transformer is the one which is 100% efficient. This means that the power supplied at the input terminal should be exactly equal to the power supplied at the output terminal, since efficiency can only be 100% if the output poweris equal to the input power with zero energy losses. But in reality, nothing in this universe is ever ideal. Similarly, since the output power of a transformer is never exactly equal to the input power, due a number of electrical losses inside the core and windings of the transformer, so we never get to see a 100% efficient transformer.Transformer is a static device, i.e. we do not get to see any movements in its parts, so no mechanical losses exist in the transformer and only electrical losses are observed. So there are two primary types of electrical losses in the transformer. In this work, all the losses that occur in a transformer are analyzed.

1.1                                       AIM OF THE STUDY

The aim of this study is to determine the trend of losses over the life of the transformer, depending on the type of intended use (load chart, periods without load and out of service, future load growth), combining losses with the cost of energy which depend on the type of supply or generation cost. The course delivers methodology necessary to perform such life cycle cost analysis.

1.2                               OBJECTIVE OF THE STUDY

There are various types of losses in the transformer such as iron losses, copper losses, hysteresis losses, eddy current losses, stray loss, and dielectric losses. At the end of this work student involved shall be able to study and analyze these different types of loses in the transformer in detail.

1.3                                    SCOPE OF THE STUDY

Losses in transformers decide about their energy performance and although transformers are yet efficient devices the potential for energy saving is nearly 50% when considering all operating units globally. The focus will be energy losses in power transformers with particular reference to medium and low voltage units. Selecting the value of losses is a decision mainly of economic nature because losses are the main operational cost and higher the cost of electrical energy, higher the importance of losses.

1.4                      EFFECT OF LOSS ON TRANSFORMER

Since an electrical transformer has no rotating parts, it has no mechanical losses. This contributes to its high operating efficiency of over 90%. However, like any electrical device, a transformer does have losses. These losses appear in the form of heat and produce an increase in temperature and a drop in efficiency.

DEFINITION AND DESCRIPTION OF TERMS

Primary winding: The winding where incoming power supply is connected. Usually this refers to High Voltage side in distribution transformers

Secondary winding: the winding where the principal load is connected. Usually this refers to Low Voltage side in Distribution transformers.

No load loss: The losses taking place in a transformer when only primary winding is energized and all secondary windings are open. They represent constant losses in a transformer.

Dielectric loss: The losses taking place in a stressed dielectric medium (insulation) subjected to stress reversals.

 Iron losses: The losses taking place in the magnetic core. There are two types; hysterisis losses and eddy current losses.

Hysteresis losses: This loss depends upon the area of the hysteresis loop, which is depending upon the maximum flux density, the type of material and frequency. It is independent of the waveform

Eddy current losses in core: This is loss due to circulating currents induced by voltage in the thickness of core laminations. It depends upon thickness of lamination, path resistance which is depended upon the type of material, R.M.S. flux density i.e. waveform and square of frequency

Eddy losses in a conductor: For a thick conductor, the induced voltage within the conductor cross section due to self linkage and due to current in other conductor varies. The difference in induced voltage in the local path in the thickness of the conductor causes extra eddy current loss. This loss varies with square of current and square of frequency.

Stray losses: All current dependant losses in a winding other than the basic I2R losses. Stray losses include eddy loss in the conductor, eddy losses in structural paths in close proximity to outgoing conductor and the eddy loss in general in the structural parts. In dry type transformers, the last two mentioned types of stray losses are absent.

Form factor: It is the ratio of the r.m.s. value of a waveform to the average value over one half cycle. For a sine wave the value of form factor is 1.11. For distorted waves with higher peak values, the form factor is higher.

Harmonics: Frequencies other than the main fundamental frequency of current or voltage which are present in a distorted wave as multiples of base fundamental frequency.

Transformer Polarity: This refers to the relative direction of the induced voltages between the high voltage terminals and the low voltage terminals. During the AC half-cycle when the applied voltage (or current in the case of a current transformer) is from H1 to H2 the secondary induced voltage direction will be from X1 to X2. In practice, Polarity refers to the way the leads are brought out of the transformer.

Burden: The load on an instrument transformer is referred to as a “burden”.

Short circuit impedance & Impedance voltage: The impedance voltage of a transformer is the voltage required to circulate rated current through one of the two specified windings; when the other winding is short circuited with the winding connected as for rated operation. The short circuit impedance is the ratio of voltage and current under above conditions.