Maintenance of Transformers (A Detailed Checklist)

A power transformer is a critical asset in an electrical substation. A planned maintenance programme helps the owner manage safety, reliability and service life. The programme must follow the transformer’s manual, site procedures, operating history and applicable standards.

A power transformer maintenance programme combines routine inspection, condition monitoring, diagnostic testing and corrective work. Preventive tasks are scheduled. Condition-based tasks respond to trends, while corrective work addresses a confirmed defect. These are complementary parts of maintenance of transformer, not two exclusive categories.

Only qualified personnel should inspect or test a transformer. Any intrusive work requires the approved isolation, lockout, voltage-verification, grounding and discharge procedure. Online observations must stay within the site’s approach-distance and personal-protective-equipment rules.

Corrective maintenance is performed when an inspection, alarm, test result or failure identifies a need. Planned work can reduce avoidable failures, but it cannot eliminate every fault. A useful condition maintenance programme records baseline values and trends so that a change can be assessed in context.

No maintenance programme can guarantee zero breakdowns. The owner should combine condition data with transformer design, age, loading, environment, fault history and consequence of failure. Abnormal readings require engineering review rather than an automatic action based on one threshold.

The headings below give examples of daily, monthly and longer-interval tasks. They are not a universal schedule. The owner should set and document each interval from the manufacturer’s instructions, applicable standards, operating conditions and condition trends.

Monthly Basis Maintenance of Transformer

A monthly route can include the following external checks when they match the installed equipment and site maintenance plan.

1. Inspect the oil level in the cup below a conventional silica gel breather. Compare it with the mark and instructions for that breather. Use only the specified transformer oil if an authorised refill is required.
2. Inspect the breather body, inlet path, seals and desiccant indication. Do not dismantle or clean it while the work could expose personnel or the transformer to an unsafe pressure, contamination or moisture condition.
3. If the transformer has liquid-filled bushings with level indicators, record each indication and check for leakage, cracked insulation or a change from its normal temperature-related trend.

Do not top up a bushing solely because one reading appears low. Confirm the reading against temperature, previous records and the bushing manual. Investigate leakage and perform any filling only during an approved outage using the manufacturer’s procedure.

Daily Basis Maintenance and Checking

A daily operating check may cover these observable conditions:

1. Record the Magnetic Oil Gauge indication for the main tank or conservator tank, as fitted, and compare it with liquid temperature and its normal trend.
2. Record the breather’s desiccant or status indication using the installed product’s colour legend, alarm logic and service limits.
3. Look from a safe position for oil leakage, unusual noise or smell, abnormal temperature, damaged parts and active alarms.

An abnormal level or leak needs prompt assessment under the site’s defect procedure. Do not add oil until the cause, correct liquid, fill level and outage method are confirmed. Desiccant indicator colours vary by product, so a pink colour is not a universal replacement rule.

Yearly Basis Transformer Maintenance Schedule

1. Inspect and functionally test the automatic, remote and manual modes of the cooling system of transformer, including fitted pumps, fans, flow indicators and alarms. Use an approved test method that does not defeat required protection.
2. Inspect bushings for contamination, cracks, chips, leaks, abnormal level and heating. Clean only when condition requires it, with the transformer isolated and by the bushing manufacturer’s approved method.
3. For an oil-filled on-load tap changer, sample and test the separate compartment at the interval and for the properties specified by its manufacturer. Use clean sampling practice and interpret dielectric strength, moisture and other results with history, oil type and service duty before deciding on filtration or replacement.
4. Inspect fitted Buchholz relays for leakage, trapped gas indication, wiring condition and correct valve position. Test alarm and trip functions only with the approved manufacturer or site procedure.
5. Inspect marshalling boxes for water, contamination, corrosion, pests and failed door seals. Check lighting, space heaters and thermostats as fitted. Isolate control supplies before cleaning or tightening terminals, and use specified torque values.
6. Test relays, alarms, control switches and their circuits in the R&C panel and RTCC against approved schemes. Clean only where needed with a material compatible with the equipment; do not spray an unspecified cleaner onto energised devices.
7. Inspect the sensing pockets and capillaries for the OTI and WTI for leakage or damage. Verify indication and alarm or trip set points by the applicable test procedure; replenish pocket liquid only if the instrument design and manufacturer require it.
8. Check the pressure-relief device and Buchholz alarm or trip circuits with their test facilities and an approved protection-test procedure. Never bridge contacts with an improvised wire, because this can create an unintended trip, arc or bypassed protection condition.
9. Measure insulation resistance and, where applicable, polarisation index with a suitable battery-operated megger. Test voltage, connections, temperature correction and acceptance criteria depend on the transformer and test standard; 5 kV is not a universal setting.
10. Inspect transformer earth connections and risers for corrosion, damage and loose joints. Measure continuity or grounding performance with the method defined by the site’s grounding programme; a clamp meter is not suitable for every grounding arrangement.
11. Use DGA, or Dissolve Gas Analysis of transformer Oil, at a risk-based interval that establishes a baseline and supports trending. Voltage class alone does not set one universal frequency. An abnormal rate of change can require confirmation and shorter follow-up intervals.

