Magazine | Issue 2 2025 0.001 0.1 1 Insulation geometry Oil conductivity Moisture and aging of cellulose 0.01 0.001 Hz 1 Hz 1,000 Hz Dissipation Factor Frequency Figure 1: The dielectric frequency response curve provides information about different factors that influence the measurement result, such as the moisture content of the insulation. Test time FDS 5 hours 40 minutes FDS & PDC 30 minutes – 1 hour 45 minutes 33 hours FDS & PDC+ Figure 2: Thanks to the patented combination of FDS and PDC+, the DIRANA completes the dielectric analysis of a transformer insulation in 30 to 105 minutes maximum. The FDS method requires more than one day for results in the same frequency range (1 kHz to 10 μHz). The substation is operating according to plan. No unusual incidents, everything is functioning normally. And then suddenly, a loud bang can be heard as a transformer’s insulation is punctured. The most expensive asset used in power transmission and distribution has just suffered a complete failure – an asset owner’s nightmare. A precondition for safe operation Insulation failure is one of the main causes of transformer failures. Regularly monitoring a transformer’s water content can help detect impending failures early on. Water is a natural byproduct of aging cellulose insulation and, therefore, an indication of its integrity and remaining life span. When an asset’s moisture content level is known, its lifetime can be estimated and – if required – extended with targeted maintenance measures, such as drying. The measurement method of choice: dielectric frequency response Dielectric frequency response (DFR) can be used to reliably assess the moisture in a transformer’s paper insulation. The non-invasive method is scientifically proven by CIGRÉ and can determine the moisture content reliably. Determining the value directly in the cellulose, where most of the moisture is located, is what makes this method so accurate. Using dielectric analysis to determine the moisture content also avoids the pitfalls of other analysis methods, such as environmental influences or the final decommissioning requirements for an asset. Informative low frequencies Information about the insulation’s dielectric response can be gathered by measuring and analyzing the dissipation factor tan(δ) across a wide frequency range. The measurement curve has a characteristic “hump” shape at very low frequencies (Figure 1). This is due to the polarizing effects of interfaces in the oil-paper insulation. The range roughly 1 to 2 decades below this increase is primarily influenced by the dielectric response of cellulose insulation, and therefore, by the transformer’s moisture content. The crux of the measurement time The dissipation factor can be determined using several methods. Frequency Domain Spectroscopy (FDS measurement) provides fast and precise results for frequencies above 0.1 Hz. However, at lower frequencies, this method is relatively slow. This means a measurement from the kHz down to the µHz frequency range lasts hours or days. 27
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