Mechanism And Application Of Three-core Cable’s Grounding Current In Medium-voltage Distribution Network

With rebuilding of power network and application costs decreasing of power cable, overhead lines are gradually replaced with underground cables. Three-core XLPE cable is being widely used in medium-voltage distribution network. In actual operation, induction voltage on sheath is caused by the electromagnetic coupling relationship between conductor core and metal sheath. After taking certain grounding measures against induced voltage, it will raise new issue of grounding current on sheath. Because of symmetrical structure and simple grounding method, induction voltage and grounding current of three-core XLPE cable both are zero during the normal running. Many literatures focus on algorithms and applications of single-core cable’s grounding current on sheath, which generally have two defects of simplistic model and overly limited conditions. Owing to the fact that few literature analyze fault characteristic of grounding current, this paper tries another prospective way to research mechanism and application of three-core cable’s grounding current when fault occurs in medium-voltage distribution network.In the first part, relevant theories and elementary knowledge are introduced, which include the structure of three-core XLPE cable, the common cable line fault, the grounding system of Medium-voltage distribution network, etc. The self-inductance and mutual-inductance of cable core and metal sheath can be calculated by power cable parameter. After that, the relationship between induced circulating current on sheath and zero-sequence current on cable core can be derived. At then, the mesh current method is adopted for solving zero-sequence equivalent circuit when single-phase earth fault occurred on three-core cable line. Considering the influence of different factors on grounding current’s fault characteristics, mechanism of grounding current is revealed in low-resistance grounding system and arc-suppression coil grounding system. It conclude that each node’s grounding current on sheath is equal to zero-sequence current on cable core. Lastly, the theoretical derivation is validated by PSCAD simulation.On the foundation of the research of grounding current’s mechanism, two applications of three-core cable’s grounding current in medium-voltage distribution network can be obtained. One application is sheath grounding reliability monitoring system based on grounding current. It can implement open circuit fault location on sheath by master station and local node, according to the distribution rules of grounding current on each sheath. The system is elaborated from principle, implementation and evaluation. Another application is verification for thermal stability of grounding current. By choosing suitable short-circuit current on sheath, short-circuit duration and thermal stability coefficient for thermal stability calibration, minimum cross-sectional area of metal sheath which satisfy the requirement can be calculated accurately.