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This paper highlights high-voltage laboratory acceptance tests performed by Manitoba Hydro on new 230kV current transformers (CTs). In performing these tests it was revealed that a number of CTs failed to meet requirements for partial discharge (PD). For the performance of PD tests, a modern digital PD measurement instrument was used. This instrumentation proved valuable in its ability to produce phase-resolved PD pattern plots which could be analyzed for identification of the defects causing discharge. From this work, much has been learned about the nature of PD in oil-paper insulated instrument transformers. Evidence from phase-resolved PD measurements suggested the presence of gas-filled voids within the insulation producing PD. It has been observed that PD in oil-paper insulation has some peculiar qualities; PDs can incept, then extinguish, and then re-incept, all while voltage stress may be held constant. Additional evidence suggests that the discharge activity may be influenced by exposure to extended periods of overvoltage prior to test. Findings show that extended exposure to overvoltage prior to PD measurement may cause instrument transformers to falsely pass PD tests.