Some measures to reduce partial discharge in power transformers

With the rapid development of power grids and the increase in transmission voltage, power grids and power users have higher and higher requirements for the insulation reliability of large power transformers. Since the partial discharge test has no destructive effect on insulation and is very sensitive, it can effectively find the inherent defects in the insulation of the transformer or the defects that endanger safety during transportation and installation. Therefore, the on-site partial discharge test has been widely used and has been listed as a must-do handover test item for transformers with voltage levels of 72.5kV and above.

Partial discharge and its principle

Partial discharge is also called electrostatic ionization, which means the flow of static charge. Under the action of a certain external voltage, static charge in the area with a strong electric field first undergoes electrostatic ionization at the location where the insulation is weak, but does not form insulation breakdown. This phenomenon of static charge flow is called partial discharge. The partial discharge that occurs near the conductor surrounded by gas is called corona.

Partial discharge is the discharge that occurs at a local location of the insulation inside the transformer. Because the discharge is at a local location, the energy is low and it does not directly constitute a penetrating breakdown of the internal insulation.

For the partial discharge test of transformers, China implemented it on transformers of 220kV and above in the initial stage. Later, the new IEC standard stipulated that when the maximum working voltage of the equipment Um≥126kV, the transformer partial discharge measurement should be carried out. The national standard also made corresponding provisions. For transformers with the maximum working voltage Um≥72.5kV and rated capacity P≥10000kVA, if there is no other agreement, the transformer partial discharge measurement should be carried out.

The partial discharge test method shall be implemented in accordance with the provisions of GB1094.3-2003, and the partial discharge quantity standard stipulates that it should not exceed 500pC. However, in actual contracts, users 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.

The harm of partial discharge

The degree of harm of partial discharge is related to its cause, location, starting voltage and extinction voltage. The higher the starting voltage and extinction voltage, the less harm, and vice versa; in terms of discharge properties, the discharge that affects solid insulation is the most harmful to the transformer, which will reduce the insulation strength and even cause damage.

Causes of partial discharge

In addition to the lack of careful design considerations, the most common factors that cause partial discharge are caused by the manufacturing process: There are usually the following main reasons:

1. The parts have sharp corners and burrs, which cause electric field distortion and reduce the discharge starting voltage;

2. There are foreign matter and dust, which cause electric field concentration. Corona discharge or breakdown discharge occurs under the action of the external electric field


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

4. The poor contact of the suspension of metal structural parts 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 inducements. Test results show that metal particles larger than ф1.5μm may produce a discharge amount far greater than 500pC under the action of the electric field. Whether it is metallic or non-metallic dust, it will produce a concentrated electric field, which will reduce the insulation starting discharge voltage and breakdown voltage. Therefore, in the process of transformer manufacturing, it is very important to maintain a clean environment and body, 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, when flattening the wire, wrapping the wire, winding, winding set, core stacking, insulating parts manufacturing, body assembly and body finishing, foreign matter residue and dust are absolutely not allowed to enter. 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, when flattening the wire, wrapping the wire, winding, winding set, core stacking, insulating parts manufacturing, body assembly and body finishing, foreign matter residue and dust are absolutely not allowed to enter.

2. Centralized processing of insulating parts

Insulating parts are very taboo with metal dust, because once the insulating parts are attached with metal dust, it is very difficult to completely remove it. Therefore, it is necessary to centrally process in the insulation workshop and set up a mechanical processing area, which should be isolated from other dust production areas.

3. Strictly control the processing burrs of silicon steel sheets.

The transformer core sheets are formed by longitudinal shearing and transverse shearing. These shearing cuts have different degrees of burrs. Burrs can not only cause short circuits between sheets, form internal circulation, increase no-load losses, but also increase the thickness of the core, which actually reduces the number of stacked sheets. More importantly, when the core is inserted into the yoke or vibrated during operation, the burrs may fall on the body of the device and discharge. Even if the burrs fall on the bottom of the box, they may be arranged in order under the action of the electric field, causing ground potential discharge. Therefore, the burrs of the core sheets should be as few as possible and as small as possible. The burrs of the core sheets of 110KV products should not be greater than 0.03mm, and the burrs of the core sheets of 220KV products should not be greater than 0.02mm.

4. The use of cold-pressed terminals for leads

is an effective measure to reduce the amount of partial discharge. Because phosphor copper welding produces a lot of spattering slag, which is easy to scatter in the body and insulating parts. In addition, the welding boundary area needs to be separated by soaked asbestos rope, so that water will enter the insulation. If the moisture is not completely removed after the insulation wrapping, the partial discharge of the transformer will increase.


5. Rounding of the edges of parts

The purpose of rounding the edges of parts is: 1) to improve the distribution of field strength and increase the starting voltage of discharge. Therefore, the metal structural parts in the iron core, such as clamps, pull plates, pads and bracket edges, pressure plates and outlet edges, the walls of the bushing riser, and the magnetic shielding guard plates on the inner side of the box wall, should be rounded. 2) Prevent friction from generating iron filings. For example, the contact parts of the lifting holes of the clamps and the hanging ropes or hooks need to be rounded.

6. Product environment and body arrangement during general assembly

After the body is vacuum dried, the body should be arranged before packing. The larger the product and the more complex the structure, the longer the arrangement time. Since the body compression and fastening of fasteners are carried out when the body is exposed to the air, moisture absorption and dust scattering will occur during the process. Therefore, the body finishing should be carried out in a dustproof area. If the finishing time (or exposure to the air time) exceeds 8 hours, it needs to be dried again. After the body finishing is completed, the oil-saving box is buckled and the vacuum oil filling stage is carried out. Because the body insulation will absorb moisture during the body finishing stage, 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. The vacuum degree of vacuuming is determined according to the body and environmental humidity and water content standards, and the vacuuming time is determined according to the furnace release time, ambient temperature and humidity.

7. Vacuum oil

filling The purpose of vacuum oil filling is to vacuum the transformer, remove the dead corners in the product insulation structure, completely exhaust the air, and then inject transformer oil under vacuum to make the body completely soaked. The transformer after oil filling must be left for at least 72 hours before testing, 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 penetration, the less likely it is to discharge, so there must be enough static time.

8. Sealing of the oil tank and parts

The quality of the sealing structure is directly related to the leakage of the transformer. If there is a leak, water will inevitably enter the transformer, causing the transformer oil and other insulating parts to absorb moisture, which is one of the factors of partial discharge. Therefore, it is necessary to ensure the reasonable sealing performance.