Simulation model for the calculation of the equivalent impedance of the transmission network
Authors: Dilparić-Cakić Milica 
Published: August 2024

The paper presents the formation of a simulation model for the analysis of the impact of renewable energy sources (RES) on the quality of electricity in the transmission system. The paper presents a concrete example from practice. The task was to determine the equivalent impedance of the system in the frequency domain at certain points of the network. These are the points where significant power from RES is expected to be connected. The simulation model should give the system impedance matrix at selected points for different cases. In other words, it is necessary to determine the system's own impedance at selected points, for different cases and at different frequencies, the so-called frequency sweep analysis. In order to determine the impedance of the system, a complex simulation model of the entire transmission network, which includes many elements of different voltage levels, was submitted. Also there were six different operating modes, i.e. different operational scenarios related to load and generation levels in the network. Summer minimum and maximum regimes for two years, as well as winter minimum and maximum regimes for one year have been provided. Also, the submitted simulation model is provided in the Siemens PSSE software package, and the calculations are required to be done in the Digsilent PowerFactory software. For these needs, the simulation model was converted for all delivered scenarios from one software package to another. Model verification was performed by comparing the power flow results in both software. It was accepted that a sufficiently well-converted model is one for which a mismatch in active power flows is up to 3% and mismatch in reactive power flows is up to 5%. The simulation model formed in this way was additionally improved by taking into account the frequency dependence of the impedance change of lines and transformers in the network. In other words, for transmission lines and transformers, the dependence of their impedance on frequency was entered into the model. Then such impedances are included in the calculation of the total equivalent impedance of the system. It is important to note that this type of data is specific and rarely publicly available, so the general recommendations defined for these purposes in CIGRE Technical Brochure TB 766 were taken into account during modeling. They take into account the change in resistance of elements with frequency as a consequence of the skin effect, while the change in reactance with frequency is adopted as linear. A frequency sweep analysis was performed on the created simulation model for all six operating states of the network. For each scenario, in addition to the basic calculation, a calculation with outages of one of the lines or transformers in the network was taken into account. In this way, a matrix was obtained that contains the system's own impedance at a selected point in the frequency domain, for a large number of different states in the network. Therefore, the model can be used for a comprehensive assessment of the impact of RES on the power quality at the connection point. Harmonic and flicker measurements can also be integrated into the model if necessary.