A54-Knowledge tit-bits on Transformer Electrical Steels-Part 2

This is in continuation to Part-1 of this article where we found some knowledge nuggets on grade designation for CRGO silicon steel; measurement of magnetic properties; grounding of laminations in a core stack. In this part we will consider aspects such as mixing of different grades of CRGO steel; heat dissipation from lamination stack; composition of electrical steel and silicon content; insulation coating on laminations; burr height on edges and significance of magnetic polarisation in CRGO steel.

A53-Knowledge tit-bits on Transformer Electrical Steels-Part-1

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?1. ?Mixing laminations of different grades

?It is not a good engineering practice to mix steel laminations of different grades as parallel path in a core. Steel with higher permeability will carry more flux and results in higher flux density (and more losses) in it than the nominal flux density value considered. In such a case of mixing, wider sheets shall be of better grade than smaller width sheets due to the higher magnetic sensitivity of better grades to mechanical stresses from handling.

For wound core applications, core loss will be lower if the inner wraps are wound with higher permeability steel and outer wraps with lower permeability steel. The core loss will be higher if the mixing is reversed.

2.????Heat dissipation from core laminations.

?When the transformer is energized, heat will be generated continuously in the core due to eddy and hysteresis losses in the core. ?When transformer is loaded, the leakage flux from winding will enter in to core, creating local flux saturation in core limb or yoke (depending on direction of power flow) by adding leakage flux to main flux.?Typical iron loss in core in kW is about 0.30 % (10kVA), 0.07 % (20MVA), 0.04 % (100 MVA), 0.012 % (1500 MVA bank) of the transformer rating in kVA. This heat generation will not be uniform in all parts of the core. Loss per unit volume will be more at corner joints of core (150-175 %) due to the flux movement at cross direction to the lamination rolling direction. Loss in W/kg in the 45- 90-degree cross direction to the lamination rolling direction can be 4 times the loss value in the rolling direction. This increase in cross directional specific core loss will be more for better grades of steel.

In large transformers or auto-transformers with very low co-ratio, high levels of leakage flux from windings can enter the core surface and create hot spots and circulating currents on the core surface. Manufacturers take care of such situations through special means- slitting the front packets of core or copper shielding.

In core type transformers, maximum core temperature will be in the yoke, above the middle limb. The maximum permissible temperature inside core is 130 degree C during normal cyclic loading, as per Table 2 of IEC 60076-7 Loading Guide. Core lamination insulation coating can withstand much higher temperatures, but above temperature limit is stipulated to avoid gas generation (hydrogen and methane) from the breakdown of thin oil film between laminations (occurs at about 140 degree C).

Almost the entire heat generated in core is dissipated through the lamination edges and very little in the perpendicular direction as the insulation coatings on each lamination prevent the heat transfer. Hence designers always provide cooling ducts on pressboard components that directly touch the core edges (eg. lower yoke) to take away the heat through oil flow. Where ever solid insulation touches the core, surface temperature of core is limited to the maximum winding hot spot.

3.????Silicon content in CRGO steel – Material Composition of Transformer steel?????????

During the early days, transformer cores were made from thin sheets of pure wrought iron. Soft iron made from Swedish sources was popular due to their better magnetic properties.

Then around 1900, it was found that adding 3-6 % of silicon or aluminium improved the magnetic characteristics of steel. It increased the electrical resistivity, thereby reducing eddy losses in steel. Adding 0.03- 0.07 % of carbon improved the texture of hot rolled steel sheets by promoting formation of an austenite phase.

When cold rolled steel was made in early 1940’s, with grain refinement in the rolling direction, it was discovered that to get loss reduction, grain formation in direction other than the Goss direction has to be stopped. Armco (USA) found out that adding Manganese sulphide (MnS) plus aluminium nitride (AlN) inhibited such grain growth in CRGO during the rolling process. Alternate inhibitors were developed in later years eg MnSe + Sb (Kawasaki, Japan) and MnS+ BN (Alleghney, USA). Generally, for conventional CRGO, MnS inhibitors are added and for high permeability material, MnS + AIN is used.

