During operation, certain components of a transformer gradually lose their original properties under the impact of thermal, electrodynamic, mechanical, and other factors, which can eventually lead to failure.
To ensure timely detection and elimination of developing defects and to prevent emergency shutdowns, transformers undergo periodic routine and major overhauls. These works are carried out in accordance with the requirements of SOU-N EE 46.603:2007 “Overhaul of Transformers and Reactors with Voltages from 110 kV to 750 kV.”
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Major Overhaul |
Routine Maintenance |
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Disassembly of the transformer, lifting of the core and windings (or tank removal), followed by detailed internal inspection |
External inspection and elimination of detected defects that can be corrected on-site |
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Repair of the magnetic core, winding (tightening/re-pressing), on-load tap changer (OLTC), off-circuit tap changer (OCTC), and bushings |
Cleaning of insulators and tank |
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Overhaul of the cover, conservator (expansion tank), breather (silica gel unit), radiators, thermosyphon filter, valves, gaskets, and insulators |
Draining sludge from the conservator, topping up oil if necessary, checking the oil level indicator |
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Servicing of the cooling system, including repair or replacement of fans |
Inspection of the drain valve and seals |
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Cleaning, sandblasting, and repainting of the transformer tank |
Inspection and cleaning of cooling devices |
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Testing, calibration, and verification of measuring instruments, protection relays, and monitoring devices |
Inspection of gas protection system |
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Transformer oil processing: filtration, regeneration, or complete replacement |
Inspection of the exhaust pipe diaphragm integrity |
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Drying of the active part if required |
Inspection of thermosyphon filters and air dryers |
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Repair of the on-load tap changer (OLTC) with removal of circular diagrams. Repair of the winding pack if available. |
Repair of the regulating device in accordance with the manufacturer’s instructions, depending on the number of switching operations (for transformers with on-load tap changers). |
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Repair of the OLTC drive.
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Inspection for air ingress in the oil circulation system and verification of cooler tightness (for transformers with forced oil–water cooling). |
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Replacement of silica gel in thermosiphon filters. |
Repair of oil-filled bushings of transformers: oil topping-up if required, measurement of dielectric loss tangent (at least once every 6 years). |
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Repair of oil pumps (for unit transformers with oil cooling systems). |
Measurement and testing of transformer parameters. |
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Replacement of all rubber seals of the transformer. |
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Complete replacement of rubber seals of disc gate valves or their replacement, as well as replacement of the oil drain valve. |
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Replacement of electrical signal and power supply cables of the cooling system, protection and monitoring equipment. |
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Repair of oil-filled high-voltage bushings. |
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Reassembly of the transformer. |
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Performance of measurements and testing of transformer electrical parameters before, during, and after the repair. |
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Major overhauls of transformers are carried out by the company's specialists in accordance with the technical specifications of the customer.
Within the scope of these works, we perform:
- Replacement of transformer windings with new ones;
- Replacement of the film insulation/protection of oil-filled transformers;
- Replacement of the on-load tap changer (OLTC) with a new unit if repair is impossible or economically unfeasible;
- Replacement of high-voltage (HV), medium-voltage (MV), and low-voltage (LV) bushings with new ones;
- Replacement of OLTC drives;
- Replacement of obsolete equipment, monitoring and protection devices with modern units from: Qualitrol Company LLC (USA), Elektromotoren und Gerätebau Barleben GmbH (Germany), ELPROM (ex-HYUNDAI) Heavy Industries Co. (Bulgaria), HSP Hochspannungsgeräte GmbH (Germany), KONČAR Instrument Transformers (Croatia), Končar Metal Structures Ltd. (Croatia)
