IEC TR 62681 pdf free download
IEC TR 62681 pdf free download.Electromagnetic performance of high voltage direct current (HVDC) overhead transmission lines.
3 Electric field and ion current
3.1 Description of the physical phenomena
Electric fields are produced in the vicinity of a HVDC transmission line, with the highest electric fields existing at the surface of the conductor. When the electric field at the conductor surface exceeds a critical value, the air in the vicinity of the conductor becomes ionized, forming a corona discharge. Ions of both polarities are formed, but ions of opposite polarity to the conductor potential are attracted back towards the conductor, while ions of the same polarity as the conductor are repelled away from the conductor. Space charges include air ions and charged aerosols. Under the action of an electric field, space charge will move directionally and ion current will be formed. The physical phenomena of electric field and ion current are described in this clause.
The electric field and ion current in the vicinity of an HVDC transmission line are defined mainly by the operating voltage and line configuration. The voltage applied to line conductors produces an electric field distribution. Unlike High-voltage Alternating Current (HVAC) transmission lines, the electric field produced by HVDC transmission lines does not vary with time and, consequently, does not produce any significant currents in humans or objects immersed in these fields.
The electric field is another aspect of the electrical environment around an overhead HVDC transmission line. An electric field is present around any charged conductor, irrespective of whether corona discharge is taking place. However, the space charge created by corona discharge under d.c. conditions modifies the distribution of an electric field. The effect of space charge on electric fields is significant.
For the same HVDC transmission lines, the corona onset gradients of positive or negative polarities are different and the intensity and characteristics of corona discharges on positive or negative conductors are also different. Consequently, during the design of HVDC transmission lines, special consideration should be paid to the allowable values of the maximum ground-level electric field and ion current density [1]1.
Corona on a conductor of either positive or negative polarity produces ions of either the positive or negative polarities in a thin layer of air surrounding each conductor [1]. However, ions with a polarity opposite to that of the conductor are drawn to it and are neutralized on contact. Thus, a positive conductor in corona acts as a source of positive ions and vice-versa. For a unipolar d.c. transmission line, ions having the same polarity as the conductor voltage fill the entire inter-electrode space between the conductors and ground. For a bipolar d.c. transmission line, the ions generated on the conductors of each polarity are subject to an electric field driven drift motion either towards the conductor of opposite polarity or towards the ground plane, as shown in Figure 1. The influence of wind or the formation of charged aerosols are not considered at this stage. Three general space charge regions are created in this case:IEC TR 62681 pdf download.
