上海意蘭可電力電子設(shè)備有限公司Shanghai

1、精品 料推薦Selection and Rating of TransformersA). IntroductionThe number of non-linear loads which draw non-sinusoidal currents even if fed with sinusoidal voltage - connected to the power supply system is large and is continuing to grow rapidly. These currents can be defined in terms of a fundamental c

2、omponent and harmonic components of higher order.In power transformers the main consequence of harmonic currents is an increase in losses, mainly in windings, because of the deformation of the leakage fields. Higher losses mean that more heat is generated in the transformer so that the operating tem

3、perature increases, leading to deterioration of the insulation and a potential reduction in lifetime.As a result, it is necessary to reduce the maximum power load on the transformer, a practice referred to as de-rating, or to take extra care in the design of the transformer to reduce these losses.To

4、 estimate the de- rating of the transformer, the load -factor may sbeK used. This factor is calculated according to the harmonic spectrum of the load current and is an indication of the additional eddy current load losses. It reflects the excess losses experienced in a traditional wire wound transfo

5、rmer.Modern transformers use alternative winding designs such as foil windings or mixed wire/foil windings.B). Transformer lossesTransformer losses consists of no-load (or core loss) and load losses. This can be expressed by the equation below.where:PC = core or no-load loss,PLL = load lossPT = tota

6、l loss.Core or no-load loss is due to the voltage excitation of the core. Although the magnetizing current does include harmonics, these are extremely small compared with the load current and their effect on the losses is minimal. Load losses are made up of I2 R loss, eddy current loss and stray los

7、s, or in equation form:Where:I2R = loss due to load current and DC resistance of the windingsPEC = winding eddy current lossPSL = stray losses in clamps, tanks, etc.The I2R loss is due to the current flowing in the resistance of the windings. It is also called ohmic loss or dc ohmic loss. The ohmic

8、loss is proportional to the square of the magnitude of the load current, including the harmonic components, but is independent of the frequency. It is determined by measuring the dc resistance and calculating the resulting loss using the winding currents at full load.There are no test methods availa

9、ble to determine individual winding eddy current loss or to separate transformer stray loss from eddy current loss. Instead, the total stray and eddy current loss is determined by determining the total load loss and subtracting the calculated ohmic losses, i.e.,The eddy current loss is assumed to va

10、ry with the square of the rms current and the square of the frequency (harmonic order h), i.e.,Where:1精品 料推薦h = harmonic order, 1,2,3, etc.hmax = the greatest harmonic order to be consideredI h = current at harmonic orderh, amperes h，I R = rated current, amperes ，PEC,R = eddy current loss at rated c

11、urrent and frequency.The eddy current loss depends on the square of the conductor dimension perpendicular to the leakage flux field. At the ends of the winding the flux field bends and the larger dimension of the rectangular conductor is perpendicular to a vector component of the leakage flux field.

12、 Equalizing the height of die primary and secondary windings, which can be achieved with any winding design, reduces the concentrated eddy loss at the winding ends. Using Foil windings or multiple strands per winding reduces both eddy current loss.Stray loss occurs due to the stray flux which introd

13、uces losses in the core, clamps, tank and other iron parts. Stray loss may raise the temperatures of the structural parts of the transformer. For dry-type transformers increased temperatures in these regions do not contribute to an increase in the winding hot spot temperature. Stray losses are diffi

14、cult to calculate and it is common to assume that the losses will vary as the square of the current times the frequency (harmonic order), as shown by:C). Transformers and K-factorThere are different approaches to accounting for additional losses in selecting a transformer. Here we would suggest the

15、way applied by transformer manufacturers in conjunction with Underwriters Laboratories in the United States, it is to calculate the factor increase in eddy current loss and specify a transformer designed to cope; this is known as the -Kfactor .Where:h = harmonic numberI h = the fraction of total rms

16、 load current at harmonic numberhMany power quality meters read the K-factor of the load current directly. Once the K-factor of the load isknown, it is a simple matter to specify a transformer with a higher K-rating from the standard range of 4, 9, 13, 20, 30, 40, 50.Note that a pure linear load one

17、 that draws a sinusoidal current would have a K-factor of unity. A higher K-factor indicates that the eddy current loss in the transformer will be K times the value at the fundamental frequency. -Krated transformers are therefore designed to have very low eddy current loss at fundamental frequency.D

18、). Transformer design considerationMany transformer manufacturers have developed designs rated for non-sinusoidal load currents while optimizing their production costs. The design process involves an analysis of the eddy current loss distribution in the windings and calculation of the hot spot tempe

19、rature rise. Analysis of the eddy loss distribution may be performed using finite element or other type computer programs. Thermal studies should be conducted using embedded thermocouples installed in test windings of prototype transformers to measure hot spot temperature to refine mathematical mode

20、ls to calculate the hot spot temperature.2精品 料推薦The subject of harmonics has received much publicity in recent times leading to the belief that the industry is only just beginning to understand the effect of harmonics and to calculate the increased eddy current losses. In fact, the study of the effe

21、cts is quite old, with eddy current losses in conductors in a magnetic field dating from 1906. Many early investigations were highly mathematical and the flux plots given in these early papers were every bit as detailed, and probably as accurate, as those produced by modern computer programs.Althoug

22、h hot spot temperature is an important performance parameter to be met by the manufacturer, there are currently no defined test methods, nor is there a requirement that this parameter be measured on production or prototype transformers. This is important since temperature is fundamental in determini

23、ng the life of the equipment.The hot spot temperature in dry-type transformers is sometimes a contentious issue. Hot spots, the positions of the highest temperature, occur naturally due to the non-uniform heat generation and the fact that the rate of heat transfer to the environment is not uniform;E). K-13 or K-20 Transformer?Transformers shall be capable of operating within the specified temperatur