One, What is partial discharge in transformer
Partial Discharge is referred to as PD (Partial Discharge), which refers to the insulation structure in the transformer, there may be some insulation weaknesses, it will occur under the action of a certain external voltage local repeated breakdown and extinction phenomenon, but does not immediately form the entire insulation penetration breakdown. This discharge limited to the local position of the insulation (weak point) is called partial discharge. Local discharge power is relatively weak, it is difficult to use the intuition of people to feel, need to use professional measuring equipment to measure.
Two, Causes of partial discharge in distribution transformer
There are many reasons for transformer partial discharge, involving many aspects, mainly including the following categories:
1. From the insulation material and the structure itself,
(1) defects such as bubbles, impurities and cracks will affect the insulation performance of the material, resulting in partial discharge.
(2) The dielectric coefficient of the bubble is usually lower than that of the insulator, so the electric field strength of the bubble is much higher than that of its adjacent insulating material, and it is easier to be broken down, resulting in discharge.
(3) The insulation structure design is unreasonable. When the insulation structure of dry type transformer is designed, the field strength between layers or turns is too high, such as unreasonable insulation structure design, etc., it may also lead to partial discharge.
(4) The insulators and metal bodies in the transformer often have some sharp corners and burrs, which will cause the electric field strength to be concentrated in a specific location, thereby causing partial discharge.
2. From the point of view of the operation process
(1) long-term operation and load changes will cause the insulation material inside the transformer to age and wear, lose the original insulation performance, and easily cause partial discharge.
(2) Overvoltage. Overvoltage in power system is one of the main causes of partial discharge in transformer. Overvoltage can be caused by external factors (such as lightning strikes) or internal factors (such as switching operation). Partial discharge occurs when the voltage exceeds the breakdown voltage of the material.
3. From the point of view of installation and process
(1) the production process is imperfect, such as the vacuum degree in the drying process is not well controlled, the drying time and temperature do not meet the requirements, resulting in residual gas in the coil.
(2) The transformer insulation may be damaged during installation, or the idle time is too long, resulting in excessive water content of the insulation material, and the overall moisture of the body, which may affect the volume of the office.
(3) The winding pouring process level is not in place, the assembly process level is not well assembled (such as the production of high and low pressure leads with burrs or improper distance) and other process problems may also affect the volume of the office.
Three, Hazards of partial discharge
Although the partial discharge of the transformer is small in scope, small in energy and short in time, it will also cause harm for a long time, which is mainly reflected in the destruction of insulation materials.
First, partial discharge directly bombards insulation, causing local insulation damage, and gradually develops may lead to insulation breakdown. The electrolysis produced by the discharge will produce ozone, nitric oxide and other active gases. Corrodes local insulation, gradually causing insulation damage.
Second, the heat generated by partial discharge will also increase the local temperature of the medium, and even carbonize.
Four, Detection method of partial discharge
The detection methods of partial discharge generally include electrical measurement, ultrasonic measurement, chemical measurement and so on.
Electrical measurement method refers to the use of oscilloscope, local discharge instrument or radio interference meter to find the waveform of discharge or the degree of radio interference.
Ultrasonic measurement method refers to: the use of ultrasonic waves in the detection of discharge, and the sound wave is converted into electrical signals for analysis.
Chemical measurement method refers to the detection of the content of various gases in dissolved oil and the law of change, usually called "chromatographic analysis".
Five, Preventive measures for partial discharge
In order to avoid the harm of partial discharge, some preventive measures need to be taken,
such as: (1) Select insulation materials with good insulation properties.
(2) Regularly check and maintain the insulation materials of the transformer, and timely replace the aging and severely worn parts.
(3) Install overvoltage protection devices to detect and suppress overvoltage in time.
(4) Install a temperature monitoring device to regularly detect changes in the surface temperature of the transformer.
