1. Field of application and purpose
This manual defines a single technical procedure for performing scheduled technical maintenance and diagnostics of oil-immersed power transformers on industrial enterprises in Ukraine. The document covers three critical diagnostic stages: sampling of transformer oil, interpretation of results of gas chromatographic analysis of dissolved gases (DGA), and measurement of winding insulation resistance.
The procedure applies to two-winding and three-winding power transformers with voltage ratings from 6 kV to 110 kV and power ratings from 630 kVA and above, installed at production sites, substations and distribution points. Works are carried out during planned equipment shutdowns, seasonal preventive inspections or upon detection of deviations in the operation of cooling and protection systems.
2. Safety Measures
DANGER OF HIGH VOLTAGE AND TOXIC SUBSTANCES!
1. Lockout and Tagout (LOTO): Before starting any work involving connection to measuring circuits or oil sampling from a transformer, it must be completely taken out of service, disconnected from the high voltage (HV) and low voltage (LV) sides with a visible disconnection of switching devices. Install grounding clamps on all incoming connections. 2. PPE: Mandatory use of dielectric gloves (class 3 according to EN 60903), protective goggles with side protection, heat-resistant protective clothing (EN ISO 11612) and special footwear with oil and benzene-resistant soles. 3. Environmental Safety: Transformer oil contains decomposition products and may be contaminated with traces of PCBs (polychlorinated biphenyls) in old transformers. Use spill containment trays with a capacity of no less than 110% of the sampling valve volume. Comply with the requirements of DSP 2.012-99 regarding handling hazardous waste.
3. Required Tools and Materials
| Tool / Material Name | Technical specification | Quantity |
|---|---|---|
| Megohmmeter | Test voltage up to 2500 V, measurement range up to 100 GΩ (for example, Fluke 1555 or equivalent) | 1 pc. |
| Torque Wrench Set | Range 5–50 Nm and 20–200 Nm with accuracy ±4% | 2 pcs. |
| Oil sample bottles | Special bottles of type Schott Duran with sealed stoppers, volume 1 L | 3 pcs. |
| Special syringes (for DGA) | Luer Lock syringes with a volume of 50 ml with three-way valves | 2 pcs. |
| Infrared pyrometer / thermal imager | Range -20 °C ... +350 °C, accuracy ±2 °C (for example, Testo 883) | 1 pc. |
| Flexible silicone hose | Diameter 8-10 mm, resistant to petroleum products | 3 m |
| Technical spirit (isopropanol) | 99.7% purity for degreasing | 1 l |
| Woven paper towels | Industrial rolls | 1 pc. |
4. Pre-inspection Checklist
| View Element | What to check | Acceptance / Rejection Criterion | Notes |
|---|---|---|---|
| Transformer main tank | Integrity of welded joints, absence of oil leak traces | Acceptable: Dry surfaces. Deviation: Presence of oil stains or droplets. |
Make photo fixation of any defects. |
| Oil expander | Oil level relative to the temperature scale | The oil level should correspond to the ambient temperature (for example, +20 °C within the marking). | Check the condition of the silica gel desiccator. |
| Silica gel plug | Color of the indicator silica gel | Acceptable: Blue or bright pink (depending on type). Deviation: Pale pink or dirty yellow (saturated with moisture). |
Replacement of the grain is carried out when the color changes by more than 50%. |
| High Voltage (insulators) and Low Voltage (insulators) | Condition of porcelain or polymer surface | Absence of scratches, cracks, signs of surface discharges (scorch marks). | Clean dust and contaminants. |
| Earthing of the housing | Reliability of bolted connections in the grounding loop | Transition resistance not more than 0.1 Ohm. Bolt tightening according to the torque. | Clean to a metallic shine. |
5. Step-by-step procedure for performing the work
Preparation of the work area and ensuring LOTO
- Obtain authorization for performing high-risk work and a work permit.
- Perform full de-energization of the transformer. Apply mechanical locks on switching devices and display warning signs "Do not switch on! People are working."
- Make sure there is no voltage on the HV and LV inputs using a portable voltage indicator of the appropriate class.
- Apply portable earthings to all wires of the transformer inputs.
Visual completion indicator: Visible contact break of circuit breakers, presence of grounding cables on the inputs.
Common errors: Starting work without checking the residual capacitance voltage on high-voltage inputs.
-
Відбір проби трансформаторної олії для лабораторного аналізу та DGA
- Clean the lower valve of the transformer oil sampling unit from dust and dirt using a lint-free cloth and isopropyl alcohol.
- Pour the initial amount of oil of approximately 0.5 liters into the prepared container (this allows removing the stagnant oil from the valve branch).
- Flush the valve and connect the silicone hose, ensuring no air bubbles enter the oil flow.
- Fill special glass bottles (or DGA syringes) with a laminar stream from bottom to top, washing and completely filling the container without leaving air gaps. Seal the bottles with stoppers.
Visual end indicator: Complete absence of air bubbles inside the glass container or syringe, seal integrity.
Поширені помилки: Контакт зразка олії з атмосферним повітрям або вологою, використання відкритих пластикових пляшок замість герметичного скла.
Insulation resistance measurement of windings (R60 / R15)
- Connect the megohmmeter to the test terminals. Use the following diagrams: HV relative to (LV + Enclosure), LV relative to (HV + Enclosure), HV relative to LV.
