Commissioning activities recommended by the factory of the transformer (Prior to HV power energization)
a- Measurements
• Electric strength of the oil (IEC test): minimum value 60kV,
• Winding insulation resistance to ground: at 20°C, The insulation resistance should reach a value of some hundreds MΩ for windings up to 72.5 kV, and some thousands MΩ for windings owning higher rated voltage.
b- Checks
Before energizing the transformer all the following checks must be done:
• Visible check of general installation and proper fastening of all components,
• Proper earthing of tank,
• Oil level in the conservator according to ambient temperature,
• Condition of oil preservation system (air bag),
• Oil level in the bushings,
• Color of hygroscopic salts inside air drier(s),
• Good operation of Buchholz relay alarm and tripping circuits,
• Minimum setting of max. current relays, taking into account the inrush current,
• Correct operation of main circuit breakers and relevant control circuits,
• Correct operation of pressure relief valve(s),
• Proper operation of alarm and tripping circuits relevant to protective devices, like temperature indicators (oil thermometer, winding temperature indicator, if any, etc.) and oil filling in the relevant pockets,
• Correct operation of control and protective circuits of on load tap changer, if any;
• "Open" position of interception valves on piping connecting tank and conservator, as well as tank and cooling plant,
• Vent of all bushings and piping running below the oil level in the conservator, so as to avoid that air or gas is trapped inside and be sure that all cavities are filled with oil,
• Absence of foreign bodies on the transformer, particularly on the cover,
• Cleanness of bushing porcelains,
• Proper setting of arcing horns on bushings,if any,
• Grounding caps fitted on bushings provided with taps for power factor measurement,
• Current transformers mounted on bushings: short circuit on secondary side or proper connection and earthing,
• Proper connection to overhead lines or cables and effective grounding of terminals to be earthed permanently (e.g. neutral ends),
• Removal of all earthing connections put for safety during erection,
• Verify the painting of the main unit and of all equipment, retouch the parts eventually deteriorated during transport or erection Before energizing it is important also to check that the unit is connected correctly for the relevant voltage. Only the connections shown on the rating plate are allowed.
If possible the unit should be energized by applying increasing voltage according to the following
procedure:
• 50% of rated voltage is applied all at once and maintained for few minutes,
• The voltage is slowly increased up to 75% of rated value and maintained for few minutes,
• The voltage is slowly increased up to 100% of rated value. Opening and closing of the line circuit breakers should be performed before putting the unit under load.
Thanks to everyone tryied to help.
