Single-Phase Exciting Current Components Extraction
Enhancing Diagnostic Power of Exciting Current and Loss Test on Power Transformers
Introduced in 2017, the extraction of exciting current components opens the door for a further enhancement of electromagnetic circuits’ diagnostics. Traditionally, analysis of single-phase exciting current and loss data relies on total current, loss and power factor. For some units, the prominence of the capacitive current component masks the behavior of the inductive component, yielding to uncertainties of the diagnostic criteria. The method allows the separation of the essential constituents of the total measured current and the report of all three, namely, IR, IL and IC components.
The total measured current (Imeas) during an exciting current and loss test is comprised of three current components, identified in Fig. 1 as IR, IL and IC. In most units with lagging Imeas, the patterns can be predicted by knowing the core type and inspecting the electrical diagram on the nameplate. In some cases, however, the presence of capacitive loading distorts the expected current patterns, making the diagnostic conclusions less certain. The method that extracts IL and IC, allows the assessment of electromagnetic circuit free from the effects of capacitive loading.
Figure 1. Equivalent circuit of a transformer under no-load conditions
Origin of Current Components
Understanding the phenomena that dictates the behavior of the current components is essential for the data analysis. A brief synopsis is given below.
Inductive Current – IL
The inductive current is a function of total inductive loading present during the single-phase exciting current and loss test. The objectives of IL are twofold: 1) to maintain the core magnetized; hence, it is influenced by changes in reluctance encountered by the flux in the core; and 2) to supply internal inductive loading.
Resistive Current – IR
The purpose of IR is to supply losses dissipated in the transformer during the test. These losses are mainly driven by hysteresis and eddy current losses in magnetized cores. For the most part, the pattern of IR, qualitatively, follows IL. IR is also unaffected by IC. Therefore, when interpretation of Imeas is challenged by pattern distortions due to the relative magnitudes of IL and IC, IR can serve as a useful diagnostic indicator.
Capacitive Current – IC
The objective of IC is to supply the internal capacitive load. This includes current accounting for the total inductively-coupled capacitive load and the total current leaking to ground.
Exciting Current Components Extraction Featured by Doble Test Assistant
Historically, the empirical data reported by Doble Test Assistant (DTA) included the total current, loss and an indication of whether the current is lagging or leading. The new generation of DTA allows an automatic extraction of total current components through an embedded algorithm with additional data now including inductive, resistive, and capacitive currents, as well as power factor and phase angle (Fig. 2). With that, the diagnostic power of the test is improved, as the data can now be evaluated without impact of capacitive loading.
For instance, the analysis of IL and IR should follow the guidelines for lagging Imeas. That is, the expected phase pattern on three- and five-legged core-type and shell-type units is of two high similar readings and a lower reading (2H1L), with the latter obtained on the phase located on the middle leg of the core. Other core types and winding configurations might lead to different patterns.
Figure 2. Exciting current components extraction featured by Doble Test Assistant
Advantages of Single-Phase Exciting Current Components Extraction
- The use of extracted exciting current components enhances our ability to evaluate the condition of electromagnetic system without the influence of capacitive loading.
- Further investigation using the method is expected to allow for a better understanding of the impact the location and the nature of the fault have on each of the current components.
- The latter will ultimately improve the diagnostic power of the exciting current and loss test on power transformers.
More Information:
- Product: DTA Software
- Article: Doble Test Assistant 8 Delivers Greater Diagnostic Power and Instrument Support for Comprehensive Field Testing and Analysis
