Complete Handbook

10. 96 / TDT / E

TRANSPORTATION OF TRANSFORMERS

Page 1/3

Distribution Transformers are transported after they are filled with oil and their wheels are dismounted. Generally there is no need for a special packaging. Only the bushings and certain parts on the cover plate are protected by means of a wooden case. In open type breather transformers, in order to prevent oil-spoiling of the silica gel, the oil in the oil container of the breather should be drained off. In open type transformers with no breathers, on the other hand, a gasket should be put under the filling lid; but this should be removed again, when putting the transformer into operation. In order to prevent shaking during transportation, the transformers should be fixed onto the frame of the vehicle. Due care should be taken during transportation and the vehicle should not exceed the normal cruising speed of 50 km/h, and excessive shakings should be avoided. One should be also careful when unloading and placing the transformer onto its foundation and measures should be taken against any probable damage.

The transformer is lifted as shown Figure 1:

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10. 96 / TDT / E

TRANSPORTATION OF TRANSFORMERS

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FIGURE 1 If the transformer has lifting lugs for inner part, lifting lugs for whole transformer and jacking lugs are as shown below :

ABB Elektrik Sanayi A.Ş.

10. 96 / TDT / E

TRANSPORTATION OF TRANSFORMERS

FIGURE 2 When levering the transformer, force must be directed as shown in Figure 3 :

FIGURE 3

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Page 3/3

1LTR-9544-102 10. 96 / TDT / E

CONTROLS TO BE CARRIED OUT AFTER TRANSPORTATION

Page 1/1

The transformers should be checked for any damages during transportation, when they arrive at the erection site. Areas where painted surface has been scratched out should immediately be cleaned and repainted. If there are small oil leakages, these can be prevented by tightening the gasket connection. If an important damage or oil leakage is observed, this should be reported to ABB ESAŞ. The normal oil level in the 20°C temperature environment has been scaled on the oil level indicator which is located on the expansion vessel. Oil level should be checked, by considering the environmental temperature where the transformer has been placed, and if necessary, degassed and dehumidified oil of same type should be added. If there is a dehydrating breather, the colour of silica gel should be checked and if 2/3 of it has turned to pink, it should be replaced or reactivated in accordance with the directions given in its manual. In addition to this, oil container of the dehydrating breather should be filled up to the specified level.

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1LTR-9544-103 10. 96 / TDT / E

STORAGE

Page 1/1

If the transformer is subject to a long period of storage (3-6 months) before it is taken into operation, proper measures should be taken to prevent wear due to environmental conditions. The transformer should not be placed, close to the ground where water might accumulate; it should be also kept clean and no damage should be allowed. Furthermore, the oil indicator and dehydrating breather should be checked monthly and the silica gel should be reactivated whenever necessary. For transformers, which are not under operation for a long time period, periodic maintenance should be carried out, and before they are taken into operation all necessary controls and checks explained in the section “T a k i n g I n t o O p e r a t i o n ” should be done accordingly.

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1LTR-9544-104 10. 96 / TDT / E

ERECTION

Page 1/1

Since distribution transformers are transported in general as to-be-yet-erected and they are filled with oil, their placement and operation does not require much effort. The wheels of transformers, excluding those which are to be erected onto poles, should be selected in accordance with the supplied drawings. Transformers to be erected onto poles are normally placed and fixed without wheels. Before making onto the electrical connections, the insulation resistance of low voltage and high voltage windings with respect to each other and to the tank should be measured by means of Megger. During this test, all the bushings should be dry and clean. In oil-transformers, the lower limit for insulation resistance is calculated by the following formula : R = 1.5 x E / √N

Where ; ♦ R : is the lower limit (MΩ) for the value to be taken at the end of a one-minute period with Megger test at a 20°C ambient temperature. ♦ E : is the voltage (kV) between phases for Delta connections or between phase and neutral for Star connection. ♦ N : is the rated power (kVA) of the transformer to be tested. The test results should be compared with the calculated value above; if it’s lower, ABB ESAÞ should be contacted. Transformers with proper insulation resistance are ready to be mounted. For this purpose, the high voltage main terminals are connected to the high voltage bushings of the transformer and the low voltage bars are connected to the low voltage bushing connection pads; cabeling of the protective relay and other components are done and grounding terminal is connected to the system grounding.

