This paper introduces the reference measurement systems for calibrating instrument transformers under power quality phenomena and their associated novel uncertainties. The systems are lately developed at the national metrology institutes in European counties within the frame of the European project 19NRM05 "IT4PQ - Measurement methods and test procedures for assessing the accuracy of instrument transformers for power quality measurements". It's worth noting that the setups, as presented in the paper, are an improved extension of the system already developed in terms of generated waveforms and system characterization level. Especially, the detailed system setups and methods for combined instrument transformer calibrations are debuted. Moreover, new characterization tests were performed by using all the presented reference measurement systems