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Some Measures To Reduce Partial Discharge Of Power Transformer.
- May 26, 2018 -

Some measures to reduce partial discharge of power transformer.


With the rapid development of the power grid and the increase of transmission voltage, the demand for the insulation reliability of large power transformers is higher and higher.


Due to partial discharge test of insulation without damaging effect and very sensitive, can effectively find the transformer insulation inside the inherent defects or transportation and installation of the defect is formed in which the partial discharge test is widely used and has been listed as a 72.5 kV transformer voltage level above the scene will do the handover test project.


Partial discharge and its principle.

Partial discharge, also known as electrostatic dissociation, is the meaning of static charge flow.
Under certain external voltage, the static charge in the field with strong electric field is in the position where the insulation is weak, the static electricity is free, but the insulation breakdown is not formed.
This phenomenon of static charge flow is called partial discharge.
The partial discharge that occurs near a conductor surrounded by gas is called corona.

Partial discharge is the discharge in the internal insulation of the transformer, because the discharge is in a local position and the energy is low, which does not directly constitute the penetrating breakdown of internal insulation.

The partial discharge test of transformer is carried out at the initial stage of 220kV and above transformers.
Later, the new IEC standard stipulates that when the maximum working voltage of the equipment is greater than 126kV, the partial discharge measurement of the transformer is required.


The national standard has also made corresponding provisions. The maximum working voltage of the equipment is greater than 72.5kv, and the rated capacity of the transformer with a capacity of more than 10000kVA is required. If no other agreement is made, the partial discharge measurement of the transformer should be carried out.


The local discharge test method shall be carried out according to the provisions of gb1094.3-2003, and the local discharge standard shall not exceed 500pC.
However, in the actual contract, users often require less than or equal to 300pC or less than or equal to 100pC. This technology agreement requires the transformer manufacturer to have higher product technical standards.

Damage of partial discharge.

The damage degree of partial discharge is related to the cause, location, starting voltage and the extinction voltage, the higher the initial voltage and the extinction voltage, the smaller the damage, and the larger the other.
In terms of discharge properties, the discharge of solid insulation is the most harmful to the transformer, which can reduce the insulation strength and even cause damage.

The cause of partial discharge.

In addition to the 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 component structure has sharp Angle and burr, which causes the electric field distortion and the initial voltage of discharge is reduced;

2. There are foreign body and dust, which causes the concentration of electric field.
The electric corona discharge or breakdown discharge occurs under the action of external electric field;
Used in production and sales of dry type transformer, transformer, station transformer, earthing transformer, arc suppression coil, traction transformer, transformer, single-phase transformers, isolation transformers, serial reactor, plateau type dry type transformer, oil-immersed power transformers, control transformers, split transformer, special transformer.
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3. Water or bubbles.
Because of the low dielectric coefficient of water and gas, the discharge first occurs under the action of electric field.

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

Measures to reduce partial discharge.

1. Dust control.

Foreign matter and dust are very important inducements to produce partial discharge.
The experimental results show that the metal particles with the above 1.5 m and above are likely to produce more power than 500pC under the action of electric field.
Whether metal or non-metallic dust exists, the concentrated electric field will be produced, which will reduce the initial discharge voltage of the insulation and reduce the breakdown voltage.
Therefore, in the process of transformer manufacturing, it is very important to keep the clean environment and body, and the dustproof control must be strictly enforced.
According to the manufacturing process, the product may be controlled strictly by the influence of dust, and the sealing and dustproof workshop shall be established.
For example, in flat wire, wire wrap paper, windings winding, winding set, iron core stack, insulation parts manufacturing, body assembly and body finishing, no foreign body residues and dust should be allowed to enter.
According to the manufacturing process, the product may be controlled strictly by the influence of dust, and the sealing and dustproof workshop shall be established.
For example, in flat wire, wire wrap paper, windings winding, winding set, iron core stack, insulation parts manufacturing, body assembly and body finishing, no foreign body residues and dust should be allowed to enter.

