transformer basics for om review

2 October 2012 Copyright 2012 TPC Solutions 1

An Introduction to Transformer Design

Review

By Ir. Thum Peng Chew

B.E.(Hons), M.Eng.Sc.,FIEM, P.Eng


Parameters in Transformer Design


Core and Winding Assembly

Windings

Core

Core Clamp

2 October 2012 Copyright 2012 TPC Solutions 4 Assembled Tank

HV Bushings

LV Bushings

Neutral Bushing

Oil Conservator tank

Buchholz relay

Valves OLTC Control Box

Oil Gauge

Pressure Relief Device

Bushing CT Chamber

OLTC


Transformer Tank Parts

Radiators made of anti-corrosion material with filling plugs and drain plugs and directly connected to tank.

Bushings Leads from winding connections are brought out externally through the bushings appropriately rated for the

winding voltages. Pressure-Relief Device operating at a static pressure of less

than the test pressure of the tank with means to prevent ingress of rain and dust.

Terminal Box A weather-proof steel box mounted on the transformer tank for connection of all wiring.

Oil-Preservation Conservator Tank with drain valve, oil gauge, piping connection to the tank via Buchholz relay.


Transformer Accessories

Dial-type thermometer gauges for oil and winding temperature. Oil gauge

Filter and drain valves Nameplate

Handholds on cover or tank Lifting eyes and lugs

Jack pad, ladder, ski-base


Transformer Design Parameters ONAN capacity, S MVA

Voltage Transformation, kV/kV Operating Frequency, f Hz

Maximum Flux Density, Bm Tesla Winding Current Density, J A/mm2

Guaranteed Losses, No-Load and Load Losses, kW Impedance, %Z = %R + j %X

Vector Group Tap-Change Range and Step-Size


Typical Causes of Failure Initial Causes

Design Defects

Manufacturing Problems Material Defects

Other Causes Poor Maintenance Lightning Surges

Short-circuits

Failure, %

35% 29% 13% 11% 6% 4% 2%


Transformer Basics


Winding Voltages and Currents

If the ONAN capacity of a 3-phase transformer is S MVA,

the power rating of one winding is S MVA. The transformers rated line voltage is Vr and the

rated line current is Ir; S = 3VrIr. The winding voltage and current ratings are:

Winding

Connection Winding Voltage Rating, V volts.

Winding Current Rating, I amperes

Star Vr /3 Ir

Delta Vr Ir /3


Voltage Induction on Winding

V = - N d/dt = mcos(2ft)

V = 2fN msin(2ft) Vrms = 2fNm

Vrms = 4.444fNBm AFe or Vrms/N = 4.444fBm AFe The winding voltage, Vrms is sinusoidal implies that

the magnetic flux density, B in the core of area, AFe is sinusoidal and Bm is the peak flux density

in Tesla. Since the B-H curve is non-linear, in keeping the

flux sinusiodal, the magnetising current is non-linear.


Magnetising Current

0t +

i(t + )

i(t)

e

t

iexc

Non-sinusoidal exciting current

Sinusoidal flux

Induced sinusoidal voltage


The Winding Ampere-turn

Given the transformer capacity is S MVA and that the power rating per winding is S = S MVA,

S = VI = 4.444fNBm AFe I x10-6

NI = Sx106/4.444fBm AFe I is the windings rated current, S is known and Bm

is fixed, the number of winding turns, N is proportional to the core area, AFe.


Impedance Definitions

Definition based on rms values: Impedance Voltage, Vz = {VR2 + Vx2}

Percentage Impedance, %Z = 100Vz/V obtained by measuring exciting voltage, Vz to force rated

current flow with the winding shorted.

Derived from Losses PL and rated winding capacity, S

%R = 100PL/S %X = {%Z2 - %R2}


Loss Components

No-Load Loss, PNL = Iron Loss + Losses at joints, bolts and burrs + (also core vibrations and noise)

Load Loss, PLL = I2R Loss + Winding Eddy Current

Loss + Stray Losses

Copper Loss, Pcu = I2R Loss + Winding Eddy Current Loss

Thus, Load Loss, PLL = Copper Loss + Stray Losses


Components of Losses

Losses

No-Load Loss

Load Loss

Iron Loss

Copper Loss

Stray Loss

Joint Loss

Vibration Noise

I2R Loss

Eddy Current Loss


Iron Loss

Iron loss depends on the quality of steel

Expressed as Specific Iron Loss in terms of watts per kg mass.

For a typical design Bm, the specific loss is around

1.1 W/kg. Amorphous Steel Low Loss


Core Characteristics to Reduce Vibration

Reduce cross fluxing by suitable core dimensioning and design.

Flux distortion minimised.

Use flat steel of low magneto-striction. Careful annealing of steel lamination to align the grains.

Laser treatment. Reduce mechanical stress on core.


Winding I2R Loss

The resistance, R per winding is given by: R = Ns/a

where = resistivity of winding conductor, ohm-m N = No. of turns

a =cross-sectional area of winding, m2 s = mean length per turn, m

Current density, J = I/a A/mm2 and Volume of conductor, Nsa = m/ (mass/density) m3.

The winding loss = I2R = I2Ns/a = (I/a)2(Nsa) = J2m/

Specific I2R loss, W/kg = J2/


Minimizing I2R Loss

I2R Losses are attributed to current flow in the windings and are based on the IEC standard

mean working temperature of 75C for Class A, B and E insulations.

If the current density in the low-voltage winding is

J1 and that in the high-voltage winding is J2,

The I2R loss is minimized when J1 = J2


Estimating Specific I2R loss Given the electrical resistivity and density of material, the specific I2R loss, W/kg = J2/ x106

For copper windings at 75C, cu = 21.4 x10-9 m cu = 8,890 kgm-3

The specific I2R loss, pe = 2.41J2 W/kg with I2R loss

minimization.

If the winding mass, mcu = (Nas) is known, the winding I2R loss can be estimated.


Winding Cross-sectional Area

Core Winding with N turns

Duct space

s

Cross-sectional area per turn, a

Total Cross-sectional Area = Na Total Volume = Nas


Estimating Eddy Current Loss

The specific eddy current loss in thin sheets in a sinusoidal field at the maximum flux density, Bm

Tesla, frequency, f Hz and sheet thickness, t metre with electrical resistivity, and mass

density,

pe = 2Bm2f2t2/6

For copper windings, Specific eddy current loss = 9Bm2f2t2 x103 W/kg This is about 5% to 15 % of the winding I2R loss


Ratio of Eddy Current Loss to I2R Loss

pe/pR: 3.8(ftBmax/J)2 x 10-9 W/kg.

Reduce eddy current loss by reducing t, the

winding thickness. To be effective, transpose insulated multi-strand

windings pe/pR is 0.05 for small transformers and increases to

about 0.15 for large transformers.


Copper Loss and Stray Loss

Copper Loss per phase, Pcu = I2R loss per phase + Eddy Current Loss per phase

Load loss, PLL = 3Pcu + Stray Loss

Total Pcu = (1+%P1eddy/100)I12R1 + (1+%P2eddy/100)I22R2

%Pcu = 100Pcu/S %R = 100PLL/S

This formula is used in the test to calculate R and then convert to the standard temperature.


SUMMARY OF LOSSES No-Load Loss -

Hysterisis Loss proportional to frequency and peak magnetic flux density to the power of 1.6 to 2.5.

Loss at joints, bolts and burrs. Stray Loss at end-edges

Load Loss

Copper loss (I2R) Eddy Current Loss from circulating current induced by

magnetic leakage flux and proportional to square of frequency, peak magnetic flux and lamination thickness.

Stray Loss by magnetic leakage to clamps and tank walls, etc.


