Connecting a transformer to the network – is a very important event that requires a series of preliminary actions, such as:
- Check expansion and contraction of transformer oil;
- Check the trigger device in the cooling system;
- Check the position of voltage switches;
- Check security system;
- Check arc suppression coil.
If a power transformer is after repair, it is also necessary to check the clean connections and absence of short-circuiting. Backup transformers can be incorporated into the work without prior inspection.
After connecting a transformer, it is beginning to show the load which is determined by the load on the busbars of electrical substation. If the transformer has a cooling class M or D, turning it on to nominal load is only possible if the temperature of insulating oil is not lower than -25 ° C. When the temperature of upper layers of oil is below the set value, it is necessary to increase it – by switching the transformer on idle mode or under load not exceeding 40-50% of nominal voltage. These conditions do not apply to emergency situations in which a transformer can be switched on directly on nominal load at any temperature. At such moments there is a large temperature difference between the oil and the windings. Due to the high viscosity of the insulating liquid – it does not damage the transformer, but it deteriorates the isolation.
The current transformer load is controlled by an ammeter. There is a special red mark on the scale – corresponding to the nominal load for each winding. Voltmeters are used to control voltage applied to transformer, and voltage of secondary windings.
Transformer voltage could be higher than nominal, but only within a small range:
- for a long time by 5% of nominal load;
- for a short time by 10% and not more than 25% of nominal load.
Thermal regime of power transformers is controlled by regular measurement of temperature in upper layers of oil with a special glass thermometer which is immersed in the thermometer pocket at the top of transformer. Also used are: distant reading thermometers and remote temperature indicators and digital signals transmitters of oil temperature.
Regular inspection of transformers, should be carried out according to the existing requirements:
- Once a day for main step-down substation transformers;
- Once a month for the rest of electrical transformers with permanent and non-permanent service personnel;
- Once a month at transformer points.
Reason for out of turn examination of power transformers are:
- Adverse weather conditions (temperature jumps, thunderstorms, high winds, etc.);
- Activated of gas protection;
- Transformer disconnected by gas or differential protection.
Basis for emergency switching off:
- Strong irregular noise and crackling inside transformer;
- Abnormal and increasing heat of transformer with a low load and normal operation of cooling device;
- Release of oil from the expander;
- Tear in exhaust pipe;
- Oil leak, lowering the level of oil in the tank below the mark.
Also transformers are taken out of operation, when laboratory tests show an immediate necessity in replacement of insulating oil.