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1LTR-9544-105 10. 96 / TDT / E

TAKING INTO OPERATION

Page 1/3

For taking into operation of transformers which are newly built or have been out of operation for a long time, the specified test and controls should be done in the following sequence. 1. Check the arching horns located on the high voltage bushings if there is any deformation. Measure the distance between the tips of the horns and compare the results with the table given below; if necessary, make calibrations. INSULATION CLASS 10N 20N 30N

A (mm) * 85 155 220

* : Acc. to DIN 42531

2. Check the pockets where the thermometers are located and if they are empty, fill them with oil. The oil necessary for this task can be taken by loosening the bolt under the oil level indicator. 3. Check the oil container of the dehydrating breather and silica gel. The oil container should be filled with oil up to the marked level; silica gel should be blue and if it is not, it should be replaced or reactivated. ( S e e Operations Instruction for the Breather) 4. Check the vector group of the transformer and the ratio for each tap. 5. Test the transformer oil for breakdown voltage in accordance with IEC 156 or TS 623 specifications. If this value lower than 40 kV, the oil is heated, degassed and filtered by passing it through an oil treatment plant. Oil samples for test purposes should be always taken from oil draining valves and to prevent contamination of the sample oil, oil should be taken after a little oil is drained off.

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1LTR-9544-105 10. 96 / TDT / E

TAKING INTO OPERATION

Page 2/3

6. Check the oil level in the expansion vessel; if necessary, add dehumidified and degassed oil. The added oil should have the same properties with the oil in the transformer and its breakdown voltage should not be less than 40 kV. 7. Check and calibrate the signal contacts of the dial contact thermometer. It is recommended that the alarm contact is calibrated at 80°C and trip contact at 90°C. However, considering the environmental conditions the user can select for other optimum values for an effective operation. 8. Check the alarm and trip circuits of the Buchholz relay by pressing the test button. At the same time, examine also the circuit breaker of the transformer and check if it operates correctly or not. ( S e e O p e r a t i o n s I n s t r u c t i o n f o r B u c h h o l z R e l a y ) 9. After min. six hours waiting, release the air accumulated in the bushings and Buchholz relay chamber. For this purpose, in bushings up to the 250 Ampere, the nut of the bolt is loosened and the bolt is pushed into the bushing for about 2-4 mm. In larger bushings, there are special air removing screws. After the accumulated air is completely released and oil starts coming out from these places, the bolts should be retightened. 10. Check if there are oil leakages on the transformers or not. If there are small oil leakages, these can be eliminated by tightening the gasket connections, slightly. Care should be taken during tightening that yield limit of the gasket is not exceeded. 11. Clean and paint the area where the paint is degraded or scratched. 12. Check all the electrical connections once more. Avoid any faulty or loose connections. 13. Check if the protective relays on the transformer correctly function. 14. Considering any voltage drop in the mains supply to which the transformer is connected, change the tap changer to a correct position. To secure good and correct contact between the contacts of the tap changer the knob should properly be inserted into its location.

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1LTR-9544-105 10. 96 / TDT / E

TAKING INTO OPERATION

Page 3/3

15. Check if the valve position is O.K. 16. On-load tap changer must be tested separately. 17. Cts are also checked. (resistance, polarity) After all the above mentioned tasks are completed and proper conditions are secured, the transformer can be energized. However, for a certain period the operation of the transformer is observed and in the case of any abnormality, it should immediately be taken out of operation and if necessary, ABB ESAÞ is informed. Neoprene rubber and cork is used for sealing at the joint between tank and cover, and nitrile rubber or rubber cork for sealing bushings to cover. These gaskets are shrinkable and need to be retightened after some days operation of transformer. Maximal tightening torque for bushings is shown in the table below, column 4. In column 5 is shown max. torque when tightening a cable lug between two nuts. For the three largest listed current it is the torque used to tighten the terminal pad. 1 2 3 4 5 RATED RATED THREAD TIGHTENING TIGHTENING VOLTAGE CURRENT ON TORQUE TORQUE FOR Nm ELECTRICAL max. kV FOR STEM STEM Amp. mm CONNECTIONS Nm 12 24 36 1 1 1

