Distribution Transformer

The voltage induced in the secondary coil is directly proportional to its number of turns.
The current induced in the secondary coil is inversely proportional to its number of turns.

Main parts of Power Transformer:

CORE (Laminations made of soft magnetic material made of silicon steel of 0.5 to 1 mm Thick)
WINDING AND INSULATION SYSTEM
TANK
BUSHINGS
RADIATORS
CONSERVATOR
TAP CHANGER- OLTC
PRV
BREATHER
MARSHALLING BOX
BUCHHOLTZ RELAY
OSR

Different types of Losses:

Winding Resistance Loss or Copper Loss. - caused due to Resistance of the winding.
Hysterisis Loss – caused due to magnetic reversal.
Eddy Current Loss - caused due to Circulation of Eddy current within the core normal to the Flux.
Magneto Restriction - caused due to Physical expansion & contraction of the core due to magnetic flux. ( Causes Buzzing)
Mechanical Loss - caused due to presence of Alternating magnetic field which causes EMF between Primary & Secondary windings. (Buzzing)
Stray Loss – caused due to Leakage Flux i.e. the path of the flux in unintended route.

Reason for Failures in Transformers:

a.Defective Manufacturing:

Defective Manufacturing (Poor design, faulty materials & bad workmanship)
Defective or Abnormal Operation (Careless dry out, careless installation, inadequate supervision & maintenance)

b.Failures due to Mechanical Damages:

Oil Leaks
Flash over of bushings due to deposits of coal dust, saline or chemicals on the bushings
Application of silicon compound
Wrong paralleling of transformers

c.Failure in Dielectric Circuits

Moisture Entering the tank by “breathing action” resulting in inter turn shorting
Deterioration of oil due to prolonged overloading of the transformer.
formation of sludge, water and acid is accelerated at high temperatures.
Safe value for oils

Topping up of oil to cover evaporation and oxidation
Improper dried wooden duct for taking terminal leads
Increase in acidity value of oil – promote oxidation

d.Failures in Electric Circuits:

Poor impregnation of coils
Presence of sharp edges on copper conductors
Dislodging of turns
Improper drying out
Sudden changes in load
Poor soldering
Sustained heavy overloads
Wrong operation of tap changer switches
Operating transformer when one HG fuse blows off
Looseness in bolted joints
Lighting surges

e.Failures in Magnetic Circuits:

Loosening of bolts connecting cores and yokes.
Insulation failure between lamination.
Insulation failure between yoke and clamping bolts.
Locking of core clamp bolts.
Avoiding metallic filling between lamination.

TESTS ON TRANSFORMERS:

a.Open circuit Test:

It is used to determine core losses
Usually performed on the low voltage side
The test is performed at rated voltage and frequency under no load

b.Short circuit Test:

It is used to determine full load copper loss
Usually performed on the high voltage side
This test is performed at reduced voltage and rated frequency with the output of the low voltage winding short circuited such that rated current flows on the high voltage side.

c.Transformer Regulation Test:

Loading changes the output voltage of a transformer.

d.Winding Resistance Test

This test is performed to find out the resistance of the winding By applying small DC voltage to the winding and measuring the Corresponding current . This is more feasible for HV winding.

e.Polarity Test:

This is required for identifying the primary and secondary phase polarities

f.Load Test:

Load test is requred to find out total loss that takes place when A transformer is loaded . In this test actual temperature rise ,voltage regulation and efficiency of the Transformer can be obtained

g.Ratio Test:

This test is performed to find out % of winding ( normally on primary side ) Included / Excluded from the circuit When a tap is changed This is one of the Important pre commissioning test

h.Insulation test of Transformer (Meggering)

i.BDV Tests on transformer oil

j.D G A Analysis of Transformer oil

Tips to reduce Failures in Distribution Transformer:

Providing perfect earthling by providing a new earth electrode.
Quality control in maintenance sheds and private repair sheds (measuring neutral currents, magnetizing currents, quality of winding wire, IR values at 60oC, load test etc.).
Providing spacers for LT lines with loose spans.
Restoration of lighting arrestors.
Re-adjustments of loads on DTs.
The filtering should be done with the oil at temperature of 60oC to 65oC. It is to be noted higher oil temperature deteriorates the quality of oil.
The number of turns of HV and LV windings of transformers should be ascertained if necessary from the manufacturers and the same are to be maintained during repairs. The total number of turns should not be reduced than the designed number of turns.
All joints and formation of delta and neutral are to be made with lugs properly crimped.

Transformer Maintainence

Cleaning of bushings and external surface of tank cooling pipes	Monthly
Checking of oil levels in the conservator and gauge glass	Monthly
Checking of silica gel in the breather and replacement is necessary	Monthly
Checking of oil level in the oil seal of breather and top up if necessary	Monthly
Checking of HG fuse and LT fuse and renew if necessary (correct gauge shall be maintained)	Monthly
Checking breaker operation	Monthly
Checking of vent pipe diaphragm	Monthly
Checking of terminal loose connections is any and tightening the same	Monthly
Checking for any oil leaks and rectification (including replacement of oil seals if required)	Monthly
Taking tong tester reading during peak load hours and remedial action whenever load exceed 80% rated capacity	Quarterly
Noting down neutral currents and load balancing in all the three phase	Quarterly
Measurement of IR values	Half Yearly
Testing of oil for BDV, acidity	Half Yearly
Checking of lightening arrestors and replacement if required	Half Yearly
Measurement of earth resistance checking of earthing system and rectification if required	Half Yearly
Overhaul of Transformer	Once in 5 years