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The presence of water in the electro-insulating system of power devices is a serious operational problem. This problem refers first of all to devices insulated with cellulose materials impregnated with electro-insulating liquids [4, 5]. Examples of such devices are power transformers and instrument transformers. This problem also refers to transformer bushing insulators. With passing years of operation of a given device, its insulation moisture rises. This problem has been discussed in different scientific publications many times [2, 4, 15], mainly in terms of oil-paper insulation moisture of power transformers. Authors of these articles notice that water is not only a decomposition product of cellulose insulation but it also contributes to this decomposition. As a result of water presence in the insulating system and exposure to high temperature, the process of cellulose depolimerization takes place, which results in a decrease of its mechanical strength [1, 12, 17]. Other disadvantageous consequences of water presence in the insulating system are: electric strength drop of the insulation, lowering the inception voltage of partial discharges [19], and the threat of the bubble effect appearance [8, 11], which increases with more moistened cellulose insulation. High insulation moisture forces restrictions of the device load. One of positive aspects of water presence in solid insulation is the increase of its thermal conductivity, which to a certain extent improves cooling of the transformer windings [7]. However, negative results of water presence are predominant and there is a trend to reduce the insulation moisture of power devices as much as possible. Due to long-lasting operation of power devices, the problem of moisture is serious and concerns not only the Polish electric power system but also the systems of most countries. The problem of insulation moisture has been known to the international community for a few decades and it is still a current issue, which is confirmed by the newest publication of the worldwide range: they concern, among others, moisture measurement methods of transformer insulation [6, 13, 20], also problems connected with forms of water presence [3, 13, 21] and its migration in the cellulose−electro-insulating liquid system [2, 15]. Nowadays, the activities of the international CIGRE Working Group WG D1.52 Moisture measurement in insulating fluids and transformer insulation are in progress. Their purpose is, among others, to improve reliability of water content measurement methods in electro-insulating liquids.

Application of capacitive sensor for measuring water content in electro-insulating liquids