Routine Tests of Transformer and their Purpose

• Visual Inspection
• To check the cracks, rust or chipping.
• To check the Serial Number.
• To check the Level of Oil.
• To check the leakage/seepage of Oil.
• To check the Markings on transformers.
• To check the colour coding of LT/HT connections.
• To check the colour of Oil.
• To check the locking Washers at the end of HT/LT eye bolts.
• Power Frequency High Voltage Test
• This test is intended to check the adequacy of main insulation to earth and between windings.
• Induced Over Voltage withstand Test
• To check the insulation of HT & LT winding and the insulation between different turns of winding.
• Turn Ratio Test
• This test is carried out to ensure the correctness of Voltage Ratio of Transformer between HT & LT windings on each tapping.
• The voltage ratio of transformer is equal to the turn ratio therefore to verify the voltage ratio we check the turn ratio.
• Measurement of No Load Losses
• This test is carried out to check the quality of the silicon steel sheet (core) used in the transformer.
• No Load losses are very important in this respect that these are permanent and are independent of load.
• These losses should be as minimum as possible and this can be ensured by using good  quality  silicon steel sheet.
• Measurement of Load Losses & Impedance
• In order to ensure the quality of copper, copper losses are measured. 
• Copper losses comprises the sum of I2R losses in the winding and the stray losses due to eddy current in conductor, clamps and tank.

• Good quality copper not only increases the life of transformer but also reduces the system losses.
• As the stray losses vary with the frequency therefore it is important to give the supply to transformer at rated frequency.
• In distribution transformers stray losses are usually negligible.
• Bird Protection Test
• To prevent flashover caused by large birds, transformer cover shall be effectively coated with a weather proof permanent insulation material having a dry dielectric withstand voltage of not less than 8kV.
• Verification of Vector Group
• There are different types of connections according to which, limbs of HT/LT windings are connected. WAPDA has adopted DYn-11 and to verify this, vector group test is carried out.
• Tank Pressure Test
• Lot of welding is involved in the fabrication & manufacturing of transformer due to which there are welding joints at top, cooling tubes & base which can cause leakage/seepage of Oil.
• In order to ensure no leakage/seepage of Oil during normal operation of transformer, pressure test is conducted.