- Set the test voltage according to the transformer passport (recommended 2500 V for windings with voltage above 10 kV).
- Measure for 60 seconds, recording the resistance value at the 15th second ($R_{15}$) and at the 60th second ($R_{60}$). Calculate the absorption coefficient $K_{абс} = R_{60} / R_{15}$.
- After completing the measurement, discharge the winding capacitance to the housing for at least 2 minutes.
Visual indicator of completion: Stabilization of the pointer or digital megohmmeter reading at the 60th second, absorption coefficient value $K_{abs} \ge 1.3$ at a temperature of +20 °C.
Common errors: Insufficient discharge time of windings after testing, leading to damage of the measuring instrument or electric shock.
Tightening of contact connections at terminals and busbars
- Check the condition of the bolted connections on the contact clamps of the high voltage and low voltage inputs.
- Use a torque wrench to tighten nuts and bolts to the nominal values of torque moments. For thread M12 set the torque to 45–50 Nm, for M10 — 25–30 Nm.
- Apply a layer of protective grease (for example, technical vaseline or zinc paste) on exposed copper-aluminum or brass connections to prevent oxidation.
Visual indicator of completion: Clicking the torque wrench mechanism on each connection, presence of marking with a torque control marker.
Поширені помилки: Перетягування різьбових з'єднань на фарфорових ізоляторах вводів, що призводить до сколу порцеляни.
6. Post-repair inspection checklist
| Testing / Inspection | Expected result | Actual Result | Status (Passed / Not Passed) |
|---|---|---|---|
| Insulation resistance of windings (HV) | $R_{60} \ge 1000$ MOhm at +20 °C | [ ] Passed | |
| Absorption coefficient ($K_{abs}$) | $\ge 1.3$ (according to COU-N EE 20.302:2007) | [ ] Passed | |
| DGA Oil Diagnosis | Fuel gas content within IEEE C57.104 limits | [ ] Passed | |
| Breakdown voltage of oil | At least 50 kV in the standard spark plug | [ ] Passed | |
| Contact connections of the inputs | Tightened to specification, without play | [ ] Passed |
7. Troubleshooting Guide
| Symptom | Likely cause | Correction action |
|---|---|---|
| Low insulation resistance of the windings ($R_{60} < 300$ MΩ) and high absorption coefficient ($K_{abs} \approx 1.0$) | Moisture in solid or liquid insulation of the transformer, presence of dirt on the bushing surface. | Dry the active part of the transformer with a stream of hot air or perform vacuum drying. Clean the terminals with a solvent. |
| High hydrogen ($H_2$) and acetylene ($C_2H_2$) content according to DGA | Presence of internal partial discharges or electric arc in an oil medium. | Reduce the transformer load, carry out an unscheduled inspection of the tap changer (TBC or RPN), and schedule a full laboratory test. |
| Continuous reduction of oil level in the expander without visible external leaks | Oil leakage through internal seals or cracks in the cooling radiators, damage to the gas relay oil circuit. | Perform overpressure purging of the tank with excess nitrogen (up to 0.3 bar), locate the leakage point, and replace the sealing gaskets. |
| Darkening and presence of sediment in the transformer oil sample | Thermal aging of oil, oxidation due to overheating of the active part of the windings. | Perform full oil regeneration or its replacement with new oil according to IEC 60296 with tank cleaning. |
8. Recommended Maintenance Schedule
| Task | Frequency | Estimated duration | Qualification Level |
|---|---|---|---|
| Visual inspection, oil level check and silica gel check | Monthly | 0.5 years | Shift Electrician (Group III) |
| Thermal imaging inspection of contact connections | Quarterly | 1 year | Diagnostic Engineer (Group IV) |
| Oil Sample Taking and DGA Analysis | Annually | 2 years | Test Specialist (Group IV) |
| Insulation resistance measurement and oil characteristic testing | Once every 2 years | 4 years | Test Laboratory Team |
| Capital repair (inspection of the active part, replacement of seals) | Every 8–12 years | 5–7 days | Specialized service team |
9. Spare Parts Catalogue
| Description of the part | Typical specification | Category UNITEC |
|---|---|---|
| Silica gel indicator | GOST 8984-75, granules 3-5 mm, blue | Transformer Consumables |
| Oil-resistant seal set | Material: oil and fuel resistant rubber NBR / FKM, resistant to -40 °C ... +100 °C | Sealing elements and gaskets |
| Insert high-voltage porcelain | VST-10/630, VST-35/1000 according to DIN / GOST | Power Inputs and Insulators |
| Gas relay | Type BUCHL, Buchholz relay 2-contact | Protective equipment and relays |
| Sampling valve | Thread G 1/2", brass ball valve with fitting | Valves and Piping |
For selection and ordering of original components, visit our electronic catalog: https://www.unitecd.com/e-catalog/.
10. Normative References
- DSTU EN 60076 (Power Transformers)
- IEC 60296 (Specification for Fresh Mineral Transformer Oils)
- IEEE C57.104 (Guide for Interpretation of Dissolved Gases in Transformer Oil)
- SOU-N EE 20.302:2007 (Testing Norms for Electrical Equipment)
- EN 50160 (Voltage Characteristics in General Purpose Electrical Networks)
