Basics of transformer partial discharges

1 Overview According to national standards, 110kV and above, large power transformers to do partial discharge test, now the contract requires the transformer high voltage local discharge less than 100PC. Local discharge on the insulation of the impact, one is the discharge point of mass on the insulation of the direct bombardment caused by local insulation damage, the gradual development of insulation breakdown; two is the insulation of the internal local discharge, although the formation of penetrating channels, but the heat generated by the discharge, so that the medium appears to be a local temperature increase, or even carbonisation. Due to the electrolytic effect of the discharge, will produce ozone, nitric oxide and some other reactive gases, so that the local insulation is corroded, and gradually cause damage to the insulation, and finally lead to thermal breakdown. Usually, the destruction of electrical insulation or local aging, mostly from the local discharge, so the harm of partial discharge is to make the transformer insulation life reduced, affecting the safe operation of the transformer.

 2 What is partial discharge For transformer insulation structure, there may be some insulation weaknesses, it will be the first in a certain external voltage will discharge, but does not immediately form the whole insulation penetrating breakdown. This is only limited to the insulation local position (weakness) at the discharge is called partial discharge. The purpose of partial discharge test: is to assess the transformer in the long-term working voltage, its product insulation can be long-term safe operation performance, found the transformer structure and manufacturing process defects. Such as:

 (1) insulation structure in the local electric field intensity is too high, may be local insulation (such as oil gap or solid insulation) breakdown or discharge along the solid insulation surface;

 (2) insulation mixed with impurities or local with defects; such as insulating paper tube, laminated cardboard, laminated board, etc., due to poorly handled hot pressing and drying process, it will be formed in its internal cavity, when immersed in oil, transformer oil often can not be immersed in this cavity, thus forming the air pockets. If immersed in the transformer oil treatment is not good, there will be bubbles in the oil, while the presence of water and impurities, under the action of the electric field, impurities will form a “small bridge”, the leakage current through the place will make the heat is serious, prompting the vaporisation of water, the formation of bubbles; at the same time, it will also make the oil decomposition of the gas generated. Insulation within the existence of these cavities (bubbles), its dielectric constant than the dielectric constant of the insulating material to be small, so the cavities on the electric field strength than the neighbouring insulating material on the electric field strength to be high. The insulating strength of gases (especially air) is lower than that of insulating materials. In this way, when the applied voltage reaches a certain value, the field strength contained in the insulation on the gas hole will first reach the degree of breakdown, so that the gas hole first discharge. 

(3) Metal parts have sharp corners; tip discharge. 

(4) products within the metal grounding components, poor electrical connection between the conductor, etc., in order to eliminate these defects, to prevent localised discharges on the insulation damage. 

3 Key factors in the generation of partial discharge Partial discharge link, generally in the electric field concentration and insulation weakness. There are many factors affecting partial discharge, and there are three main points in combination:

 (1) The material of the insulating material; 

(2) the insulation structure of the product design; (not much of a problem)

 (3) the degree of refinement of the production and processing manufacturing process. From the test point of view to analyse the causes and parts of the partial discharge, there are five key factors that cause partial discharge:

 (1) The sharp corner burrs of the conductive and non-conductive body; 

(2) Cavities and voids in the gaps of solid insulation and trace air bubbles in the oil;

 (3) Metallics that produce suspended potentials under high electric fields; 

(4) Dust, dirt and foreign matter on the surface of the insulator.

 (5) Poor insulation drying treatment and surface moisture. The above key factors should be strictly controlled by taking measures from design, process, operation and management. Two body insulation and partial discharge related knowledge Our transformer commonly used insulation materials are: transformer oil, cardboard, moulding parts and oil-paper composite insulation. Oil-immersed transformer insulation structure used in the main insulating material is transformer oil and insulating paper, that is, oil and paper insulation structure. Transformer oil and insulating paper combined with high electrical strength. Than the two separate separate oil and paper any kind of material is much higher, can produce 1 + 1 > 2 effect. Under good conditions of oil immersion, the degree of elimination of impurities determines the actual oil-paper insulation may achieve the electrical strength, so the actual production process control is of great importance. The disadvantage of oil and paper insulation is that both are easy to be contaminated, containing a few per cent of the impact of impurities is very serious, therefore, in the process, as far as possible, to obtain a more pure oil and paper, and accordingly select the appropriate working field strength, to ensure the reliability of the transformer insulation structure.

 (Emphasis on purification, combined with 6S work) Transformer production should be carried out in the fully closed structure of the purification workshop, ventilation equipment to install air filtration devices; the use of paraffin vapour-phase dry manic equipment, the equipment is put into use can make the transformer body cleaner, more thorough drying. 

Basically: do not bring the foreign matter into the body; production process does not produce foreign matter; operation does not appear new foreign matter. Pay attention to control the metal particles scattered by the friction of silicon steel sheet at the yoke when inserting iron to ensure the cleanliness of the body. Unless the paper is completely saturated with oil, there will be voids of air or other gases in the paper. This will undoubtedly make the paper’s electrical strength is reduced, at this time the gap on the voltage borne on the paper and much higher than the gap breakdown does not mean that the insulation is damaged, this part of the discharge will produce the appearance of local discharge, will gradually corrode the insulation, and ultimately can lead to damage to the insulation, so the transformer’s immersion in oil and the static discharge time must be strictly in accordance with the requirements of the process documents. 

Prevention of local discharge problems in several issues that require special attention: 

1, wedge-shaped oil gap discharge problems. Examples: pressure plate cracking, cushion plate cracking, end circle cushion block cracking, lead pinch wood, small oil gaps between segments and turns, line turns and cushion block contact, cracking, very easy to produce local discharge. Wedge-shaped oil gap breakdown strength is reduced, is the insulation weakness, in the higher electric field will first occur under the action of partial discharge. 

