How can you validate cable and overhead line models for EMT simulations?
The time-domain response of a cable system and overhead line model can be validated based on impedance frequency sweep calculations.
With the frequency sweep command your can calculate the transfer function of the line in the frequency domain. The calculation is based in the line equations for R, L and C (see Technical Reference) and therefore correspond to the correct response of the line.
The time-domain EMT models for cable systems and overhead line (lumped PI--model, Bergeron model, J. Marti model or phase-domain ULM model) are an approximation of the repose in the frequency domain. The time-domain response can be validated as follows:
1. Connect the line (cable system or overhead line) to a voltage source (ElmVac) that injects a multiple frequency voltage into the line (as demonstrated in the attached project).
2. Run an EMT simulation with the line being in steady state.
3. Record the voltage and current at the sending end of the cable.
4. Execute a Fast Fourier Transform (FFT) on the current and voltage signal to obtain the voltages and currents in the frequnecy domain.
5. Calculate the impedance Z(f) = U(f) / I(f) for each harmonic order using the FFT results.
6. Compare the frequency sweep calculations with the EMT/FFT simulation. The EMT/FFT results should approximately match to the calculation in the frequency domain (frequency sweep calculations).
Attached with this FAQ are two PowerFactory project, one containing a cable system, the other an overhead line system. For both cases, the frequency sweep calculations are compared with the response of the cable in the time domain.
Based on the comparison of the frequency domain and EMT time domain response of the model, the line model can be tuned and its response in the time domain optimised.