ENGRAM Engineering Ltd.

Insulation distances arround the active part in the tank

End-Distances in the core window

Main insulation distances between the windings

Phase to phase distances in core window

Spacer thicknesses in the winding

The insulation distances between the outer winding and the tank wall on the non regulated low voltage side

Insulation level Minimum distance

up to 12 kV 80 mm

up to 24 kV 90 mm

up to 36 kV 100 mm

up to 45 kV 110 mm

over 45 kV 120 mm

The above distances are to be increased by 40 mm if the HV insulation level is over 120 kV

The insulation distances between the outer winding and the flux collector covered tank wall on the regulated high voltage side

Insulation level Minimum distance

up to 36 kV 120 mm

up to 52 kV 160 mm

up to 72.5 kV 180 mm

up to 100 kV 200 mm

up to 123 kV 220 mm

up to 145 kV 240 mm

up to 170 kV 270 mm

up to 245 kV 300 mm

up to 362 kV 320 mm

up to 420 kV 340 mm

up to 525 kV 390 mm

The HV side distances shall be used for distance between the tank wall and OLTC and for the OLTC and outer winding.

The LV side distances shall be used for the distance between the outer winding and the short side of the tank.

The given distances can be modified by the followings.

the non usual arrangement of the leads

LV side regulation

high currents on LV side

booster, auxiliary transformer or reactor inside the tank

more tap changer in the tank

etc

Under the core to the bottom of the tank:

65 mm

Above the core to the tank cover:

80 100 mm

The given distances are with the consideration of 30 40 mm pressure ring

In case of ODAF cooling the end distances are changed

LI AC UD + LD X Y

325 140 200 60 90

350 140 200 60 90

380 152 210 65 95

450 185 220 70 100

550 230 240 75 105

650 275 260 80 115

750 325 280 115 150

850 360 300 125 165

900 395 320 140 175

950 395 340 150 185

1050 460 360 195 240

Radial clearances in the same phase

Oil duct at the winding is 6 mm maximumEF EndFed, CF CenterFed, CR CourseRegulation, FR - FineRegulation

BIL 75 95 125 170 250 325 380 450

AC kV 30 38 50 70 100 140 152 185

Main gapEF 16 16 16 16 17 24 25 32

CF 16 16 16 16 17 22 24 29

HV CR/FREF 16 16 16 16 17 25 27 32

CF 16 16 16 16 17 22 24 29

CR

-FR

no ZnO

20% 16 16 16 16 16 16 16 20

10% 16 16 16 16 16 16 16 16

with ZnO

20% 16 16 16 16 16 16 16 16

10% 16 16 16 16 16 16 16 16

BIL 550 650 750 850 900 950 1050 1300 1425

AC kV 230 275 325 360 395 395 460 570 630

Main gapEF 38 46 54 60 66 66 76 96 106

CF 36 44 50 56 62 62 72 90 100

HV CR/FR

EF 38 46 54 60 66 66 76 96 106

CF 36 44 50 56 62 62 72 90 100

CR

-FR

no ZnO

20% 22 26 31 34 38 38 42 52 58

10% 16 16 16 17 20 20 22 26 29

with ZnO

20% 16 17 20 24 25 25 29 36 38

10% 16 16 16 16 16 16 16 17 20

No switching impulse test

BIL 75 95 125 170 250 325 380 450

AC kV 30 38 50 70 100 140 152 185

FR

-F

R

Y connected

20% 20 20 20 20 20 20 20 20

10% 20 20 20 20 20 20 20 20

Delta connected

20 20 20 20 20 26 28 33

HV - HV 20 20 20 20 20 26 28 33

No switching impulse test

BIL 550 650 750 850 900 950 1050 1300 1425

AC kV 230 275 325 360 395 395 460 570 630

FR

-F

R

Y connect.

20% 24 29 33 38 40 42 47 58 63

10% 20 20 20 20 20 21 23 29 32

Delta connected

41 48 56 63 67 70 78 96 106

HV - HV 41 48 56 63 67 70 78 96 106

The given minimum values are for the average spacer thickness in the winding.

At internal directed oil flow the minimum spacer thickness is 3 mm.

For position changing of parallel conductors the appropriate place should be ensured. This is the sum of conductor height and the spacer thickness.

BIL of the winding end Minimum thickness of the spacers

BIL 450 kV 3 mm

450 kV < BIL 550 kV 4 mm

550 kV < BIL 750 kV 4.5 mm

750 kV < BIL 950 kV 5 mm

950 kV < BIL 6 mm

DIL factors

Weidmann curve safety margin

T50

Spacer thickness

Combined stress

Point stress

Turn to turn

The reference voltage kVAC is calculated by dividing the actual occurring voltage by the corresponding DIL factor

DIL factor

BIL Chopped wave 2.75

BIL full wave 1.2/50 s 2.50

Switching impulse 1.90

1 hour voltage 0.80

AC test voltage 1.00

Nominal voltage 0.59

Maximum allowed electrical stress in the oil ducts is given by the Weidmann curves (degassed oil):

Adjacent to a winding:

Between cylinders:

Safety margin with respect to the curves of degassed oil: 10%

Safety margin defined by

Safety margin values (minimum):

Regular transformer: 10 % (Limit = curve * 0.9)

Large Power, Special, Critical, : 20 % (Limit = curve * 0.8)

37.0

max *5.17 dE

37.0

max *5.21 dE

With d: width of the oil duct

strength

stressSF 1

Weidmann curves

Bare electrodes + cooling ducts adjacent to windings

1: degassed oil -> E = 17.539 * d-0.3581

2: gas saturated oil -> E = 14.196 * d-0.3690

Insulated electrodes

1: degassed oil -> E = 21.407 * d-0.3684

2: gas saturated oil -> E = 18.557 * d-0.3769

Use only T50 factor for situation between discs where:Safety margin is not metSafety margin can not easily be increased by standard measures.Well defined half-value time. Well defined peak.