250 250 250 250 630 1000

M12 M12 M12 M12 M20 M30

15 15 15 15 30 75

40 40 40 40 120 30 1)

1

2000

M42

95

30 1)

1

3150

M48

110

30 1)

1

) M12 Screw for tightening terminal pad.

ABB Elektrik Sanayi A.Ş.

1LTR-9544-106 10. 96 / TDT / E

OPERATION & MAINTENANCE

Page 1/2

The life of a transformer depends, largely, on operational conditions. By means of a proper operation and regular maintenance, mechanical and thermal wear due to the environmental conditions, electrical, mechanical and thermic stresses can be brought down to a minimum. For this purpose, following measures should be taken during operation and the transformer should be checked frequently in such a way that preventive measure is taken before problem occurrence. 1. The voltage adjustment in the distribution transformers is carried out by means of “Off-Load Tap Changer” which is driven by the handle on the cover plate.

WARNING Care must be taken that tap changer can only be operated when the transformer is off-circuited, and the transformer should not be put into operation unless the tap changer handle is secured by means of a pin or another locking mechanism. 2. The LV and HV connections must be disconnected before the transformer is taken out of operation or maintained. It is important for life security. 3. One of the major issue in the transformers is the status of oil. The oil level should always be checked. If the oil level is below the minimum level, it should be recovered with the oil of same type. If there is a continuous drop in the oil level within a few days, the transformer tank should be closely examined and any oil leakage should be removed. It should be examined by taking oil samples at regular intervals (not more than one year) and breakdown voltage should be tested, too. If the breakdown voltage, measured in compliance with IEC 156 or TS 623 specifications, is less than 25 kV, the oil should be dehumidified and filtered. If the color of the oil has darkened and there is sludge formation in the transformer, it is recommended that the oil should completely be changed. Before taking oil sample for test purposes, to remove the precipitation in the valve, oil should be allowed to flow freely for a while. In the case that new oil is added to the transformer, care should be taken that it has the same quality. Detailed information about oil maintenance in the transformer can be taken from IEC 422.

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1LTR-9544-106 10. 96 / TDT / E

OPERATION & MAINTENANCE

Page 2/2

4. If there is a breather on the transformer, it should be frequently checked and if the color of the silica gel has turned out into pink, it should be replaced or reactivated in accordance with the directions given in the related documents. 5. The oil temperature should be constantly checked. A temperature increase can be caused by excessive loading, but it may also result from an internal failure. If any temperature increase beyond normal ranges is observed, its causes should be examined. If there is no excessive load or any failure, oil may be stiff and the sludge or particles have blocked the oil channels. In this case, oil should be also examined, and if necessary, it should be replaced, and the active part should be flushed by new oil to clean the sludge and particles. 6. Any gas accumulation in the Buchholz relay chamber should also be checked at least once a month and any such accumulation should be removed. Furthermore, any alarm and trip contacts should be tested as explained in the related specifications. 7. The bushings should be cleaned at regular intervals. 8. The transformer should be examined at regular intervals for general condition and if there is any oil leakage and degradation on its paint, all the necessary measures should be taken and wheel axis should be greased. 9. All the electrical connections, cables, signalling and operational circuits of the auxiliary devices should be examined at regular intervals and care should be taken that protective relays should remain operative all the time. Beside the constant examinations, the transformer should be taken out of operation at regular intervals and taken into general maintenance. During the general maintenance, the oil is tested, and if necessary, it is dehumidified and filtered by a treatment plant or replaced; the operation of the accessory devices are examined; all the necessary calibration is done; electrical circuits are checked; oil leakages are removed; bushings and the tank is cleaned; those areas where the paint is degraded are repainted.

