Evaluation and characterisation of test signals in the time and frequency domain for the frequency response analysis of voltage measurement systems

The power grid is undergoing a rapid transition driven by the large-scale integration of renewable energy sources and the adoption of SF6 -free technologies. Converter-interfaced renewable generation, long-distance HVDC transmission, and increasingly frequent switching operations are contributing to elevated levels of high-frequency distortion across modern networks. These non-fundamental voltage components result in complex signal environments that require advanced measurement and monitoring to assess their origin and impact on grid assets. At the same time, new equipment such as vacuum circuit breakers, SF6 -free instrument transformers, and SF6 -free gas-insulated switchgear is being deployed with limited long-term experience, highlighting the need for a deeper understanding of the frequency-dependent behaviour of both existing and emerging technologies. This need is further reinforced by the introduction of wideband performance classes in IEC 61869-1:2023 Ed. 2.0, which explicitly addresses high-frequency disturbance conditions. Together, these developments underscore the importance of comprehensive high-frequency characterisation to ensure the reliability and longevity of assets in the evolving power grid. This paper presents the most appropriate test signals which can be used to determine the frequency response of power system equipment, such as instrument transformers. The most reasonable test signals in both the time and frequency domains, depending on the required wideband class, are evaluated. The focus is on the advantages and challenges of generating the test signal, the impact on the test setup itself, and how to measure the primary and secondary voltages precisely. Physical dependencies are listed and discussed. Depending on the different approaches, the paper provides suggestions on the conditions and environment under which the specific test signals offer the highest precision.