Influence of Current and Voltage Transformer Errors on Harmonic Compliance Assessment in Power Quality Measurements

Power quality measurements in medium- and high-voltage networks are often performed indirectly using current transformers (CTs) and voltage transformers (VTs). Even when the power quality instrument itself complies with relevant standards, frequency-dependent transformer errors may affect harmonic quantities used for compliance assessment. This paper proposes an interval-based compliance-decision framework for stationary harmonic states to evaluate the influence of CT and VT magnitude errors on harmonic compliance decisions. The framework converts admissible transformer errors into lower and upper bounds of assessed harmonic quantities and compares these intervals with normative limits. The approach is applied to voltage harmonic limits according to EN 50160 and to current harmonic and total demand distortion limits according to IEEE Std 519-2022. The results show that transformer errors are most relevant in near-limit cases, where the error interval may cross the compliance boundary and lead to possible false compliance or possible false non-compliance. An illustrative application using measured CT current-error data demonstrates how transformer-specific frequency-response data can be introduced into the proposed framework. The proposed method provides a decision-oriented assessment of instrument-transformer error contributions in indirect harmonic compliance studies.