The following tasks are sometimes placed on a two-year cycle, but the actual interval must follow the asset plan:

1. Check or calibrate OTI and WTI instruments, alarm contacts and trip contacts at the documented interval and compare results with the allowed tolerances.
2. Measure bushing capacitance and tan & delta; or power factor using the applicable test method. Compare corrected results with nameplate, baseline and previous values rather than applying a two-year rule to every bushing.

Maintenance of Transformer on Half Yearly Basis

A liquid-testing programme may include dielectric strength, water content, acidity, interfacial tension, dissipation factor, resistivity and other tests appropriate to the insulating liquid. Test selection and interval depend on liquid type, voltage class, condition history and the applicable standard; every property does not need a six-month test on every transformer.

A distribution transformer still requires inspection and maintenance. Its interval can be scaled to design, criticality, loading, environment and observed condition, but light average load does not make it maintenance-free.

Maintenance of Current Transformer

A Current Transformer, or CT, supplies scaled signals to metering and protection equipment in an electrical substation. A CT or its secondary circuit fault can impair current measurement and protection of the electrical power system. Never open-circuit a CT secondary while primary current can flow. The maintenance plan and test connections must follow the CT nameplate, drawings, manufacturer and applicable standard. The yearly items below are examples, not a universal interval.

1. Measure CT insulation resistance only after the equipment is isolated, proved de-energised, grounded and prepared for the test. Primary-to-ground, primary-to-secondary and secondary-circuit tests stress different insulation systems. Select each test voltage from the CT rating, manufacturer and test standard; 2.5 kV, 5 kV and 500 V are examples, not default values. Control secondary shorting and grounding links. Disconnect sensitive devices as required and discharge the tested insulation afterwards.
2. Use infrared thermography to compare primary terminals and the CT body under representative load. A qualified person must perform online scanning from a safe position and trend results against load, ambient conditions and comparable phases.
3. During an approved outage, inspect CT secondary boxes, junction boxes, wiring, shorting links, grounding and terminal condition. Clean where needed and tighten to the specified torque. Confirm the secondary circuit is complete and correctly grounded before return to service.

A condition-based external inspection can include these items at the site’s chosen interval:

1. Inspect porcelain or composite housings for cracks, chips, tracking, contamination, oil leakage and damaged grading parts. Escalate any structural crack or active leak to the manufacturer or responsible engineer.
2. Clean external insulation only when contamination requires it. Use an approved outage and the method and materials allowed for that housing. A cotton cloth is not a universal cleaning specification.

A monthly operating route may include the following non-intrusive checks where the site plan requires them.

1. Inspect an oil-filled CT from a safe position for leakage at joints, flanges and indicators. Record and assess a leak, then use an approved outage and repair procedure if corrective work is required.
2. Inspect the secondary terminal box externally for oil or water ingress, damaged seals and corrosion. Do not open or disturb an energised CT secondary circuit to investigate a leak.

Capacitance and tan delta or power-factor testing can help assess the insulation of suitable high-voltage CTs. Applicability, test connection and interval depend on CT design and condition. Compare results with nameplate, baseline, temperature-corrected history and similar units.

Dissolve Gas Analysis applies only to CT designs with a suitable oil volume and sampling provision. Use the manufacturer’s limits and trend confirmed laboratory results. An abnormal result calls for engineering assessment and follow-up; it does not by itself mean that the insulating oil must be replaced.

Maintenance of Voltage Transformer and Capacitor Voltage Transformer

An inductive voltage transformer and a capacitor voltage transformer, or CVT, are not the same construction. A CVT includes a capacitive divider, compensation reactor and intermediate transformer. Their common inspection items can overlap, but diagnostic tests and limits must match the installed design.

A voltage transformer or CVT still needs a documented inspection and test plan. Online visual checks and thermography may be possible without an outage. Intrusive work requires planned isolation because taking an instrument transformer out of service can affect protection, metering and carrier communication. A yearly interval is not sufficient for every asset or condition.

Yearly Maintenance of Voltage Transformer or Capacitor Voltage Transformer

1. Inspect porcelain or composite insulation for contamination, cracks, chips, tracking and leakage. Clean only when needed, during an approved outage, using the manufacturer’s method.
2. On a CVT fitted with a spark gap or ferroresonance-suppression circuit, inspect and test the assembly as its manual specifies. Do not remove parts or abrade electrodes with emery paper unless the manufacturer explicitly requires that procedure.
3. Inspect high-frequency grounding and carrier-coupling connections against the drawings, whether or not power-line carrier communication is in service. Do not alter a grounding link without an approved scheme and outage.
4. Use infrared thermography under representative load to compare capacitor-stack units, terminals and connections. A qualified person should assess any temperature difference against load, ambient conditions and similar phases.
5. During an approved outage, inspect the VT junction box, secondary terminals, fuses or links, grounding, seals and wiring. Clean where needed and tighten connections to specified torque before confirming the secondary circuits are correct.
6. Inspect gasket joints and pressure or oil indicators for leakage or damage. Confirm the leak source and follow the manufacturer’s repair procedure rather than replacing a gasket without assessing the sealed system.

Capacitance and tan delta or power-factor tests can support condition assessment of suitable VTs and CVTs. The interval and test connection depend on design and risk, not a universal three-year rule. Trend each corrected result against nameplate, baseline and previous values. A change in tan delta or capacitance can indicate insulation change, contamination, leakage or moisture, but the cause must be confirmed with the complete test record.