Question

Hi.

I'm writing a project about overhead lines, and have a question regarding AC and DC in overhead line.

Is there any different in the electric field if you look at AC and DC

And do you know anything about the simulation program FEMM

Hope you can help me, good day

Answer 1

In direct current (DC), the electric charge (current) only flows in one direction. Electric charge in alternating current (AC), on the other hand, changes direction periodically. The voltage in AC circuits also periodically reverses because the current changes direction.

Both the AC and DC in overhead lines have their own behaviour that are being utilized as per convenience and suitability.The major difference between both of these are mentioned herewith.

1 for an AC transmission the dissipation through dielectric loss becomes significant above 765 kV. The dielectric losses  are caused when dipoles in matter align with a changing local electric field. As the polar structures turn to follow the field, the movement causes local heating. This case is not seen in case of DC transmission

2. overhead DC power lines can transport significantly more power for greater distances than AC lines, mainly because of two reasons: the effective voltage can be higher, and the wires can be bigger. This is not feasible for ac lines due to skin effect that restrict an AC current from penetrating to the center of a large wire there by reducing the power carrying ability.

3. Another important property that differentiates AC from DC power lines is that for an AC line, the line power must be synchronized with the local AC grid at both ends of the line, whereas DC power can bridge between two different synchronized AC grids that are not synchronized with each other. For this reason, DC power lines are often referred to as “asynchronous links” by power engineers.

FEMM

The finite element method (FEM) is a computational method that can be applied to obtain solutions to a variety of problems in engineering and science. Steady, transient, linear and nonlinear problems in electromagnetics, structural analysis, and fluid dynamics may be analyzed and solved with it. It combines geometrical adaptability and material generality for modeling arbitrary geometries and materials of any composition without a need to alter the formulation of the computer code that implements it. The idea of the method is to divide the problem domain into a large number of subdomains, called finite elements, each with a simple geometry resulting in the transformation of the initial problem from a small but difficult to solve into a big but an easy to solve.