Increase withstand by multiplying with adequate factor in function of the half-value time.

T50 FACTOR

Time, microsecondUnshrunk paper covering, mm

0,65 0,75 0,80 0,85 1,00 1,15 1,25 1,50 2,00 2,50 3,00

1 1,514 1,507 1,503 1,500 1,489 1,478 1,471 1,454 1,419 1,385 1,351

2 1,343 1,341 1,340 1,340 1,332 1,323 1,319 1,309 1,289 1,269 1,249

3 1,266 1,264 1,262 1,261 1,257 1,253 1,250 1,243 1,228 1,213 1,198

4 1,220 1,218 1,217 1,216 1,213 1,210 1,208 1,202 1,191 1,180 1,167

5 1,188 1,186 1,186 1,185 1,182 1,180 1,179 1,174 1,165 1,156 1,146

7 1,146 1,145 1,145 1,144 1,142 1,141 1,140 1,137 1,130 1,123 1,116

10 1,109 1,108 1,108 1,107 1,107 1,106 1,105 1,103 1,098 1,093 1,088

15 1,073 1,073 1,072 1,072 1,072 1,071 1,070 1,069 1,067 1,064 1,061

20 1,052 1,052 1,051 1,051 1,051 1,051 1,050 1,049 1,048 1,046 1,044

25 1,037 1,037 1,037 1,037 1,037 1,036 1,036 1,035 1,034 1,033 1,032

30 1,026 1,026 1,026 1,026 1,026 1,026 1,025 1,025 1,024 1,024 1,022

35 1,017 1,017 1,017 1,017 1,017 1,014 1,006 1,000 1,017 1,016 1,016

40 1,011 1,011 1,011 1,011 1,010 1,010 1,010 1,010 1,010 1,010 1,009

45 1,005 1,005 1,005 1,005 1,005 1,005 1,005 1,005 1,005 1,005 1,004

50 1,000 1,000 1,000 1,000 1,000 1,000 1,000 1,000 1,000 1,000 1,000

55 0,996 0,996 0,996 0,996 0,996 0,996 0,996 0,996 0,996 0,998 0,996

60 0,992 0,992 0,992 0,992 0,992 0,992 0,992 0,992 0,992 0,993 0,993

65 0,989 0,989 0,989 0,989 0,989 0,989 0,989 0,989 0,989 0,990 0,990

70 0,986 0,986 0,986 0,986 0,986 0,986 0,986 0,986 0,986 0,987 0,987

75 0,983 0,983 0,983 0,983 0,984 0,984 0,984 0,984 0,984 0,984 0,985

The target is the partial discharge free arrangement

Partial discharge free curve as formula:

Umax=(0.375*sp+15.75)*wire+9.5*sp+57

The minimum paper insulation on normal rectangular conductor is 0.45 mm.

For Continously Transposed Cable this is 0.6 mm.

The thickness of enamel on CTC individual strands depends on the CTC manufacturer. Typical value is arround 0.1 mm.

In windings without key spacers the allowable maximum impulse voltage between wire:

Umax=84.5* wire+10 [kV]

In case of a winding with internal directed oil flow the following minimum spacer thicknesses should be used.

3 mm up to 60 mm winding radial dimension

4 mm up to 100 mm winding radial dimension

6 mm up to 140 mm winding radial dimension

7.5 mm winding radial dimension over 140 mm

4 mm is the minimum spacer thickness in case of CTC winding

The above thicknesses must be increased for electrical reasons

AVERAGE COMBINED STRESS

The equivalent radial gap clearence:

t = oil clearance + pressboard clearance/2 + paper clearance/1.5 [mm]

The equivalent axial gap clearance

uncompressed thickness of key spacers + (thickness of paper insulation * (2.2/3.5)) [mm]

Axial stress calculated maximum lghtning impulse voltage between discs

Radial stress calculated maximum radial lightning impulse voltage to neighbouring winding

Max. allowed value:

BIL full wave: < 25 kV / mm

BIL chopped wave: < 27.5 kV / mm

22 ) . .

()

. .

(..

gapaxeq

stressax

gapradeq

stressradStressCombAv

POINT STRESS

On critical points (if the average combined stress seems to be critical), electric field calculation should be made for checking the critical regions.

The field calculation practical boundary conditions:Axi-symmetric

Max voltage in time to be used, in axial and radial direction, independent from each other.

Max radial voltage at same position of considered discs.

Max allowed Point stress < 35 kV / mm (max. 6 mm duct at the winding)

TURN TO TURN IN DISC

Strength is the 70% of the curve on page 303 (fig.5.79) from Large Power Transformers, Karsai-Kernyi-Kiss

Continuous service

Turn-to-turn voltage at nominal voltage < 2.5 kV / mm

Have a good job!