Oil-immersed transformer overhaul and small maintains

Oil-immersed transformer overhaul and small maintains 

What is a transformer?

Transformer is a transmission and distribution system in an extremely important electrical equipment, but for various reasons, sometimes in operation will break down. According to the nature of maintenance work, transformer maintenance is divided into overhaul and repair.

Oil-immersed transformer: any need to drain the oil lifting core (or lifting open bell) for overhaul is called overhaul; if you don’t need to drain the oil, not lifting core (lifting open bell), only in the external overhaul or make up the oil, oil treatment is called minor repair.

Conventional transformer overhaul process

1 Clean the casing, check the porcelain sleeve has no discharge marks and damage phenomenon.

2 Check whether the top fastening bolt of casing is loose or overheating.

3 Clean the transformer oil tank and oil storage cabinet, safety airway and pressure regulator and other accessories.

4 Check the dust and other debris deposited in the cover of the pressure relief valve.

5 Check the oil level of the oil storage cabinet is normal, oil level gauge is intact, clean, and discharge the oil in the collection basin.

6 Check the breather, replace the failed and discoloured desiccant.

7 Replenish the insulating oil of transformer body and oil-filled casing.

8 Check the radiator for oil seepage, and whether the cooling fan is normal.

9 Check the thermometer for measuring the upper oil temperature.

10 Check the gas relay for oil seepage, whether the valve opening and closing is flexible and reliable, and whether the control cable insulation is good.

11 Check the transformer shell grounding wire and neutral point grounding device.

12 Check the on-load tap-changer operating control circuit, the drive section and its contact action, and clean the inside of the operating box.

13 Check the on-load tap-changer operating control circuit, drive section and its contact action, and clean the inside of the operating box.

14 Dispose of defects that can be eliminated locally, such as oil leakage or seepage.

15 Carry out electrical tests on the specified items.

When the electrical test and the oil test show unqualified results, it is necessary to carry out overhaul and judge whether the transformer has the conditions for core lifting inspection according to the site conditions, and then carry out the relevant overhaul operation.

In addition to this reason in the following three cases need to carry out hanging cover inspection:

1 The transformer is generally recommended to carry out the lifting cover inspection within 5 years after it is put into operation.

2 The main transformer operating in the power system when subjected to outlet short circuit, after comprehensive diagnosis and analysis, can be considered in advance of overhaul.

3 Transformers in operation should be overhauled in advance when abnormal conditions are found or when internal faults are judged by tests.

Transformer overhaul project

1 Lift the bell or lifting out the body overhaul.

2 Overhaul of coil, lead wire and magnetic (electric) shielding device.

3 Iron core, iron core fasteners (screws, clamps, straps, straps, etc.), pressure nails, connecting piece and grounding piece of the inspection.

4 Overhaul of oil tanks and accessories, including sleeves, moisture absorbers, etc.

5 Overhaul of ancillary equipment such as coolers, oil pumps, water pumps, fans, valves and pipework.

6 Overhaul of safety protection devices.

7 Calibration of temperature measuring device, calibration of gas relay.

8 Overhaul and test of operation control box.

9 Overhaul of the non-excited or on-load tap-changer.

10 Replacement of all sealing gaskets and leakage testing of components.

11 Drying of the body insulation if necessary.

12 Transformer oil treatment or oil change.

13 Testing and commissioning after overhaul.

14 Technical modifications that can be carried out in conjunction with the overhaul of the transformer (e.g. reinforcement of the mechanical strength of the oil tank, change of the internal earthing device of the body to external earthing, improvement of the oil level gauge, installation of sealing devices in the oil conservator).

15 Finally, the oil tank is cleaned and sprayed with paint.

Overhaul process flow

Removal of leads → electrical, oil preparation test, insulation judgement → part of the oil removal dismantling accessories and overhaul → exhaust oil and processing, removal of tap changer connectors → hanging bellows (body) body inspection, overhaul and test insulation → moisture then drying treatment → oil injection according to the prescribed oil injection method → installation of bushings, coolers and other accessories → sealing test → oil level adjustment → electrical, oil laboratory → end.

Transformer overhaul overhaul of components

Winding overhaul

1 check the phase partition and screen (appropriate disintegration of a phase), the screen should be clean and undamaged, tied tightly intact, tap lead outlet closed good, no screen deformation, heat and dendritic discharge. If abnormalities are found, the other two phases of the screen should be opened for inspection, and the partition between the phases should be complete and firmly fixed.

2 check the winding surface should be free of grease and deformation, the whole winding without tilt and displacement, wire spoke to no obvious protruding phenomenon, turn insulation is not broken.

3 check all parts of the winding pads are not loose, pads should be arranged neatly, spoke spacing is equal, support firm and appropriate compression force.

4 check the winding insulation is not damaged, the oil channel is not insulating paper, grease or debris blocking phenomenon, if necessary, can be used to soft brush (or with a silk cloth, foam) gently wipe; winding turns surface, wire, if any damage to the exposed should be wrapped processing.

Leads and insulation bracket overhaul

1 Check the insulation wrapping of the lead and stress cone for deformation, brittleness, damage, broken strands, twisted lead, lead and lead joints welded for good condition and overheating.

2 Check the length of the leads from the winding to the on-load tap-changer, the thickness of the insulation wrapping, the welding (or connection) of the lead joints, the insulation distance of the leads from each part, and the fixing of the leads.

3 Check the insulation bracket for looseness and damage, displacement, check the fixing of the leads in the insulation bracket, the fixing bolts should have anti-loosening measures, and the inner side of the clamps for fixing the leads should be padded with additional insulation in order to prevent the insulation of the leads from being injured.

4 check whether the insulation distance between the lead and each part meets the specified requirements, high-current leads (copper or aluminium row) and the box wall spacing should not be less than 100mm in general, in order to prevent leakage of heat.