Winding Current Densities and Power Capacity

Winding cross-sectional area = aCu Irms = JrmsaCu

IHV = JrmsaHV ; ILV = JrmsaLV IHVNHV = ILVNLV

For minimum I2R loss, JHV JLV The total Cu area, NHVaHV Cu = NLVaLV Cu = Acu

The power capacity per winding = VrmsIrms = 2.222fN(Bm AFe)(Jrms ACu)


Frame Size The Frame Size in MVA, S = 3S MVA = VI x 106 is

related to the physical size defined by physical dimensions, Afe and ACu .

S = 2.222f(BmAFe)(JACu) per phase where f = operating frequency, Hz (50)

Bm = Peak Flux Density, T(1.6 1.8) J = winding current density, A/mm2(3.0 3.5)

AFe = nett magnetic core leg cross-sectional area, m2 , m=BmAFe

ACu = total copper cross-sectional area, m2


Voltage per Turn The Voltage per turn

V/N = 4.444fBmAFe = 4.444f m = {(8.888fBm/J)(AFe/ACu)} S

For a given S and constant Bm & J, V/N = f(AFe/ACu)

Thus, the winding output coefficient, KVS = (V/N)/S = {(8.888fBm/J)(AFe/ACu)}

where V/N is the voltage per winding turn does not change much Generator transformer, KVS = 26.8

Primary transmission transformer, KVS = 19 Secondary transmission transformer, KVS = 20

Distribution transformer, KVS = 19.6


Winding Arrangement Dimensions

Mean lengths per turn Core

Core HV Winding LV Winding

rLV

rm

rHV

h

bLV

bLH

bHV

sLV = 2 rLV

sm = 2 rm

sHV = 2 rHV

Mean turn Radius

Area of Leakage Flux

ALV = sLV bLV

Am = sm bm = (sLVbLV +sHVbHV)

AHV = sHV bHV

bm = bLV + bHV

bL0 bH0

hw

hH0 hL0

bFe


Reactance The percentage reactance, %X

= 0.79SfN2cw/V2h Where h = winding axial length, mm

c = all windings mean turn length (sLV + sHV), mm w = the reactive window width through which the

total leakage flux passes, determined from bLH + (bLV + bHV).

h = length of leakage flux.

%X = 0.79Sfw/{(V/N)2h/c}


Impedance Variation

For a given core, capacity and voltage transformation ratio, a lower reactance requires a

larger magnetic core cross-section reflected in the windings mean turn length.

As the reactance is increased, the core cross-section decreases and the iron loss also decreases but the copper loss increases.

The ratio of Cu/Fe losses increases.


Output Coefficient, KAS The core output coefficient is approximately

constant and given by: KAS = AFe/S

Practical KAS Values:

Generator = 0.071 Primary Transmission = 0.055

Secondary Transmission = 0.058 Distribution = 0.057


Temperature Rise

Losses give rise to Temperature Rise - Heating up the Windings

- Heating up the Insulation particularly the Oil

Thus, there are allowable maximum temperatures of windings and insulation (oil).

If the ambient design temperature for oil is 30C,

then the oil maximum temperature rise is 75C.


Temperature Rise Limits Insulation System Temp. Average Winding Temp. Rise All Classes immersed in oil 60K but for top oil = 55K Above but hermetically sealed 65K but for top oil = 50K All Classes in bituminous compound 50K Classes not in oil or bituminous compound

105(A) 60 K 120(E) 75 K 130(B) 80 K 155(F) 100 K 180(H) 125 K 200 135 K 220 150 K


Winding Temperature Rise Limits

Winding Insulation Material limits the maximum Temperature Rise in the

windings and transformer oil. Hence, a transformers capacity rating is

determined by winding temperature rise

Class A Insulation Oil-immersed 40 to 105C

Dry 45 to 105C


Hot Spot Temperatures The hottest spot temperature of the insulation is

the limiting factor to transformer loading. Excessive temperature shortens life.

Thus, winding temperature is critical and depends

on load, cooling efficiency, coolant temperature and time of application of load.

Transformers have many ratings but is given a

nominal rating for convenience.


Capacity Ratings Measured at the Outputs

The insulation has a large thermal capacity which

takes a long time to heat up. Thus, transformers have long thermal time-constants

approximately 3 hours.

The capacity ratings at the output of transformers are dependent on temperature rise.


Cooling With losses heating up the insulation and oil, a

transformer has to dissipate the heat to the outside environment so that its maximum

temperature rise is not exceeded. It is done by natural cooling of the tank surface. At larger capacities, additional cooling surfaces

has to be added with radiators which are detachable from the tank.

Assisted cooling can be achieved by adding fans to blow air across the radiators.

Cooling Class Letter Description


Table 2.1.2 lists the code letters that are used to make up the four-letter designation. This system of identification has come about through standardization between different international

standards organizations and represents a change from what has traditionally been used in the U.S. WhereOA classified a transformer as liquid-immersed self-cooled in the past, it is now designated by the newsystem as ONAN. Similarly, the previous FA classification is now identified as ONAF. FOA could be OFAFor ODAF, depending on whether directed oil flow is employed or not. In some cases, there are trans-formers with directed flow in windings without forced circulation through cooling equipment.

An example of multiple ratings would be ONAN/ONAF/ONAF, where the transformer has a baserating where it is cooled by natural convection and two supplemental ratings where groups of fans areturned on to provide additional cooling so that the transformer will be capable of supplying additionalkVA. This rating would have been designated OA/FA/FA per past standards.

2.1.3 Short-Circuit DutyA transformer supplying a load current will have a complicated network of internal forces acting on andstressing the conductors, support structures, and insulation structures. These forces are fundamental tothe interaction of current-carrying conductors within magnetic fields involving an alternating-currentsource. Increases in current result in increases in the magnitude of the forces proportional to the squareof the current. Severe overloads, particularly through-fault currents resulting from external short-circuitevents, involve significant increases in the current above rated current and can result in tremendousforces inside the transformer.

Since the fault current is a transient event, it will have the asymmetrical sinusoidal waveshape decayingwith time based on the time constant of the equivalent circuit that is characteristic of switching events.The amplitude of the symmetrical component of the sine wave is determined from the formula,

Isc = Irated/(Zxfmr + Zsys) (2.1.1)

where Zxfmr and Zsys are the transformer and system impedances, respectively, expressed in terms of perunit on the transformer base, and Isc and Irated are the resulting short-circuit (through-fault) current andthe transformer rated current, respectively. An offset factor, K, multiplied by Isc determines the magnitudeof the first peak of the transient asymmetrical current. This offset factor is derived from the equivalenttransient circuit. However, standards give values that must be used based on the ratio of the effective ac(alternating current) reactance (x) and resistance (r), x/r. K typically varies in the range of 1.5 to 2.8.

As indicated by Equation 2.1.1, the short-circuit current is primarily limited by the internal impedanceof the transformer, but it may be further reduced by impedances of adjacent equipment, such as current-limiting reactors or by system power-delivery limitations. Existing standards define the maximum mag-nitude and duration of the fault current based on the rating of the transformer.

TABLE 2.1.2 Cooling Class Letter Description

Code Letter DescriptionInternal First Letter

(Cooling medium)O Liquid with flash point less than or equal to 300CK Liquid with flash point greater than 300CL Liquid with no measurable flash point

Second Letter (Cooling mechanism)

N Natural convection through cooling equipment and windings

F Forced circulation through cooling equipment, natural convection in windings

D Forced circulation through cooling equipment, directed flow in man windings

External Third letter (Cooling medium)

A AirW Water

Fourth letter (Cooling medium)

N Natural convectionF Forced circulation

2004 by CRC Press LLC

Cooling Code Internal External


Tank Vibrations The vibrations transmitted to the tank are altered

by the tanks vibration modes before they are emitted as audible noise. Thus, no rigid

connections between core and tank.

The tank design/manufacture has to take into account of resonance modes and reflection

patterns around the tank. Panel mass-to-stiffness ratio changes the resonance frequency, vibration patterns and

noise amplitude - stiffeners.