2, partial discharge specific occurrence of typical parts. In the transformer insulation structure according to the first local discharge insulation medium, can be divided into bubble and oil discharge; and local discharge specific typical parts can be divided into: solid media cavities, electrode sharp corners, oil partition insulation in the oil gap, oil wedge, and oil along the surface of the solid medium, where the electrode and the solid medium in contact with the surface of the medium is very. 

3, on the potential and field strength. Partial discharge starting discharge voltage is determined by the discharge part of the local field strength, so the insulation structure should be determined according to the concept of field strength. In the transformer may appear in a variety of different parts of the higher field strength and lead to local discharge, most of these parts in some oil gap, oil wedge, air gap, there is a suspended potential of the metal conductor, conductor sharp corners and solid surfaces. Higher field strengths are not always found at high potentials. Low potential or ground potential may also appear higher field strength. That is, not only in the transformer in the high potential can appear on the local discharge, in the low potential or even ground potential may also appear on the local discharge. For example, the ground screen is effectively and reliably earthed. For example, if the structure of the fixture is not well chamfered, the field strength is high between the fixture and the lead wire, then the local discharge occurs on the fixture rather than on the lead wire. 

4, pressure plate, pad and other structural components in the anti-local discharge design. 

(1) chamfer (2) small oil-soaked holes (3) small grooves (4) large sheets of cardboard (5) to ensure that the pressure (6) the use of adhesive, spread evenly to prevent bubbles. 

5, on the end of the electrostatic board. The treatment of electrostatic board end insulation should be very careful. The place is the highest field strength, poorly handled, highly prone to localised discharge. Requirements in strict accordance with the drawings, alignment, do not hurt the electrostatic ring, to maintain a high degree of purification. 

6, diagonal ring processing. It can be generally believed that all parts of the corner ring, are considered to be the key parts of the insulation structure, to be treated with special care. The role of the corner ring is to split the oil gap (large oil gap into a small oil gap, improve electrical strength), the second is to increase the creepage distance (reduce the creepage field). Angle ring placed to strive for accurate location, to ensure the thickness, service in place, no duck neck phenomenon. Designed to leave a pressure service margin. 

7, on the ground screen. Ground screen is one of the key parts of the high-voltage power transformer. Plays a role in improving the shape of the electrode. Hollow conductor if there is no net charge in the cavity, the electric field in the electrostatic equilibrium state, the remaining charge can only be distributed on the outside surface, the conductor and the cavity at any point in the field strength is zero. Therefore, if any object is placed into the cavity of a hollow conductor, the object is not affected by any external electric field. Conductor core placed in a metal shell ground screen, due to electrostatic induction, in the ground screen will appear on the inner and outer surfaces of an equal amount of anisotropic induced charge, the ground screen of the outer surface of the charge generated by the electric field will have an effect on the outside world. In order to eliminate the impact of the ground screen can be grounded, the outer surface of the induced charge due to grounding and be neutralised, the corresponding electric field disappears. It can be seen, for the grounding of the ground screen, the external electric field will not affect the ground screen within the core, the core will not affect the external electric field. The above theory also applies to the shielding of welding heads, coil outlets, etc., which are widely used in power transformers. Other simple knowledge in transformer insulation Transformer body insulation is the main insulation, which is the insulation from the coil to the grounded part of the core and tank (mainly the end insulation), and the insulation from the coil to the other coils (mainly the main insulation between coils of the same phase). The body insulation of a transformer is made up of a series of insulating parts which are made up of specific insulating materials. The life of the insulating material determines the life of the transformer, and the same insulating structure relies heavily on the properties of the insulating material, so understanding the insulating material is important for further understanding of the insulating structure. For a certain voltage level transformer insulation needs, in the transformer body insulation, respectively, to take certain insulation measures. 

Mainly the following two parts.

 1, the main insulation between the coils The main insulation between coils is oil – partition insulation, partition for the insulating paper tube. According to the insulating properties of the material can determine the electrical strength of the main insulation between the coils, the most important is the electrical strength of the oil gap immediately adjacent to the surface of the coil. Transformers generally use thin paper tube small oil gap structure. Thin paper tube small oil gap electrical strength: coil main insulation distance between the paper tube ≤ 4mm, oil gap ≤ 15mm when called thin paper tube small oil gap structure.

 2、Electrical strength of end insulation End insulation refers to the coil end to the yoke and adjacent coil end insulation. End of the electric field distribution is complex, the maximum field strength in the high-voltage coil end of the inner oil gap. The maximum field strength can be weakened if the end section wire is rounded or if there is additional insulation, or if an electrostatic ring is added. If and increase the insulation spacer, angle ring, then again make the creepage distance increase. At present, China’s production of oil-immersed transformer insulation structure, mainly using oil a partition insulation structure type. The following oil a partition structure of the basic laws and its characteristics of the outline of the analysis. Effective volume effect of oil-immersed resistive strength. That is, in the uniform electric field and a little uneven electric field, transformer oil resistance strength with the voltage of the oil gap volume decreases and increases, that is, the transformer oil has a “volume effect”. In the transformer insulation structure, is the use of ordinary insulating cardboard or corrugated cardboard will be separated into a number of small oil gap interval (volume) corrugated cardboard height is exactly the oil a partition of the main insulation structure of the oil gap width. In the high-voltage transformer insulation structure, the number of partitions with the increase in voltage level and increase. But the oil gap can not be too small, because the oil flow if blocked, heat dissipation difficulties, so the selection of oil gap size should be appropriate, in the transformer insulation structure in the axial oil channel, generally should not be less than 6mm.