The theoretical basis of power cable insulation
Discharge is an important cause of insulation breakdown.
What is discharge?
Between two conductors with a potential difference, when the performance of the insulating material decreases, electron energy transfer occurs between the two conductors. For example, the ignition between the high-voltage live wire and the ground wire is discharge, and the complete discharge is the instant of discharge. A complete arc channel is formed between the electrodes.
The special case of discharge-partial discharge
Case 1: There is insulation between two conductors. When there are defects in the insulation material, such as impurities, voids, and the tip of the conductor, it will cause the internal electric field of the insulation to diverge and cause pulse discharges to be generated inside the insulation.
Case 2: Corona generated by external discharge is also a type of partial discharge. There is air insulation between the high potential and the ground. When the electric field around the conductor is particularly concentrated at a certain point, such as a wire burr, it causes a pulse discharge in the air. , And did not form a short circuit to the ground, forming a corona.
Characteristics of Partial Discharge
Partial discharge also has the basic characteristics of discharge, that is, there is the migration of electron energy. Due to the small discharge energy and the barrier of insulating materials, a complete arc channel may not be formed between the two electrodes. Once such a channel appears, it will be aggravated. Partial discharge, until the formation of a two-pole connection, a short-circuit discharge failure will occur.
Causes of partial discharge
The presence of air gaps in the main insulation can cause partial discharges. Because the relative dielectric constant of the air gap is much smaller than that of the cable insulation, under the action of the power frequency electric field, the air gap has to withstand greater electric field strength, resulting in partial discharge. With the multiple discharges of the air gap, the air gap path continues to expand, and the discharge The amount gradually increases until breakdown occurs, causing damage to the cable.
The presence of impurities in the main insulation can cause partial discharge. The breakdown strength of impurities is much smaller than that of insulating materials. Under the action of an electric field, the impurities first undergo discharge, carbonization and gasification, generating air gaps, and causing partial discharges.
The tip and burr of the conductor can cause partial discharge. Since the tip will increase the intensity of the electric field, the insulating material around the tip will discharge first and then develop into a breakdown. This is what we often call the tip effect.
The influence of moisture on cable insulation
In the production process of cross-linked cable, there will be water molecules in the insulating material. Under the action of electric field and temperature, water branches will be formed. The water branches will grow and migrate during long-term operation, and gradually evolve into air gaps. Discharge, damage the insulation.
In addition, after the cable is formed, the outer sheath is damaged and water enters, and moisture exists outside the core and insulation, which will also reduce the insulation characteristics of the cable and form a discharge channel. During construction, the inner and outer sheaths must be protected to prevent water from entering the core.
The influence of temperature on cable insulation
The performance of cable insulation materials is closely related to temperature. With the increase of temperature, the insulation performance decreases, the insulation resistance decreases, the breakdown field strength decreases, and the temperature increases. Dismutation, severe thermal breakdown will occur, so the working temperature of the cable must be strictly controlled, and the cable must not be overloaded.
The influence of semiconductor interface on insulation
The semiconductor shielding layer is processed in the production of cable terminations and joints, which is the key to the quality of the joints. This is the place where the field strength changes suddenly. If the processing level is not high, the insulation will be damaged after being put into operation. In severe cases, breakdown will occur in the completion test.
Effects of damage to insulating materials
During the installation of the cable connector, the semi-conductive shield should be stripped. If damage is caused to key parts, such as knife marks, internal creeping discharge channels will also be formed.
This is cable knowledge from Henan Tano Cable Co.,Ltd. I hope it will be helpful to you.