Power Transformer Condition Monitoring Based on Evaluating Oil Properties

Abstract

The authors review the techniques applied to diagnose oil aging. Further, the authors put forward a new diagnostic method. It stipulates for an additional high-frequency measuring loop formed in an operating transformer. This dielcometric measuring capacitor cell includes a stray capacitance generated by the transformer winding and core. The monitoring of the dependence between the physico–technological oil parameters and the measuring cell capacity is fundamental for the procedures for determining the composition and properties of the transformer oil filling this cell. High-frequency low-voltage is the output signal. To prevent the cross-impact of low-frequency high-voltage and high-frequency low-voltage circuits, the pilot high-frequency low-voltage is excited by a special coupling capacitor; the output to the power feeder is conducted through an appropriate low-frequency choke, where the measuring capacitor cell does not disturb the normal transformer operation. The key physical processes used for the monitoring are analyzed and described in detail. The authors develop an algorithm to compute the current resistances of both the transformer oil and its impurities. The transformer state is estimated by comparing the parameters specified with preset permissible limits. A structure flowchart based on two synchronous quadrature detectors is proposed for a high-frequency measuring loop. The monitoring system considered allows for determining the following insulating oil properties by using the algorithm for processing the recorded data: moisture content; dielectric losses due to the accumulation of aging products in the oil and its pollution; and the content of dissolved gases in the oil. The monitoring system operability and efficiency are confirmed by appropriate experimental studies. The experiments are conducted using a TM-25-6/0.4 oil-filled transformer with a capacity of 25 kVA in a steady-state operating mode at a load current of 25 A. It is found that the proposed control system allows for identifying a critical defect of increased moisture content in the oil with no more than 10% error, and a sensitivity threshold in the order of tenths of ppm.

Determining the Remaining Functional Life of Power Transformers Using Multiple Methods of Diagnosing the Operating Condition Based on SVM Classification Algorithms

“…The acid number is the mass of potassium hydroxide (KOH) required to neutralise the acids in the oil sample and is expressed in mg KOH/g. To eliminate the problems caused by sludge formation and deposition, it is recommended to treat (regenerate) transformer oil when the acid index is between 0.20 and 0.40 mg KOH/g [31,32]. Normal and critical values of the acid index are also specified in IEC 60422:2015, IEEE Std.…”

“…The second method described is the determination of the condition of the cellulose insulation based on furan compounds, low molecular weight alcohols and the moisture content of the paper-oil insulation system. The third method described is the condition of the transformer according to the condition of the insulating oil, as determined by the oil quality index [30][31][32].…”