Novel Condition Monitoring techniques for Large Oil/Paper insulated Transformers : Online Dielectric Response Measurements.

  • Novel Condition Monitoring techniques for Large Oil/Paper insulated Transformers : Online Dielectric Response Measurements.

Overview

Dielectric Response measurements are useful for determining the age of the cellulose material (paper and press bord) in a power transformer through the moisture content of the paper. These tests require the transformer to de-energised and disconnected from the network, along with the long duration of the test, can result in a outage of 1 day minimum. The preparation time for the test results in the transformer cooling down and hence the distribution of the moisture in the oil and the paper will change, resulting in test inaccuracies. A system has been developed whereby a dielectric response can be performed online with the transformer energised and drawing load. The technique has been verified in the laboratory on a 200kVA 11kV/433 3-phase transformer. A Chirp excitation voltage is used to speed up the measurement time, with a Chirp-matched filter to extract magnitude and phase.

Outcomes

(a) Experimental connection of the 200 kVA three phase transformer under investigation in the laboratory for measurement of the dielectric response online. The connection of the VT’s is shown to create the “virtual neutral” for use in the measurement circuit. Resistor Rn is used to measure the response current. (b) Measured dissipation factor for the tested transformer as well as the complex capacitance responses for the transformer under test. (C’: real part of complex capacitance; C”: imaginary part of complex capacitance; DE: de-energized; E: energized)

 

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