Noise Measurement

At 0.3m away from the transformer surface for ONAN noise.

At 0.6 m way from the transformer surface

for ONAF noise

TRANSFORMER CORE



Contributions to Iron Loss Specific Iron Loss of an assembled core is measured

empirically and expressed as watts per kilogram of material.

This is made up of: l Hysteresis Loss

l Losses due to uneven flux distribution due to joints l Losses due bolt holes

l Losses due to burrs on lamination edges

Iron Loss = Specific loss (W/kg) x mass of core (kg)


Hysteresis and Eddy Current Loss

Hysteresis Loss: Proportional to frequency, f , Bm1.6 to Bm2.5.

Eddy Current Loss:

Proportional to f2, Bm, and steel lamination thickness

To keep the eddy current loss low: l Laminations are thin

l Electrically insulated from one another.

Core Materials


www.leonardo-energy.org CHAPTER 2. TECHNICAL ASPECTS

3.0

2.0

1.0

Co

re lo

ss W

17

/50

(W

/kg

)

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 Year

Laser irradiated HiB 0.23

Start of HiB productionHiB 0.30

CGO 0.30

CGO 0.35

Start of CGO production

CGO 0.23

HiB 0.23

Start of domain refining

Figure 2.2: Dierent types of magnetic steel

1. Around 1900, hot-rolled steel became the basic material for the core, made up ofindividual sheets separated by insulating layers to reduce no-load losses. Cold-rolledsteel and more sophisticated insulation techniques were progressively developed forimproving the performance.

2. Cold-rolled grain oriented silicon steels (CGO) became available in the 1950s andwere the first big leap forward in the reduction of no-load losses.

3. Various processing and coating techniques and a reduced silicon content led to thecreation of high permeability grain oriented steels (HiB). They remain the currentstandard material for manufacturing distribution transformers in Europe.

4. During the 80s, techniques were introduced to refine the domains of the iron crystalsby laser etching.

5. More recently, the development of amorphous iron introduced a significant newevolution for reducing iron losses.

Next to the choice of the steel, the way in which distribution transformer cores are de-signed, cut, fabricated and assembled, plays an important role in energy eciency. In-creasing the size of the core reduces the density of the magnetic field, and in this wayimproves energy eciency.

Amorphous iron deserves a special mention. Distribution transformers built with amor-phous iron cores can have more than 70% lower no-load losses compared to the bestconventional designs, and achieving up to 99.7% eciency for 1000 kVA units. Amor-phous iron became commercially available in the early 1980s. These transformers havecores wound with amorphous ribbon made from a ferric metal alloy produced by veryrapid quenching to avoid crystallisation. This technology has been used in several hun-

February, 2005 Page 18 of 50

1950s Cold-rolled grain-orientated steel, CGO 1980s Amorphous steel

High permeability grain-orientated steel, HiB

Grade Losses of CRGO Steel


Thickness, mm Grade Core Loss @ 1.7T/50Hz, W/kg 0.23 M3 0.90 0.27 M4 1.12 0.30 M5 1.30 0.35 M6 1.45 0.23 23ZDKH85 0.85 0.27 27ZDKH90 0.90 0.23 23M-OH 1.00 0.23 TCH-0 0.90 0.27 TCH-1 1.00


Mitred Joints

Vanished Yoke Laminations

Centre Limb

End Limb Yoke

Building the Core of Distribution Transformer

Magnetic Flux flow

Flux changes direction at joint


MAGNETIC CORE STRUCTURE

Usually 3 circular cross-sectional limbs forming a Rectangular Frame on which concentric windings are

arranged. The magnetic circuit is mitred-joined and completed by the

horizontal yoke pieces at the top and bottom of the limbs The magnetic core is made up of thin vanish-insulated

laminations of cold-roll grain-orientated steel or amorphous steel for KVA capacity.

Core is earthed through the tank body


The 3-Limb Magnetic Core

Circular Core

Clamp

Core Diameter, d To be Surrounded by

Concentric Windings


Requirements of Core Steel Laminations

l Hysteresis loss is reduced by cold-rolled grain orientation of silicon steel such that the magnetic flux

flow in the direction of rolling for least loss. l When the flux turn at the corners of the rectangular

frame, the loss increases . Hence, use mitred core joints to reduce loss.

Thus, significant loss reduction is made at the joints. l Eddy current loss is kept to a minimum by reducing

the thickness of the laminations and providing adequate insulation between the laminations when

they are packed together.


Treating the Core Laminations Lamination sheets come in rolled drums. They are

coated with hot-oil resistant bonding insulation. l The laminations are slit to width reducing burrs formation at the edges to avoid inter-laminar contact.

l Cut the laminations to length and mitred joint angle. l Use belt to apply evenly slowly rising temperature up

to 800C on lamination in nitrogen and hydrogen atmosphere to relieve stress and slowly cool over

temperature-controlled zones - annealing. l Apply coating to lamination surface and check for

coating thickness and insulation resistance.


Preparing the Laminations

Direction of Roll

Cuts to Width Cuts to Mitred Joint Angle

Steel Lamination from Drum


Core Building l Lamination are laid down flat on a building jig to

avoid deformation. l Identical laminations are laid on top of one another

before changing to the complementary lamination for interleaving.

l The smaller the number, the lower the loss but longer time required to build.

l Add laminations until given stack dimension is reached.

Stacking factor > 0.97 l Jig is turned for core to stand vertically.


Building the Core of Power Transformer

Vanished Core Laminations

Stepped Pattern Arrangement of Different-width Core Lamination

Interleaved Mitred Joints


Stepped Pattern and Space Factor

Laminated Stack

Stepped Patterns

Space Factor ~ 1.0


Shape of Core Limbs and Yokes Usually cylindrical with cross-section approximating a

circular contour made by strips with different width in a stepped pattern.

Thus, the aim is to get the filled area as close as possible to that of a circle i.e. fill factor or core space

factor 1.0. Usually the fill-factor is > 0.95 using 9 steps and above

to approximate half the circle. The other half is a mirror image.

Yoke and Limb widths are equal.


Core Joints Usually interleave/ overlapping mitred joints between

limbs and yokes to alternating disposition of different lengths of lamination to reduce magnetic reluctance,

vibrations and improve mechanical strength.

Minimise the gaps between the abutting plates to reduce transfer of flux and hence extra eddy current

loss in the steel. Hence, limit the number of identical plates before

stacking the overlapping plates.


Stacking and Interleaving

Mitered Joint

Stacking

Interleave Abutting

Identical Plates

Lamination

Flitch Plate to hold Limb Lamination Together


The magnetic field impinging on the flitch plates induces eddy currents. The losses are relatively small but the

magnitude of flux density is highest at the top and bottom of the flitch plates, where hot-spots are formed. To avoid, slots are provided at top and bottom of the flitch plates.

Based on the magnetic field and eddy current density, the losses are calculated for principal and extreme tap positions in the flitch plates as shown in Table VII below.

TABLE VIISTRAY LOSS IN THE FLITCH PLATES

Mode Stray loss, kW Max. Tap 0.65 Nor. Tap 0.62 Min. Tap 0.52

Fig. 8. Temperature profile in Flitch Plate

The temperature profile in the flitch plate is estimated by specifying heat transfer co-efficient and using 3-D FEM.

Well, in absolute terms, the stray losses in flitch plates may not form a significant part of the total losses of the transformer [3]. Nevertheless, it deserves designers attention as it could cause abnormal local hotspot rise in the flitch plates, and that in-turn disintegration of oil in the close vicinity, and consequential generation of fault gases, which could be misconstrued as fault / defect in the transformer.

