Article 1: CRGO Steel - Handle with care - by Saif Qureishi (CEO - KRYFS Power Components Ltd.) Paper Presented at TRAFOSEM 2004

Article 2: CRGO-In-house vs. Outsourcing - by Saif Qureishi (CEO - KRYFS Power Components Ltd.) Paper Presented at TRAFOSEM 2006.

Article 3: Reducing Building Factor by Using Step Lap (SL) Laminations – by Saif Qureishi (CEO - KRYFS Power Components Ltd.) Paper Presented at TRAFOSEM 2008.

 

CRGO-In-house vs. Outsourcing

CRGO Laminations- Inhouse manufacture or Outsourcing?

A debate has been triggered by the decision of some State Electricity Boards (SEBs) decision to specify as a tender condition that Transformer manufacturers (TMs) should have their own Transformer Lamination

manufacturing facility. SEBs are now imposing tender conditions to the effect that "ONLY those Transformer Manufacturers who have their own core cutting facility would be eligible to participate in the

tender".

This move ostensibly is to prevent the use of Secondary/Defective and old and used CRGO Silicon Electrical Steel in Transformers which supposedly leads to increase in No- Load Losses, Magnetising

Current drawn and results in the failure of Transformers.

It is therefore necessary to analyse whether, this condition suggested by some TMs who have their own core cutting facility and disguised as a "magic pill" that would lead to better quality of Transformers being

manufactured is valid and the implications thereof.

At the very outset let me state that, as the Managing Director of a Transformer Lamination facility, my Company is an interested party in this debate, however, the analysis given herein is from an impartial

perspective to genuinely assess the contention that, "an in-house core cutting facility automatically leads to better quality of Transformer Laminations and thus better Transformers".

The importance of Transformer Lamination on Transformers is evident from the name the component has been given by the Transformer industry - "the core" - as it is considered to be the heart of every

Transformer.

Failures in Transformers due to magnetic core or circuit can be due to various reasons. The J&P Transformer Book ( 11th Edition , published by Butterman Heinworth, General Editor C.A.Worth) accepted

by Electrical Engineers worldwide to be a standard reference for Transformer design, Chapter 24 deals with this issue. Amongst the 13 reasons stated therein for the failure of Transformer due to failures in the

magnetic circuit, the following 5 reasons seem to be of relevance for this discussion:

1. Failure of insulation between Laminations and of the insulation between the yoke and the yoke bolts producing large eddy currents, generating considerable amount of heat.

2. Burrs developed during manufacture resulting in local short circuits, eddy currents and consequently abnormal heating occurring.

3. Presence of metallic fillings or turnings present between the Laminations are liable to produce

local eddy currents and excessive heating of the core. 4. Abnormal gaps left between the cores and the yoke would result in severe eddy currents and

burning of the cores and yoke in the vicinity of the gaps. 5. In older Transformers ageing of the core plates may take place and result in increase of iron loss and

rise in temperature of the Transformer which may result in partial or complete destruction of the coil insulation and sludging of the coil.

In view of the above, let us analyse if these reasons for failure could be definitely avoided if the producers of Transformers were to manufacture the Transformer Laminations in-house instead of buying from a

Lamination manufacturer (outsourcing).

Reason Nos. 1 (weak insulation) and 5 ( old and used Laminations) above are related to the quality of raw material used and therefore if the Laminations are made from inferior quality of CRGO material then

definitely there is a possibility that these reasons would be applicable. However a TM with an in-house facility to produce Lamination is as prone to usage of inferior quality of raw material ( to save on material costs) as a TM who out sources this activity. The counter argument here maybe that a TM who buys from

outside may not be aware of the quality of raw material used by the manufacturer of Laminations, who may purposely use inferior quality of raw material without informing the TM. However if the TM has an inward material receipt inspection system, this fact would certainly be brought to notice and the TM must take

appropriate action. If the TM does not have an inward material inspection system then not much can be said about that TM's quality of Transformer, in any case.

Reason Nos. 2 (Burrs in cutting), 3 ( turnings or steel residue between Laminations) and 4 ( gaps between yoke and core plates) are precisely the reason why the activity of manufacturing Laminations should be

outsourced and not done in-house.

This is because, manufacturing of Laminations though a seemingly simple job of shearing, cutting and notching, is in reality a high precision, high accuracy job. The thickness of the sheet being handled and cut is only 0.23 mm to 0.35 mm or 230 microns to 350 microns. Also the sheet should not be bent, dented or damaged during handling as this directly affects the core loss and the magnetic property of the resultant core. The dimensional accuracies in terms of length , breadth and the angles ( 45 degrees or 90 degrees as the case maybe) have to be within the tolerance, the V- Notch in the yoke has to be precisely done so

as to accommodate the yokes without airgaps, holes have to be accurately punched so that the clearances of the bolts are adequate, the slitting has to be perfect to avoid camber and variation, the burrs have to be

within the specified tolerance. These are just some of the parameters to be controlled during the manufacture of Transformer Laminations.

A TM, whose main production activity and expertise is in the manufacture of Transformers and or electrical equipments would not only have to be aware of the nuances of the manufacture of Transformer

Laminations but also develop the expertise to implement these checks and controls.

A Quality conscious manufacturer of Transformer Laminations would certainly be aware of all these aspects and developed the requisite expertise for production control of these crucial parameters. Further a

Quality conscious manufacturer of Transformer Lamination would also have the trace ability of the materials used and therefore it would be possible to check at any time from production records maintained

the raw material used.

