Transformer is a device that converts AC voltage, current and impedance. The working principle of transformer is that it is a static electrical appliance made by the principle of electromagnetic induction. There are many classifications of it. According to its non-common functions, it can be divided into pulse power transformer, intermediate frequency transformer, trigger transformer, shielding transformer, etc.
Transformer is an electrical appliance that transfers electric energy or transmits signals from one circuit to another circuit by using the principle of electromagnetic induction. It is an important component for electric energy transmission or signal transmission. Transformer is a device that converts AC voltage, current and impedance. When AC current flows through the primary coil, AC magnetic flux is generated in the iron core (or magnetic core), so that voltage (or current) is induced in the secondary coil. Transformer consists of iron core (or magnetic core) and coil. The coil has two or more windings, of which the winding connected to the power supply is called primary coil, and the remaining windings are called secondary coils.
1. Allowable temperature
When the transformer is running, its coil and iron core generate copper loss and iron loss. These losses are converted into heat energy, which causes the temperature of the transformer's iron core and coil to rise. If the temperature exceeds the allowable value for a long time, the insulation will gradually lose its mechanical elasticity and age.
The temperature of each part of the transformer is different during operation. The temperature of the coil is the highest, followed by the temperature of the core. The temperature of the insulating oil is lower than that of the coil and the core. The upper oil temperature of the transformer is higher than the lower oil temperature. The allowable temperature during transformer operation is checked according to the upper oil temperature. For a Class A insulation transformer in normal operation, when the ambient air temperature is up to 40°C, the limit operating temperature of the transformer winding is 105°C. Since the temperature of the winding is 10°C higher than the oil temperature, in order to prevent oil quality deterioration, the upper oil temperature of the transformer is stipulated not to exceed 95°C. Under normal circumstances, in order to prevent the insulating oil from over-oxidizing, the upper oil temperature should not exceed 85°C. For transformers that use forced oil circulation water cooling and air cooling, the upper oil temperature should not often exceed 75°C. (The maximum allowable value of the upper oil temperature of this transformer is 80°C)
II. Allowable temperature rise
Only monitoring the upper oil temperature during transformer operation cannot guarantee the safe operation of the transformer. It is also necessary to monitor the temperature difference between the upper oil temperature and the cooling air - that is, the temperature rise. The difference between the transformer temperature and the ambient air temperature is called the transformer temperature rise. For Class A insulation transformers, when the ambient maximum temperature is 40°C, the national standard stipulates that the winding temperature rise is 65°C and the upper oil temperature is allowed to rise to 55°C. As long as the transformer temperature rise does not exceed the specified value, the transformer can be guaranteed to operate safely within the specified operating life under rated load. (The transformer can operate continuously for 20 years with rated load during normal operation)
III. Reasonable capacity
In normal operation, the power load borne by the transformer should be around 75-90% of the rated capacity of the transformer.
IV. Reasonable current range
The maximum unbalanced current of the transformer low voltage shall not exceed 25% of the rated value; the allowable range of transformer power supply voltage change is plus or minus 5% of the rated voltage.
If it exceeds this range, the tap switch should be used to adjust the voltage to the specified range. (Adjustment should be carried out during power outage) Voltage regulation is usually achieved by changing the position of the tap of the primary winding. The device that connects and switches the tap position is called a tap changer, which adjusts the transformation ratio by changing the number of turns of the high-voltage winding of the transformer. Low voltage has no effect on the transformer itself, only reduces some output, but has an impact on electrical equipment; voltage increases, magnetic flux increases, iron core saturation, iron core loss increases, and transformer temperature increases.
V. Overload
Overload is divided into normal overload and accident overload. Normal overload is caused by increased user power consumption under normal power supply conditions. It will increase the temperature of the transformer, causing the transformer insulation to age faster and reduce its service life. Therefore, overload operation is generally not allowed. In special cases, the transformer can be overloaded for a short period of time, but it must not exceed 30% of the rated load in winter and 15% of the rated load in summer. In addition, the overload capacity of the transformer should be determined based on the temperature rise of the transformer and the manufacturer's regulations.
When an accident occurs in the power system or user substation, in order to ensure continuous power supply to important equipment, the transformer is allowed to operate under overload for a short period of time, that is, accident overload. Accident overload will cause the coil temperature to exceed the allowable value, so the insulation will age faster than under normal conditions. However, the chance of accident overload is small, and in general, the transformer is underloaded, so a short overload will damage the insulation of the transformer. The time and multiple of accident overload should be implemented according to the manufacturer's regulations.