The electric field stress distribution in AC cables is uniform, and the focus of cable insulation materials is on the dielectric constant, which is not affected by temperature. In contrast, the stress distribution in DC cables is highest at the inner layer of the insulation and is influenced by the resistivity of the insulation material. Insulation materials exhibit a negative temperature coefficient, meaning that as temperature increases, resistivity decreases.
When a cable is in operation, the core losses cause the temperature to rise, leading to changes in the resistivity of the insulation material. This, in turn, causes the electric field stress within the insulation layer to vary. In other words, for the same thickness of insulation, the breakdown voltage decreases as the temperature rises. For DC trunk lines in distributed power stations, the aging rate of the insulation material is significantly faster due to fluctuations in ambient temperature compared to buried cables, which is a critical point to note.
During the production of cable insulation layers, impurities are inevitably introduced. These impurities have relatively lower insulation resistivity and are unevenly distributed along the radial direction of the insulation layer. This results in varying volume resistivity at different locations. Under DC voltage, the electric field within the insulation layer will also vary, causing the areas with the lowest volume resistivity to age faster and become potential points of failure.
AC cables do not exhibit this phenomenon. In simple terms, the stress on AC cable materials is uniformly distributed, while in DC cables, the insulation stress is always concentrated at the weakest points. Therefore, the manufacturing processes and standards for AC and DC cables should be managed differently.
Cross-linked polyethylene (XLPE) insulated cables are widely used in AC applications due to their excellent dielectric and physical properties, as well as their high cost-performance ratio. However, when used as DC cables, they face a significant challenge related to space charge, which is particularly critical in high-voltage DC cables. When polymers are used as DC cable insulation, a large number of localized traps within the insulation layer cause the accumulation of space charges. The impact of space charges on insulation materials is mainly reflected in two aspects: electric field distortion and non-electric field distortion effects, both of which are highly detrimental to the insulation material.
Space charge refers to the excess charge beyond electrical neutrality within a structural unit of a macroscopic material. In solids, positive or negative space charges are bound to localized energy levels, providing polarization effects in the form of bound polarons. Space charge polarization occurs when free ions are present in a dielectric material. Due to ion movement, negative ions accumulate at the interface near the positive electrode, and positive ions accumulate at the interface near the negative electrode. In an AC electric field, the migration of positive and negative charges cannot keep up with the rapid changes in the power frequency electric field, so space charge effects do not occur. In a DC electric field, however, the electric field distributes according to resistivity, leading to the formation of space charges and affecting the electric field distribution. XLPE insulation contains a large number of localized states, making space charge effects particularly severe.
XLPE insulation is chemically cross-linked, forming an integrated cross-linked structure. As a non-polar polymer, the cable itself can be likened to a large capacitor. When DC transmission stops, it is equivalent to charging a capacitor. Although the conductor core is grounded, effective discharge does not occur, leaving a significant amount of DC energy stored in the cable as space charges. Unlike AC power cables, where space charges are dissipated through dielectric losses, these charges accumulate at defects in the cable.
Over time, with frequent power interruptions or fluctuations in current strength, XLPE insulated cables accumulate more and more space charges, accelerating the aging of the insulation layer and reducing the cable’s service life.
Post time: Mar-10-2025