Thirdly from an economic standpoint it is never competitive to manufacture the components of any equipment in-house,. The Transformer industry is comparable to any assembled product manufacturing

industry like the automobile industry or the computer hardware industry. Very few components of the assembled products are manufactured by the original equipment manufacturer (OEM) themselves. Almost all the major components as well as minor components are outsourced to vendors as it is acknowledged

that a reliable, quality conscious manufacturer of components would be in a position to supply better quality of components at a very competitive price. Therefore many OEMs work closely with their vendors to

develop their manufacturing as well as managerial capabilities.

The reason for this is economic as well as managerial. The economic reason is that a component manufacturer, like a manufacturer of Laminations would have better economies of scale in the

procurement of raw material as well as better management of inventories. As an example consider that, a TM who manufactures Power Transformers upto 50 MVA would typically have a requirement of a large

quantity of say 500 mm to 600 mm width of CRGO coils for manufacturing of their core. Now CRGO producing mills produce CRGO coils in the width ranging from 863 mm (A.K. Steel, USA ) to 1000 mm ( most other mills) . If the TM were to manufacture the core in-house, they would have to keep in stock

balance "left over" coils generated after the use of the larger widths by them. These "left over coils" would then have to be disposed at a far lesser price than the cost of raw material to manufacturers of either

Transformer Laminations or Transformers who would have some use for them. Alternatively, the TM would have to hold the material in stock as till the time that they have some order where the same could be utilized. As the TM is basically a Power Transformer manufacturer it would be very difficult for them to utilise this material or they may be forced to take some orders for a lower rating at a lower price just to

utilise this "left over" material!

Further CRGO comes in at least 15 different "grades' with different core losses and thick nesses. Expecting a TM to stock all or most of these grades also makes no economic sense. A Lamination

manufacturer whose business it is to manufacture Laminations is in a much better position to forecast the stock requirement and stock the same for supply on time

From personal experience we find that our customers give us their monthly production planning and operate on "Just in time" inventory as far as the Lamination requirement goes. This enables them to plan

their working capital in a much better way and also improves the overall efficiency of their operations.

Ofcourse from an economic standpoint, the cost of the above in-built inefficiencies in in-house manufacturing by TMs would be loaded on to the final selling price of the Transformer that the TM

produces. So the SEB would be forced to buy at a higher price from a TM who has an in-house facility to manufacture Laminations if they insist on the in-house manufacturing condition!

From a managerial standpoint the Law of "Focus on your core competencies" needs no elaboration. A Quality conscious Lamination manufacturer, would definitely be able to mange the manufacturing

operations, the wastages and reduce the inefficiencies far better than a TM can. This is because a Quality conscious Lamination manufacturer would "know the business" and therefore better placed to run a leaner outfit producing at a lower cost than a manufacturer of electrical equipments. In a competitive environment,

this would translate to a more economically price of Transformer Laminations and ultimately more economical Transformers.

However the problem of SEBs and TMs maybe that there aren't that many "Quality conscious" Transformer Lamination manufacturers. The problem would also be compounded by the large influx of

secondary, defective and old and used CRGO material into the country which is then reused in new Transformers, thereby leading to the problems enumerated at the beginning.

So what is the solution?

The solution, according to me, is two fold:

1. Ensure that the Lamination manufacturers that the TMs buy from are approved by the SEBs, or

are accredited by some international recognized certification body like ISO 9000 etc. to ensure that a minimum quality parameters and tolerances are maintained.

2. The TMs, SEBs and the Quality conscious Lamination manufacturers jointly take up the issue of restricting the imports of secondary , defective and old and used discarded Laminations in the country in the interest of the nation and also to ensure that these materials are not used in the

manufacture of Transformers.

In conclusion it can be said that, expecting better quality of Transformers by simply stipulating a tender condition requiring TMs to have an in-house core cutting facility is naïve and counterproductive

economically as explained above. 

CRGO Steel - Handle with care

A) HISTORY

B) TERMS AND DEFINITIONS

C) PROPERTIES OF GRAINS, DOMAINS AND UNDERSTANDING OF HYSTERESES LOSSES

D) PROCESSING OF CRGO STEEL INTO LaminationS

E) DESIGN LOSSES VERSUS ACTUAL LOSSES

F) GRADES, NOMENCLATURES AND MATERIALS

G) CONCLUSION

 

A) HISTORY

The earliest process to manufacture Cold Rolled Grain Oriented Electrical Steel, popularly known as CRGO, was first patented 70 years ago in 1933 in USA. The earliest grades of CRGO were known as M10, (approx. 1.00 watts/lb. at 1.5T/60Hz) and M9 (approx.0.90 watts/lb. at 1.5T/60Hz)

By 1947, the first catalogue containing design curves and other essential information on grain oriented steels was published.

In 1955, grades M7 (approx.0.7w/lb at 1.5T/60Hz) and M6 (approx.0.6w/lb at 1.5T/60Hz) were developed and were the most widely used grades of CRGO.

However, the first Conventional Grain Oriented Steel (CGOS) grades known popularly today as M3, M4 and M5 were developed in the late sixties and the Hi-B Grain Oriented Steel grades (HG-OS) were developed in the early seventies whilst laser scribed material in the mid-eighties.

ARMCO, USA, (now known as A.K. Steel) were the pioneers in development of CGOS grades whilst the Japanese mills Nippon Steel and Kawasaki Steel, the pioneers in development of HGO grades and laser scribed grades.