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1LTR-9544-107 10. 96 / TDT / E

OPENING / RESEALING HERMETIC SEALED TYPE TRANSFORMERS

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Sealed transformers must not be opened unnecessarily as this could cause the oil to come out air to go in depending on the temperature of the oil. This results in disturbances of the normal pressure variations causing increased stress on the tank due to under-pressure or overpressure. When opening the transformer, for example to replace a bushing, opening and resealing must be performed according to these instructions.

Opening ♦ If the temperature of the transformer oil is higher than +20 ..... 25°C, approx. 5% of oil must be drained through the drain valve to relieve the overpressure. ♦ If the temperature of the transformer oil is below +20°C no draining is needed as there is an under-pressure inside the tank. ♦ The filling plug can then be opened. If necessary the oil level can be lowered by draining through the drain valve before carrying out the planned maintenance. (If the transformer is provided with an oil level indicator, it also acts as the filling plug.)

Resealing When resealing the transformer, the temperature of the oil must be within +20 .... 25°C to ensure the normal pressure inside the tank during operation. ♦ Fill in oil through the filling plug until the oil level is above cover level, then close the filling plug. ♦ Loosen the upper nuts and upper gaskets of all bushings and possible de-airing plugs. ♦ Fill with oil through high voltage bushings until it runs out through the low voltage bushings and possible de-airing plugs.

ABB Elektrik Sanayi A.Ş.

1LTR-9544-107 10. 96 / TDT / E

OPENING / RESEALING HERMETIC SEALED TYPE TRANSFORMERS

♦ Tighten the upper nuts of low voltage bushings and de-airing plugs. ♦ Continue filling with oil until the high voltage bushings are completely filled. ♦ Tighten the upper nuts of the high voltage bushings.

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Page 2/2

1LTR-9544-108 10. 96 / TDT / E

TRANSFORMER OVERLOAD CAPACITY Page 1/2

A. Permissible Continuous Loading Transformers can be loaded up to the rated power continuously under ambient conditions where air temperature is not exceeding.

Ambient Ambient Ambient

60/65 °C

55/60 °C

50/55 °C

45/50 °C

Trf. design temp. rise

40 °C 30 °C 20 °C

45 °C 35 °C 25 °C

50 °C 40 °C 30 °C

55 °C 45 °C 35 °C

as the maximum reached as the average of any day as the average of any year

The dimensioning and the mentioned limits are based on IEC Publication 76 Power Transformers. B. Permissible Short Time Loading - Continuous and Cyclic Duty ( A c c . t o IE C 3 5 4 ) In practice the transformer is very seldom continuously loaded at the same load. The load and the temperature primarily fluctuate according to the time of the day. If the transformer loading is, part of the time, lower than the continuous load capacity at the relevant permissible ambient air temperature, it can be correspondingly loaded more at other times even though the ageing remains normal during the whole time (e.g. during 24 hours). IEC’s loading guide 354 also gives short-term loads on this basis. TABLE 1 shows permissible continuous or cyclic duty as a function of the ambient air temperature and the loading time, provided the loading capacity at other times of the day does not exceed 25%, 50% or 90% of the rated power. The hot-spot temperature in winding is limited to 140°C and oil temperature to 115°C. Note max. permitted power is 1.5 times the rated power. Also daily use of life (V) is 1 or less, which means that ageing of the transformer is normal.

