Todays development trends in power systems are characterized by a continuously growing share of power electronics in all voltage levels. This has resulted in an increased awareness for voltage quality not only at distribution level but also in transmission systems. To measure voltage quality parameters at high and extra high voltage, instrument transformers can significantly affect the overall accuracy of the measurements due to their frequency-dependent transfer characteristic. The paper starts with a short introduction of the new challenges and resulting needs related to Power Quality and Electromagnetic Compatibility in transmission systems. Limits of voltage instrument transformers for the accurate measurement of voltage harmonics and general methods for its determination are described and illustrated by examples. Finally, available solutions to ensure accurate voltage harmonic measurements are presented briefly.