In the development of Grain Oriented Steels over the past 70 years, not only have the hystereses losses been significantly reduced from the earliest grades of GO developed, but the thickness has been significantly reduced, thereby reducing the eddy current losses. The insulation coating has been significantly improved to keep inter-laminar losses at a minimum.

These improvements in GOS have led to an ever increasing demand of this grade of steel which, though being classified as a "steel" is very rarely impacted by the international price movement or other factors influencing mild steels or other categories of steel products. GOS has provided the opportunity to reduce the size of magnetic cores in electrical equipments as also reducing other materials and thereby reducing the cost whilst improving the efficiency of electrical equipments.

There have also been no serious challenges in terms of replacements of GOS for the application in core material in Transformers and there is hardly any new material on the horizon either. Potential challengers like Metglass Amorphous Boron Strip / Mu Metal /Nickel Iron etc. have proven to be not quite useful in replacing GOS due to various technical problems and have already been relegated for use in special purpose applications (mainly high frequency) only in developed countries. That the producers of these materials have tried to dump this technology on developing countries is another matter altogether, which needs to be discussed separately.

Therefore, a comprehensive understanding of GO steels is necessary, especially in the Indian context where CRGO steel is seen from the following perspectives by Transformer Manufacturers (TMs)

1. A final balancing item in the costing of Transformers. As the competitive pressure on prices of Transformers increases, the only maneuverable "A" class item of significant value is CRGO core, where costs can be reduced. Therefore, TMs are forced to downgrade their core to reduce cost.

2. A large quantity of seconds, defectives and used GO materials are available, thereby complicating the design and purchase decision further. In fact India is known to be one of the largest markets worldwide, for secondary GOS.

3. A lack of sufficient information regarding the design parameters, latest materials, nomenclature leading to outdated core designs, which are rarely upgraded or reviewed.

4. A tendency by SEBs to specify the "best" HGO available and stringent documentation and inspection procedures in a bid to improve the core quality.

In view of the above, this paper attempts to first explain the various terms associated with GO steels, the relevant properties, the best processes for fabrication and the relevant check points and some suggestions and conclusions to ensure better core quality.

 

(B) TERMS AND DEFINITIONS

1. AISI - American Iron and Steel Institute which gave the nomenclature for CGO materials with M as a prefix and a number following (eg. M4, M5, M6 etc.) M indicates magnetic material, and the number following approximately indicated 10 times the core loss of earliest CRGO material in watts per lb. at 1.5T and 60 cycles. Today however, this number is not relevant, but still denotes the accepted grade and popularly used throughout the world (e.g.

M4 denoted magnetic material having core loss of approx.0.4W/lb at 1.5T/60Hz). 2. Core Loss: It is the electrical power lost in terms of heat within the core of electrical

equipment, when cores are subjected to AC magnetising force. It is composed of several types of losses - Hystereses loss, eddy current loss within individual Laminations and inter-laminar losses that may arise if Laminations are not sufficiently insulated from each other.

3. Eddy Current Loss: This component of core loss is the energy lost by the circulating current induced in the metal by the variation of magnetic fields in the metal. Therefore, more uniform the magnetic field in the metal, lower the eddy current losses.

4. Hystereses Loss: The power expended in a magnetic material as a result of the lack of correspondence between the changes in induction resulting from the increase or decrease of magnetising force (which is a result of it being cyclic, i.e. alternating) (explained in detail later on in this paper).

5. Inter-laminar Loss: The power expended in a stacked or wound core as a result of weak insulation resistance between Laminations resulting in the flow of eddy current within a core, across Lamination sheets.

6. Surface Insulation Resistance: The resistance of a unit area of surface coating measured perpendicular to the surface usually expressed in ohm-Cm2 per Lamination. Surface insulation resistance is considered adequate if the inter-laminar loss is restricted to less than 2% of total core loss. In absolute values it should be greater than or equal to 10 ohms Cm2 and it is measured by the Franklin test method.

7. Saturation Induction: The maximum excess of induction possible in given material above that produced in a vacuum by a given magnetising force. It is numerically equal to the maximum induction expressed in gausses minus the magnetising force in Oersteds (B minus H).

8. Stacking factor: The proportion of steel that would be found when Lamination sheets are stacked on top of each other as compared to a solid steel section for the same volume. It varies between 95% to 97% for CRGO steel coils, however it reduces with fabrication if there are "burrs" developed. This is turn would increase the overall core loss of the electrical equipment. The balance percentage of stacking factor (3 to 5 %) is air!

9. Burrs: The residual steel on the edge of steel sheet where shearing or punching during fabrication has taken place, thereby increasing the thickness on the edge and reducing the stacking factor. Burrs can be reduced by accurate and precise fabrication and having cutting blades and tools well sharpened at all times. They can also be reduced by deburring and stress relief annealing.

 

(C) PROPERTIES OF GRAINS, DOMAINS AND UNDERSTANDING OF HYSTERESES LOSSES

Every type of steel has "grains" which consist of "domains". These "domains" are nothing but electrical charges oriented in any random direction. Therefore if a Transformer were to be made of Mild Steel used as core material, the core loss would be approx. 16 to 17 w/kg at 1.5T/50Hz and the size of the Transformer would be approx. 18 to 20 times the size of a Transformer manufactured with GO steels.

The main difference between regular "carbon" steels and GO steels are:

1. The size of the "grains" in GO steels are purposely "grown" and made bigger and are about 10 times the size of the grains in regular steel, thereby reducing the hystereses losses. The size of grains in CGOS is 2 mm to 5mm and HGOS is 5mm to 20mm. In regular steels the size of a grain is less than 0.5mm.