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1LTR-9544-108 10. 96 / TDT / E

TRANSFORMER OVERLOAD CAPACITY Page 2/2

TABLE 1. Permissible Continuous Short Time Loading - Continuous and Cyclic Duty, Vmax=1 Trf. design temp. rise

Permissible Continuous Short Time Loading. Preload=25% AmbC Amb.C Amb.C Amb.C

T2 (h) 0.50 1.00 2.00 4.00 6.00 8.00 12.00

-5 -10 -15 -20 S% 150 150 150 150 150 147 140

5 0 -5 -10 V 0.00 0.00 0.00 0.00 0.51 1.00 1.00

S% 150 150 150 150 147 140 133

15 10 5 0 V 0.00 0.00 0.01 0.25 1.00 1.00 1.00

S% 150 150 150 150 139 133 126

25 20 15 10 V 0.00 0.00 0.03 0.80 1.00 1.00 1.00

S% 150 150 150 143 131 125 118

35 30 25 20 V 0.00 0.01 0.11 1.00 1.00 1.00 1.00

45 40 35 30

45/50°C 50/55°C 55/60°C 60/65°C

55 50 45 40

S% 150 150 150 134 123 117 110

V 0.01 0.03 0.34 1.00 1.00 1.00 1.00

S% 150 150 149 125 114 108 102

V 0.02 0.10 1.00 1.00 1.00 1.00 1.00

S% 150 150 138 115 105

V 0.07 0.30 1.00 1.00 1.00

S% 150 150 150 131 121 116 110

V 0.02 0.07 0.63 1.00 1.00 1.00 1.00

S% 150 150 143 122 112 107 101

V 0.06 0.22 1.00 1.00 1.00 1.00 1.00

S% 150 150 131 111 102

V 0.19 0.71 1.00 1.00 1.00

S% 150 150 132 118 112 109 105

V 0.48 0.97 1.00 1.00 1.00 1.00 1.00

S% 123 110 101

V 1.00 1.00 1.00

S%

V

Permissible Continuous Short Time Loading. Preload=50% T2 (h) 0.50 1.00 2.00 4.00 6.00 8.00 12.00

S% 150 150 150 150 150 146 140

V 0.00 0.00 0.01 0.11 0.62 1.00 1.00

S% 150 150 150 150 146 139 133

V 0.00 0.00 0.02 0.36 1.00 1.00 1.00

S% 150 150 150 149 138 132 126

V 0.00 0.01 0.06 1.00 1.00 1.00 1.00

S% 150 150 150 140 130 124 118

V 0.01 0.02 0.20 1.00 1.00 1.00 1.00

Permissible Continuous Short Time Loading. Preload=90% T2 (h) 0.50 1.00 2.00 4.00 6.00 8.00 12.00

S% 150 150 150 150 149 144 138

V 0.00 0.01 0.04 0.32 1.00 1.00 1.00

S% 150 150 150 150 141 136 131

V 0.01 0.03 0.12 1.00 1.00 1.00 1.00

S% 150 150 150 141 133 128 123

V 0.05 0.10 0.39 1.00 1.00 1.00 1.00

S% 150 150 147 131 124 119 115

V 0.15 0.30 1.00 1.00 1.00 1.00 1.00

T2=Duration of the peak load, V=Relative rate of using life, S=Loading as percent

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1LTR-9544-109 10. 96 / TDT / E

VENTILATION OF TRANSFORMER CELLS

Page 1/4

If transformers intended for natural cooling are installed indoors, e.g. in cells, sufficiently large ventilation openings must be provided above and below the transformer so that natural air movement is sure to remove the heat losses. For transformers of normal construction one can assume a rise in air temperature of Δϑ≈12K, which is equivalent to an air throughput of 4 to 5 m3/min. per kW of heat losses. The rate of heat flow is governed by the size of the air openings, the height of the exhaust shaft and resistances in the air path. The resistance of the air path is generally: R = R1 + m2.R2 Here, R1 resistance and acceleration coefficients in the inlet duct, R2 resistance and acceleration coefficients in the exhaust duct, m ratio of inlet duct cross-section A1 to exhaust duct cross-section A2. Commonly used arrangements are shown in Fig. 1. The total resistance is made up of components. The following values can be used for the respective resistance and acceleration coefficients: Acceleration Right-angled elbow Rounded elbow Elbow of 135° Gradual change of direction Wire grille Louvres Divergent cross-section

1)

1.0 1.5 1.0 0.6 0...0.6 0.5...1 2.5...3.5 0.25...0.9 1)

The lower value is for a ratio of inlet area to shaft area of1:2, the higher value for 1:10.

