A59-Knowledge tit-bits on Transformer Electrical Steels-Part 3

?In previous parts of this article, we covered topics such as CRGO silicon steel grade designation; measurement of magnetic properties; grounding of laminations in a core stack; mixing of different grades of CRGO steel; heat dissipation from lamination stack; composition of electrical steel; insulation coating on laminations; burr height on lamination edges and magnetic polarization in CRGO steel. In this part, let us discuss about iron loss building factor; magnetic ageing of core; steel grades in transformer specifications; magnetostriction and noise.

A 53-Knowledge tit-bits on Transformer Electrical Steels-Part-1

https://www.dhirubhai.net/pulse/a53-knowledge-tit-bits-transformer-electrical-p-ramachandran

A 54-Knowledge tit-bits on Transformer Electrical Steels-Part 2

https://www.dhirubhai.net/pulse/a54-knowledge-tit-bits-transformer-electrical-2-p-ramachandran

1)???Core loss Building Factor

It is the ratio between W/kg from transformer test bed (measured no load loss / net weight of silicon steel used) to the W/kg from actual Epstein test value or the value given in test report from CRGO maker. Building factor will be different in limb, yoke and at joints of the core.?At?joints, it will be several times higher than in the limbs. But average value as estimated above is sufficient for quality control, validating the designs or improving core design and construction.

Core loss building factor = Total measured core loss (W) / {specific iron loss as per Epstein test for the working flux density and frequency (W/kg) x Core weight (kg)}

Core loss building factor will be always more than one and affected by both magnetic and manufacturing factors.

Magnetic Factors- Depending on the core construction, non-uniform flux distribution and distorted flux waves; Grade of steel (superior the grade, more is building factor); holes in laminations; type of mitred joint at corners (step-lap joints reduce building factor); core dimension (a higher limb height results in lower BF; Flux density, core construction (wound core have lower BF than stacked core; a 3 phase 5 limbed core has higher BF than a 3 phase 3 limbed core)

Manufacturing Factors– mechanical stress from slitting (wider the lamination, less is building factor), cutting and handling of the laminations; building of the core; burr height at edges, insulation damage at slit/cut edges; Very high or very low core clamping pressure increases the building factor.

????2) ?Magnetic Ageing of CRGO

Magnetic “ageing” was a troublesome phenomenon in transformers till 1900 when???change over from soft iron sheets to silicon steel started in core. Hysteresis loss in iron sheets used to increase by two to three times within a short time after putting the transformer in to service. This used to cause overheating of transformers and “core fires” in dry type transformers was common. This was overcome with the introduction of silicon steels.

Today’s transformer cores have stable magnetic characteristics and iron loss will not increase with service age, thanks to better silicon steel and superior insulation coatings developed. Some transformer manufacturers may claim that no-load losses will increase with service, but this is not true or acceptable.

To check the stability of magnetic characteristics, a magnetic ageing test is used. When samples for Epstein test are aged at 150 degrees C heating for 216 hours (9 days) the increase in iron loss shall be less than 3 % when tested in 25 cm Epstein frame. Equivalent accelerated ageing tests are 225 degrees C for 24 hours or 200 degrees C for 100 hours.

3)???Should I specify CRGO grade while buying transformer?

From a user’s angle, there is no need to specify any specific grade of CRGO in specifications. Of course, he can indicate or prefer any specific make at the time of order or design review.?Designer selects the grade based on the working flux density selected, marginal savings in W/kg versus marginal increase in price for better grade. User is benefited only by lower losses and not by using a better grade of core steel.

When Hi B grades were introduced in 1970s, many users were carried over by these new grades and started specifying Hi B grades while keeping the maximum flux density as 1.55 T in their specifications. At such low flux densities W/kg reduction with ?better grades is negligible but extra cost is incurred without any compensating ?reduction in total iron loss.

4)???Magnetostriction

Magnetostriction is one of the magnetic properties that accompany ferromagnetism. It causes reversible deformations of a material body due to the magnetization arising from an applied magnetic field. Magnetostriction of electrical steels is recognized as the main cause for the acoustic noise generated from transformers. ?Standardization of magnetostriction measurement methods ?is necessary ?to find means to mitigate it and there by transformer noise. Transformer noise is considered as an environmental pollution, especially in urban areas.

?When a core is magnetized by alternating magnetic field, core laminations increase in length in the direction of magnetization and contract at right angles there to. This change is called magnetostriction. This dimensional change can be positive or negative. Magnetostriction is a magneto-mechanical phenomenon which accompanies the change of the volume fraction of magnetic domains which have a certain magnetic orientation with respect to the direction of the applied magnetic field. This phenomenon is intrinsically sensitive to stress. The stress sensitivity is dependent on material conditions such as grain orientation, residual stress, and coating tension.

The magnetostriction of electrical steel is increased by compressive stresses in the magnetizing direction rather than tensile stresses. Its magnitude depends on type and grade of electrical steel, the induction level, silicon content in steel, direction of magnetization with respect to direction of steel rolling. A silicon content of 6.5 % reduces magnetostriction to almost zero but such high silicon content affects workability. In normal CRGO with silicon content of less than 3 %, magnetostriction depends on the quality of steel. A low loss steel has small and negative magnetostriction, while high loss grade steels have larger and mainly positive magnetostriction. A high degree of preferred grain orientation, flatness, and uniformity of magnetic properties in steel give lower magnetostriction.

Magnetostriction measurement on ?lamination samples can be done by any of these standards- IEC TR 62581 ed1.0-2010 - by Single sheet and Epstein test specimens using optical sensors and accelerometers.; IEC 60404-17:2021 - Measurement of magnetostriction by means of a single sheet tester and an optical sensor

?????5) ??Excitation Current Ratio

??Some users ask for the ratio of transformer excitation currents at 100 % and 110 % of rated voltage. Earlier days this factor was 2-3 due to the low flux densities used (due to limitations of CRGO available then) and 90 degrees over lapped joints at the corners. Today this factor is 4-10 due to the higher flux densities employed (compared to the saturation flux density), step-lap mitred joints at corners for power transformers and single-phase wound cores for distribution transformers. This factor will be less with Hi B and domain refined core grades compared to conventional steels. Some users consider this factor as a means of indirectly checking the margin of working flux density to the saturation flux density of the core steel used.

????6) Units & Conversions

Field Strength:

1Oersted = 80 A/m

Flux Density:

1Tesla (Wb/m2) =10,000 gauss (lines/cm2)

(2022-01-29)