2. The grains in GO steels are all aligned almost parallel to the direction of rolling of the steel

(i.e. the length of the steel). The angle of mis-orientation (i.e. deviation from the rolling direction) is maximum 7% for conventional GO and less than 3% for Hi-B GO steels. This reduces the hystereses losses as "switching" (explained later) becomes easier within the domains.

3. The chemical composition of the GO steels has about 3.2% of Silicon as an alloy, thereby increasing the specified volume resistivity of the steel, thereby reducing the eddy currents. GO Steels are also decarbonised and have no more than 0.06% of carbon in their chemical composition, which prevents aeging of the steel.

4. There is a special carlite insulation coating on the steel, which reduces the inter-laminar eddy current losses within the core.

Let us understand how exactly hystereses losses are developed with respect to GO electrical steels: The microstructure of the steel, as mentioned before, consists of numerous "grains" each of which have domains. The magnified diagram would look like this:

O = Angle of misorientation from Rolling direction Grains which is less than 7% for CGOS and less Than 3% for HGOS

The typical picture inside any "grain" would consist of domains like this:

A domain when expanded would look like this:

H1

V1 V2

H2

Thus, every domain is nothing but a closed magnetic circuit as shown in the figure above.

Now consider what happens when an alternating current of 50 cycles is applied. The domains "switch" to and fro 50 times in a second. Therefore the domain looks like this as the current alternates 50 times and the diagrams below represent the direction of the domain as the current alternates.

H1

V1 V2

H2

    

And so on ….. 50 times every second

It is relatively very easy for the vertical switches (V1 and V2) to occur but very hard for the horizontal (H1 and H2) switches to occur.

The horizontal switches require more energy to be completed and also "lag" behind the vertical switches, and this results in heat, which results in the hystereses loss within the steel. The sum total of the energy required for the horizontal switches to occur are the total hystereses losses of the steel. Thus the larger the grains, the lower the losses as there are less total number of grains in the steel and therefore less number of "switches" and low hystereses losses.

  

(D) PROCESSING OF CRGO STEEL INTO LaminationS

CRGO steel is a "delicate" steel to be handled with care. As the magnetic property of the steel and not the tensile strength (as is the case with most other steels) is the important quality required, it is imperative that we understand the nuances in handling, storing and processing of this steel. If these are not done properly, it ultimately leads to higher losses and the results are not as per design.

Stresses are of two types, elastic stress and plastic stress. An elastic stress is a temporary stress which any GO steel may be subjected to like some load on top of the coil or a slight force to decoil. The moment the stress is removed, the original magnetic properties of the material are restored and these are no longer damaged.

However, a plastic deformation due to winding into cores or pulling or stretching or bending GOS as shown below, can only be rectified by a stress relief annealing at around 820ºC.

1. Storage of CRGO coils has to be done properly as improper storage may result in excessive stresses unintentionally. This type of stress can be elastic or plastic depending on the severity of the wrong storage and the resulting deformation in coil shape (if any).

2. Improper handling of strip, sheets or long Laminations as shown in the diagram below, can introduce stresses that can distort magnetic properties. These stresses are usually plastic

stresses.

Tests conducted at the plant of M/s. Kryfs Power Component Ltd., Kherdi, on Soken Single Sheet Tester showed a deterioration of 7% in core loss for material that was bent. However after stress relief annealing at 820ºC, the deterioration was only 2% and most of the original magnetic properties (with respect to core loss) of the material were restored.

3. Processing operations like slitting, shearing, notching, holing etc. all damage the grain structure of the GO material around the area of fabrication and working. Most of these induced stresses are plastic stresses that can only be removed by stress relief annealing. To determine the effect of annealing, two stacks of Epstein samples measuring 30mm x 305mm were fabricated from M4 grade CRGO steel coils. Stack 1 was cut and annealed in a fast single sheet roller hearth annealing furnace at a temperature of 820ºC and stack 2 was left unannealed. Both the stacks were sent to ERDA, Vadodra for evaluation of specific core loss and B-H curves. The report is attached in Annexure 1 but the brief results are as under:

 Core loss at 1.5T/50hz(w/kg)

Core loss at 1.7T/50Hz (w/kg)

Stack 1 (annealed) 0.82 1.36

Stack 2 (unanealed) 1.00 1.61

Values as per Mill T.C

0.81 -

4.This clearly shows that stress relief annealing significantly restores the original magnetic value of the material and removes both elastic and plastic stresses. This is especially true when the width of the strip being worked with is extremely narrow.

5. Burrs are developed during fabrication which are unavoidable in any steel fabrication operation. Burrs decrease the stacking factor (see the definition of Burrs) In Indian conditions where most of the fabrication processes are performed manually and carbide blades are not used, burrs are easily developed and can dramatically increase the overall losses of the GO steels. Therefore the Laminations need to be deburred (to reduce / remove the burr) and also stress relief annealed thereafter as it creates an oxide film on the burrs, thereby reducing the conductivity of burr contact and minimising losses.

6. The method of holding the Laminations in a core assembly and the mechanical pressure applied to the core assembly also affects the total core loss. Uninsulated bolts or assembly by welding, would provide a low resistance path and increase eddy current losses and should therefore be avoided. High assembly pressures decrease the surface resistance and

increase the inter-laminar losses and increase the total core losses. Therefore excessive clamping on the core must be avoided as the resistance of surface insulation is inversely proportional to the pressure applied. A high clamping pressure leads to breakdown of surface insulation resistivity and higher inter-laminar losses.