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1LTR-9544-109 10. 96 / TDT / E

VENTILATION OF TRANSFORMER CELLS

Page 2/4

Figure 1

Section through transformer cells: a) Air inlet at ground level, air extracted via shaft. b) As a) but without shaft c) Air inlet below ground. Air extracted through an opening in the wall. d) Transformer bay with fan. A1=air intake area, A2=air exhaust area, H=“shaft height”, 1=fan, 2=exhaust louvres, 3= inlet grille or louvres, 4=skirtings, 5=baffle plate.

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1LTR-9544-109 10. 96 / TDT / E

VENTILATION OF TRANSFORMER CELLS

Page 3/4

For the case shown in Fig.1a), we arrive at air intake :

acceleration grille divergent cross-section gradual change of direction

1 0.75 0.55 0.60 R1 = 2.90

exhaust air :

acceleration right-angled elbow louvres

1 1.50 3 R2 = 5.5

If the exhaust duct is made 10% larger than the inlet duct, then m= A1/A2 = 1/1.1 = 0.91 and m2 = 0.83, and hence R=2.9 + 0.83 . 5.5 = 7.5 The ventilation conditions can be calculated with the formula; (Δϑ)3 . H = 13.2(P2v/A21).(R1 + m2R2) Numerical equations with Δϑ in K, H in m, Pv in kW and A1 in m2. Example1: Transformer losses Pv = 10 kW, Δϑ = 12K, R = 7.5 and H = 6 m gives us: A1 ≈ 1 m2 Experience has shown that the ventilation cross-sections can be smaller if the transformer is not permanently under full load, the bay faces north or cooling is favoured by other factors. A small amount of the heat losses is also dissipated through the walls. For exact calculation, see DIN 4701.

Forced-Flow Ventilation of Transformer Cells

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1LTR-9544-109 10. 96 / TDT / E

VENTILATION OF TRANSFORMER CELLS

Page 4/4

If natural ventilation is not adequate, or the required flow area would be too large or the shaft too high, a fan must be provided. This must be appropriate for the required flow rate and head. With air of specific heat Cp 1008 Ws/kg.K, a density at ϑ50 °C of = 1.1 kg/m3 and Pv in kW, the required cooling air flow rate is: Q = 54.Pv / Δϑ in m3/min. The fan must provide not only the static pressure needed to overcome resistances in the air path, but also the dynamic or discharge pressure. Static and dynamic pressure together amount to ≈0.2 ... 0.4 mbar. The power to drive the fan is then; PL = Q.P / η Example 2: If the circumstances are as in Example 1, but now the bay is to contain a transformer of 1600 kVA with losses of 20 kW, the required air flow rate becomes; Q = 54x20/12 = 90 m3/min. and the fan rating (efficiency η = 0.2) is PL = (90m3/min x 0.35mbar) / 0.2 = 0.263 kW Dust filters (p ≈ 0.4 - 1.2 mbar) call for special attention. To ensure adequate air circulation the distance between transformers and walls should if possible not be less than 400 mm.

ABB Elektrik Sanayi A.Ş.

1LTR-9544-109 10. 96 / TDT / E

VENTILATION OF TRANSFORMER CELLS

Page 1/4

If transformers intended for natural cooling are installed indoors, e.g. in cells, sufficiently large ventilation openings must be provided above and below the transformer so that natural air movement is sure to remove the heat losses. For transformers of normal construction one can assume a rise in air temperature of Δϑ≈12K, which is equivalent to an air throughput of 4 to 5 m3/min. per kW of heat losses. The rate of heat flow is governed by the size of the air openings, the height of the exhaust shaft and resistances in the air path. The resistance of the air path is generally: R = R1 + m2.R2 Here, R1 resistance and acceleration coefficients in the inlet duct, R2 resistance and acceleration coefficients in the exhaust duct, m ratio of inlet duct cross-section A1 to exhaust duct cross-section A2. Commonly used arrangements are shown in Fig. 1. The total resistance is made up of components. The following values can be used for the respective resistance and acceleration coefficients: Acceleration Right-angled elbow Rounded elbow Elbow of 135° Gradual change of direction Wire grille Louvres Divergent cross-section

