Some measures to reduce partial discharge in power transformer
Some measures to reduce partial discharge in power transformer
With the rapid development of the power grid and the increase of the transmission voltage, the grid and power users have higher and higher requirements for the insulation reliability of large Power Transformers. Because the partial discharge test has no destructive effect on the insulation and is very sensitive, it can effectively find the inherent defects in the transformer insulation or the safety and safety defects in the transportation and installation engineering. Therefore, the field partial discharge test has been widely used, which has been listed as 72.5kV. And the above-mentioned voltage level transformer must be done at the site of the handover test project.
Partial discharge and its principle
Partial discharge, also known as electrostatic discharge, means the flow of static charge. Under a certain applied voltage, in the region where the electric field is strong, the electrostatic charge firstly dissipates static electricity at a position where the insulation is weak, but does not form insulation breakdown. This phenomenon of static charge flow is called partial discharge. A partial discharge occurring near a conductor surrounded by a gas is called a corona.
Partial discharge is a discharge that occurs at a local insulation position inside the transformer. Because the discharge is in a local position, the energy is low and does not directly constitute a penetrating breakdown of the internal insulation.
For the partial discharge test of the transformer, China implemented the transformer of 220kV class and above in the initial stage. Later, the new IEC standard stipulated that when the maximum working voltage of the device is Um≥126kV, the partial discharge measurement of the transformer is to be performed. The national standard has also made corresponding regulations. For transformers with the highest working voltage Um≥72.5kV and rated capacity P≥10000kVA, if there is no other agreement, the partial discharge measurement of the transformer should be carried out.
The partial discharge test method shall be carried out in accordance with the provisions of GB1094.3-2003, and the partial discharge quantity standard shall be no more than 500pC. However, users in the actual contract often require less than or equal to 300pC or less than or equal to 100pC. This technical agreement requires transformer manufacturers to have higher product technical standards.
Partial discharge hazard
The degree of damage of partial discharge is related to the cause, location and starting voltage and extinguishing voltage. The higher the initial voltage and the extinguishing voltage, the smaller the harm, and vice versa. In terms of the discharge property, the discharge pair affecting the solid insulation Transformers have the greatest hazard, which can reduce the insulation strength and even cause damage.
Causes of partial discharge
In addition to the design considerations that are not well-considered, the most common factors are caused by the manufacturing process: there are usually several main reasons:
1. The structure of the parts has sharp corners and burrs, causing electric field distortion and lowering of the initial voltage of discharge;
2. There are foreign objects and dust, which cause electric field concentration. Corona discharge or breakdown discharge occurs under the action of an external electric field
3. There are moisture or bubbles. Due to the low dielectric constant of water and gas, the discharge occurs first under the action of an electric field;
4. Poor contact of the metal structural member suspension agent forms an electric field concentration or a spark discharge.
Measures to reduce partial discharge
1, dust control
Among the factors that cause partial discharge, foreign matter and dust are very important incentives. The test results show that metal particles above φ1.5μm may generate discharges much larger than 500pC under the action of electric field. Whether metal or non-metallic dust is present, a concentrated electric field is generated, which lowers the initial discharge voltage of the insulation and lowers the breakdown voltage. Therefore, it is very important to maintain a clean environment and body during the transformer manufacturing process, and dust control must be strictly implemented. Strictly control the degree to which the product may be affected by dust during the manufacturing process, and establish a sealed dust-proof workshop. For example, in the case of flat wires, wire wraps, winding windings, winding sets, core stacking, insulation manufacturing, body assembly, and body finishing, foreign matter residue and dust entry are absolutely not allowed. Strictly control the degree to which the product may be affected by dust during the manufacturing process, and establish a sealed dust-proof workshop. For example, in the case of flat wires, wire wraps, winding windings, winding sets, core stacking, insulation manufacturing, body assembly, and body finishing, foreign matter residue and dust entry are absolutely not allowed.
2, the insulation processing
Insulation is very difficult to carry with metal dust, because once the metal dust adheres to the insulation, it is very difficult to completely remove it. Therefore, it is necessary to centrally process in the insulation workshop and set up a machining area, which should be isolated from other areas where dust is produced.
3. Strictly control the processing burrs of silicon steel sheets
The transformer core piece is formed by slitting and shearing shearing, and these shearing cuts have different degrees of burrs. The burr not only causes short circuit between the sheets, but also forms an internal circulation, which increases the no-load loss, and also increases the thickness of the core, actually reducing the number of laminations. More importantly: when the iron core is inserted into the yoke or subjected to vibration during operation, the burr may fall on the body and discharge. Even if the burr falls on the bottom of the box, it may be arranged in an orderly manner under the action of the electric field, causing the ground potential to discharge. Therefore, the core piece burr should be as small as possible and as small as possible. The burr of the core piece of 110KV product should be no more than 0.03mm, and the burr of the core piece of 220KV product should be no more than 0.02mm.
4, the lead is cold pressed terminal
The use of a lead cold-press terminal form is an effective measure to reduce the amount of partial discharge. Because of the use of phosphor bronze welding, a lot of splash slag is generated, which is easy to be scattered in the body and the insulation. In addition, the welding boundary zone needs to be separated by soaked asbestos rope so that water enters the insulation. If the moisture is not completely removed after the insulation is wrapped, the partial discharge of the transformer is increased.
5, the rounding of the edge of the parts
The purpose of rounding the edge of the component is to: 1) improve the distribution of the field strength and increase the initial voltage of the discharge. Therefore, the metal structural members in the core such as the clips, the pull plates, the legs and the edge of the bracket, the edge of the pressure plate and the outlet, the wall of the riser of the casing, and the magnetic shield of the inside of the tank should be rounded. 2) Prevent friction from generating iron filings. If the lifting hole of the clip is in contact with the lanyard or the hook, it needs to be rounded.
6. Product environment and body finishing during total assembly
After the body is vacuum dried, the body should be finished before packing. The larger the product, the more complex the structure, the longer the finishing time. Since the body is tightly pressed and the fasteners are tightened, the body is exposed to the air, and moisture absorption and dust scattering occur during the process. Therefore, the body should be arranged in the dust-proof area, such as finishing time (or exposed to the air). The time) is more than 8 hours and needs to be re-dried. After the body is finished, the fuel tank is vacuumed and oiled. Because the insulation of the body during the body finishing stage will absorb moisture, it is necessary to dehumidify the body. This is an important measure to ensure the insulation strength of high-voltage products. The method adopted is to vacuum the product. According to the body and the ambient humidity and water content standards, the vacuum degree of vacuuming is determined, and the vacuuming time is determined according to the furnace time, ambient temperature and humidity.
7, vacuum oiling
The purpose of vacuum oiling is to completely remove the dead air by vacuuming the transformer, remove the dead space in the insulation structure of the product, and then inject the transformer oil under vacuum to completely soak the body. The oil-filled transformer can be tested at least for 72 hours. This is 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 time of oil immersion. The better the degree of soaking, the less likely it is to discharge, so there must be enough rest time.
8, the seal of the fuel tank and parts
The quality of the sealing structure is directly related to the leakage of the transformer. If a leak occurs, moisture will inevitably enter the inside of the transformer, which will cause moisture absorption of the transformer oil and other insulating parts, which is one of the factors of partial discharge. Therefore, it is necessary to ensure the sealing performance is reasonable.