7. Inaccurately cut angles in mitred cores also result in a distortion of flux and increase in overall core losses. Air gaps at joints can drastically alter the values of t he total core loss.

8. Variation in thickness in the same width step of material not only results in problems in core building, but also increases the overall core loss of the material as it increases the air gaps during the assembly.

9. Residual material on Lamination surfaces like oil, dust etc. also adversely affects the stacking factor and increases the total core loss.

10. The method of assembly of core, i.e. one piece at a time or two pieces or three pieces also marginally increases or reduces the core loss (lower number of sheets in assembly results in lower core loss).

 

(E) DESIGN LOSSES VERSUS ACTUAL LOSSES

A regular complaint of Transformer designers is that though individual losses on single sheet tester are within the guaranteed parameters, the total no load core loss of the material on assembled core are not matching the theoretically derived no load losses.

In the light of the above discussions, it is clear that there are various other factors affecting the total no load core loss besides the intrinsic value of the core loss of the GOS material alone.

It must also be mentioned that SOKEN (Japanese) single sheet tester which is mentioned in Nippon Steel Catalogues and is known to display consistent readings and results over a number of years, requires regular calibration which is often ignored. Much cheaper locally (Indian make) versions of the single sheet tester, whilst reliable for non-grain oriented and lower grade of electrical steels are not consistent in their results and cannot be relied upon to provide accurate measurements for Grain Oriented Steels. This observation is made from practical experience.

Further, designers would be well advised to develop their own data on the points mentioned above as there is no universal standard on most of these points and the practices differ with different Transformer Manufacturers.

However, a guideline on dimensional and other tolerances extracted from major international standards from finished Transformer Laminations is given below as a quick reference guide:

ATTRIBUTE TOLERANCE PERMISSABLE

Length

Upto 315mm) +0/-0.4 mm(From 315mm to 1000 mm) +0/- 0.6mm(From 1000 mm to 2000 mm) +0/- 1 mm(From 2000 mm to 4000 mm) +0/- 1.6mm

WidthUpto 150 mm) +0/- 0.25 mm(From 150 mm to 500 mm) +0/- 0.3 mm(More than 500 mm) )/- 0.5mm

Angle + / - 5 minutes

Edge Camber Max.1.5mm in 2000 mm length (as per BS 60 1)

Burr 25 Microns Max. or 10% of thickness, whichever is less

Stacking Factor95.5% (for M3)96% ( for M4 & M5)96.5% (for M6) (as per major International standards)

Thickness +/- 0.03 mm (as per major International standards)

Insulation Resistivity Min.10 Ohm / sq.Cm. as per Franklin method

  

(F) GRADES, NOMENCLATURES AND MATERIALS

Different mills have different brand names and nomenclatures whilst producing GOS and HGOS. Many a times this creates confusion in the mind of the customer regarding the exact requirement of the material. Designers use outdated nomenclatures from old catalogues of mills which are no longer valid and this causes some confusion in the material being asked for and supplied by the fabricator.

Most mills have now switched over to the following method of grading Grain Oriented Steels: (Thickness) (Brand Name) (Core loss at 1.7T/50Hz)

For eg. Nippon Steel grade 23ZH100 means thickness 0.23mm, ZH is the brand name for Hi-B for Nippon Steel and 100 means 1.00W/kg at 1.7T/50Hz.

Similarly 23 RGH100 IS Kawasaki Steel nomenclature for the same material and 23ORSIH100, the Thyssen Krupp Eklectrical St eel (TKES) nomenclature for the same material.

Therefore TMs would be well advised to use these latest nomenclatures whilst specifying GOS requirements to avoid confusion. Even if a TM is looking for a particular core loss at 1.55T or 1.6T, then the GOS which gives the required core loss (these intermediate losses can only be derived from standard core loss curves of mills as no mill guarantees losses at intermediate flux densities) and specify the core loss of the grade of GOS required at 1.7T in the purchase order. Rather than specifying old nomenclatures like MOH, MIH or MZH which are neither precise nor convey adequate information, new nomenclatures conveying precise thickness and core loss information to the fabricator should be used.

Another important question is how to ensure the quantity of the material being used is prime? Many SEBs have initiated stage inspections of material during fabrication of the Laminations to ensure that only Prime material is being used. Though this is a step in the right direction, it is a tedious and time consuming process but due to lack of a better solution at the moment, a generally accepted practice.

One more solution could be for Central Electricity Authority to approve fabricators of Laminations who comply with certain specified quality procedures and methods as "Approved Fabricators" who could be entrusted the work of ensuring the required quality, for certain jobs where quality cannot be

compromised.

 

(G) CONCLUSION

Though the processing of CRGO steels appears to be a simple engineering activity of fabrication of steel into desired shapes as per the design provided, in reality it is one of the most demanding and precision jobs in the engineering industry. Therefore, it is imperative that TM have the basic knowledge of this delicately important raw material which forms the core of their Transformer. The information provided in this paper attempts to provide this basic foundation.

