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Power Transformer To Reduce Partial Discharge-ERG
- May 12, 2018 -

With the rapid development of the power grid and the increase of transmission voltage, the grid and power users have higher and higher requirements for the insulation reliability of large-scale 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 projects. Therefore, the partial discharge test on site has been widely used, and has been listed as 72.5kV. And above the voltage level transformer must do the transfer test project on site.

Partial discharge and its principle

Partial discharge, also known as electrostatic discharge, means the flow of static electricity. Under a certain applied voltage, the electrostatic charge firstly occurs in the weaker insulation location in the area where the electric field is strong, but no insulation breakdown is formed. This phenomenon of static charge flow is called partial discharge. The partial discharge occurring near the conductor surrounded by gas is called corona.

Partial discharge is the discharge that occurs in the local insulation of the transformer. Because the discharge is in a local position, the energy is low, and it does not directly constitute the penetration of the internal insulation.

For the partial discharge test of transformers, in the early stages of our country, we performed the transformers of 220kV and above. Later, the new IEC standard stipulated that when the maximum working voltage of the equipment Um ≥ 126kV, the transformer partial discharge measurement should be done. National standards have also made corresponding regulations. For transformers with the maximum working voltage Um≥72.5kV and rated capacity P≥10000kVA, if there is no other agreement, transformer partial discharge measurement should be carried out.

Partial discharge test method according to the provisions of GB1094.3-2003, the provisions of partial discharge standards should not exceed 500pC. However, users in actual contracts often require less than or equal to 300 pC or less than or equal to 100 pC. This technical agreement requires transformer manufacturers to have higher product technical standards.

Partial discharge hazards

The degree of damage of partial discharge is related to its cause, location and starting voltage and the level of extinguishing voltage. The higher the starting voltage and extinguishing voltage, the smaller the hazard, and vice versa; in terms of discharge properties, the discharge pair that affects solid insulation is affected. Transformers are the most harmful and will reduce the dielectric strength and even cause damage.

Partial discharge causes

The factors that cause partial discharge, apart from poor design consideration, are the most common ones caused by the manufacturing process: There are usually several main reasons:

1. The component structure has sharp corners and burrs that cause electric field distortion and decrease the discharge starting voltage;

2. Foreign matter and dust cause electric field concentration. Corona discharge or breakdown discharge under the action of an external electric field

3, there is moisture or bubbles. Because of the low dielectric constant of water and gas, discharge occurs first under the action of the electric field.

4. The poor contact of the metal structure suspending agent forms electric field concentration or 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 with a diameter of more than 1.5 μm may generate a discharge of more than 500 pC under the action of an electric field. Whether metallic or non-metallic dust is present, a concentrated electric field is generated, the initial discharge voltage of the insulation is reduced, and the breakdown voltage is reduced. Therefore, it is very important to maintain the clean environment and the body during the transformer manufacturing process. It is necessary to strictly implement dust control. According to the extent of the product may be affected by the dust in the manufacturing process to strictly control the establishment of a sealed dust-proof plant. For example, in the formation of flat conductors, wire wrapping paper, winding winding, winding sets, core stacking, insulation manufacturing, body assembly and body finishing, foreign material residues and dust ingress are absolutely not allowed. According to the extent of the product may be affected by the dust in the manufacturing process to strictly control the establishment of a sealed dust-proof plant. For example, in the formation of flat conductors, wire wrapping paper, winding winding, winding sets, core stacking, insulation manufacturing, body assembly and body finishing, foreign material residues and dust ingress are absolutely not allowed.

2. Concentrated processing of insulating parts

Insulators are very taboo with metal dust, because once the insulation is attached to metal dust, it is very difficult to completely clean. Therefore, it is necessary to centralize the processing in the insulation workshop and set up a machining area, which should be separated from other dust-producing areas.

3, strict control of silicon steel processing burr

Transformer core pieces are formed by slitting and shearing. These shearing cuts all have different degrees of burrs. Glitches can not only cause short-circuits between the plates, form internal circulation, increase no-load losses, but also increase the core thickness, which in fact reduces the number of laminations. More importantly, the yoke may cause the burr to fall on the device and cause electric discharge when the yoke is inserted into the core or it is shaken during operation. 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 discharge of the ground potential. Therefore, the core burrs should be as small as possible and as small as possible. The core burr of 110KV product should be no more than 0.03mm, and the burr of 220KV product core piece should be no more than 0.02mm.

4, the use of cold pressure terminals

The use of lead cold-press terminal forms is an effective measure to reduce the partial discharge. Because the use of phosphor bronze welding to produce a lot of splash slag, easily scattered in the body and insulation. In addition, the welding boundary zone needs to be separated by a soaked asbestos rope so that the water will enter the insulation. If the moisture is not completely removed after the insulation is wrapped, the partial discharge of the transformer will increase.

5, the rounding of the edge of the component

The purpose of rounding the edges of components is to: 1) Improve the distribution of the field strength and increase the starting voltage of the discharge. Therefore, metal structures in iron cores such as clamps, pull plates, feet and edges of brackets, edges of pressure plates and outlet openings, walls of raised casings, and magnetic shielding shields on the inside of the casing walls should be rounded. 2), to prevent the generation of iron shavings. If the clip's lifting hole and the lanyard or hook contact parts are rounded.

6, product assembly environment and body finishing

After the body is vacuum-dried, the body must be trimmed before packing. The larger the product, the more complex the structure, the longer the finishing time. Because the body is pressed and the fasteners are fastened, the body is exposed to the air, and moisture absorption and dust scattering occur during the process. Therefore, the body must be finished in the dust area, such as finishing time (or exposure to air). The time is more than 8 hours and it needs to be re-dried. After the finishing of the body, the fuel-saving phase of the fuel-saving tank is affixed. Because the body insulation during the body trimming process will absorb moisture, the body needs to be dehumidified. 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, water content standards to determine the degree of vacuum vacuum, and according to the furnace time, ambient temperature, humidity to determine the vacuum time.

7, vacuum injection

The purpose of vacuum oil injection is to vacuum the transformer, remove the dead angle in the insulation structure of the product, exhaust the air completely, and then inject the transformer oil under vacuum to fully saturate the body. The oil-filled transformer must be tested for at least 72 hours before it can be tested. 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 the oil immersion. The better the degree of soaking, the less likely it is to discharge, so there must be enough quiet time.

8. Sealing of fuel tanks and components

The quality of the seal structure is directly related to the leakage of the transformer. If there is a leak, moisture will inevitably enter the interior of the transformer, resulting in moisture absorption of the transformer oil and other insulating parts, which is one of the factors of partial discharge. Therefore, we must ensure that the sealing performance is reasonable.