Analysis of the influence of loop spacing on short-circuit electrodynamics of power cables

1. Introduction

With the increasing transmission voltage level and transmission capacity of power cables in recent years, high-voltage large-section cables have been widely used in power systems. In tunnels, high-voltage large-section cables are generally laid in a serpentine shape to reduce the thermal expansion and contraction of the cables and effectively alleviate the harm caused by the mechanical effect of the cables. The three phases of the cable are arranged horizontally or in a type pattern. There is a certain phase spacing between the horizontally laid cables, which is conducive to the dissipation of heat; the three phases of the type-laid cables are close together, which is conducive to reducing the induced voltage on the metal sheath. With the increasing number of cable loops laid in tunnels, the electromagnetic fields generated by each phase affect each other, making the electrodynamic analysis of the cable more complicated. Common short-circuit faults in cable systems include three-phase short circuit, two-phase short circuit, two-phase ground short circuit, and single-phase ground short circuit. The consequences of three-phase short circuit faults are more serious than other types, and the short-circuit current can reach more than ten times the normal working current. The energized conductor is in an alternating electromagnetic field and is subjected to huge electrodynamic forces. The cable will vibrate under the action of the alternating electrodynamic forces. If the mechanical strength of the cable and its accessories is insufficient, it may cause large mechanical stress in the cable body, joints and clamps, which may destroy the entire transmission line in severe cases. Therefore, accurate analysis and calculation of the electromotive force of high-voltage large-section cables can provide a theoretical basis for cable laying and hardware design to avoid failures.

2. Establishment of cable model

High-voltage large-section cables are usually laid in two ways: horizontal and type. A cable tunnel can contain multiple circuits such as double circuits and triple circuits. In this paper, the ZC-YJLW02 type AC 330kV cross-linked polyethylene insulated cable shown in Figure 1 is taken as the research object. The nominal cross-section of the cable conductor is 2500mm².

The short-circuit electromotive force received by the cable during operation is generated by the conductor in the magnetic field under the short-circuit current. The other layer structures of the cable have little effect on the numerical results of the electromotive force. Therefore, the conductor shield and insulation shield with smaller thickness are merged into the cross-linked polyethylene (XLPE) insulation layer with similar performance. Finally, a cable electromagnetic finite element model with a five-layer structure is established, which is the copper conductor layer, XLPE insulation layer, buffer water barrier layer, aluminum sheath layer and outer sheath layer. Combined with the actual laying of a section of high-voltage large-section cable in a tunnel in Xi'an, the cable span is set to 6m, the cable axis is a vertical serpentine structure, the distance from the crest to the trough is 0.26m, and the three-phase horizontal and type cable model is established as shown in Figure 2. The material parameters used for cable electromagnetic calculation are shown in Table 1.

When the short-circuit current flows through the conductor, an electromagnetic field will be generated in space. Considering the attenuation of the vector magnetic potential in space, sufficient calculation domains are set around the cable to ensure the accuracy of the short-circuit electromotive force calculation. The material of the calculation domain is air, and the vector magnetic potential at the outer boundary of the calculation domain is set to 0 (the first type of boundary condition), and the boundary at the junction of the cable end face and the calculation domain is set to magnetic insulation. The short-circuit electromotive force solution is a transient electromagnetic characteristic analysis, and the solution termination time is set to 60ms, that is, it contains 3 cycles, and the step size is 0.4ms. Under alternating current, the influence of the conductor skin effect on the electromotive force calculation must be considered. In order to ensure the speed and accuracy of calculation, the software adaptive meshing and manual setting of meshing size are combined to mesh the cable with denser meshes and the computational domain with relatively sparse meshes, as shown in Figure 3.

III. Conclusion

This paper uses electromagnetic coupling finite element method to establish the electrodynamic model of horizontal and type-laying high-voltage large-section cables, analyzes the electromagnetic distribution and short-circuit electrodynamic characteristics of the two laying methods under three-phase short circuit, and focuses on the influence of loop spacing on the short-circuit electrodynamics of double-circuit cables. The following conclusions are drawn:

(i) After a three-phase short circuit occurs, in the radial spatial dimension of the cable, the magnetic field intensity is the largest at the conductor surface and gradually decays outward. In the time dimension, the magnetic field oscillates and decays, which is reflected as a repetitive saddle-shaped surface in the space-time diagram; in the cable axial direction, the magnetic field intensity remains unchanged. The short-circuit electrodynamics of the cable changes periodically with time, and the period is approximately in multiples of the power frequency period. The peak value of the short-circuit electrodynamics gradually decays with the increase of the period. The maximum values of the short-circuit electrodynamics of horizontal and type-laying cables appear in phase B and phase A respectively.

(II) In the case of double-circuit horizontally laid cables, the electromotive force is exponentially related to the circuit spacing. As the circuit spacing increases, the electromotive force first drops sharply and then tends to be flat. An inflection point can be found, which can be used as a reference for the compromise value of the circuit spacing. The same method can also be used to find the inflection point for the change in electromotive force and circuit spacing of double-circuit type-laid cables. For ZC-YJLW02 type AC 330kV cross-linked polyethylene insulated cables, considering the short-circuit electromotive force and tunnel utilization rate, it is recommended that the horizontal double-circuit spacing be no less than 800mm, and the type-laid double-circuit spacing be no less than 600mm.