Capacitive voltage transformers - electrical performance and effective diagnostic measures

The electrical performance of capacitive voltage transformers (CVT) and thus the accuracy of the device is not only dependent on the transformation ratio of the intermediate inductive transformer but also on the capacitive divider, the compensating coil and the damping devices. All components of the CVT are prone to ageing when they are in operation over a long lifetime, especially under harsh environmental conditions. The accuracy requirements as well as the equipment condition can be affected during the overall lifetime but generally no diagnostic measurements are performed on CVTs. As a result, the failures or errors remain hidden. Failures and errors of CVT may not only result in an economical damage as the voltage is not measured correctly. It can also result in malfunction of the protection system or outages and technical damages of the CVT and the surroundings when the devices explode. This paper discusses the relevant design criteria and the electrical behavior of CVTs for typical failure modes. A model-based testing solution is discussed to assess the electrical performance and the root cause of a possible fault indication. The diagnostic measures presented in this paper can therefore accurately determine and assess the electrical performance of CVTs.