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Daily maintenance of transformer
Jan 29, 2018

                               Daily maintenance of transformer

First, allow the temperature

When the transformer is in operation, its coils and iron cores produce copper and iron losses. These losses become thermal energy, which increases the temperature of the core and the coil of the transformer. If the temperature exceeds the allowable value for a long time, the insulation will gradually lose its mechanical elasticity and cause the insulation to deteriorate.

The temperature of each part during the operation of the transformer is not the same, the temperature of the coil is the highest, followed by the temperature of the iron core, and the temperature of the insulating oil is lower than the temperature of the coil and the iron core. The upper oil temperature of the transformer is higher than the lower oil temperature. Permissible temperature in transformer operation is checked against the upper oil temperature. For Class A insulated transformers in normal operation, when the ambient air temperature is up to 400C, the transformer winding's limit operating temperature is 1050C. Since the winding temperature is 100C higher than the oil temperature, in order to prevent deterioration of the oil quality, the upper oil temperature of the transformer is not required to exceed 950C. Under normal conditions, in order to prevent over-speed oxidation of the insulating oil, the upper oil temperature should not exceed 850C. For transformers that use forced oil circulating water cooling and air cooling, the upper oil temperature should not exceed 750C.

Second, allow temperature rise

Monitoring only the upper oil temperature in the transformer operation does not guarantee the safe operation of the transformer. It must also monitor the temperature difference between the upper oil temperature and the cooling air—that is, temperature rise. The difference between the transformer temperature and the ambient air temperature is called the temperature rise of the transformer. For Class A insulated transformers, when the ambient maximum temperature is 400C, the national standard specifies that the temperature rise of the windings is 650C, and the allowable temperature rise of the upper oil temperature is 550C. As long as the temperature rise of the transformer does not exceed the specified value, the transformer can be guaranteed to operate safely within the specified operating year under rated load. (Transformer can operate continuously for 20 years with rated load during normal operation)

Third, reasonable capacity

In normal operation, the transformer should withstand a load of 75-90% of the transformer's rated capacity.

Fourth, the transformer unbalanced current maximum voltage shall not exceed 25% of the rated value; Transformer power supply voltage change allowed range is 5% of the rated voltage.

If more than this range should be used to adjust the tap switch, so that the voltage reaches the specified range. It is usually to change the position of the primary tap tap to achieve voltage regulation. The device that connects and switches the tap position is called the tap changer. It adjusts the ratio by changing the number of turns of the high voltage winding of the transformer. The low voltage has no effect on the transformer itself, but only reduces some output, but has an impact on the power equipment; the voltage increases, the magnetic flux increases, the iron core saturates, the core loss increases, and the transformer temperature increases.

V. Overload

The overload is divided into two cases: normal overload and accident overload. Normal overload is caused by the increase in electricity consumption of the user under normal power supply conditions. It will cause the temperature of the transformer to rise, resulting in accelerated aging of the transformer insulation and reduced service life. Therefore, under normal circumstances, no overload operation is allowed. In special circumstances, the transformer may operate under overload for a short period of time. However, the rated load may not exceed 30% in winter and 15% of the rated load in summer. In addition, the transformer's overload capacity should be determined based on the transformer's temperature rise and manufacturer's regulations.

When an accident occurs in the power system or the user substation, in order to ensure continuous power supply to important equipment, the transformer is allowed to run for a short period of time, that is, the accident is overloaded. When the accident is overloaded, the coil temperature will exceed the permissible value. Therefore, for the insulation, Aging faster than normal conditions. However, there is little chance of accident overload. Under normal circumstances, the transformer is under-loaded, so short-term overload causes damage to the insulation of the transformer. The time and multiple of the accident overload should be implemented according to the manufacturer's regulations.

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