The effect of using non-magnetic material (stainless steel) for flitch plate with following combination of slots was studied and the results obtained are shown in Table VIII below.

a) Flitch plate without slot b) Flitch plate with slots at top and bottom c) Flitch plate with slot(s) throughout winding height

TABLE VIIISTRAY LOSS IN FLITCH PLATE WITH DIFFERENT DESIGNS

Stray loss, kW

Mode MS Plate

with slots at top &

bottom

SS Plate without

slot

SS Plate with slots at

top & bottom

SS Plate with slot(s)

throughout winding height

Max. Tap 0.65 1.416 0.485 0.291 Nor. Tap 0.62 1.324 0.458 0.286 Min. Tap 0.52 1.248 0.425 0.252

From the above, it is observed that for a given design of flitch plate,

a) Loss in SS plate without any slot is the highest b) Loss in SS plate with slots at top and bottom is about

26% less than that with MS plate c) Loss in SS plate with slot(s) throughout the winding

height is about 54% less than that with MS plate

E. Estimation of stray loss in Edge Stack Stray loss in edge stack occurs due to flux impinging

normally (radially) on the outermost packet of the core. For estimation of stay loss in edge stack it is essential to

compute the 3-D magnetic field values along & across the height of the edge stack. Fig. 9 & 10 below show the plots of the modulus of flux density components (Bx, By, Bz) along and across the height of edge stack respectively, at

normal tap position on the HV side of the transformer. The curves A, B and C indicate the component of flux densities normal to the edge stack, along the width of edge stack and along the height of edge stack respectively.

As the magnitude of normal magnetic flux density is higher at the top and bottom winding edges, Fig. 9 represents first and second triangle with peak value flux densities 0.04270T & 0.04422T respectively (at winding edges) along the height of edge stack. The length covered by first and second triangle is represented by notations L1 & L2 in Fig. 9 is 635 & 710 mm respectively and distance between the peaks of two triangles represented by notation L12 in Fig. 9 is 1544 mm.

Fig. 9. Flux density variation along the height of the Edge Stack

Fig. 10. Flux density variation across the height of the Edge Stack at top winding edge position

The average value of magnetic field across the length of edge stack is computed from Fig. 10. The maximum and minimum value of magnetic field at top winding edge position across the edge stack, represented by notations Bm1 & Bm2 in Fig. 10, is 0.07285T & 0.04379T respectively. Similarly, the maximum and minimum value of magnetic field at bottom winding edge is also obtained. These magnetic field values are estimated for all phases at principal & extreme tap positions on both HV & LV sides of transformer. The stray loss based on above magnetic field values estimated in edge stack is shown in Table IX below.

Fifteenth National Power Systems Conference (NPSC), IIT Bombay, December 2008

501

Treatment of Edge Stack to Reduce Loss and Hot-Spot Temperature

TABLE IXSTRAY LOSS IN EDGE STACK


F. Total stray load losses The stray losses in winding i.e. eddy losses are also

measured as part of total stray losses during testing and are practically inseparable; hence same are calculated through another 2-D package and added to the structural losses to get the total stray losses. The total stray losses in all structural parts and windings are computed at normal and extreme tap positions and the details are as summarized in Table X below.

TABLE XTOTAL STRAY LOAD LOSSES IN TRANSFORMER

Component Stray losses, kW Sr. No. Max. Tap Nor. Tap Min. Tap1 Tank 13.60 13.48 11.95 2 Shunts 3.83 4.28 2.48 3 Frames 2.72 2.25 1.82 4 Flitch Plates 0.65 0.62 0.52 5 Edge Stack 4.90 5.40 4.38 6 Winding eddy losses 27.67 27.03 21.86 Total Stray + Eddy losses 53.37 53.06 43.02

Distribution of component stray losses, calculated as percentage of the total stray load losses at normal tap position is represented in Fig. 11 below.

Fig. 11. Component stray losses as percentage of the total stray losses The estimated values of stray losses are compared with

the tested values to validate the above results.

V. COMPARISON OF STRAY LOSS RESULTSComparison of the stray losses estimated by software

program and the measured test results is shown in Table XI below.

TABLE XICOMPARISON OF TOTAL STRAY LOSSES

Total Stray losses, kW Sr. No. Component Max. Tap Nor. Tap Min. Tap 1 Tested values 52.98 49.95 46.93 2 Estimated values 53.37 53.06 43.02 Deviation -0.74 % -6.22 % 8.33 %

The reference tested values vis--vis the estimated values show a deviation of -0.74%, -6.22% & 8.33% at maximum, normal and minimum tap positions respectively.

VI. CONTROL OF STRAY LOSSES

A. Shunt design modification Magnetic shunts are effective in controlling the structural

stray losses as they offer high permeable path to the leakage flux. The design of magnetic shunts depends on various factors, viz. length, width and height, placement with

respect to the windings, type and material. In the present case study, the height of magnetic wall shunts was increased by 645 mm on HV side of tank wall to attract larger chunk of the leakage flux entering the tank and the results obtained with above modification are shown in Table XII below.

TABLE XIICOMPARISON OF ESTIMATED TANK LOSS WITH MODIFIED SHUNT

Tank stray loss, kW Mode Standard Shunt Modified Shunt

Reduction in loss (%)

Max. Tap 13.60 12.13 10.80 Nor. Tap 13.48 11.68 13.33 Min. Tap 11.95 10.59 11.41

It is observed that increase in shunt height results in reduction in the tank loss significantly. This in turn does have the effect of increasing the loss in the shunts, which is marginal and hence ignored while reporting the total stray losses with modification.

B. Modification in Edge Stack In large transformer, the radially incident flux may cause

considerable eddy current loss in the edge stack, resulting in abnormal local hot spots, thereby increasing the risk of bubbling of oil in the local vicinity. Effect of division of the edge stack on the stray loss was studied and the estimated results are reported in Table XIII below.

TABLE XIIICOMPARISON OF LOSS IN EDGE STACK

Edge Stack stray loss, kW Mode Standard design Modified design Reduction in

loss (%) Max. Tap 4.90 2.18 55.51 Nor. Tap 5.40 2.57 52.42 Min. Tap 4.38 2.09 52.27 The temperature profile of the edge stack is also analyzed.

The losses in the core blade packets including edge stack and flitch plates are estimated and corresponding loss density values entered into the program. The various heat transfer co-efficients at outer core boundary surface are also specified to solve planar temperature field in core blade packets. Fig. 12 & 13 show the temperature profile of core cross-section without & with division of the edge stack. The temperature profile is differentiated from minimum to maximum by blue to red colour band.

Fig. 12. Temperature profile in standard edge stack design


502

Fig. 13. Temperature profile in modified edge stack design

It is observed that in the present case the stray loss is reduced by 52% at normal tap position and hotspot temperature rise is reduced by 14 K after the division of edge stack in two halves, which is quite significant.

C. Total stray losses in transformer after modification The total stray losses estimated in the transformer with

modified shunt and divided edge stack are presented in Table IXV below.

TABLE IXVTOTAL STRAY LOAD LOSSES WITH MODIFIED SHUNT

AND DIVIDED EDGE STACKStray losses, kW Sr.

No Component Max. Tap Nor. Tap Min. Tap1 Tank 12.13 11.68 10.59 2 Shunts 3.83 4.36 1.35 3 Frames 2.72 2.25 1.82 4 Flitch Plate 0.65 0.54 0.62 5 Edge Stack 2.18 2.57 2.09 6 Winding eddy losses 27.67 27.03 21.86 Total Stray + Eddy losses 49.18 48.43 38.33

D. Comparison of total stray losses after modification The comparison of stray losses after modification in shunt

and edge stack is shown in Table XV below. TABLE XV

COMPARISON OF STRAY LOSSES AFTER MODIFICATIONTotal stray losses, kW Sr

No. Design Max. Tap Nor. Tap Min. Tap 1 Standard 53.37 53.06 43.02 2 After Modification 49.18 48.43 38.33 Reduction 4.19 4.63 4.68

The results show that the modification in shunts and edge stack effect reduction in the total stray losses by 4.19 kW, 4.63 kW & 4.68 kW at maximum, normal and minimum tap positions respectively.