1)

1.0 1.5 1.0 0.6 0...0.6 0.5...1 2.5...3.5 0.25...0.9 1)

The lower value is for a ratio of inlet area to shaft area of1:2, the higher value for 1:10.

ABB Elektrik Sanayi A.Ş.

1LTR-9544-109 10. 96 / TDT / E

VENTILATION OF TRANSFORMER CELLS

Page 2/4

Figure 1

Section through transformer cells: a) Air inlet at ground level, air extracted via shaft. b) As a) but without shaft c) Air inlet below ground. Air extracted through an opening in the wall. d) Transformer bay with fan. A1=air intake area, A2=air exhaust area, H=“shaft height”, 1=fan, 2=exhaust louvres, 3= inlet grille or louvres, 4=skirtings, 5=baffle plate.

ABB Elektrik Sanayi A.Ş.

1LTR-9544-109 10. 96 / TDT / E

VENTILATION OF TRANSFORMER CELLS

Page 3/4

For the case shown in Fig.1a), we arrive at air intake :

acceleration grille divergent cross-section gradual change of direction

1 0.75 0.55 0.60 R1 = 2.90

exhaust air :

acceleration right-angled elbow louvres

1 1.50 3 R2 = 5.5

If the exhaust duct is made 10% larger than the inlet duct, then m= A1/A2 = 1/1.1 = 0.91 and m2 = 0.83, and hence R=2.9 + 0.83 . 5.5 = 7.5 The ventilation conditions can be calculated with the formula; (Δϑ)3 . H = 13.2(P2v/A21).(R1 + m2R2) Numerical equations with Δϑ in K, H in m, Pv in kW and A1 in m2. Example1: Transformer losses Pv = 10 kW, Δϑ = 12K, R = 7.5 and H = 6 m gives us: A1 ≈ 1 m2 Experience has shown that the ventilation cross-sections can be smaller if the transformer is not permanently under full load, the bay faces north or cooling is favoured by other factors. A small amount of the heat losses is also dissipated through the walls. For exact calculation, see DIN 4701.

Forced-Flow Ventilation of Transformer Cells

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1LTR-9544-109 10. 96 / TDT / E

VENTILATION OF TRANSFORMER CELLS

Page 4/4

If natural ventilation is not adequate, or the required flow area would be too large or the shaft too high, a fan must be provided. This must be appropriate for the required flow rate and head. With air of specific heat Cp 1008 Ws/kg.K, a density at ϑ50 °C of = 1.1 kg/m3 and Pv in kW, the required cooling air flow rate is: Q = 54.Pv / Δϑ in m3/min. The fan must provide not only the static pressure needed to overcome resistances in the air path, but also the dynamic or discharge pressure. Static and dynamic pressure together amount to ≈0.2 ... 0.4 mbar. The power to drive the fan is then; PL = Q.P / η Example 2: If the circumstances are as in Example 1, but now the bay is to contain a transformer of 1600 kVA with losses of 20 kW, the required air flow rate becomes; Q = 54x20/12 = 90 m3/min. and the fan rating (efficiency η = 0.2) is PL = (90m3/min x 0.35mbar) / 0.2 = 0.263 kW Dust filters (p ≈ 0.4 - 1.2 mbar) call for special attention. To ensure adequate air circulation the distance between transformers and walls should if possible not be less than 400 mm.