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Reducing Building Factor by Using Step Lap (SL) Laminations

By Saif Qureishi,  BSc (Physics), MBA (IIM, Bangalore)Managing Director, KRYFS Power Components Ltd., MumbaiEmail questions or comments on this paper to saif.qureishi@kryfs.com)

Summary: A proper value of the Building factor (BF) is  important  while calculating  the No-Load Loss (NLL) of any power or distribution transformer. It is an empirically derived factor which is based on the experience of the Transformer Manufacturer (TM) and ranges from 1.08 to 1.35 for three phase, three limb cores. Step Lap (SL) construction of transformer core  instead of conventional but type construction (or Non Step Lap (NSL) type) which is still widely used by TMs in India, has been successfully used by various TMs world over, to reduce the Building factor in Transformer cores by 5 to 8%, reduce the No Load current and the noise level relative to conventionally stacked NSL cores. This paper explains how SL laminations reduce the No Load Loss in a transformer by considering the specifics of magnetic  Flux transfer in joints areas of a  SL core versus a NSL transformer core.

The magnetic circuit is one of the most important active parts of any transformer. While the basic principle of transformation of energy has remained the same for over a century, since the first transformer was built, transformers have become more efficient due to improvements in materials and more sophisticated production processes (better manufacturing technology).

This paper examines one such production process  which reduces the NLL, No Load current and noise level of a transformer and hence improves it’s efficiency.

BF is a non dimensional parameter defined as the Ratio of (No Load Loss of a transformer / core weight ) in watts per kg to the Epstein or Single Sheet watt loss (in watts per kg).

Therefore BF= (NLL measured on transformer in watts per kg)                         (Epstein or Single Sheet Test loss in watts per kg)

BF ensures that the NLL of a transformer does not  equal to the core weight of the transformer multiplied by the Single sheet loss as defined by the producing Mill at the particular operating flux density. For example, if the core weight is 200 kgs for

a 100 KVA transformer, manufactured from M4 grade material and operating at 1.7 Tesla, if the material used is C 120 27 (i.e conventional M4 grade GO having a core loss of 1.2 Watts per kg at 1.7 Tesla) then the Maximum NLL should simply be 200 kgs  x 1.2 w/kg = 240 watts. However, in practice it is never so for a three phase, three limb core due the BF.

BF depends on various factors, some of the major ones being the following:

1. Core Geometry : BF is directly proportional to the Area of Proportion of Corner Joints to the total core area, kinds of joints (SL or NSL ) , Air gaps, overlap area at joints, etc.) 

2. Quality of workmanship of Core: Burrs in Laminations, Accuracy of dimensions especially at the corner joints (precision of angles) , flatness of the laminations, dust particles in between laminations, skills in core building (squareness of the core), clamping pressure on the core etc. 

3. Grade of Material Being used : Whether material being used is Hi- B Grain Oriented  (HGO) or Conventional Grain Oriented (CGO) which affects the stacking factor of material, Insulation resistivity values of material (IR values of coating on GO), Permeability of the material, thickness of the lamination (which effects eddy current losses). 

However the major reason contributing to BF is Core Geomtery, i.e the Area of Proportion of Corner joints to the total area of the core. Therefore in a smaller rating core (like 25 kva to 100 kva- varies from 1.25 to  1.35 ) where the BF is much higher compared to a Power transformer core like 25 MVA and above (varies from 1.08 to 1.15) where it is comparatively lower.

As it is not possible to examine all these factors in a single paper, we shall only examine the most important one, which can directly reduce the NLL significantly- using SL laminations for making transformer core and understanding the reasons for the same, mainly the  advantages in flux transfer in SL over NSL joints.

Both Hysterisis and Eddy Current losses when added make up the NLL of a transformer.  Due to the complexity of determining each individually, generally designers use either of the following two equations while calculating the NLL in a transformer:

(1)            NLL= Wt x Kt x w

OR

(2)          NLL = (Wt – Wc) x w + Wc x w x Kc

Where w is core loss in Watts per kg of the CRGO material used in core.Wt is the total weight of the coreWc is the weight of the core at Corner joints.Kt is the Building factor of the total core and Kc is the Building factor of the core at the corner joints.

A Cross sectional view of behavior of Flux in a Conventional NSL Core stack is shown in Figure 1 below. It has mitred joints and is usually built two laminations at a time with an off-centre overlap of around 10 mm in the centre leg. As can be

seen from the diagram, the joints are staggered one on either side 5 mm around the centre point.

Figure 1- A cross sectional view of the Conventional  Non Step Lap (NSL) type joints and behaviour of flux in it, which is mostly used in India.

A Cross sectional view of SL Laminations (6 steps) and the behavior of flux  is shown in Figure 2. As can be seen the joints are staggered 3 on either side, around the centre point.

Figure 2 Behavior of Flux in the 6 SL  joint.

Similar to an electrical circuit where, Electric current follows the path of least resistance, in a magnetic circuit, Magnetic flux follows the path of least reluctance (which corresponds to Highest Magnetic Permeability). In fact, the situation in an electric circuit and a magnetic circuit are analogous. Like Resistance (R= V/I)  in any electric circuit,

Reluctance ,R= F/ Φ

Where R is the Reluctance of the Magnetic Circuit in ampere turns/ weber F is the Magnetomotive Force in Ampere – turns and Φ is the Magnetic flux in webers.

As we see above in Figures 1 and 2 above, when the Flux in the transformer core approaches the air gap at the corner joint in the core, it has two options – Either to cross the Air gap at the joint (where the Magnetic Permeability is much lower (=1), as the Magnetic Reluctance of Air  is much higher .

The second option for the flux is to cut across the insulation layers of the laminations and move to the overlapping laminations in the vertical direction  (the direction perpendicular to the rolling direction of the laminations) above or below where the Magnetic Permeability is of the order of 10^4  and as a result the Magnetic Reluctance is much lower.