VII. CONCLUSIONS1. Stray losses in a transformer can be precisely estimated

using EDMAG-3D software program that is a powerful tool to aid fairly accurate 3-D field mapping of complex transformer asymmetries.

2. The loss in the bottom frames is higher than the top frames because of its close proximity with bottom edge of the winding. It was observed that lowering of the bottom frame height resulted in reduced frame losses. This is attributed to its reduced interaction with the leakage field returning to the bottom yoke.

3. The stray loss in edge stack is significant, leading to localized hotspot. Division of the edge stack effects substantial reduction in loss as well as the temperature.

4. Choosing appropriate material for flitch plate and judicious slot dimensioning could effect reduction in stray

losses in the flitch plates.

VIII. FURTHER WORKPrecise estimation of stray losses is a subject in itself. It

may not be prudent to attempt very precise simulation for computation of stray losses in routine designs disregarding the economic considerations. However, application of modern high speed and accurate computation tools offer deep insight into the complex field phenomena in asymmetric transformer geometries. There is a wide scope to exploit these tools for development of new cost-effective designs, exploring possibilities for improvements in certain areas like shunt materials, use of yoke shunts, use of width-wise wall shunts [4] etc.

ACKNOWLEDGEMENTSThe authors are grateful to the EMCO Management for granting permission to publish this paper.

REFERENCES [1] Ramaswamy E, Sarma D V S, Lakhaini V K, Design of magnetic and

non-magnetic shunts for a power transformer using EDMAG-3D, XIInternational Scientific Conference, Transformer Building-2005, September 2005, pp. 70-77.

[2] Turowski, J., Turowski, M., and Kopec, M., Method of three-dimensional network solution of leakage field of three-phase transformers, IEEE Transactions on Magnetics, Vol. 26, No. 7, September 1990, pp. 2911-2919.

[3] D A Koppikar, S V Kularni, PN Srinivas, S A Khaparde, R. Jain, Evaluation of flitch plate losses in power transformers, IEEE Transections on Power Delivery, Vol. 14, No. 3, July 1999.

[4] Prof. S V Kulkarni & Prof. S. A. Khaparde, Transformer Engineering Design and Practice, Marcel Dekker, New York 2004, pp. 169-230.

About the Authors:

Mr. Chetan C Adalja, born in April 1982, a gold medalist from Nirma University, completed his graduation in Electrical Engineering from CKPCET, Surat, South Gujarat University in 2003, followed by post-graduation in 2005 in PAS-Power Apparatus and Systems from Nirma University, Ahmedabad. He started his professional career as Lecturer at

Engineering College in Surendranagar, Gujarat.He has been associated with EMCO Limited from 2006 and working as a senior engineer in Technology Department. He has authored 3 technical papers.

Mr. M.L. Jain, born in December 1945, completed his graduation in Electrical Engineering from MNNIT, Allahabad University in 1968, followed by post-graduation in 1970 in Design and Production Engineering Heavy Electrical Equipment from MANIT Bhopal. He started his professional career as transformer design and development engineer in BHEL Bhopal in 1971.

From 1979 onwards upto 1996, Mr. Jain was associated with testing of transformers and other HV equipments. He has authored a chapter on testing of transformers and reactors in BHEL monograph Transformers published by Tata McGraw-Hill. Since 1996, Mr. Jain has been associated with EMCO Limited. Having worked as Head of Testing & Quality disciplines, he is presently Vice President Technology, responsible for up-gradation of transformer technology. He has authored over 20 technical papers in the field of transformer design analysis, testing and diagnostics. He is representing EMCO on professional bodies like BIS and CBIP, and is a member of CIGRE(I).


503


Edge Stack divided into 2 halves


Bolt Holes at Yoke

45 Mitred Joints with Interleaving

Building the Core of Power Transformer

Limbs


Core Cross-Sectional Areas

The gross core cross-sectional area is defined by the circumscribing diameter of the core circle, d

AFe,g = d2/4 And the nett core cross-sectional area,

AFe = kFeAFe,g Where kFe is the core space factor < 1.0.


Completed Core Limbs

Slotted Flitch plate


Core Bolting and Clamping Bolting:

Bolts have to be insulated and temperature resistant, thus cooling required.

Bolts are potential source of faults. Bolt hole and cooling slots increase local flux density

whose direction is changed from the roll direction and hence increase iron loss.

Increases cross-fluxing and eddy current loss. Hence, better with Boltless Core design.

Use Clamping with bands of insulating material and steel beams and tension members outside windings.

Use laminated wood to support core and to separate steel support from leakage field of windings.

Loss in Frames


The stray loss values in shunts, estimated based on above magnetic field values, are indicated in Table IV below.

TABLE IVSTRAY LOSSES IN SHUNTS

Stray loss, kW Mode HV Side LV Side Side Shunt Total Max. Tap 0.78 2.51 0.54 3.83 Nor. Tap 1.38 2.54 0.36 4.28 Min. Tap 0.76 1.47 0.25 2.48

It is observed that the stray loss values in HV side shunts are lower than those on LV side due to their smaller height and larger distance from the outer most winding.

C. Estimation of Stray Loss in Frames Frames, also called yoke beams, are made of mild steel

material and are used for clamping of yokes and supporting the windings. The frames are modeled as epures coinciding with their physical locations for magnetic field plotting and estimation of losses.

Fig. 5 & 6 below show the plots of the modulus of flux density components (Bx, By, Bz) in top & bottom frames along the height of the frame (from bottom to top) on the HV side of the transformer at normal tap position. For estimation of loss in the frames, it is essential to obtain the maximum and minimum values of flux densities occurring along the height of the frames, which is represented by notations B1 & B2 in Fig. 5 & 6.

Fig. 5. Flux density variation along the height of the Top Frame

Fig. 6. Flux density variation along the height of the Bottom Frame

It is observed that owing to the proximity effect, the maximum flux density occurs in the bottom part of top frame and the top part of bottom frame.

The maximum and minimum values of flux densities,

typically at normal tap position, obtained for top and bottom frame are as under. See Table V.

TABLE VMAGNETIC FIELD CONCENTRATION IN FRAMES

Magnetic field (B), T Top Frame Bottom Frame

Maximum value (B1) 0.00744 0.02022 Minimum value (B2) 0.00032 0.00139

The field concentration in the bottom frame is over 2.7 times of that in top frame. This is attributed to lesser distance between the winding bottom edge and the bottom frame.

Similarly, the maximum and minimum field values are obtained for top and bottom frames both for HV and LV sides of transformer at extreme tap positions. The loss in the frames calculated from magnetic field values is as shown in Table VI below.

TABLE VISTRAY LOSS IN FRAMES

Stray loss, kW Mode Top Frame Bottom Frame Total Max. Tap 0.98 1.74 2.72 Nor. Tap 0.82 1.43 2.25 Min. Tap 0.58 1.24 1.82

The loss in the bottom frame, which is higher as compared to the top frame, is commensurate with the higher flux concentration in the bottom frame.

D. Estimation of Stray Loss in Flitch Plates Flitch plates, made of MS and with slots at top and

bottom positions are used in the present case. The flitch plates are 200 mm wide and 12mm thick modeled to the scale, taking care of the slots and analysis carried out using FEM technique. It is important to note that the stray losses in such structural elements are quite low but the incident magnetic field on them can be quite high for the exposed area leading to unacceptable local hot spots. Fig. 7 & 8 shows the vector plot of eddy current density J (A/m2) and temperature rise profile (K) from minimum to maximum value differentiated by a colour band from blue to red, red being the highest.

Fig. 7. Vector plot of current density J (A/m2) in Flitch Plate

The magnetic field impinging on flitch plates induces eddy currents. The eddy current loops are shown in both solid and slotted regions in Fig. 7. The magnitude of normal flux density being the highest at top and bottom winding edges, it results in higher losses and hotspots in those regions of the flitch plates. In order to avoid such situations, the slots are provided in the flitch plates at both top and bottom locations.


