Research On Electromagnetic Environment Effect Of UHVDC Transmission Lines

Based on the actual situation of our country, UHVDC transmission is widely used in China due to its unique advantages. Relevantly, the electromagnetic environment problem along with UHVDC transmission has attracted much attention. The criteria of electromagnetic environment of UHVDC transmission lines mainly include electric field, ion current, DC magnetic field, audible noise and radio interference.The main reason causing electromagnetic environment problem is DC corona. When the electric field on the conductor surface exceeds the corona inception field strength, the corona discharge will appears around the conductor. So a calculation is first made in this paper for the electric field strength on the conductor surface. The Markt-Megle method and adaptive charge simulation method are used to calculate the surface electric field strength of conductor bundles with three, four, five and six sub-conductors, respectively. It shows that for the five or more sub-conductors, the results obtained from the adaptive charge simulation method is more accurate than those from the Markt-Megle method. The investigation is carried out on the influence of sub-conductor number, sub-conductor cross section, split spacing and conductor height on the conductor surface maximum field strength, which shows that the maximum electric field on the surfaces of±800kV DC conductors usually exceed the corona inception field strength and so the corona discharge is easy to cause.As the space-charge-free field is the important component of total electric field, the maximum space-charge-free field strength can be used as a reference index. Split conductor is equivalent to a single wire and the presence of the ground is taken account by the image method. The obtained results are compared with those from the charge simulation method and a better agreement is shown between them. Therefore, the simplified image method, other than the the complex one, is employed to calculate the ground space-charge-free field strength. The influence of unipolar HVDC and bipolar HVDC sub-conductor cross section, bundle spacing, conductor height as well as the bipolar pole spacing is also inquired on the nominal electric field distribution on the ground.A detailed formula derivation is completed and the corona inception voltage calculation method as well as the integral method of electric field to the potential are given. By using the analytic method to calculate ground electric field strength and ionic current density, the calculated results and the measured results are compared, which have verified that the proposed method and program are workable and feasible. Meanwhile, the analysis is performed for the effect of unipolar HVDC and bipolar HVDC sub-conductor cross section, bundle spacing, conductor height and bipolar polar distance on the total electric field at ground level and ion current density size distribution.For calculating the DC line radio interference and audible noise, CISPR and EPRI recommended formula is adopted.In the±800kV UHVDC Hami to Zhengzhou transmission project, six sub-conductors are chosen and the conductor section, split spacing, conductor height and polar distance are taken as900mm2,45cm,16m,22m, respectively. Furthermore, the maximum ground electric field strength, ionic current density, audible noise and radio interference in the positive conductor projection to the lateral20m can meet our UHVDC line electromagnetic environment limitation standard.The calculation results are obtained by MATLAB software and are expressed in the form of chart and table.