Do DC cables require higher corona resistance and space charge resistance?
Publish Time: 2025-11-03
In modern power transmission systems, DC cables are increasingly widely used, playing a crucial role, especially in high-voltage direct current (HVDC) transmission, new energy power generation, rail transportation, and electric vehicles. Compared to traditional AC cables, DC cables exhibit significant differences in electric field distribution, insulation aging mechanisms, and operating characteristics.1. The space charge effect is more pronounced under a DC electric field.In AC cables, the electric field direction changes periodically with the current, and charges move back and forth within the insulation layer, making it difficult for them to accumulate over a long period. However, in DC cables, the voltage polarity is constant, causing free charges to migrate towards the electrodes under the influence of a strong electric field and be captured within the insulation material or at interfaces, forming "space charges." These accumulated charges distort the original electric field distribution, significantly increasing the local electric field strength, even exceeding the material's breakdown field strength, thus triggering early insulation degradation or partial discharge. The accumulation of space charges is particularly severe under conditions of temperature gradients, material defects, or interface mismatches.2. Corona Resistance Directly Affects Cable Lifespan and SafetyCorona discharge is a localized, self-sustaining discharge phenomenon that occurs in a gaseous medium under a high electric field. It commonly occurs at cable terminations, joints, or uneven areas of the conductor shielding. Although corona primarily occurs in the air gap, the ozone, nitrogen oxides, and high-energy particles it produces can cause chemical corrosion and physical erosion of adjacent solid insulation materials, leading to embrittlement, cracking, and even breakdown. Under DC conditions, although the corona initiation voltage is usually higher than that of AC, the continuous discharge characteristics of DC corona mean that once it occurs, its destructive effect on insulation is more persistent and cumulative. Furthermore, DC corona is not easily self-extinguishing and has complex discharge modes, potentially accompanied by pulsed or steady-state discharges, further accelerating material aging. Therefore, DC cable design must strengthen electric field homogenization measures, such as using high-precision semiconductor shielding layers, optimizing stress cone structures, and selecting insulation and sheathing materials with stronger corona resistance, such as corona-resistant polyimide or fluorinated polymers.3. Targeted Optimization of Materials and Structural DesignTo address the aforementioned challenges, modern DC cables have undergone several technological upgrades in material selection and structural design. First, the insulation material not only requires high volume resistivity and dielectric strength but also excellent charge dissipation capabilities. For example, introducing polar groups or inorganic fillers into XLPE can effectively regulate the trap energy level distribution of the material, promoting the release of space charge. Second, in manufacturing processes, ultra-high cleanliness and three-layer co-extrusion technology are emphasized to reduce impurities and micropores, lowering the probability of corona initiation. Furthermore, the cable's shielding layer and conductor surface treatment are more refined to ensure uniform electric field distribution and avoid localized field concentration. For high-voltage DC cables, stepped insulation designs or composite insulation structures are often employed to balance radial and axial electric field stresses.In summary, DC cables indeed require higher corona resistance and stronger resistance to space charge accumulation than AC cables. This is determined by their unique operating mechanism and long-term electric field characteristics. Only through the coordinated advancement of material innovation, process optimization, and structural design can we ensure the safe, efficient, and long-term operation of DC cables in modern power systems.