500


























500


























500


























500

Bottom Frame has higher loss Flux density from proximity effect.

Loss is proportional to flux density.


Core Clamping

Steel Beam

Steel Support

Bands


Core Clamping

Steel Beam

Wood Laminations

Bands


Core Building Factor Calculated Core Loss = Specific Loss of Core Material

x Total Core Mass.

Core Building Factor = Measured Core Loss/ Calculated Core Loss

Mill test certificates should contain the measured

specific loss of the steel, W/kg. Typical figures: 1.07 W/kg for limbs and yokes, 1.17 W/

kg for complete core with mitred joints. Thus, the largest loss is in the joints.


Core Magnetising Current

Magnetising current is affected by: l Direction variation in the steels permeability

l Air gaps

As distinct from core loss factors


Core Temperature Rise No limit is set for Core Temperature Rise.

The IEC recommends that no part of the core should become hot enough to damage itself or its adjacent

parts.

To reduce the risk of damage to inter-laminar insulation or core bolts,

l the maximum temperature of the internal core hot-spot is 120C.

l increase the thermal conductivity of all core materials l provide cooling means by ducts.


Core Insulation

When the core assembly is completed, its insulation is measured by applying an alternating voltage at 2 kVrms

between: The Core Bolts

Side Plates And Core itself.

TRANSFORMER WINDINGS


2 October 2012 74

WINDING CONDUCTORS

Copper in oil-type Aluminum in dry-type

For economy, in the form of:

Wires low current (< 10A) but poor space factor. Strips (> 10A, J = 3.5 A/mm2)

Foils and Sheets (current limited by thickness of foil or sheet) and have high space factor and good cooling

ability.

Copyright 2012 TPC Solutions

Continuously Transposed Conductor (CTC)


FIGURE 2.1.6 Schematic of single-phase shell-form construction.

FIGURE 2.1.7 Continuously transposed cable (CTC).


circumvent the induction of different EMFs (electromotive force) in the strands due to different loopsof strands linking with the leakage flux, which would involve circulating currents and further loss.Different forms of conductor transposition have been devised for this purpose.

Ideally, each conductor element should occupy every possible position in the array of strands suchthat all elements have the same resistance and the same induced EMF. Conductor transposition, however,involves some sacrifice of winding space. If the winding depth is small, one transposition halfway throughthe winding is sufficient; or in the case of a two-layer winding, the transposition can be located at thejunction of the layers. Windings of greater depth need three or more transpositions. An example of acontinuously transposed conductor (CTC) cable, shown in Figure 1.10, is widely used in the industry.CTC cables are manufactured using transposing machines and are usually paper-insulated as part of thetransposing operation.

Stray losses can be a constraint on high-reactance designs. Losses can be controlled by using acombination of magnetic shunts and/or conducting shields to channel the flow of leakage flux externalto the windings into low-loss paths.

1.4.4 Short-Circuit ForcesForces exist between current-carrying conductors when they are in an alternating-current field. Theseforces are determined using Equation 1.15:

F = B I sin U

whereF = force on conductorB = local leakage flux densityU = angle between the leakage flux and the load current. In transformers, sin U is almost

always equal to 1

FIGURE 1.10 Continuously transposed conductor cable.


For multi-strand conductor to share same flux.

2 October 2012 76

WINDING INSULATIONS

Those within a winding to prevent electrical inter-turn breakdown by having appropriate insulation, cooling by circulation using paper board-type spacers of high

dielectric strength.

Major insulation between windings and between a winding and earth requiring good mechanical strength and electrical properties (puncture

strength) .


2 October 2012 77

Insulation Types

For wires, Synthetic Enamel for low voltage withstand.

For Strips, Paper for higher voltage.

For two or more strips in parallel and

transposed, synthetic insulation on individual and paper for overall wrapping.


2 October 2012 78

Overall Insulation Oil or SF6 gas

OIL QUALITY

Max. Viscosity = 40 mm2s-1 at 20C Max. Acidity = neutralised by 0.03 mg KOG per gram. Breakdown Voltage = 40 kV at 2 kV/s between 13 mm

dia. Spheres at 25mm apart. Same permitivity of other insulants

Maintenance: Keep out Moisture oil conservator, silica gel and

breather, drycol


2 October 2012 79

Power Frequency Test Voltage

Windings are tested for their ability to withstand power frequency test voltages according to IEC 60076.

The purpose is to prove that the windings have adequate insulation to withstand indefinitely power

frequency voltages experienced in the power system. Induced over-voltage test to prove insulation within

winding. Separate source test to prove low voltage winding

insulation withstand to other windings, and to earth.


2 October 2012 80

Winding Temperature

The Thermal Image Method Thermometer immersed in top oil and

connected to a dial-type instrument. Transformer load current injects heat through a

current transformer. The temperature indicated = top oil temperature, 0 + k c the temperature difference across the coil insulation.

k =1.1 1.5


Stray Loss Sources Winding Eddy Current Loss from

eddy current and circulating current loss

Edge Stack Loss

Structural Loss from Frame/Clamps, Flitch Plate, Wall Shunt and Tank

Body


Radial Leakage Flux in Transformer Parts


Chapter 2 Transformer Losses and Temperature Rise

10

The main parts of a transformer, which are subject to leakage flux, are illustrated in Fig 2.3.

The magnetic circuit, i.e. the iron core.

The primary, secondary and regulating winding.

The yoke clamps.

The flitch plate.

The tank and the tank shields (shunts).

Fig. 2.3 A 2-D transformer cross-section illustrates essential parts subject to leakage flux

As mentioned in the previous section, the stray flux has the effect of creating eddy current losses within the windings. The eddy current losses are concentrated in the end discs due to the radial flux. Flux shunts can be used to divert the flux to avoid too large radial components.

On high-current transformers, it is also the case that the generated stray flux can

give rise to eddy current losses in the tank. In this situation a reduction in the magnitude of losses can be obtained by the provision of a flux shunt or shields to prevent currents flowing in the tank. This will prevent an excessive temperature

Top Yoke Clamp

Flitch Plate (laminated & slotted)

LV Winding

HV Winding Regulating Winding

Iron Core

Tank Shunt

The bending of the leakage flux at the end discs produces radial fluxes that create localised hot-spots. The flux shunt diverts the flux

direction to reduce the radial flux.

997

11. STATISTICAL ANALYSIS

Orthogonal array design of experiments, a statistical technique, allows the effect of several factors on a response to be determined efficiently [4]. Analysis of relative effects of different factors can be obtained by decomposition of variance, which is commonly called Analysis of Variance (ANOVA). A cross-section of a transformer depicting a core, flitch plate, fiame, windings and tank is shown in Fig. 1. Symmetry is assumed about the center plane of the windings. The various geometrical dimensions that have a pronounced effect on flitch plate loss (response) are chosen as parameters (factors) for design of experiments,

Each factor is assigned 3 equidistant levels, as given in Table I (all dimensions in mm), to examine non-linear relationship between factors and response. The levels correspond to range of these factors for transformers of rating fkom 5 MVA to 315 MVA. Five factors viz., half winding height (xl), end clearance (x2), core-LV gap (x3), LV-HV gap (Q) and HV winding to tank clearance (x5) which affect the losses considerably, only have been chosen for the analysis, reducing the number of experiments [5]. Radial depths of windings are kept fixed for all experiments. Effect of fiame is indirectly taken, since the b e height varies in accordance with the level of factor x2. The regression model has a constant term and 20 variables (regressors) : 5 linear terms, 5 quadratic terms, and 10

TABLE I LEVELS OF FACTORS

842.5

135 250

interaction terms, The minimum number of experiments required to evaluate the 20 regression coefficients is 21, which necessitates use of b7 orthogonal array [6]. Thus, number of experiments to be carried out are reduced fiom 243 (35, 5 factors at 3 levels each) to 27 without losing out on accuracy. The LV and HV windings are defined as current driven coils with ampere-turns of IO5 and -lo5 respectively. Core symmetry axis and tank outer boundary are constrained with Dirichlet condition, so that the field lines are parallel to these boundaries. Center line of windings is left unconstrained (Neumann condition) so that field lines are perpendicular to this boundary axis. The FEM analysis was done for 27 combinations of 5 factors as per L27 orthogonal may for a mild steel flitch plate (12 mm thick). Leakage field plot for a certain combination of factors is shown in Fig. 2.

