In-situ calibration of voltage transformers on the Swedish national grid

In Sweden virtually all voltage transformers for high voltages are capacitor voltage transformers (CVTs). Concern has been voiced both nationally and internationally regarding the stability of CVTs over time. The program for in situ calibrations has been in operation since 1989. The Swedish measurement system is based on the use of a very accurate multi-ratio inductive voltage transformer rated for the full dielectric requirements on the 400 kV grid. The output voltage from the voltage transformer under test is compared to the output from the reference transformer in an automatic bridge circuit. A computer is used to read the data from the bridge and to prepare the calibration certificate. A six decade precision inductive voltage transformer, an inductive voltage divider, can be employed as a ratio matching element