Power transformers routine inspection and tests (1)

Safety Precaution 

• A work permit must be issued and the qualified maintenance staff should carry the suitable safety category,
• Wear your safety helmet and safety harnesses.
• Transformer isolation and earthing on all its sides is required
• There is a danger of falling into the oil containment pit when working near large power transformers.
• A proper well fixed ladder should be used for climbing on top of the transformer tank.

Tools and Equipment

• Safety helmet, safety belt, and tool set
• Voltage indicator, Ladder
• Portable earth for discharging,
• Fine cloth for cleaning purposes, Silicagel,
• Complete tool kit/set, including a torque wrench.

Environmental Recommendation

• Before starting the cleaning of all porcelain surfaces of all transformer wear your breathing mask to protect lung tissues from the dust particulate.
• Don't burn the cloth used in cleaning, collect it and send it to the dump.
• Fix any oil leaks to prevent pollution and save money of wasted oil.
• Oil leaks to the ground have very bad impact on the environment, mainly on soil and ground water and eventually on the ecosystem. "It takes only I liter of hydrocarbon spilled in an aquifer to render 1,000,000 liters of potable water useless for human consumption.

Work to be carried out

1. Check  for signs of oil leaks, correct if possible.
2. Inspect all painted surfaces of the transformer, remove any signs of rust and repaint bare spots 
3. Check condition of earthing bus, all bolted connections must be checked for tightness. Thoroughly Clean all porcelain surfaces on all bushings.
4. Check for correct oil level in all transformer bushings, top up if necessary.
5. Open bleed valve on buchholz relay until a small amount of oil escapes, test alarm and trip circuit using the test button next to the vent, valve, check all electrical connections for tightness for tightness.
6. Check the breather, if required replace or regenerate the silicagel.
7. Check the pressure Relieve device on main transformer tank and if fitted on ULTC compartment.
8. Inspect condition of all cooling fans, check that screens have no obstructions, check for cracks in fan blades, test run all motors and check for abnormal bearing noise. It is recommended to change motor bearings after 3000 hours of running time or every 4 years, whichever comes first.
9. Interval may be adjusted by technical services unit.
10. Inspect transformer control cabinet, blow out dust, check condition of door seal, check all electrical connections for tightness, check proper operation of space heaters.
11. Use water pressure to clean radiators and transformer tank walls, do not direct the stream of water directly at the cooling fans. It is recommended to use the transformer deluge system to perform the cleaning operation, it will provide a functioning test for the system at the same time the unsealed wiring must be covered by plastic cover to avoid short circuit in wiring system.
12. Take and record insulation resistance readings (Megger readings), high voltage winding to low voltage winding, high voltage to tertiary winding , low voltage to tertiary winding, high voltage to earth, low voltage to earth, tertiary winding to earth, Compare readings to historical data.
13. Take and record capacitance and dissipation factor      %, Compare reading to historical data.
14. Perform ratio tests in lowest, highest and neutral tap position, compare each high voltage to neutral winding to corresponding low voltage to neutral winding, reading taken at all points of tap changer. Compare readings to historical data
15. Take and record winding resistance readings, each high voltage phase to neutral, Each low voltage phase to neutral, tertiary delta winding phase to phase Reading taken at all points of Tap changer, Compare reading to historical data
16. Inspect and clean tap changer control cabinet, check all electrical connections for tightness, check functioning of cabinet heater and check door seals, check motor operating mechanism
17. It is important to perform a complete tap changer (electrical operation) from highest to lowest step and back to the position it was found in, this action will clean all internal contacts of the tap changer
18. Inspect condition of external tap changer drive shaft, tighten all couplings and bolted connections
19. Take and record final reading on tap changer operations counter
20. Take oil samples for laboratory analysis
21. Be sure that all tools have been taken off the transformer
22. Clean and apply Graphite Grease to all contact surfaces of all neutral disconnect switches, lubricate all bearing points
23. Check for correct spacing of are horns on all bushings
24. Clean, grease and tighten all couplings of drive shaft to neutral disconnect switches
25. Place transformer back into normal operating condition