Single-phase exciting current components extraction

Introduced in 2017, the extraction of exciting current components opens the door for a further enhancement of electromagnetic circuits’ diagnostics.

byRoberto Borges


When first proposed in 1967, analysis of exciting current test results employed only the total current [1]. Advances in test equipment, allowed the measurement of exciting current as a phasor and the test became known as exciting current and loss. Current diagnostic criteria are based on the evaluation of a two-dimensional matrix of currents and watts and the identification of DETC, LTC and phase patterns, which are defined by various phenomena, other than failure modes, influencing the data. The physics behind the data was reviewed in [2]. An equivalent circuit of a transformer under no-load conditions is shown in Fig. 1. The circuit accounts for the energy loss and storage processes when power travels from one winding to another. The leakage impedance, due to magnetically coupled loops (e.g., series units, preventive autotransformers), is not included in the circuit.

Losses in the core, mostly due to hysteresis and eddy currents, are accounted for by Rm. The electrical energy stored in the turn-to-turn capacitances as well as in the capacitances to other windings and between windings and ground (all acting as loads) are accounted for by C. The magnetic energy stored in the core is accounted for by Lm.

The total measured current (Imeas) 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, introduced in [4, 5], allows the assessment of electromagnetic circuits free from the effects of capacitive loading.

 

 

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