Symptoms: Oil or winding temperature alarms come up, the cooling fans run all the time and the tank is hotter than usual.
Load above the transformer rating
Measure the load current on each phase over a full shift and move or shed load, or plan a larger unit if the plant has grown.
Radiators or cooling fans blocked or failed
Clean dust and debris from the radiator fins and check that every cooling fan starts at its set temperature.
Transformer room poorly ventilated
Check the room vents and extract fans, and keep stored material away from the airflow paths.
Harmonic currents from drives and rectifiers
Have the harmonic content measured, because harmonics heat the windings more than the load current suggests, and fit filters if needed.
Prevention: Load and temperature readings trended monthly, and a thermal survey once a year.
Louder humming or buzzing
Hits OEE: Availability
Symptoms: The transformer hum is louder than it used to be, or a rattle or vibration can be heard and felt on the tank.
Loose core clamping or mounting bolts
Have the clamping and mounting checked and tightened by authorised electrical staff during a planned outage.
Supply voltage high or tap position wrong
Measure the secondary voltage and check the tap setting against the supply voltage.
Covers, panels or fittings resonating
Find the loose cover or panel and tighten or pad it.
Harmonic or direct current content in the load
Have the load measured for harmonics and direct current offset, which raise core noise.
Prevention: Noise and vibration noted on every inspection round so a change is noticed early.
Oil leaking from the tank
Hits OEE: Availability
Symptoms: Oil stains or drips appear around gaskets, valves, radiators or the drain plug, and the oil level gauge falls.
Gaskets hardened or shrunk with age
Replace the leaking gaskets during a planned outage, with the transformer isolated and drained to the level the work needs.
Valves or bolted flanges loose
Tighten the flange bolts evenly and check that drain and sample valves are fully closed and capped.
Cracked weld on a radiator or the tank
Have the crack repaired by a specialist, and keep the oil level topped up with the correct oil until then.
Pressure rise from overheating
Find and fix the cause of the overheating, since hot oil expands and pushes oil past weak seals.
Prevention: Oil level and the area under the transformer checked on every round; bunding kept clean so leaks are visible.
Gas relay or pressure relief alarm
Hits OEE: Availability
Symptoms: The gas relay alarms or trips, or the pressure relief device operates.
Internal fault producing gas
Do not re-energise until the gas and oil have been sampled and tested by a specialist. Only authorised electrical staff work on it, after isolation and earthing under your site electrical safety rules.
Air trapped after maintenance or an oil top-up
Have the relay vented and the gas tested; air alone points to trapped air, not a fault.
Low oil level from a leak
Find the leak and restore the oil level before the unit is returned to service.
Relay wiring or float fault
Test the relay and its wiring once a real fault has been ruled out.
Prevention: Gas relay and pressure relief device tested on the protection maintenance schedule.
Oil test results getting worse
Hits OEE: Availability
Symptoms: The dissolved gas analysis or oil quality test shows rising gases, moisture or acidity compared with earlier samples.
Overheated connections or core hot spots
Increase the sampling frequency and have a specialist interpret the gas pattern before the next outage.
Partial discharge or arcing inside the tank
Plan an internal inspection or electrical tests with a specialist, and reduce load if advised.
Moisture entering through the breather or seals
Check the breather and seals, and have the oil dried or treated if moisture keeps rising.
Insulation paper ageing
Plan oil treatment and, over time, a replacement budget based on the trend.
Prevention: Oil sampled on a fixed interval and every result compared with the previous ones.
Breather silica gel changed colour
Hits OEE: Availability
Symptoms: The silica gel in the dehydrating breather has changed colour along most of its length.
Gel not replaced on schedule
Replace or regenerate the silica gel and add it to the inspection round.
Humid site or wash-down area nearby
Shorten the replacement interval, or fit a maintenance-free breather where the site allows it.
Oil cup at the bottom of the breather empty
Top up the oil cup to the mark so incoming air is filtered and sealed.
Leaking seals drawing in moist air
Find and fix leaking seals so air only enters through the breather.
Prevention: Breather colour checked on every inspection round.
Secondary voltage too high or too low
Hits OEE: Quality
Symptoms: Equipment downstream trips on over or under voltage, motors run hot or lights flicker when large loads start.
Tap position not suited to the supply
Measure the voltage over a day and have the off-load tap changed with the transformer isolated.
Supply voltage varying
Log the incoming voltage and discuss persistent variation with the utility.
Voltage drop on long or undersized cables
Measure at the load as well as at the transformer, and resize cables if the drop is in the cabling.
Loose connection on a phase
Check the terminations with thermography under load and tighten them during an outage.
Prevention: Voltage logged at the main switchboard after any change in the plant load.
Tripping when switched on
Hits OEE: Availability
Symptoms: The upstream protection trips when the transformer is energised, sometimes only on some attempts.
Protection settings not allowing for inrush current
Have the relay settings reviewed by a protection engineer against the transformer inrush.
Internal fault
Do not keep re-trying; have insulation and winding tests done before the next attempt.
Settings changed during a recent modification
Compare the relay settings with the last approved settings record.
Cable or termination fault on the feeder
Test the feeder cable insulation before re-energising.
Prevention: Protection settings records kept up to date after every change.
Hot bushing or cable connection
Hits OEE: Availability
Symptoms: A thermal survey shows one bushing, cable lug or link much hotter than the others.
Bolted joint loose
Retighten or remake the joint during a planned outage under a permit to work.
Corroded or oxidised contact faces
Clean the contact faces and use the jointing compound the maker specifies.
Lug or cable undersized for the load
Check the cable and lug ratings against the measured load and replace if undersized.
Dirt or moisture on the bushing surface
Clean the bushings and look for tracking marks, which need a specialist check.
Prevention: Thermal survey of all connections under load once a year.
Guidance only. Always follow lockout/tagout and your site's safe-work procedures, and verify against OEM manuals before acting.
Recurring transformer stops usually trace to a cause you cannot see by hand. The partner we recommend is Fabrico: it tracks OEE and downtime live from PLCs, IoT sensors or AI cameras; where cameras are fitted, the video of a stop sits next to its machine data so the cause can be reviewed; and a technician opens the work order from the stop in the same system. Data is hosted in the AWS EU region, and Fabrico and the company behind it hold ISO/IEC 27001, ISO/IEC 20000-1 and ISO 9001.