Electrical Power Transformer Reduce Partial Discharge

With the rapid development of the power grid and the increase of the transmission voltage, the power grid 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 the insulation and is very sensitive, it can effectively find the inherent defects in the insulation of the electrical power transformer or the defects that endanger safety in the transportation and installation projects. Therefore, the on-site partial discharge test is widely used, which has been listed as 72.5kV. The handover test items that must be done on site for transformers with voltage levels above and above.

Partial discharge and its principle

Partial discharge, also known as electrostatic dissociation, is the flow of electrostatic charges. Under the action of a certain applied voltage, the electrostatic charge in the area with strong electric field will first generate electrostatic dissociation at the position with weak insulation, but no insulation breakdown will be formed. This phenomenon of electrostatic charge flow is called partial discharge. Partial discharges that occur near conductors surrounded by gas are called coronas.

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

For the partial discharge test of the electrical power transformer, our country is carried out on the transformer of 220kV and above in the initial stage. Later, the new IEC standard stipulates that when the maximum working voltage of the equipment is Um ≥ 126kV, it is necessary to measure the partial discharge of the transformer. The national standard has also made corresponding provisions. For transformers with a maximum working voltage of Um ≥ 72.5kV and a rated capacity of 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 implemented in accordance with the provisions in GB1094.3-2003, and the partial discharge standard shall 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.

Hazards of Partial Discharge

The degree of hazard of partial discharge is related to its cause, location, and the level of initial voltage and extinguishing voltage. The transformer is the most harmful, which will reduce the insulation strength and even cause damage.

Causes of partial discharge

In addition to the poor design considerations, the most common factors causing partial discharge are caused by the manufacturing process: usually there are the following main reasons:

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

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 is moisture or air bubbles. Due to the low dielectric coefficients of water and gas, discharge occurs first under the action of the electric field;

4. The poor contact of the suspension agent of the metal structure causes electric field concentration or spark discharge.

Measures to reduce partial discharge

1. Dust control

Among the factors that generate partial discharge, foreign matter and dust are very important inducements. The test results show that metal particles with a diameter of ф1.5μm may generate discharges far greater than 500pC under the action of an electric field. Whether metal or non-metal dust exists, a concentrated electric field will be generated, which will reduce the initial discharge voltage of the insulation and reduce the breakdown voltage. Therefore, in the transformer manufacturing process, it is very important to maintain a clean environment and body, and dust control must be strictly implemented. Strictly control the degree to which products may be affected by dust during the manufacturing process, and establish a sealed and dust-proof workshop. For example, in the process of flattening the wire, wrapping the wire, winding winding, winding set, core stacking, manufacturing of insulating parts, body assembly and body finishing, foreign matter residue and dust are absolutely not allowed to enter. Strictly control the degree to which products may be affected by dust during the manufacturing process, and establish a sealed and dust-proof workshop. For example, in the process of flattening the wire, wrapping the wire, winding winding, winding set, core stacking, manufacturing of insulating parts, body assembly and body finishing, foreign matter residue and dust are absolutely not allowed to enter.

2. Centralized processing of insulating parts

It is very taboo for insulating parts to contain metal dust, because once the insulating parts are attached to metal dust, it is very difficult to completely remove it. Therefore, it is necessary to centralize processing in the insulation workshop, and set up a machining area, which should be isolated from other areas that produce dust.

3. Strictly control the processing burrs of silicon steel sheets

Transformer core pieces are formed by longitudinal shearing and transverse shearing. These shearing cuts have different degrees of burrs. The burr can not only cause short circuit between chips, form internal circulation, increase no-load loss, but also increase the thickness of the iron core, which actually reduces the number of laminated chips. 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 may occur. 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 discharge. Therefore, the core burr should be as small as possible. The burr of the core piece of 110KV product should not be larger than 0.03mm, and the burr of the core piece of 220KV product should not be larger than 0.02mm.

4. The lead adopts cold-pressed terminal

The use of lead cold-pressed terminals is an effective measure to reduce the amount of partial discharge. Because a lot of spatter welding slag is generated when using phosphor copper welding, it is easy to be scattered in the body and insulating parts. In addition, the welding boundary area needs to be separated by water soaked asbestos rope, so that water can enter the insulation. If the moisture is not completely removed after the insulation wrapping, it will increase the partial discharge of the transformer.

5. Rounding of component edges

The purpose of rounding the edge of the parts is: 1) To improve the distribution of the field strength and increase the initial voltage of the discharge. Therefore, the metal structural parts in the iron core, such as clips, pull plates, pads and bracket edges, pressure plates and wire outlet edges, the wall of the casing riser, and the magnetic shielding shield on the inner side of the box wall, should be rounded. 2), to prevent friction from producing iron filings. 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 finishing during final assembly

After the body is vacuum-dried, the body should be sorted out before packing. The larger the product and the more complex the structure, the longer the finishing time will be. Since the body is compressed and the fasteners are tightened, the body is exposed to the air, and moisture absorption and dust scattering will occur during the period. Therefore, the body should be cleaned in a dust-proof area, such as finishing time (or exposure to the air). time) more than 8 hours, it needs to be dried again. After the finishing of the body is completed, the stage of vacuuming and filling the fuel tank is carried out. Because the insulation of the body will absorb moisture during the body finishing stage, 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. Determine the vacuum degree of vacuuming according to the body, ambient humidity and water content standards, and determine the vacuuming time according to the release time, ambient temperature and humidity.

7. Vacuum oiling

The purpose of vacuum oil injection is to remove the dead angle in the insulation structure of the product by evacuating the transformer, exhaust the air completely, and then inject the transformer oil in a vacuum state to completely soak the body of the transformer. The transformer after oil injection can be tested after standing for 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 time of immersion in the oil. The better the degree of penetration, the less possibility of discharge, so be sure to allow enough rest time.

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 inside 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, it is necessary to ensure the reasonable sealing performance.