A transformer is a crucial electrical device in power transmission and distribution systems. However, due to various reasons, it may experience malfunctions during operation. Transformer maintenance is classified into major overhauls (major repairs) and minor overhauls (minor repairs) based on the nature of the maintenance work.
Oil-Immersed Transformers: Major and Minor Overhauls
For oil-immersed transformers, major overhauls involve draining the oil and hoisting the core (or hoisting the cover), while minor overhauls do not require oil drainage or core hoisting. Minor overhauls involve external inspections, oil replenishment, and oil treatment without the need for oil draining or core hoisting.
Routine Transformer Maintenance Procedure:
Clean the bushing and check for discharge marks and damage on porcelain bushings.
Check if the bolts securing the top of the bushing are loose or overheated.
Clean the transformer tank, oil storage tank, safety passage, and pressure relief device accessories.
Inspect the dust and debris deposited inside the pressure relief valve cover.
Check the oil level in the oil storage tank, the condition of the oil level gauge, and drain oil sludge from the oil drain pan.
Check the respirator and replace the discolored desiccant.
Top up the insulation oil in the transformer body and oil-filled bushings.
Check for oil leakage in the radiator and ensure the cooling fan is functioning properly.
Inspect the temperature gauge measuring the upper oil temperature.
Check for oil leakage in the gas relay, ensure flexible valve opening and closing, and verify the insulation of control cable.
Inspect the grounding wire of the transformer casing and neutral point grounding device.
Inspect the on-load tap changer control circuit, drive parts, and contact operation, and clean the inside of the control box.
Check the control circuit, drive parts, and contact operation of the on-load tap changer, and clean the inside of the control box.
Address defects such as oil seepage and leakage that can be rectified on-site.
Conduct specified electrical tests after maintenance.
Major Overhaul Projects for Transformers:
Hoist the cover or body for inspection.
Overhaul coils, leads, and magnetic (electric) shielding devices.
Inspect the core, core fasteners (core screws, clamps, tie straps, etc.), press pins, connecting pieces, and grounding plates.
Overhaul the tank and accessories, including bushings, moisture absorbers, etc.
Overhaul auxiliary equipment such as coolers, oil pumps, water pumps, fans, valves, and pipes.
Overhaul safety protection devices.
Calibrate temperature measurement devices, gas relays.
Overhaul the operation control box and conduct tests.
Overhaul the no-load tap changer or on-load tap changer.
Replace all sealing gaskets and perform component leak tests.
Dry the insulation of the transformer body if necessary.
Process or replace transformer oil.
Conduct tests and trial operation after major overhaul.
Technical upgrade projects that can be carried out in conjunction with transformer major overhaul (e.g., strengthening mechanical strength of the oil tank, converting internal grounding devices to external grounding, improving oil level gauges, and adding sealing devices to the oil storage tank).
Finally, clean the tank and apply paint.
Major Overhaul Process:
Dismantle leads → Electrical and oil preliminary tests, insulation assessment → Partial oil drainage, disassembly of accessories, and inspection → Complete oil drainage and treatment, removal of on-load tap changer connecting parts → Hoist the cover (or body) for inspection, overhaul, and insulation testing → Dry if moist → Oil filling according to specified method → Install accessories such as bushings and coolers → Sealing test → Adjust oil level → Electrical and oil tests → Completion.
Overhaul of Transformer Components:
Transformer winding Overhaul:
Inspect phase spacing plates and shields (disassemble one phase if necessary), ensuring cleanliness, integrity, secure bundling, and proper closure at the exit of phase separation wires.
Inspect the coil surfaces for cleanliness and absence of oil residue, deformities, tilting, or displacement. No obvious protrusions should be present radially along the conductors, and coil insulation should be intact.
Lead and Insulation Support Overhaul:
Inspect the insulation wrapping of leads and stress cones for deformations, brittleness, and damage. Check for broken strands, twisting, and ensure good welding at lead joints. Examine for any signs of overheating.
Check the length of leads from the winding to the tap changer, the thickness of insulation wrapping, the welding (or connection) of lead joints, the insulation distance between leads and various components, and the fixation of leads.
Inspect insulation supports for any looseness, damage, or displacement. Check the fixation of leads in insulation supports, and ensure that the fixing bolts have anti-loosening measures. The inner side of the clamps securing the leads should be padded with additional insulation to prevent damage to lead insulation.
Verify if the insulation distance between leads and various components meets the specified requirements. The distance between high-current leads (copper or aluminum bars) and the box wall should generally not be less than 100mm to prevent magnetic leakage and heating.
