Diagnostic Frequency Response Analysis (FRA) Techniques for Faults in Transformers

摘要

When power transformers are operated for many years, it becomes necessary to grasp their internal conditions in order to confirm how viable they are in order to prevent malfunction in advance. It is very important to establish diagnostic technologies to check inner conditions effectively. A method of Frequency Response Analysis (FRA) has been researched as a new approach to discover any deformation of the inner assembly of a transformer and any malfunctions such as contact defect by the external diagnosis. Such an approach contains, however, some unclear aspects including phenomena and diagnostic approach. Although an FRA measurement method is specified in IEC60076-18, it is intended mainly for users as a guideline for macroscopic diagnosis. It is, therefore, generally difficult to perform detailed diagnosis to identify the abnormal parts. Under such technical circumstances, we investigated the applicability of FRA through actual measurements and analysis. For this purpose, we adopted two fault models for aged transformers; grounding lead wire disconnection in the electrostatic shield (Case 1) and double grounding in the core (Case 2). In Case 1, we confirmed that it is possible to locate a faulty section through a 3-phase comparison and analytical comparison of FRA characteristics, even though initial data was lost and not available. As a result, the effectiveness of FRA measurements was proven in regard to fault diagnosis on winding systems. In Case 2, we confirmed that diagnosis by comparison is possible with the use of initial data of FRA characteristics in a low-frequency domain. This implies that FRA measurements by external diagnosis on transformer cores are still effective.