Obviously the flux will chose  option 2 i.e to cut across insulation layers and transfer to the laminations overlapping above or below it. However as CRGO saturates at a flux density of approximately 2 Tesla  this is the constraining factor for the “ready to transfer flux” approaching the air gap at the corner joint.

Let us consider a core which is operating at a flux density of 1.7 Tesla. As the flux  approaches the “air gap” at the corner joint, it has to decide between option 1 and 2 above. If all the flux  transfers  to the overlapping laminations above or below, in a NSL type joint where there are just two overlapping laminations, then the flux in the overlapping laminations will be 5.1 Tesla/ 2 = 2.65 Tesla per lamination which creates “overcrowding of flux” and also much over the saturation limit of the CRGO (which is approx 2 Tesla). Thus in a NSL  type joint, some of the flux will get transferred to the adjoining overlapping laminations, however some part of the flux will also have to jump across the air gap (which is option 1). This is what is shown in Figure 1 above. Even the flux which gets transferred to the adjoining laminations in a NSL, increases the flux density in the lamination above the  saturation level which automatically contributes to the saturation of the material at joints and therefore higher NLL.

The flux which crosses the air gap contributes to “wasting of flux” and therefore requires a higher no load current to achieve the required calculated flux density. Further the over-saturation of flux at the corner joints also leads to higher magnetostriction of the core which is the main cause of noise level in a transformer. This transfer of flux in a Conventional type (NSL type) core is more explicitly shown below once again for better understanding in Figure 3:

Figure 3: Diagrammatic representation of flux transfer to overlapped sheets in a Non Step Lap joint

However the situation is different for SLcore. The “ready to transfer flux” approaching the air gap has many more options as can be seen in figure 2 above , simply because there are more layers of laminations “available” for distributing this “ready to transfer flux”. As can be seen in the diagrammatic representation of the 6 Step Lap core, the flux has six options to jump instead of just two and therefore there is a more balanced distribution of flux over the adjoining laminations which results in very little flux jumping the air gap thereby also contributing to lower losses at the corner joints, as the flux density of the corner joints remains around the saturation flux density of 2 Tesla. This is explained diagrammatically below in Figure 4 :

Figure 4: Diagrammatic representation of flux transfer to overlapped sheets in a Step Lap joint.

It has been determined by Mechler and Girgis in (1) that the Flux density in NSL joints at the air gap is high as compared to in a SL joint and the flux density in the adjoining laminations in a NSL joint is much higher, as compared to SL joint which is discussed more in detail below with the help of diagrams and plots.

The distribution of this flux in NSL  is represented below in Figures 5,6,7 and 8

Figure 5: Magnetic flux lines in a NSL  core joint at 1.7 T induction.

Figure 6: Corresponding Sketch of Figure 5  to identify lines for line plots (not to scale).

Figure 7 : Magnetic flux density in the steel lamination along line 1 of Fig. 6

Figure 8 : Magnetic flux density along line 3 of Fig. 6

As can be clearly seen from the above, the flux density in the steel laminations along line 1 after the Air Gap reaches 2.7 Tesla and this is approximately the same amount of flux found along line 3 (which is normal or perpendicular to the direction of rolling) in the steel laminations. It should also be noticed that the Flux density in the air gap is as high as 0.7 Tesla.

Mechler and Girgis then repeated the same experiment with a SL core and the results are shown below in Figures 9,10, 11 and 12.

Figure 9 : Flux lines in step-lap core joint at B (overall) = 1.7 T.

Figure 10 : Sketch for identification of lines in a Step Lap Joint

Figure 11 : Flux density distribution along line 9.

TECHNICAL DETAILS : CRGO or Cold Rolled Grain Oriented Steel is available in various grades (generally called M3, M4, M5 & M6). Major international standards such as Japanese (JIS), American (ASTM), German (DIN) and British Standards are given in table 1 which specify grade, thickness, Watt Losses and Magnetic Flux density.

IMPORTANT ELECTRICAL PROPERTIES OF CRGO Table 1 - Grain Oriented Electrical Steel strips

Japanese

JIS C 2553 (1986) Classification

Density(kg/dm)

Iron Loss(W/kg)W17/50

MagneticFlux Density (T) B Symbol

Thickness

mm 27 P 100

0.27

7.65

1.00 max.1.85

27 P 100 1.00 max.27 G 120 1.20 max.

1.7827 G 130 1.30 max.27 G 140 1.40 max. 1.7530 P 110

0.30

1.10 max.1.85

30 P 120 1.20 max.30 G 130 1.30 max.

1.7830 G 140 1.40 max.30 G 150 1.50 max. 1.7535 P 125

0.35

1.25 max.1.85

35 P 135 1.35 max.35 G 145 1.45 max.

1.7835 G 155 1.55 max.35 G 165 1.655 max. 1.75

 

British

BS 601 Part 2 (1973)

Maximum specific total loss at a peak magnetic flux density of 1.5T and a frequency of 50 Hz.

