Instrument transformers are crucial elements in the metering infrastructure of high voltage grids. With the evolvement of smart grids and the liberalization of the electricity market, new billing points can arise in the grids where electrical energy is transferred between different parties. When these new billing points are at existing places in the grid, it is strongly preferred to use the available instrument transformers for realization of a revenue metering system rather than installing new CTs and PTs. However, these available instrument transformers do not always meet the accuracy class requirements prescribed by legislation like electricity metering codes. In this paper, we present the results of an extensive performance evaluation of the ratio error of high voltage CTs under actual operating conditions. The aim of this evaluation was to find out to what extent CTs that do not meet the required accuracy class can nevertheless be used for revenue metering systems when their ratio errors are measured and corrected for. The study has a significant impact on the expansion of the metering infrastructure within smart grids, since it paves the way to a significant cost saving in the realization of new revenue metering setups.