| With the increasing lack of land availability,transmission corridors are growingly scarce,and therefore the multi-circuit transmission lines on the same tower which save land occupation have become the trend for power grids.The corona effect induced by multi-circuit transmission lines on the same tower is more complex owing to mutual coupling of lines,diversified phase-sequence arrangements,and construction of taller transmission towers.As an important part of the corona effect,corona loss(CL)is an important factor for measuring the operation economy and reliability of transmission lines.The CL evaluation of 750 kV AC four-circuit transmission lines on the same tower in dusty and sandy northwestern areas at high altitudes is an important link in the transmission design,lending significance to this research.For this reason,sponsored by the State Grid Corporation's Science and Technology Project,“Research on Key Technologies of 750 kV Four-circuit Transmission on the same Tower”,this paper research on evaluation of CL in 750 kV four-circuit transmission lines on the same tower considering complex meteorological conditions.The main research contents are as follows:The average maximum field strength of each phase conductor of 750 kV four-circuit transmission line on the same tower was researched.By using the FEM method,under various towers and phase sequences,the average maximum electric field strengths of transmission lines of six-and four-cross-arm towers in this transmission project are in the range of 14.33~15.70 kV/cm,and 12.29 ~15.90 kV/cm,respectively.Because of the interaction of electric fields between any two phases,the average maximum field strength of six-and four-cross-arm towers in the middle phase of the tower is larger than that of its side phase.Because of compact arrangement of the four-cross-arm tower,the field strength of the mesophase is more serious,and it can be reduced by phase sequence arrangement.Compared with single-circuit and double-circuit transmission lines on the same tower,the surface electric field strength of 750 kV four-circuit transmission lines on the same tower increases under rated voltage.The experimental data of CL in corona cage of 6×LGJ-500 at different altitudes under heavy rain were studied.The higher the test voltage,the stronger the field strength and the higher the altitude,the more obvious the impact on CL.Based on the LSM,the CL test data of 6×LGJ-500 bundle conductors under various field strengths were fitted and corrected in Exp-linear form,and the CL test data of that in heavy rain and sandy-dusty weather(after altitude correction)were acquired.The CL in 750 kV four-circuit transmission lines on the same Tower was evaluated under various towers and phase sequences.Firstly,the CL equivalence coefficients of the corona cage equivalent to the actual line were calculated.Next,considering fair,rainy,snowy,foggy weather,and in particular sandy-dusty weather,the CL from four-circuit transmission lines on the same tower under various phase sequences of the two tower modes were evaluated to thus acquire the mean annual CL and the maximum CL.The mean annual CL and maximum CL of six-cross-arm towers are 65.92 kW/km and 299.04 kW/km,respectively;the mean annual CL and maximum CL of four-cross-arm towers are in the range of 34.63~55.54 kW/km and 149.16~238.90 kW/km,respectively.The optimal arrangement(the four-cross-arm tower mode under II phase sequence)was obtained based on the evaluation of CL.Then,by comparing the CL and resistance loss of transmission lines under the optimal and the worst phase sequence arrangements in the project involving 750 kV four-circuit transmission lines on the same tower,it is found that the annual average CL was 25% to 47% of the resistance loss.Moreover,the maximum CL from such transmission lines is 107% to 215% of the resistance loss.Finally,the relationship between CL and altitude was analyzed. |
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