Finite-Difference Time-Domain Code
The FDTD simulation space consists of a mesh of small cubic cells. Each of these can be designated as a conductor, a dielectric or free space in order to define the structure to be simulated.

FDTD simulations can be used for:

   • Design feasibility studies
   • Antenna optimisation
   • Determining frequency response
   • Improving PD detection
   • Modelling energy transfer

Capabilities
Data output can take many forms, including:

   • 2D snapshots of electric / magnetic fields
   • Time-domain sampling of fields at any number of points within the structure
   • Currents and surface charges

The simulation volume is truncated using absorbing boundaries. Spatial resolution is limited only by the available memory and processing power of the computer workstation. The number of cells is typically in the range 106 – 107 and simulation times can vary from fifteen minutes to several hours.

Example - Modelling transient radiation from a gas-switched HV pulse transmitter
Simulation volume:   2.4 x 2.4 x 8.0 m
Mesh spacing:         20 mm
Time step:                38 ps

The transient wavefront is dominated by the temporal characteristics of the gas discharge, while subsequent oscillations are governed by the electrode geometry.

Cylindrical co-ordinates
The software can also operate in a cylindrical co-ordinate system. In this form, it has been used to study discharges in the gaseous insulation between coaxial conductors.

These plots show the integrated energy density of the radiated electric field components over the coaxial cross-section of a 400 kV gas insulated busbar. The signal source is a partial discharge located at the outer conductor, 1.2 m from this plane.

time (2 ns per div)

The time-domain comparison above illustrates the level of agreement that can be achieved between simulation and measurement.

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