Modern day CRGO grades contain 2.9 -3.2 % of Silicon. Higher % of silicon is not preferred due to difficulties in rolling as steel becomes more brittle with higher silicon content.

Japanese have developed special silicon steel with silicon content of 6.5 %. The chemical vapor deposition of the silicon allows higher silicon content without the problems of brittleness during the rolling operation. Apart from reducing eddy loss, high silicon content also brings down magnetostriction, the cause for transformer noise.

4.????Insulation on laminations

During the first three decades of transformer engineering, 0.5 mm thick soft iron sheets were used in transformer cores with paper insulation between laminations. ?Later laquers or varnishes were used for insulation. By 1950s an inorganic insulation coating comprised of magnesium silicate was developed. This is formed on the lamination surface by the reaction of annealing separator with the steel surface during high temperature annealing.?This coating is sometimes called as ‘mill glass’ or Forsterite glass film coating, even though the coating is not strictly glass. This EC-2 coating (designation as per IEC 60404-1-1) is sufficient for wound cores but an additional magnesium phosphate or silicate-based coating is required for stacked core laminations of power transformers. This is termed as EC-5-G coating over EC-2 (C5 coating over C2 as per ASTM 976). This can withstand stress relief annealing treatment at a temperature up to 845-degree C. The coating thickness should be sufficient for the induced voltage per turn but it should not be so thick as to reduce the space factor of core. At a particular flux density, adequacy of surface insulation for steel is a function of lamination width. Wider laminations require a better insulation coating. Modern insulation coatings create a stretch tension on lamination surface that reduces sensitivity of steel to mechanical stresses, reduces iron losses and magnetostriction.

Adherence of insulation coating to the base metal is checked by a bend test (ASTM A 720). A strip is bent at least 90 degrees around a 20-22 mm diameter mandrel and insulation looseness on concave surface shall be less than 15 %.

The insulation coating resistance expressed in Ω. mm2 represents the electrical resistance offered to the passage of current through the coating. The measured insulation coating resistance before or after the possible application of a stress relief heat treatment shall be not less than 500 Ω. mm2 (Clause 7.3.5 Insulation coating Resistance - IEC 60404-8-7-2017 CRGO Strips and sheets; Test Method IEC 60404-11 & ASTM A 717 &737)

5.????Permissible Burr height in lamination edges?

The determination of the burr height applies only to slit coils delivered in the width in which they will finally be used. The measured burr height shall not exceed 0,025 mm. (25 micro meters) (Clause 7.2.7 Burr height - IEC 604-8-7 -2017 “CRGO strips and sheets”) A high burr height will cause shorting of adjacent laminations, resulting in circulating currents and extra eddy losses.

Nearly half a century back, slitting cutters were of high carbon steel and the burr heights on the slitted coils were excessively high. My company then used to grind the edges of slit laminations to remove burrs and anneal each lamination in a continuous annealing oven to relieve the stress. Today silicon carbide or diamond cutters are used for slitting and typical burr height values for good laminations are of the order- 90 % of laminations with less than 10 micro-meters and 100 % less than 20 micro- meters. Measurement is made at lamination edges, at every 50mm on a sample (one metre long), using a hand-held micro meter.?Measurement method is as per IEC60404-9

??6. ???Magnetic Polarization

In National Standards for CRGO (Cold Rolled Grain Oriented Silicon Steel) like IEC 60404-8-7-2017, there is a requirement of “Minimum magnetic polarization for Peak magnetic field strength H=800 A/m “

The word polarization means magnetization ie flux content (similar to current density in electric circuit) at a particular magnetic field strength (similar to voltage). So, when standard asks for a minimum value of polarization at a particular field strength, it indirectly gives the magnetic conductivity (permeability) of the electrical steel. So normal CRGO steel has a minimum magnetic polarization of 1.78- 1.8 T and high permeability CRGO steel has 1.85 -1.88 T at a peak magnetic field strength of 800 A/m. Looking it another way, it means you require less magnetizing force with high permeability cores to achieve the same flux density.

B= μ0 (permeability of vacuum) x μr (relative permeability) x H and polarization J = B - μ0 (ie magnetic constant or the permeability of vacuum) x H