Research On The Electrical Characteristics Of Coupled Transmission Lines And Analysis Software Development

The power transmission line is extremely important in the power system. The shortage in land resource brings new challenge to the power transmission lines. The underground power cables are more widely adopted in city grid and there are more occasions that overhead transmission lines are sharing the same corridors. These two occasions both bring the multiconductor coupling problems. Current is induced in the cable sheath and it will cause thermal problems, limiting the cable’s ampacity. More and more HVDC circuits are built onto the existing corridor, alongside the HVAC transmission lines. The steady state induction of fundamental frequency voltages and consequent circulating currents into the DC circuit will cause problems to the converters. A lot of work has been done in these multiconductor coupling problems. However, there are also some shortage in the existing research:1) The sheath loss calculation algorithm is not efficient enough; 2) The characteristic of the connecting impedance strategy to reduce sheath loss is not fully revealed; 3) The DC bias of MMC under fundamental frequency circulating current has not been studied; 4) The coupling effect on the LCC-MMC HVDC system has not been studied. According to the shortage of existing research listed above, we studied several coupled line issues, which can be concluded as following:(1) The computer efficient algorithm is established to fulfill the loss calculation task, which is based on the improved coupled line model. The key of the coupled line model is to use series expansion to calculate the admittance matrix without diagonalizing the complex propagation matrix of the cable line. The dimension of the nodal equation is also greatly reduced. The algorithm is as accurate as the traditional multiconductor methods while improving the efficiency by 80%. Based on the computer efficient algorithm, we develop the cable sheath analysis software.(2) We carry out a systematic research on the sheath loss of cables with different arrangements, including single bonding, solid bonding and cross bonding. For cross bonded cables, we study how the cross bonding joints number and segment length deviation influence the sheath loss.(3) To reduce the sheath loss, this paper studies a promising measure that uses connecting impedance in the cross bonding joints. The characteristic of the connecting impedance is fully revealed. We find that inductor does much better than resister here.(4) The DC bias of MMC under fundamental frequency circulating current is first studied in this paper, which shows refined consistency between theoretical analysis and simulation results. The DC bias of MMC is much smaller than that of LCC under the same conditions.(5) Comprehensive studies are carried out on the possible coupling effect of LCC-MMC system from the nearby AC lines. The parameters of the coupled system are studied on the coupling effect. The relationship between separate distance and parallel length of AC and DC lines is revealed.