Explain The Theoretical Basis Of Power Cable Insulation For You

An important cause of insulation breakdown is discharge.

What is discharge?

Between two conductors with a potential difference, electron energy migrates between the two conductors as the performance of the insulating material degrades. For example, ignition between the high voltage live wire and ground wire is a discharge, and a full discharge is the instantaneous formation of a complete arc path between the two electrodes.

One of these situations is when there is insulation between two conductors. When the insulating material has internal defects such as impurities, gaps, conductor tips, etc., the insulating material will cause electric field divergence within the insulating material and pulse discharge within the insulating material.

The second case: the corona generated by the external discharge is also a partial discharge. There is air insulation between high potential and ground. When the electric field around a conductor is particularly concentrated at a certain point, such as a conductor burr, it can cause a pulsed discharge in the air and will not form a short to ground and corona.

Characteristics of partial discharge

Partial discharge also has the basic characteristic of discharge, that is, the migration of electron energy. Due to the small discharge energy and clogging of the insulating material, a complete arc path may not be formed between the two electrodes. Once such a channel occurs, the partial discharge will intensify until a two-pole connection is formed and a short-circuit discharge failure will occur.

Causes of partial discharge

Air gaps in main insulators can cause partial discharges. Since the relative permittivity of the air gap is much smaller than the relative permittivity of the cable insulation, under the action of the power frequency electric field, the air gap should withstand a large 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 volume gradually increases until failure occurs, resulting in cable damage.

Impurities in the main insulation can cause partial discharges. The breakdown strength of impurities is much smaller than that of insulating materials. Under the action of the electric field, the impurities are first discharged, carbonized and vaporized, creating air gaps and causing partial discharges.

At the tip of the conductor, burrs can cause partial discharges. As the tip increases the strength of the electric field, the insulating material around the tip first discharges and then develops breakdown, which is often referred to as the tip effect.

Summary of test results

It can be seen from the results of the above needle plate electrode test that the initial voltage of partial discharge and electric tree in the insulating material is closely related to the radius of curvature of the electrode. The larger the radius of curvature, the higher the initial voltage of PD and electric tree; on the contrary, the smaller the radius of curvature, the lower the initial voltage.

The needle tip crack creates an air gap, and the relative permittivity in the air gap is much smaller than that of the solid insulating material. The air gap should withstand large electric field strengths, resulting in partial discharges at very low voltages.

The effect of water on cable insulation

During the production process, there will be water molecules in the insulation material of the cross-linked cable. Under the action of electric field and temperature, water branches will form, and water branches will grow and migrate during long-term operation, and gradually evolve into air gaps, forming discharges and damaging the insulation.

In addition, after the cable is formed, the outer sheath is damaged by water, and there is moisture outside the core and insulation, which will also reduce the insulation properties 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.

Temperature effects of cable insulation

The properties of cable insulation are closely related to temperature. As the temperature increases, the insulation performance decreases, the insulation resistance decreases, the breakdown field strength decreases, the temperature increases, and the insulation accelerates aging. Exceeding the maximum working temperature will also lead to cable deformation, resulting in cable deformation, field strength distribution and polarization, which will seriously lead to thermal breakdown. Therefore, the working temperature of the cable should be strictly controlled, and the cable should not be overloaded.

Influence of the semiconductor interface in the face of insulation

During the manufacture of cable terminations and butt joints, there is a semiconductor shielding layer, which is the key to the quality of the joint. This is the part of the sudden change in field strength. If the level of processing technology is not high, the insulation will be damaged after being put into operation, and serious situations will occur in the completion test.

Affect the damage of insulating materials

In the process of installing the head of the cable connector, the outer semi-conductive shield should be stripped off. If damage is caused to the key parts, such as knife marks, an internal creepage flash discharge channel will also be formed.