Operation of power transformers involves such processes as external inspection, measuring oil temperature and load, phasing, oil testing, corrective and preventive maintenance, overhauls etc.
Inspection is performed once every six months. It includes checking the level of oil, checking for leaks and inspecting insulators.
Each transformer should be equipped with a thermometer measuring the temperature of the insulation liquid. This parameter should also be recorded into a special logbook.
If the transformer operates long enough, the temperature of the upper oil layers must not exceed 95ºC. At the same time the difference between the temperature of of the oil and the ambient temperature should not be more than 60ºC.
If the transformer is equipped with a thermosiphon filter, adsorbent color should be noted. It is most usually silica gel. Blue color means good adsorption ability, pink means that the sorbent is near exhausted.
Load is measured twice a year. First measurement is taken in May or June when power consumption is minimal, the second measurement is taken in December, at maximum loads.
Before engaging the transformer, the dielectric strength of the transformer oil and its chemical composition must be checked. Also, resistance of the winding to direct current and winding insulation resistance should be checked, breakers and relay protection is tested; the possibility for parallel operation is checked, along with phasing, secondary connection circuits etc.
Parallel operation of power transformers is possible when the following conditions are met:
- identical phase displacement groups;
- equal transformer ratios;
- equal impedance voltage;
- power ratio below 3:1.
Preventive maintenance is performed at least once every four years, including:
- external inspection and repair of possible problems;
- cleaning of insulation and the tank;
- removal of dirt from expansion tank and its washing;
- adding oil;
- checking of oil level indicator;
- changing of silica gel in the thermosyphon filter, if necessary;
- checking of drain valve and seals;
- checking grounding;
- taking oil samples;
- measuring insulation resistance and testing of insulation by increased voltage.
Overhaul involves core extraction and oil change. It is performed if necessary. The decision is made based on the results of inspection and tests. Overhaul is inly performed in special repair shops.
The most usual power transformer damage is short circuit of windings, insulation breakdown, mechanical damage to insulation in case of through fault and transformer oil leaks.
Every transformer should have a logbook with factory specifications, primary operation parameters and test results. Repair and servicing dates are also recorded.