Calculation of the Electric Field below Hybrid Overhead Lines

Abstract

The demand for increased transmission capacities in Germany will be covered in part by high voltage direct current (HVDC) lines. In order to reduce the need for new corridors, hybrid systems with AC and DC circuits together on the same tower are planned. Therefore, the characteristics of such arrangements need to be studied. In the first part of this paper, electromagnetic coupling mechanisms between overhead lines are summarized. Next, the method of image charges as a way to calculate the electric field around overhead lines is presented. The method is then used to analyze the electric field on ground level below three different hybrid line configurations. KeywordsHVDC; hybrid line; electric field I. HYBRID LINES FOR THE GERMAN “ENERGIEWENDE” The remaining nuclear power plants in Germany are scheduled to be shut down in a few years according to the agreements of the Energiewende. As they constitute the largest generating units in southern Germany today, a considerable amount of power plant capacity will have to be substituted. Furthermore, continuing expansion of volatile renewable power generation with large regional differences and often far from load centers will lead to increased load flows over large distances. In order to retain network stability and reliability in spite of these challenges, the transmission capacities of the electrical grid have to be increased. For the first time in Germany, four high voltage direct current (HVDC) lines are among the projected measures [1]. Finding new line corridors is often extremely difficult, therefore the possibility of operating hybrid AC/DC power lines on existing towers is currently investigated by transmission grid operators (TSOs). The TSOs Amprion and TransnetBW plan to put into operation a 340 km hybrid line from Osterath to Philippsburg in 2019, when the Philippsburg nuclear plant will be decommissioned [2]. While the mechanical design of hybrid overhead lines is not very different to that of conventional ones, the close proximity of AC and DC conductors causes electrical coupling effects that have to be studied precisely. Also, it must be ensured that electromagnetic fields around hybrid lines do not exceed the permissible values [3]. In the first part of this paper, coupling mechanisms and their consequences are discussed in a general way. Next, the method of image charges as an approach to calculate the electrostatic field around overhead lines is explained in detail. Finally, this method is used to calculate the electric field below hybrid overhead lines. Three different tower types and the influence of different placement of the DC poles are considered regarding the maximum field values on ground level. For one configuration, the instantaneous field distribution along an AC cycle is discussed. II. COUPLING MECHANISMS BETWEEN OVERHEAD LINE CIRCUITS In general, three different coupling mechanisms can be distinguished: inductive, (quasi-) ohmic and capacitive coupling. A. Inductive Coupling Magnetic fields resulting from time-dependent current flows in one conductor produce longitudinal voltages in adjacent conductors according to Maxwell’s laws. Thus, high transients in one system will induce large overvoltages in systems nearby. In closed circuits, these voltages will also cause currents. On a hybrid line, stationary load flow in the AC conductors results in an alternating current component in the DC system because the DC sources appear as a short circuit to alternating currents. In addition to an increased voltage drop across a DC reactor, these currents can be harmful especially to any equipment with iron cores. Line commutated HVDC converters will turn the induced fundamental frequency current mainly into a second harmonic and a DC component on the AC side. In the worst case, this offset can lead to core saturation and endanger a safe transformer operation [4]. B. Ohmic Coupling High electric field strength on the surface of conductors leads to ionization of surrounding air molecules, the so called corona discharge. While ionized field charges from AC conductors stay close to the wires, field charges originating from DC conductors can move over large distances and thus reach neighboring systems. AC conductors collect the free charges, resulting in a small DC current being injected. The problems from such undesired current components have been discussed in the previous chapter.