FDTD modeling of fast transient currents in high voltage cables

A novel approach to the modeling of sub-nanosecond partial discharge (PD) phenomena in high voltage cables is presented to illustrate the capabilities of the finite-difference time-domain (FDTD) simulation method for modeling transients and their effects measured at a distance. FDTD modeling computes the three-dimensional electric and magnetic fields throughout the simulation volume. This numerically intensive process has the advantage that time-varying quantities such as voltages and conducted currents can be recorded at any position within the model. For modeling PDs in cables, a current source is introduced within the insulating medium and its time-domain waveform defined using digitally sampled data from an actual PD current pulse measurement. High frequency current transformers (HFCTs) are commonly used for on-site detection and location of PD in HV distribution cables. The HFCT itself cannot be modeled directly using FDTD because its geometrical detail is too small for the FDTD mesh. However, a hybrid model can be implemented by recording magnetic field values on path enclosing a conductor, applying Ampere's circuital law to determine the PD current and applying the responses of real HFCTs that have been characterized experimentally. The method for simulating localized insulation breakdown is presented, showing how this can be used to launch a PD pulse into a cable model and predict the output response of an HFCT some distance away. This process is illustrated through a parametric study of variations in the PD source parameters and by comparison of the data with the measured propagation properties of PD signals in an 11 kV EPR-insulated cable sample. Examples are also given in which the FDTD software is used to model time-domain reflectometry measurements that can be useful for locating damage within the same type of HV cable. Similar techniques could be applied to much larger insulation breakdown currents, for example, in gas switches.