Analysis Of Operation Characteristics And Relay Protection Applicability Of Parallel Cables

With the development of China’s national economy and urbanization,the electricity load of the city is growing rapidly,which puts forward higher requirements for the transmission capacity of the cable line.To make full use of the transmission capacity of existing cables and reduce the costs of expansion,paralleling existing cables with new ones has been gradually applied in engineering practice.Due to the near-location among multi-loop parallel cables,the electromagnetic coupling is strong,especially the special structure of common-phase parallel,which may bring many new problems and challenges to the operation characteristics and relay protection of parallel cables.Combined with the engineering application demand,systemic research has been carried out to analyze and optimize the operation characteristics and relay protection of parallel cables.The results are important in theory and practice for improving the operating efficiency of parallel cables,ensuring the safety of cables,and improving the reliability of power supply.When operating in parallel,the difference of coupling effects among parallel cables may lead to uneven distribution of load current and reduce the load capacity utilization of parallel cables.Based on the structure of parallel cables,a calculation method of load capacity utilization is proposed.On this basis,the phase sequence arrangement with the highest load capacity utilization under different layouts is analyzed theoretically based on the equal-current matrix,which is checked by numerical simulation.The conclusion plays an important guiding role in improving the transmission efficiency of parallel cables.When flows through cable,the load current will induce metal sheath voltage and the voltage will be superimposed by different circuits under parallel,which may exceed the requirements of engineering specifications.According to the shortcomings of existing studies,this paper proposes an improved calculation method for metal sheath induced voltage of multi-loop parallel cables by considering load transmission capacity and uneven current distribution under different layouts.Based on this method,the factors affecting the mutual inductance voltage of metals are analyzed,and some suggestions for improving the voltage are given.It’s difficult to transpose for cable,whose asymmetry is higher.The unreasonable arrangement will further aggravate the asymmetry under parallel,which will affect the power supply quality and equipment safety of the power grid.In this paper,the ratio of negative/zero sequence current to positive sequence current is used to reflect the asymmetry of parallel cables,and the weights are determined by the possible harm of negative/zero sequence current to the power grid.On this basis,a method for calculating the asymmetry of parallel cables is proposed,and the effects of different layouts,phase sequence arrangements,metal sheath grounding modes,cable circuit spacing and phase spacing on parallel cable asymmetry are studied.Some measures to improve the asymmetry of parallel cables are proposed.The operation characteristics of parallel cables involve many aspects such as load capacity utilization,sheath induced voltage,and asymmetry,which are all affected by the cable layouts.In this paper,the analytic hierarchy process is used to construct the judgment matrix and weights according to the influence degree of each operation characteristic parameter on parallel cables and power grid.Then,the comprehensive evaluation index is established.On this basis,combined with the engineering practice,the operation performance under different layouts and phase sequence arrangements is systematically analyzed based on double-loop parallel cables.The analysis conclusion can provide an important reference for the optimal design of parallel cables.In reality,parallel cables are often used in the common-phase parallel structures to reduce the cost of capacity expansion.Since current transformers measure the confluence of the parallel cables,when faults occur inside parallel cables,the augmenting or shunt function of sound circuits will directly affect the operation performance of relay protection.According to the typical protection configuration scheme of parallel cables,this paper systematically analyzes the influence of the special structure of common-phase parallel cables on distance protection and differential protection by theoretical research and digital simulation.Aiming at the existing problems,improved schemes are proposed to improve the performance of protection and ensure the operation safety of parallel cables.