Loss is calculated as the integral of pJ2 (yresistivity, J= current density) over the volume of conducting flitch plate material in which eddy currents are produced. The ANOVA was subsequently carried out to quantify the effect of each factor on the flitch plate loss. Results of ANOVA are shown in Fig. 3. It can be seen that only fhctor Q (LV-HV gap) has a more or less linear relation to the loss. Variation of factor x5 (HV winding to tank clearance) has a relatively less effect on the loss as compared to other factors. For fixed ampere turns, as axial length'of winding reduces, leakage field increases correspondingly. Also, as the axial height of windings is reduced (with other factors unchanged) the radial leakage field incident on the flitch plate will increase. Hence there is an increase in flitch plate loss with reduction of winding height. Similar explanation can be given for the effect of variation of other factors on the loss. Regression analysis was subsequently carried out to compute regression coefficients of the quadratic surface.

The quadratic surface generated can be used by designer for a quick estimate of loss in the flitch plate after correcting

XI x2 x3 x4 M Fadorlevds - Fig. 3. ANOVA results

Fig. 2. Leakage field plot

Authorized licensed use limited to: INDIAN INSTITUTE OF TECHNOLOGY BOMBAY. Downloaded on October 25, 2008 at 01:26 from IEEE Xplore. Restrictions apply.

Radial Flux

Slotted Flitch Plate and laminated tank

shunt reduce losses

Stray Losses The Total Stray Loss =

20% 25% of Total Load Loss

But can be reduced to 8% 10% of Total Load Loss

By flux control methods magnetic shunts


Stray Loss Components


TABLE IXSTRAY LOSS IN EDGE STACK


F. Total stray load losses The stray losses in winding i.e. eddy losses are also

measured as part of total stray losses during testing and are practically inseparable; hence same are calculated through another 2-D package and added to the structural losses to get the total stray losses. The total stray losses in all structural parts and windings are computed at normal and extreme tap positions and the details are as summarized in Table X below.

TABLE XTOTAL STRAY LOAD LOSSES IN TRANSFORMER

Component Stray losses, kW Sr. No. Max. Tap Nor. Tap Min. Tap1 Tank 13.60 13.48 11.95 2 Shunts 3.83 4.28 2.48 3 Frames 2.72 2.25 1.82 4 Flitch Plates 0.65 0.62 0.52 5 Edge Stack 4.90 5.40 4.38 6 Winding eddy losses 27.67 27.03 21.86 Total Stray + Eddy losses 53.37 53.06 43.02

Distribution of component stray losses, calculated as percentage of the total stray load losses at normal tap position is represented in Fig. 11 below.

Fig. 11. Component stray losses as percentage of the total stray losses The estimated values of stray losses are compared with

the tested values to validate the above results.

V. COMPARISON OF STRAY LOSS RESULTSComparison of the stray losses estimated by software

program and the measured test results is shown in Table XI below.

TABLE XICOMPARISON OF TOTAL STRAY LOSSES

Total Stray losses, kW Sr. No. Component Max. Tap Nor. Tap Min. Tap 1 Tested values 52.98 49.95 46.93 2 Estimated values 53.37 53.06 43.02 Deviation -0.74 % -6.22 % 8.33 %

The reference tested values vis--vis the estimated values show a deviation of -0.74%, -6.22% & 8.33% at maximum, normal and minimum tap positions respectively.

VI. CONTROL OF STRAY LOSSES

A. Shunt design modification Magnetic shunts are effective in controlling the structural

stray losses as they offer high permeable path to the leakage flux. The design of magnetic shunts depends on various factors, viz. length, width and height, placement with

respect to the windings, type and material. In the present case study, the height of magnetic wall shunts was increased by 645 mm on HV side of tank wall to attract larger chunk of the leakage flux entering the tank and the results obtained with above modification are shown in Table XII below.

TABLE XIICOMPARISON OF ESTIMATED TANK LOSS WITH MODIFIED SHUNT

Tank stray loss, kW Mode Standard Shunt Modified Shunt

Reduction in loss (%)

Max. Tap 13.60 12.13 10.80 Nor. Tap 13.48 11.68 13.33 Min. Tap 11.95 10.59 11.41

It is observed that increase in shunt height results in reduction in the tank loss significantly. This in turn does have the effect of increasing the loss in the shunts, which is marginal and hence ignored while reporting the total stray losses with modification.

B. Modification in Edge Stack In large transformer, the radially incident flux may cause

considerable eddy current loss in the edge stack, resulting in abnormal local hot spots, thereby increasing the risk of bubbling of oil in the local vicinity. Effect of division of the edge stack on the stray loss was studied and the estimated results are reported in Table XIII below.

TABLE XIIICOMPARISON OF LOSS IN EDGE STACK

Edge Stack stray loss, kW Mode Standard design Modified design Reduction in

loss (%) Max. Tap 4.90 2.18 55.51 Nor. Tap 5.40 2.57 52.42 Min. Tap 4.38 2.09 52.27 The temperature profile of the edge stack is also analyzed.

The losses in the core blade packets including edge stack and flitch plates are estimated and corresponding loss density values entered into the program. The various heat transfer co-efficients at outer core boundary surface are also specified to solve planar temperature field in core blade packets. Fig. 12 & 13 show the temperature profile of core cross-section without & with division of the edge stack. The temperature profile is differentiated from minimum to maximum by blue to red colour band.

Fig. 12. Temperature profile in standard edge stack design


502

Stray Loss Control Use laminated material

Use high resistivity material Reduce Flux Density by using material of lower

permeability Reduce Flux Density by parallel magnetic path of low

reluctance Reduce Flux Density by using a magnetic shielding

plate of high conductivity


Reduction of Stray Losses


Item No. Source of Loss Loss Reduction

1 Tank Provide shunts of right material and adequate dimensions.

2 Edge Stack Slit the core lamination into 2 or 3 parts to separate edge stacks from middle stack

3 Flitch Plate Laminate, Slot and Stainless Steel

4 Frames Aluminium or non-magnetic inserts (stainless steel)

Magnetic Shunts


Stray Losses in Structural Components 209

A practical formula for calculation of thickness of width-wise tank shunt isgiven in [6,32] with reference to figure 5.16:

(5.34)

where B, the flux density value to be limited in shunts, can be taken as 1.7 Tesla forCRGO material and 1.4 Tesla for CRNGO material. Since the effectivepermeability of the width-wise shunt is less due to inter-laminar non-magneticgaps, it is preferable to take values of B about 20% lower than the above values(i.e., 1.4 Tesla for CRGO and 1.15 Tesla for CRNGO). Under overloadingconditions, shunts may get saturated and become ineffective; hence a design witha lower value of flux density also helps under overloading conditions.

The other type of magnetic shunt, edge-wise shunt, is better than width-wiseshunt because the flux is incident on the thickness (edge) of laminations resultingin negligible eddy loss in them. A typical edge-wise shunt is shown in figure 5.30.The effective permeability of laminations as seen by the incident flux is much

Figure 5.28 Width-wise shunt Figure 5.29 Optimum width-wise shunt

Copyright 2004 by Marcel Dekker, Inc.

