Short Circuit and Open Circuit Natural Frequencies of 3-Φ Transformers: Derived Analytical Expressions and its Applications

Abstract

Frequency Response Analysis (FRA) method is perhaps the most sensitive tool that can detect even the slightest of winding/core movements. High sensitivity, non-invasiveness, non-destructiveness, and on-site capability are some salient features and#8722; making it an ideal monitoring and detection tool. The existence of Standards (IEEE, IEC, and CIGRE) is ample testimony of its global acceptance and superior detection capabilities. The detection principle is based on observing a deviation between two measured FRAs which implies a possible fault. Naturally, the next logical step is to analyze these deviations to determine the type of fault, estimate the extent of damage and its severity, and as a bonus, predict its location, if possible. However, even after three decades, arriving at these inferences is still at the research level. Even though there is a consensus among all the standards on FRA test/measurement procedures, best-suited terminal connections, cable layout, grounding practices, etc., they remain largely silent regarding interpretation and diagnostics. A detailed analysis of literature compiled in Chapter 1 reveals that lack of a mathematical foundation might be one reason for the present plight of FRA. So, developing a generic mathematical-based approach for interpretation and location of incipient mechanical winding damages in actual 3-and#934; transformer windings using measured FRA, is imperative. Development of such a generic method necessitates derivation of closedform expressions, which can directly link measured FRA quantities to the electrical parameters of the winding. For assessing damage severity, the challenge is to identify a quantity which is not only extractable from measured FRA, but also be sensitive, monotonic, and traceable to the fault. Driven by this philosophy, this thesis aims to address the following - 1. Propose a unified and general approach to derive closed-form analytical expressions (for each multiphase winding) to link the measured open and short circuit natural frequencies to electr...