Understanding of HV Current Transformer Failures During Short-Time Current Tests - An Insight of Current Density Calculations Beyond IEC Standards

Abnormal current is the result of a power system fault for which all components must be well designed. And also it is highly difficult to estimate the number of such occurrences in the network. As a part of this, it is very important to design an instrument transformer in a manner that it should withstand short-time thermal current and dynamic currents. In general, these tests have been considered as type tests as per IEC and IEEE standards. Short circuit tests on inductive current transformers shall be made on the primary conductor with all secondary windings short-circuited so that the required thermal energy and peak current is created. Dynamic peak test can be combined with thermal test also but major peak current must be at least rated dynamic current (I
_dyn
). Therefore, primary conductor design is to be given a key attention with regard to short circuit withstand capability. Enough cross sectional area along with its support structure decides the performance during these tests. Otherwise it leads to failure at various instants viz. opening of tank in case of oil filled inductive current transformers, cracking of body in case of dry type current transformers and failure during post accuracy measurements. Further as per IEC standard it is mandatory that the immediate insulation covered on conductor must not show any kind of deterioration. In this paper methodology of short-time thermal current and dynamic current tests as per IEC and IEEE standards have been described. Insights of temperature calculations as per IEEE standards have considered for arriving critical current densities. Further, optimum current densities are proposed for oil filled as well as dry type current transformers. Visual inspection is clearly reviewed and necessary recommendations were made. Few failure case studies were analyzed which further helps to manufacturers to improvise the primary conductor designs and its mechanical structure.