Chapter 5210

higher for this shunt as compared to the width-wise shunt since the flux does notencounter any non-magnetic gaps once it enters the shunt. In the width-wiseshunt, due to non-magnetic gaps (however small they be), the effectivepermeability at the entry point reduces making it less effective as compared to theedge-wise shunt. The flux distribution at the entry point is quite complicated. Thepresence of inter-laminar non-magnetic gap reduces the average permeability inthe direction normal to the laminations to a low value, hence the flux tends to staywithin a particular lamination until it saturates. The flux finds its way through thenext lamination when the earlier lamination saturates and so on. Thus, it can beseen that the effectiveness of the width-wise shunt is less as compared to the edge-wise shunt.

The manufacturing process of edge-wise shunts is quite elaborate. In one of theforms, a set of laminations are epoxy moulded (like that of laminated flitchplates). In another design, it can be made into a wound form. The loss advantagewith the edge-wise shunts has to be assessed vis--vis their higher cost andmanufacturing time as compared to the width-wise shunts. The performances ofthese two types of shunts are compared in [64] by finding their effectiveanisotropic permeability. A substantial reduction in tank stray losses is reported in[26] by the use of edge-wise shunts. It is preferable to experimentally check thequantum of stray loss reduction before standardizing the use of edge-wise shunts.

Figure 5.30 Edge-wise shunt


Width-wise Shunt Optimized Width-wise Shunt Edge-wise Shunt

Magnetic Shunt Combinations



Usually, the tank shapes are not so conducive for the placement of magneticshunts and eddy current shields in such an ideal manner.

5.9.2 Eddy current shielding

Aluminum or copper shields are used for shielding structural components fromthe high current and leakage fields. Eddy currents induced in them repel theincident field reducing the losses in structural components. As discussed inSection 5.1, the thickness of these shields should be adequate for theireffectiveness and for reducing the loss in shields themselves. In most of the cases,the loss in the structural component and eddy current shield is more than that ofthe structural component and magnetic shunt. However, the eddy current shieldshave the advantage that they can be fitted on odd shapes of the tank unlikemagnetic shunts. The weight of the eddy current shield is also usually lower thanthe magnetic shunt. For shielding a tank from the high current field, the eddycurrent shields are better than the magnetic shunts. This is because there are gapsbetween magnetic shunts reducing their effectiveness as shields.

An analytical formulation is given in [72] for calculating loss in the eddycurrent shield and the tank shielded by it. The paper has used a two-dimensionalapproximation and has first outlined the method of calculation for eddy loss of atank, shielded by an aluminum shield, due to a line current. The method is thenextended to transformer windings, wherein the windings are replaced by aninfinite array of line currents by using the theory of images. The eddy current lossin the shields used in air core reactors is evaluated by the image method usingFourier-Bessel integral in [73]. For the finite dimensions of shields, 2-Dapproximations and end effects make the analytical formulations inaccurate andsuch problems can be simulated by 3-D numerical techniques.

Figure 5.32 Combination of vertical and horizontal magnetic shunts


Types of Flitch Plates



5.5 Stray Loss in Flitch PlatesStray flux departing radially through the inner surface of windings hits fittingssuch as flitch plates mounted on the core. On the surface of the flitch plate (lyingon the outermost core-step of limbs for holding core laminations togethervertically), the stray flux density may be much higher than that on the tank. Hence,although the losses occurring in a flitch plate may not form a significant part of thetotal load loss of a transformer, the local temperature rise can be much higher dueto high value of incident flux density and poorer cooling conditions. The lossdensity may attain levels that may lead to a hazardous local temperature rise if thematerial and type of flitch plate are not selected properly. The higher temperaturerise can cause deterioration of insulation in the vicinity of flitch plate, therebyseriously affecting the transformer life.

There are a variety of flitch plate designs being used in power transformers asshown in figure 5.8. For small transformers, mild steel flitch plate without anyslots is generally used because the incident field is not large enough to cause hotspots. As the incident field increases in larger transformers, a plate with slots at thetop and bottom ends can be used (where the incident leakage field is higher).Sometimes, flitch plates are provided with slots in the part corresponding to thetap zone in taps-in-body designs. These slots of limited length may be adequate ifthe incident field on the flitch plates is not high. Fully slotted plates are evenbetter, but they are weak mechanically, and their manufacturing process is a bitmore complicated. The plates can be made of non-magnetic stainless steel havinghigh resistivity only if their thickness is small as explained in Section 5.1.1. Whenthe incident leakage field on the flitch plate is very high, as in large generatortransformers, the best option would be to use a laminated flitch plate. It consists ofa stack of CRGO laminations, which are usually held together by epoxy moldingto make the assembly mechanically strong. The top and bottom ends oflaminations are welded to solid (non-magnetic) steel pads which are then locked

Figure 5.8 Types of flitch plates


Chapter 5188

to the frames. A laminated flitch plate not only minimizes its own eddy loss but italso acts as a magnetic shunt reducing the loss in the first step of the core.

The literature available on the analysis of flitch plate loss is quite scarce. Anapproximate but practical method for calculation of the loss and temperature rise ofa flitch plate is given in [4], which makes certain approximations based on theexperimental data given in [33]. The field strength at the inner edge of LV windingis assumed to vary periodically with a sinusoidal distribution in the space along theheight of the winding, and the non-sinusoidal nature is accounted by multiplyingthe loss by a factor. The eddy current reaction is neglected in this analyticalformulation. For a fully slotted flitch plate, the formulation is modified by consideringthat the plate is split into distinct parts. A more accurate 2-D/3-D FEM analysis isreported in [34], in which many limitations of analytical formulations are overcome.The paper describes details of statistical analysis, orthogonal array design ofexperiments, used in conjunction with 2-D FEM for quantifying the effect of variousfactors influencing the flitch plate loss. This Section contains results of authorspaper [34] 1999 IEEE. Reprinted, with permission, from IEEE Transactions onPower Delivery, Vol. 14, No. 3, July 1999, pp. 9961001. The dependence of flitchplate loss on the axial length of windings, core-LV gap, winding to yoke clearanceand LV-HV gap is observed to be high. The flitch plate loss varies almost linearlywith LV-HV gap. A quadratic surface derived by multiple regression analysis canbe used by designers for a quick but approximate estimation of the flitch plate loss.The loss value obtained can be used to decide type (with slots/without slots) andmaterial (magnetic mild steel/non-magnetic stainless steel) of the flitch plate to controlits loss and avoid hot spots. The effectiveness of number and length of slots in reducinglosses can be ascertained accurately by 3-D field calculations. In the paper, in-depthanalysis of eddy current paths has been reported for slotted mild steel and stainlesssteel flitch plates, having dimensions of 1535 mm200 mm12 mm, used in a single-phase 33 MVA, 220/132/11 kV autotransformer.

For this analysis, a mild steel (MS) flitch plate with r=1000 and V= 4106mho/m has been studied. The corresponding skin depth is 1.1 mm at 50 Hz. Theresults obtained are summarized in table 5.1. The loss values shown are for onefourth of the complete plate.

Case number Description Loss in watts1 No slots 1202 1 slot throughout 923 3 slots throughout 454 7 slots throughout 325 1 slot of 400mm length 1006 3 slots of 400mm length 527 7 slots of 400mm length 45

Table 5.1 Loss in MS flitch plate



The direction of eddy currents indicates the predominance of radial field at thecross section, 0.5 mm from the surface. There are no eddy current loops inthickness of the plate (see figure 5.14). These are the reasons for the effectivenessof slots in the SS plate. The eddy current loops are parallel to the surface (on whichthe flux in incident) indicating that the eddy loss in the SS plate is predominantlydue to the radial field. Hence, the slots in the SS plate are more effective ascompared to the MS plate. This means that the loss should reduce approximatelyby a factor of (n+1). From the first two results given in table 5.2, we see that thereduction in the loss is more (12 times) than expected (8 times). This may be dueto fact that each slot is 5 mm wide causing a further reduction

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