OIL TYPE TRANSFORMER
DRY TYPE TRANSFORMER
TRANSFORMER BASIC FORMULAS
3 phase power = sqrt3*V*I
Transformer ratio = (V1/V2) = (N1/N2) = (I2/I1) = K
TO SELECT kVA OF A TRANSFORMER
- Find the expected MD (maximum demand) i.e., Load*Diversity factor. (Diversity factor is always greater than 1)
- Load requirement in kVA at x PF (power factor) i.e., PF=(kW/kVA)
- Considering upto 85% loading of rated transformer capacity.
SELECTION OF METAL OXIDE VARISTOR
1.5 times of phase voltage
Let us consider (11/.433) kV transformer
(11kV/sqrt3)*1.5=9.5kV
operation < 9.5kV NO & >9.5kV NC (surge voltage)
MULTIPLICATION FACTOR
MP = (CT ratio) * (PT ratio)
for eg: CT ratio 30A/5A & PT ratio 11kV/110V
then, MP = 600
INDIVIDUAL & COMBINED VALUE OF EARTHING
Body earthing
Neutral earthing
Surge arrester earthing
"The combined value should be always less than that of the individual value".
WINDING TEMPERATURE OF A TRANSFORMER
55'C Ventilation fan is switched on
70'C Alarm is activated
80'C Trip signal is activated
LIMIT SWITCH
With the help of limit switch trip signal is activated if any one of the door seems to be opened.
PROTECTION OF TRANSFORMER FROM LT SIDE ACB
- Overload protection 80% of full load
- Ground protection 30% of full load @ 400 ms
- Short circuit protection 3.5 times of full load @ 50 ms
- Instantaneous protection 7.19 times of full load
FAULTS IN TRANSFORMER
- Phase fault (L-L or 3 Phase )
- Earth fault (L-G or L-L-G) Must sever case because core is connected to earth.
- Some internal troubles of oil type transformer are:
- Buchholz alarm
- Buchholz trip
- Winding temperature indicator alarm (30'C - 150'C)
- Winding temperature idicator trip (30'C - 150'C)
- Oil temperature indicator alarm (30'C - 150'C)
- Oil temperature indicator trip (30'C - 150'C)
- Pressure relief valve alarm (OP.PR. - .42 kg/Cm2)
- Pressure relief valve trip (OP.PR. - .42 kg/Cm2)
- Minimum oil gauge alarm
TESTING OF A TRANSFORMER
- To check insulation resistance use Megger
- To check winding resistance use clamp meter or multimeter
1kV = 10 Mohm Good insulation
Atleast 50% voltage rating of the equipment should be selected as megger voltage
TAP CHANGING OF A TRANSFORMER
- On load tap changer
- Off load tap changer
TYPES OF TRANSFORMER
CT primary current & secondary current.
CT is a step up transformer.
CT burden is the connected load like meters & relays.
CT types are 2 wire, 3 wire & 4 wire.
CT on no load is called zero burden.
CT on limited load is called typical burden.
CT on over load is called high burden.
CT burden calculation includes pilot wire , meters & relays.
CT secondary should not be in open circuit on no load condition as it leads to explode due to over voltages in the secondary side.
- Potential transformer
- Lightning transformer
- Isolation transformer
- Distribution transformer
- Power transformer
- Dry type transformer
- Oil type transformer
BDV TEST OF A TRANSFORMER OIL
- A high voltage is applied in the range from 0 kV to 100 kV to the copper electrodes by placing these two electrodes by a gap of 2.5mm in a sample of the transformer oil(naphthenic mineral oil).
- Withstanding voltage of oil should be minimum 3 times the operating voltage of the transformer. So, it should withstand upto 40 kV for about 1 minute.
- Tripping voltage of oil should be checked by injecting slowly from 40 kV to 90 kV of mamimum range.
- Two cycle of testing is to be followed one is from main tank another is from OLTC tank.
- Injecting temperature of oil after BDV test must be above 70'C.
SILICA JEL
Silica gel is used as a visual indicator of moisture in the transformer oil. Colour of new silica gel used in transformer is blue. After absorbing moisture they lose the blue colour and finally turn to pink.
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