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The condition assessment of oil–paper insulation is a vital issue in transformer management. To investigate the effect of moisture content on the accuracy of the prediction model, accelerated ageing experiments were performed with different initial moisture content (IMC) in laboratory condition. Two types of multi-parameter lifetime prediction models, e.g. with and without consideration of IMC, were presented for comparison purpose. The authors found that a better accuracy for its prediction results could be achieved by establishing the prediction models with the consideration of IMC. Besides, models considering IMC have better economic feasibility than that without considering IMC(non-IMC) as its total cost is cheaper according to life cycle cost (LCC) calculations. In addition, failure probability based on the proposed prediction models was also calculated as a direct indicator for evaluating the oil–paper insulation status. [ABSTRACT FROM AUTHOR]

The measurement of water inside a transformer is essential to maintain its reliable operation, as the water content can increase as the transformer ages. This measurement is frequently performed by a utility as a part of its periodic maintenance activities. In this paper, the application of optical fiber sensors as an online method to measure water content of paper inside an operating transformer is presented. There are several advantages of using these sensors, including their size, lightweight, flexibility, multiplexing, no-electromagnetic interference, etc. A unique benefit of fiber optic technology is that sensors can be embedded into the winding structure and operate in areas of high electric field strength. For this work, fiber optic water and temperature sensors are investigated by comparing their measurements to a commercial water activity probe immersed in oil. The study found that the determination of water content by fiber optic sensors compared closely with other techniques, which lead to the conclusion that they can be an effective technology for water measurement in an operational transformer. [ABSTRACT FROM AUTHOR]

In conventional frequency domain spectroscopy (FDS) measurement, sinusoidal excitation is applied to a composite oil‐paper insulation under test, for a minimum of two cycles over a wide range of frequencies typically starting from 1 kHz to 1 mHz or even upto 0.1 mHz. The frequency sweep down to 1 mHz, is inevitably time consuming especially in lower frequency ranges. Considering the aforesaid fact, the main aim of the present study is to reduce overall FDS measurement time. The insulation under test is subjected to some fraction of one full cycle of excitation, and the corresponding dielectric response current is measured for each fraction. Using least square curve‐fitting technique, response current over two cycles is predicted from which dielectric dissipation factor (tanδ) is calculated. For this contribution, sinusoidal and non‐sinusoidal excitations in form of triangular waveform with five different initial slopes are used as excitation voltage. The technique is experimentally validated on three samples with different preset moisture contents prepared in the laboratory. The dielectric dissipation factor obtained by using the proposed technique is compared with the conventional method using two cycles, which show negligible differences. Most important of all is the overall test time, which can be brought down significantly when compared with conventional FDS measurement. [ABSTRACT FROM AUTHOR]

The water concentration in transformer paper insulation must be monitored because, if it becomes too high, the paper insulation will age rapidly [1]-[4] and the transformer may fail on overload [5]-[9]. A failure can occur on overload if the temperature of the paper rises sufficiently to cause evolution of water bubbles. If these bubbles cause dielectric failure of the oil gap, the transformer may fail catastrophically. Paper insulation becomes brittle as it ages, and eventually becomes too brittle to act as an effective insulator. Oxygen increases the aging rate because the acids created by chemical reactions occurring in the oil, paper, and oxygen react further with water [10]-[12]. A study of the thermal aging of Kraft paper found that wet oxygenated paper degrades 40 times faster than the same paper heated in a dry oxygen-free environment [4]. The paper within a transformer is likely to degrade fastest at the top of the winding, where it is hottest [13]. Since the water concentration in the paper significantly influences its aging, it is important that the water concentration be measured accurately at the top of the winding. [ABSTRACT FROM PUBLISHER]

Frequency domain spectroscopy (FDS) is being used to assess the insulation condition of oil–paper power transformers. However, it has to date only been implemented on de‐energised transformers, which requires the transformers to be shut down for an extended period and may cause significant costs. To solve this issue, a newly improved monitoring method based on the FDS principle is proposed to implement the dielectric measurement on energised transformers. Moreover, a chirp waveform excitation and its novel processing method are introduced. Compared with the conventional FDS results, dielectric results from the energised insulation system have higher tanδ values because of the increased losses. To further understand the insulation ageing process, the effects of the geometric capacitance are removed from the measured imaginary admittance of the insulation system to enhance the ageing signature. The resulting imaginary admittance is then shown to correlate well with the central time constant in return voltage measurements results. The proposed methods address the issues on techniques used on energised transformers and provide a clue for on‐line FDS diagnostic application. [ABSTRACT FROM AUTHOR]