Grade Maximum specific total loss W/kg 35M6 1.11

30M6 1.07

30M5 0.97

28M5 0.95

28M4 0.89

    

USA

 

  

 

 

 

AISI ( 1983 ) MAXIMUM CORE LOSSES - Electrical Steels Grain Oriented Full

Processed ASTM A665

ASTMType

FormerAISIType

Thickness Maximum Core Loss at 15

kg (1.5 T) W / lb W /kg

Inch mm 60Hz 50Hz 60Hz 50H

z27G053 M-4 0.0106 0.27 0.53 0.40 1.17 0.8930G058 M-5 0.0118 0.30 0.58 0.44 1.28 0.9735G066 M-6 0.0138 0.35 0.66 0.50 1.46 1.11

IMPORTANT PHYSICAL PROPERTIES OF CRGO

Density gm/c3 7.65Silicon content % 3.10Resistivity micro Ohm-centimeter 48.00Ultimate Tensile Strength 0° to Rolling Direction Kg/mm2 32.60Ultimate Tensile Strength 90° to Rolling Direction Kg/mm2 38.20 Stacking factor % M4 (.27 mm) 96.00Stacking factor % M5 (.30 mm) 96.50

Stacking factor % M6 (.35 mm) 97.00

CRGO materials come either in the form of coils or sheets. Table 2 gives details of dimensions and tolerances as per JIS C 3553.

DIMENSIONS & TOLERANCES

COILS

Thickness

0.18 mm (0.0071 in. )0.20 mm (0.0079 in.), 0.23 mm (0.0091 in. )0.27 mm (0.0106 in.), 0.30 mm (0.0118 in. )0.35 mm (0.0138 in.)

Width(Standard width available

with range)

914 mm (36 in.), and 1000 mm (39 in. )from 50 mm(2 in.), to 1.050mm (41 in. )

Inside Coil Diameter 508 mm (20 in. )

Sheets

Thickness0.30 mm (0.0118 in.), 0.35 mm (0.0138 in. )

Width914 mm (36 in.), and 1000 mm (39 in. )

LengthLength will be available according to negotiation

Tolerances in Dimensions & Shape - conform to JIS C 2553.

Widthmm

Thickness mm

TOLERANCEThickness mm

Deviation of thickness in transverse direction

Widthmm

Camber in any 2 metres(Slit

Shear Burr mm

 

 

 

mm Products)mm

150or under

0.180.200.230.270.300.35

+0.02+0.02+0.02+0.03+0.03+0.03

0.02 or under

+0.20

1.0 or under

  0.05or unde

over 150to 400

0.180.200.230.270.300.35

  +0.02 +0.02+0.02+0.03+0.03+0.03

0.02or under

+0.30

over 400to 750

0.180.200.230.270.300.35

+0.02+0.02+0.02+0.03+0.03+0.03

0.03or under

+0.50

over 750

0.180.200.230.270.300.35

+0.02+0.02+0.02+0.03+0.03+0.03

0.03or under

+0.60

Besides the Watt Losses at specific flux densities of 1.5 T and 1.7 T CRGO manufacturers also give curves of indicating Watt Losses ad A.C. Magnetization at various flux densities. These curves are of immense help to Transformer designers, and available on request.

Conventional CRGO materials (M4, M5, M6) are used regularly for cores in Transformers. However recently due to environmental protection, energy savings are becoming a very important factor and minimizing care loss in Transformers is becoming a must. Nippon Steel Corporation has come out with low loss Hi-B materials, which guarantee low Watt Losses at 1.5 Tesla flux density. Such materials are called Hi-B materials. Table 3 gives magnetic properties of Hi-B material. Popular Hi-B grades used in India are 23 MOH & 27 MOH Watt.

Hi - B CRGO MATERIALS :

Thickness

Grade

Core Loss Laminati

on Factor

Max. Typical Typic

al Typical

mm milW

(W/Kg)

W(W/K

g)

W(W/K

g)

W(W/K

g)

W (W/K

g)B (T) %

0.23 9 23ZDKH85

0.85 0.57 0.78 0.34 0.46 1.9197.5

23ZDKH90

0.90 0.58 0.80 0.35 0.48 1.91

23ZDMH85

0.85 0.57 0.78 0.34 0.46 1.9197.4

23ZDMH90

0.90 0.59 0.81 0.35 0.48 1.91

23ZH90 0.90 0.63 0.87 0.37 0.51 1.9297.7 23ZH95 0.95 0.64 0.90 0.38 0.53 1.92

23M-OH 1.00 0.66 0.93 0.39 0.54 1.92

0.27 11

27ZDKH90

0.90 0.62 0.84 0.38 0.53 1.9298.0

27ZDKH95

0.95 0.65 0.88 0.39 0.52 1.91

27ZDMH90

0.90 0.62 0.84 0.38 0.53 1.9197.9

27ZDMH95

0.95 0.65 0.88 0.39 0.53 1.91

27ZH95 0.95 0.69 0.93 0.41 0.55 1.9198.1 27M-OH 1.03 0.72 0.99 0.43 0.59 1.91

27M-1H 1.09 0.74 1.03 0.44 0.61 1.91

0.30 1230ZH10

01.00 0.73 0.98 0.44 0.58 1.92

98.330M-OH 1.05 0.74 1.01 0.44 0.60 1.91

0.35 11 35M-1H 1.16 0.85 1.13 0.52 0.68 1.92

98.5 35M-2H 1.22 0.90 1.19 0.54 0.73 1.92

A. K. Steel Corp. of USA ( formally ARMCO) has also come out with their own brand of low loss CRGO called Trancor H-0 and Trancor H-1. The Watt Losses are as follows :

CORE LOSSES

ARMCO TRAN. CORE

(TC)

THICKNESS INCHES (mm)

WATTS PER KG 60

Hz 1.7 T

WATTS PER KG 50

Hz 1.7 T

TYPICAL WATTS PER

KG @ 1.7 T/50

Hz T

TC H0 .009 ( .23 ) 1.32 0.90 0.80

TC H1 .011 ( .27 ) 1.46 1.00 0.90

TC H2 .012 ( .30 )   1.05 0.95