The Research On Optimal Layout And Relay Protection Of Same-Phase Parallel Cables

With the rapid growth of urban load demand, in order to reduce the cost of equipment and due to the limitation of technical conditions, the operation mode, multi-cables of same-phase in parallel, has been highly valued which can increase line capacity. At the same time, its application is also gradually increasing. However, using the parallel operation mode, if its cable laying methods is improper, it will cause a lot of outstanding problems such as the increasing nonuniform extent of current distribution between same-phase parallel cables, higher sheath induced voltage and the higher parameter asymmetry. What's more, it may lead to the deterioration of the performance of relay protection, endanger the safety of cables and the operation of the power network. Based on the engineering application demand, this paper focuses on parameter calculation of multi-cables sharing the same phase, optimal layout design, performance of relay protection analysis and other aspects. Systematic theory and simulation research was carried out. This research has important theoretical significance and engineering application value for ensuring the safe operation of cables and power grid and improving the reliability of power supply.According to the structure and operation characteristics of same-phase parallel cables, in this paper, The internal mechanism and the influencing factors of the uneven current distribution of the parallel cable are analyzed. The equivalent circuit models of parallel cable transmission system is established. It also deduces the general calculation method of load-current distribution and sheath circulation current of parallel cable lines under different sheath grounding modes. Based on the law of electromagnetic induction, a universal method is proposed for the calculation of parallel cable sheath induced voltage. The basic analysis unit is that the sheath and the core wire are respectively connected with the ground to form the current circuit. Based on it, the calculation model of sequence impedance parameters and parameter symmetry evaluation indexes of parallel cables are established. On the basis of the above theoretical study, using analytic hierarchy process, it proposes a kind of evaluation system and method of parallel cable optimal layout which comprehensively considers multi factors such as load current distribution, sheath induction voltage and line parameter symmetry. It is also good for optimization design of parallel cable layout manner.According to the practical application requirement, it develops a set of calculation software for same-phase parallel cables operation parameters. According to the actual needs, the software can calculate some important operation parameters such as sequence impedance parameter, parameter symmetry index, sheath induction voltage, current distribution and sheath circulation current under different sheath grounding modes, different layout manner and different number of circuits for different kinds of power cables. Compared with simulation results, it shows that the developed software has high accuracy and can meet the requirements of engineering design.It uses double-circuits cables in same-phase parallel as an example which is most widely used. Under this situation, it analyses the effect on current distribution, sheath circulating current, sheath induction voltage and parameter symmetry caused by multi factors such as different sheath grounding modes, different layout manner and others. What's more, in order to guide engineering practice, it also proposes the best layout manner for double-circuits cables under different sheath grounding modes and several typical laying conditions.When a short circuit fault occurs inside same-phase parallel cables, the internal circulation current between cables may bring negative effects on the protective action performance. From different aspects such as theoretical research and digital simulation, it analyses the applicability of distance protection, directional pilot protection and current differential protection for parallel cables. At the same time, it gives related application suggestions. The research conclusion gives guidance and reference for the protection selection, setting and application for same-phase parallel cables.