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1, the electric field stress distribution of AC cable is balanced, cable insulation material is focused on dielectric constant, dielectric is not affected by temperature, and the stress distribution of DC cable is the largest cable, the cable insulation material resistance coefficient, insulation material has a negative temperature coefficient phenomenon, that is, temperature increased, resistance to small When the cable is running, the core loss will cause the temperature to rise, and the electrical impedance of the insulating material will change, which will cause the electric field stress of the insulating layer to change, that is to say, the same thickness of insulating layer, because of the temperature rise, its breakdown voltage becomes smaller.
For some distributed power stations, due to the changes in the ambient temperature, the insulation of the cable is much faster than the buried cable, which should be noted in particular.
2, the cable insulation layer production process, inevitably will dissolve some impurities, they have relatively small insulation resistivity, the radial distribution along the insulating layer is uneven, this will also lead to different parts of the volume resistivity, in the DC voltage, cable insulation layer of the electric field will be different, so that The minimum insulation volume resistivity will be aging faster, becoming the first to be penetrated by the hidden point. And the AC cable does not have this phenomenon. In layman's parlance, the material of AC cable is balanced by the stress impact, while the insulation stress of DC cable always has the biggest impact in weakest place.
Therefore, the cable manufacturing link in the AC/DC cable should have different management and standards.
3, XLPE insulated cable in the AC cable has been widely used, it has very good media performance and physical performance, cost-effective, but as a DC cable, it has a difficult to solve the space charge problem, which is highly valued in high-voltage DC cable. When the polymer is insulated by DC cable, there are a large number of local traps in the insulating layer, which results in the space charge agglomeration and the influence of the space charge on the insulating material, which is mainly embodied in two aspects of the electric field distortion effect and the non electric field distortion effect. The so-called space charge, refers to the macroscopic matter in a structural unit more than the electricity neutral part of the charge, in the solid, positive or negative space charge is bound to a certain local energy level and in the form of bound polarization to provide polarization effect. The so-called space charge polarization, is when in the dielectric containing free ions, because of the ion movement, on the positive electrode side of the interface accumulation of negative ions on the negative electrode side of the interface accumulation of positive ions process. In an alternating electric field, the transfer of the positive and negative charge of the material is unable to change with the working frequency electric field rapidly, so the space charge effect is not generated, but in the DC electric field, the electric field is distributed according to the resistivity, which will form the space charge and influence the electric field distribution, the polyethylene insulation has a lot of local Cross-linked polyethylene insulation layer is chemically crosslinked, is the monolithic cross-linked structure, belongs to the Non-polar high polymer, from the cable entire structure looks, the cable itself is like a larger capacitor, the DC power transmission stops, is equivalent has already a capacitor charging completes, although the conductor core has the ground treatment, but, has not been able to discharge effectively, A large amount of DC power still exists in the cable, this is called Space charge, which is not consumed by the dielectric loss as the AC power cable does, but is enriched at the defect of the cable, and the XLPE insulated cable gathers more and more space charge with the prolonged or frequent outage of the use time and the current strength and weakness.
Accelerate the aging speed of insulating layer, thus affecting service life.
Therefore, the DC cable insulation performance and AC cable difference is still very large.