Methodologies for Defining Harmonic Specifications in Transmission Systems at Future Points of Connection of Energy Resources

The integration of renewables into transmission networks introduces significant challenges for maintaining power quality, particularly in greenfield substations where harmonic observability is absent. TSOs must define harmonic specifications for new PoCs, including permissible distortion limits and Thevenin equivalent parameters, early in the planning process. This paper presents and compares two approaches for forecasting harmonic background levels at transmission-level PoCs. The first, currently applied by the Polish TSO, uses a deterministic method based on a steady-state harmonic model and nodal admittance relationships, producing fixed distortion values for selected scenarios and contingencies. The second, under development, introduces a probabilistic framework that leverages historical measurements, impedance-based weighting, and amplification factors derived from frequency-domain scans, capturing variability and resonance effects. A case study on the Polish transmission network demonstrates that the probabilistic method offers richer insights into harmonic behaviour, while the deterministic approach remains computationally efficient and suitable for sensitivity analysis. Future work will focus on hybrid solutions and machine learning enhancements to improve forecasting accuracy and adaptability in power-electronics-dominated grids.