Research On Voltage Unbalance Of Four-circuit Transmission Lines With Different Voltage Levels On The Same Tower

With the development of economy and urbanization,in order to solve the contradiction between the increase of users’ demand for electricity and the tension of transmission line corridors,multi-circuit lines on the same tower is widely used because of its strong transmission capacity,small area of corridor occupation and low construction cost.In the four-circuit lines with different voltage levels on the same tower,there are many circuit numbers,complex coupling relationships between conductors,and asymmetric line parameters,which may cause voltage unbalance.After the power grid of a certain area in Shandong Province was transformed into four-circuit lines(two 35 k V lines and two 220 k V lines)with different voltage levels on the same tower,serious voltage unbalance occurred,resulting in the failure of normal power transmission operation.Taking this as the research background,this paper makes a systematic study on the voltage unbalance problem of four-circuit transmission lines with different voltage levels on the same tower,which has both neutral point direct grounding mode and neutral point non effective grounding mode,and puts forward the corresponding improvement measures.The main work of this paper is as follows:(1)The complex coupling relationship between four-circuit lines on the same tower is analyzed.Based on the tower structure data,the line parameters are studied.Based on the study of the mode of lines on the same tower and the line length distribution,the lumped parameter π model is determined as the line model,and the simulation models of four-circuit lines with different neutral grounding modes are established.(2)Based on the theory of parameter calculation of four-circuit lines with different voltage levels on the same tower,the mathematical expression of voltage unbalance degree of neutral point ungrounded system on the same tower is derived.The influencing factors of voltage unbalance degree are analyzed,and the simulation verification is carried out based on the established simulation model.The mathematical expression of voltage unbalance degree of the system with neutral point grounded through arc suppression coil on the same tower is derived,and the influence of detuning degree and damping rate on voltage unbalance degree is simulated and analyzed.(3)Take the four circuit transmission lines on the same tower of a certain regional power grid in Shandong Province as an example.According to the reconstructed line parameters,the corresponding simulation model is established.The results show that the simulation results are completely consistent with the measured data at that time.On the one hand,the correctness of the model established in this paper is verified;on the other hand,the root cause of the voltage unbalance problem is revealed: the 35 k V line is transformed into a part of four-circuit lines on the same tower,which is affected by the electrostatic coupling effect of the upper 220 k V line,and the parameters of the line on the same tower are asymmetric,resulting in the increase of the neutral point displacement voltage of the 35 k V system.(4)Based on the above simulation analysis and example verification,combined with the actual situation,three measures to improve the voltage unbalance are proposed: the optimal phase sequence arrangement,increasing the capacitance to the ground,cyclic transposition.The effectiveness of the improvement measures is verified by simulation.The paper has 30 pictures,23 tables,and 85 references.