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1LTR-9544-110 10. 96 / TDT / E

PARALLEL CONNECTIONS

Page 1/4

1. Conditions For Parallel Operation If two or more transformers have to be connected in parallel, it is important to remember that only transformers bearing indentical indices (See TABLE 1) can be run in parallel. The index is stamped on the transformer rating plate. Transformers with indices 5 and 11 can also be run in parallel if the conductors of both the LV and HV side are suitably crossed. The connections which are possible in such cases are shown in TABLE 2. Fig. 3 shows one of the possible alternatives.

Other conditions for parallel running are : ♦ Poles having the same polarity on the HV side and LV side shall be connected in parallel (except with transformers bearing the indices 5 and 11 as stated in the above paragraph), ♦ Transformers shall have the same transformation ratio, ♦ The short-time withstand voltage shall have to be the same (within ±10% accuracy), ♦ The rated outputs of the transformers must not deviate from each other more than 1:3.

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PARALLEL CONNECTIONS

Page 2/4

INDEX

0

5

6

11

CONNECTION

Dd0 Yy0 Dz0

Dy5 Yd5 Yz5

Dd6 Yy6 Dz6

Dy11 Yd11 Yz11

TABLE 1. Indices and connection groups

HV SIDE

LV SIDE

Main Busbar

L1 L2 L3

L1 L2 L3

Transformer which is in connection Dy5, Yd5, Yz5

A

B C

a

b

c

B C or C B A or B A C

c

b

a

b

a

c

a

c

b

A

Transformer which is in connection Dy11, Yd11, Yz11

TABLE 2. Connection in parallel of transformers with indices 5 and 11.

ABB Elektrik Sanayi A.Ş.

1LTR-9544-110 10. 96 / TDT / E

PARALLEL CONNECTIONS

Page 3/4

Figure 3 : Example of connection in parallel when the transformer indices are 5 and 11.

The above matters can be clarified on the basis of the data printed on the transformer rating plate. Before connecting the transformers in parallel for the first time, it is vital to check the voltages. For this purpose the HV sides of the transformers are connected in parallel and similarly the star points of the LV sides or two corresponding phases. When high voltage is applied to the HV side, no voltage must exist between the poles of the LV side which are intended to be connected in parallel, whereas normal voltage must exist

ABB Elektrik Sanayi A.Ş.

1LTR-9544-110 10. 96 / TDT / E

PARALLEL CONNECTIONS

Page 4/4

between the various phases. Should this not be the case, then the transformers are incorrectly connected. It is recommended after the transformers have been put into operation, to measure the currents in the different transformers. They should be distributed approximately in proportion to the rated outputs.

2. Load Distribution Between Parallelly Running Transformers To enable the calculation of load distribution the rated output SN and the short-time withstand voltage zk of each transformer running in parallel must be known. During parallel running it is common practice for the transformer with the lowest shorttime withstand voltage zkmin to be the most heavily loaded. When this transformer is loaded at its rated output, the maximum total output Smax of all the transformers operating in parallel can be obtained. The output can be calculated by means of the following equation, where each term is indicating the load of the corresponding transformer. An example for the parallel operation of three transformers:

ABB Elektrik Sanayi A.Ş.

1LTR-9544-110 10. 96 / TDT / E

PARALLEL CONNECTIONS Smax = (zkmin/zk1).SN1 + (zkmin/zk2).SN2 + .... An example of the parallel operation of three transformers: Transformer 1.............. SN1 = 500 kVA................... zk1 = 5.0% Transformer 2.............. SN2 = 800 kVA................... zk2 = 5.4% Transformer 3.............. SN3 = 1000 kVA................. zk3 = 6.0% Total Rated Output :

SN = 2300 kVA zkmin= 5%

S1

= (5.0/5.0).500 kVA

= 500 kVA

S2

= (5.0/5.4).800 kVA

= 740 kVA

S3

= (5.0/6.0).1000 kVA

= 830 kVA

Smax

= S1 + S2 + S3

= 2070 kVA

i.e. approx. 90% of the total rated output.

ABB Elektrik Sanayi A.Ş.

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