The effect of partial discharge on insulation is as follows: first, the direct bombardment of discharge particles on insulation causes the damage of local insulation, and gradually develops the breakdown of insulation; Second, although the partial discharge inside the insulation does not form a penetrating channel, the heat generated by the discharge makes the medium appear local temperature rise, and even carbonization. Due to the electrolytic effect of discharge, some active gases such as ozone and nitric oxide are produced, which corrodes local insulation, gradually causes insulation damage, and finally leads to thermal breakdown. Usually, the damage or local aging of electrical insulation starts from partial discharge, so the harm of partial discharge is to reduce the insulation life of the transformer and affect the safe operation of the transformer.
What is partial discharge?
For the transformer insulation structure, there may be some insulation weaknesses, which will first discharge under a certain applied voltage, but will not immediately form the entire insulation penetration breakdown. This discharge limited to the local position of the insulation (weak point) is called partial discharge.
The purpose of partial discharge test is to assess the performance of the transformer under the action of long-term working voltage, whether the product insulation can run safely for a long time, and to find the defects of the transformer structure and manufacturing process.
For example: (1) the local electric field strength in the insulation structure is too high, which may be the breakdown of local insulation (such as oil gap or solid insulation) or discharge along the solid insulation surface;
(2) insulation mixed with impurities or local defects; Such as insulated paper tube, laminated cardboard, laminated wood, etc., due to the poor treatment of hot press drying process, it will form a cavity in its interior, when immersed in oil, transformer oil often can not be immersed in this cavity, thus forming an air hole. If the immersed transformer oil is not treated well, there will be bubbles in the oil, and there are water and impurities. Under the action of the electric field, the impurities will form a "small bridge", and the passing of the leakage current will make the place heat seriously, prompting the water to vaporize and form bubbles. It also causes the oil there to decompose and produce gas. The dielectric constant of these air pockets (bubbles) inside the insulation is smaller than the dielectric constant of the insulation material, so the electric field strength on the air pocket is higher than that on the adjacent insulation material. 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 on the air pocket contained in the insulation will first reach the degree of breakdown, so that the air pocket will discharge first.
3. The key factors of partial discharge
The partial discharge is generated in the part where the electric field is concentrated and the insulation is weak. There are many factors affecting partial discharge, which are mainly three points:
(1) the material of the insulation material;
(2) Insulation structure of product design; (There is no big problem)
(3) The degree of refinement of the production and processing manufacturing process.
The causes and locations of partial discharge are analyzed from the test point of view. There are five key factors that cause partial discharge: (1) sharp corner burrs of conductive and non-conductive bodies;
(2) Holes in solid insulation and gaps and trace bubbles in oil;
(3) Metal objects that produce suspension potential under high electric field;
(4) Dust, dirt and foreign matter on the surface of the insulator.
(5) The insulation is not dry and the surface is damp.
The above key factors should be strictly controlled from the aspects of design, process, operation and management.
