What is Electrical Partial Discharge?
Electrical Partial Discharge ; They are electrical discharges or sparks that occur due to the gaps in the structure of the insulating material or the problems in its continuity between two conductive electrodes, and the inability to form a full bridge. The amount of partial discharge is very weak and small and cannot be detected by the senses such as the eye and only a very sensitive partial discharge measuring instrument can detect it. Although the partial discharge time is short and the energy is low, it is very harmful. Its long-term existence causes great damage to the insulating material. First, the insulation material adjacent to the partial discharge will be directly bombarded by the discharge effect. Second, the heat generated by the discharge is the chemical action of active gases such as ozone and nitrogen oxides, which cause corrosion and aging of the local insulation, increasing conductivity and eventually leading to thermal degradation. The aging and damage of the internal insulation in the operating transformer mostly starts from the partial discharge.
What is the Importance of Electrical Partial Discharge Control?
According to the research done by IEEE; A large proportion (80%) of the faults that occur in medium voltage and high voltage systems are due to electrical partial discharges. It is generally seen as pulses of less than 1 microsecond duration. Although the pulses are of very short duration, the energy released during the pulse causes the insulating material surrounding the conductor to deteriorate. If left unchecked, it may result in insulation faults.
Partial discharge may occur due to aging-induced deterioration, thermal or excessive electrical stresses, faulty installations, faulty workmanship or faulty designs, even under normal operating conditions in devices and materials that operate with or transmit high voltages. As a result of its progress and growth in the insulating material, it may weaken the insulation sufficiently and result in a short circuit between phases or phase-earth in 3-phase systems.
How Do We Detect Transformer Partial Discharge?
- Electrical Measurement Method: Use an oscilloscope or radio interference meter to find the characteristic waveform of the discharge or the level of radio interference.
- Ultrasonic Test: The sound waves emerging in the discharge are detected and the sound waves are converted into electrical signals, recorded on a tape for analysis, and the distance between the detection point and the discharge point, and the distance between the detection point and the discharge point can be obtained by using the transmission time difference. electrical signal and audio signal.
- Chemical Test: Detect the content of various dissolved gases in the oil and the law of increase and decrease. This test finds changes in oil composition, proportions and amounts to determine the presence or absence of partial discharge (or local overheating).
Measures to reduce partial discharge of transformers
1. Dust Control
Among the factors that cause partial discharge, foreign matter and dust are very important factors. The test results show that metal particles with a diameter of ф1.5μm can produce discharges much larger than 500pC under the action of an electric field. Whether metal or non-metal powder, a concentrated electric field will be generated which will reduce the initial discharge voltage of the insulation and reduce the breakdown voltage. For this reason, it is very important to keep the environment and body clean in the transformer manufacturing process and dust control must be strictly applied. Strictly control to what extent the products can be affected by dust in the production process, and it is necessary to establish a sealed and dustproof workshop.
2. Central Processing of Insulation Parts
It is very important that it contains metal dust, because once the pieces of insulation adhere to the metal dust, it is very difficult to remove it completely. Therefore, it is necessary to centralize the processing in the isolation workshop and set up an area that must be isolated from other dust-generating areas.
3. Strictly Control the Machining Burrs of Silicon Steel Plates!
Transformer core parts are formed by longitudinal cutting and cross cutting. These cutting cuts have varying degrees of burrs. Burr not only causes a short circuit between the chips, it increases the loss, it also increases the thickness of the iron core. More importantly: when the iron core is inserted into the yoke or subjected to vibration during operation, burrs may fall on the body and discharge may occur. Even if the burr falls to the bottom of the box, it can be arranged in an orderly manner under the influence of the electric field and cause a ground potential discharge. Therefore, the core burr should be as small as possible. The burr of the core part of the 110KV product should not be larger than 0.03mm.
4. Lead, Cold Pressed Terminal
The use of lead cold press terminals is an effective measure to reduce the amount of partial discharge. When using phosphor copper welding, a lot of spatter welding slag is formed, so it is easy to disperse into the body and insulator parts. In addition, the source boundary area must be separated by a submerged asbestos thread so that water can enter the insulation. If moisture is not completely removed after the insulation winding, it will increase the partial discharge of the transformer.
5. Rounding Component Edges
The purpose of rounding the edges of the parts:
To improve the distribution of field power and increase the starting voltage of the discharge. Therefore, metal structural parts such as clips in the iron core, pull plates, pads and bracket edges, pressure plates and wire exit edges, the wall of the case riser and the magnetic shielding shield on the inside of the box wall, must be rounded.
To prevent friction from producing iron shavings. For example, the contact part between the lifting hole of the clip and the lanyard or hook should be rounded.
6. Product Environment and Body Coating During Final Assembly
After the hull is vacuum dried, the hull should be sorted before packaging. The larger the product and the more complex the structure, the longer the finishing time will be. As the stem is compressed and the fasteners tightened, the stem is exposed to air during which time moisture absorption and dust scattering occur. Therefore, the body should be cleaned in a dust-proof area such as finishing time (or exposure to air). time) more than 8 hours, it must be dried again. After the hull is finished, the vacuuming and fuel tank filling phase is started. Since the stem insulation will absorb moisture during the stem finishing phase, it is necessary to dehumidify the stem. This is an important measure to ensure the insulation strength of high-voltage products. The method to be used is to vacuum the product. Determine the vacuuming time according to body, ambient humidity and water content standards, vacuum degree, leaving time, ambient temperature and humidity.
7. Vacuum Lubrication
The purpose of vacuum oil injection is to inject transformer oil in vacuum to remove the dead angle in the insulation structure of the product by evacuating the transformer, completely evacuate the air, and then completely absorb the transformer body. After oil injection, the transformer can be tested after waiting at least 72 hours, because the degree of penetration of the insulating material is related to the thickness of the insulating material, the temperature of the insulating oil, and the oil immersion time. The better the degree of penetration, the less likely it is to ejaculate, so make sure that sufficient absorption time has passed.
8. Sealing of Fuel Tank and Parts
The quality of the sealing structure is directly related to the leakage of the transformer. If there is a leakage point, moisture will inevitably enter the transformer and cause moisture absorption of transformer oil and other insulating parts, which is one of the partial discharge factors. Therefore, it must meet reasonable sealing values.