2. Centralized processing of insulation parts.

Insulation is very taboo with metal dust, because once the insulation is attached to metal dust, it is very difficult to clean it thoroughly.
Therefore, it is necessary to centralize the processing in the insulation workshop and set up a machining area, which should be paid attention to the isolation of other dust producing areas.

3. Strictly control the processing burr of silicon steel sheet.

The core of transformer is cut through longitudinal shear and transverse shear, and these shear incisions have different degree of burrs.
The burr can not only cause the short circuit of the film, but also form the internal circulation, increase the no-load loss, and also increase the core thickness, actually reducing the number of laminated slices.
More importantly, the burr may fall on the device and discharge when it is placed in the yoke or in motion.
Even if the burr falls on the bottom of the box, it may be arranged in an orderly arrangement under the action of electric field, resulting in ground potential discharge.
Therefore, the core should be as small as possible, as small as possible.
The burr of the iron core of 110KV product should not be greater than 0.03mm, and the burr of 220KV product iron core should not be greater than 0.02mm.

4. The lead wire adopts cold pressing terminal.

It is an effective measure to reduce the local discharge of electricity by using the cold pressing terminal.
Because of the use of phosphor copper welding, it is easy to be scattered in the body and insulation parts.
In addition, the weld boundary area should be separated by an impregnated asbestos rope so that the water will enter the insulation.
If the water is not cleaned thoroughly after the insulation, it will increase the local power of the transformer.


5. Roundness of parts edges.

The purpose of the edge roundness of parts is: 1) to improve the distribution of field strength and improve the initial voltage of discharge.
So the core of metal structural parts such as clip pieces, drawing board, shoes and stent edge, pressure plate and outlet edge, elevated casing wall, magnetic shield to protect the inside of the box wall plate are all round should be handled.
2) prevent iron filings from friction.
If the lifting hole of the clamp is connected with the hanging rope or hook, it needs to be rounded.

6. Product environment and body arrangement during assembly.

After vacuum drying, the body should be arranged before boxing.
The larger the product, the more complex the structure, and the longer the processing time.
Because the body pressure, tighten fasteners are body exposed in the air, will happen during the moisture absorption and dust scattered, so the body needs to be done in the dust, such as finishing time (or time) of exposed to the air more than 8 hours, need to be dry processing again.
After the finishing of the body finishing, the oil tank pump suction stage is deducted.
Because the body insulation will absorb moisture, it is necessary to dehumidify the body.
This is an important measure to ensure the insulation strength of high-voltage products, and the method is to vacuum the product.
The vacuum degree of vacuum is determined according to the standard of body and environment humidity and moisture content, and the time of vacuuming is determined according to the time of release, ambient temperature and humidity.

7. Vacuum filling

Vacuum filling out that the purpose of transformer vacuum, clear product insulation structure in the corner, completely discharge the air, then inject transformer oil under vacuum condition, make the body completely soaked.
After filling the transformer, the test can be done for at least 72 hours, because the 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 immersion, the less likely it is to discharge, so there must be plenty of time.

8. Sealing of oil tank and parts.

The sealing structure is directly related to the leakage of transformer.
If there is a leak, there will be water entering the transformer, which will lead to the moisture absorption of transformer oil and other insulating parts, which is one of the factors of partial discharge.
Therefore, ensure the reasonable sealing performance.

Original title: some measures to reduce partial discharge of power transformer.


Thank you for your kindly attention. Hope everything goes well with you.

This is Blanca Chen, from Zhejiang ERG Technology Joint Stock Co.,Ltd. We manufacturer and design transformers accessories. 

 

Products: (Sorry that for the e-mail not to be sent back, our catalogue and web are not attached for our first contact.)
1. All kinds of transformer cooler
2. Oil pump
3. Butterfly valve
4. Oil flow relay 
5. Control cabinet

6. Terminal box etc.
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E-mail: blancachen@erg.cn

 

Best Regards,

Blanca Chen

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