Why is water killing power transformer insulation? Water is a slow but deadly poison for power transformers

Water is a threat to the insulation of oil (or ester) insulated power transformers. It leads to reduced breakdown strength of the oil and an accelerated aging of the cellulose insulation. Dielectric frequency response analysis is a suitable and accurate way to assess the water content of the insulation and it helps to determine the condition and remaining lifetime of the transformer. Prior drawbacks of this technology, such as long measurement times and complex assessment, can be eliminated using modern software. The PDC+ technique, an improved version of the polarization depolarization current (PDC) measurement, reduces the measurement time up to a factor of 20. Further automation of the whole measurement and assessment process makes the PDC+ technique easy and reliable for all users. 1 Water in power transformers Water is the foundation for life on our planet. It is essential for every living plant or creature. But there is one place where water greatly reduces life: The oil-paper insulation of power transformers. This insulation consists of paper and pressboard elements which provide the mechanical stability of the insulation. The main component of paper and pressboard is cellulose. It consists of glucose molecules which are linked and form a chain (Figure 1). The average number of glucose molecules in a cellulose chain (which is also called “degree of polymerization” or “DP” in short) is about 1200 for new paper. These large chains give the paper the mechanical strength it requires to fix the windings, even in rough conditions such as when a short circuit has occurred. Figure 1: Part of a cellulose molecule showing 12 glucose molecules (schematic) Unfortunately, these chains can be split by water molecules which reduces the mechanical force they can withstand. A small amount of moisture is always present in the paper of a power transformer, even if it is dried perfectly. At elevated temperatures, the water molecules split the cellulose chains, causing shorter chain lengths and – as a byproduct – even more water. So a self-accelerating process is taking place which increasingly reduces the mechanical stability of the transformer insulation. The speed of this process is highly dependent on the temperature but also on the moisture content (Figure 2). Although the higher temperatures displayed in the chart are unlikely to be reached by the average temperature of the transformer, local hot spots have to be considered. Figure 2 Dependency of life expectance on moisture content and temperature [1] When the average length of a cellulose chain in paper is 200 or below, it is usually considered at the end of its life because the mechanical strength is then so heavily reduced that it can’t withstand higher stresses. As aging produces more water molecules, the water content of a power transformer is a very good indicator for the age of the transformer insulation. It allows not only an assessment of the remaining lifetime but also correct condition based maintenance: A moderately wet power transformer might be dried which reduces the water content and thereby slows down the aging. If a power transformer is very wet, the paper is usually in a very aged state and drying wouldn’t help as it can’t un-age the paper. If it was dry, drying would be a waste of time and money. In a mineral oil filled power transformer, the amount of oil is about 10 times larger than the amount of cellulose insulation [2]. However, as water is barely soluble in oil, the vast majority (> 99 %) of water is located in the cellulose insulation and not in the oil. Oil samples of power transformers contain water only in the lower ppm (parts per million) range whereas the range for water in the cellulose is usually in the area of 0.3 % to 6 %. The water content of the oil is highly dependent on temperature, oil condition and impurities [2]. Small errors in sampling and handling result in high deviations of the final result [2]. The water content in the cellulose provides a much more reliable value for condition assessment, as it is barely influenced by those parameters. Unfortunately, sampling of cellulose for moisture analysis is a very difficult task as the solid insulation of a power transformer isn’t easily accessible. 2 Moisture determination via dielectric frequency response analysis A simple and reliable way to determine the amount of moisture in a power transformer is dielectric frequency response analysis (DFR) [3]. When a material absorbs water, it changes its dielectric properties, such as conductivity, capacity and dielectric losses. This principle is used, for example, in humidity sensors. Cellulose also shows such a dependency of dielectric values at different water contents. 2.1 Measurement principle In dielectric moisture analysis of power transformers, the whole main transformer insulation is used as a humidity sensor. The measurement device is connected to the bushings and determines the dielectric properties of the insulation. The connection is very simple, like in a normal power factor/tan(δ) measurement. Only one insulation has to be measured. Typically, the main insulation from the high voltage to the low voltage side (CHL) is used as it contains most cellulose. For analog to power factor/tan(δ) measurements, a guard is used to bypass unwanted influences (Figure 3). Figure 3: CHL measurement on a 2 winding transformer without and with guard As moisture influences the dielectric properties of cellulose, especially at very low frequencies, the measurement is performed down to the μHz region. Figure 4 shows the dielectric losses of four power transformers in a frequency range from the μHz region up to some hundred Hz. Each curve has a characteristic shape which includes a more or less pronounced “hump” at lower frequencies. The region below the “hump”, around 1 2 decades from the “hump” peak, is highly influenced by the water in the paper insulation. Determining this frequency region is essential. Figure 4: Broadband dielectric response of four different power transformers at 20°C 2.2 Result assessment The assessment of the curves is a complex process as many parameters are influencing the dielectric properties [2]. However, modern measurement software, which include a database of dielectric properties of pressboard at various water contents and temperatures, help users to automate this process. The principle of the assessment is to simulate a transformer insulation using the database and to model barriers and spacers with the so called X-Y-model [4]. This way, no “fingerprint” or previous measurement is required and all kinds of transformers (and also other oil-paper insulated assets) can be simulated (Figure 5). The parameters of the simulated curve are adapted until the simulated curve fits the measured curve. When this is the case, all parameters (moisture, oil conductivity, etc.) of the “fitting” simulated curve are identical to the parameters of the measured transformer. The whole procedure involves a high number of calculations but is performed within seconds by the measurement software. The DFR method has been described by CIGRÉ [5] [6], and an IEEE guide is currently under development [7]. There are no other non-invasive ways to assess moisture in a transformer that can provide comparable accuracy. Figure 5: Principle of the water content assessment of power transformers 3 Reduce the time for DFR measurements 3.1 Traditional FDS measurement method Traditionally, dielectric parameters are measured in frequency domain by applying different frequencies and measuring the response. This technique is called frequency domain spectroscopy (FDS) and is applicable for all frequencies, from the μHz area to the GHz area and above. It is quite simple in handling, but takes a lot of time to measure low frequencies as the duration of sine waves at, for example, 10 μHz, takes about 27 hours, not taking into consideration all the other frequencies which also have to be measured. 3.2 Conventional PDC measurement method Another method to measure the dielectric properties is to apply a voltage step to the tested asset and to measure the resulting polarization current for some time. This time-dependent information can be transferred to frequency-dependent dielectric properties [8]. It is called polarization depolarization current measurement (PDC) as traditionally not only the polarization, but also the depolarization current, was measured and compared. Modern algorithms only require the polarization current which halves the measurement time. By using PDC instead of FDS, the measurement time can be significantly reduced as one single PDC measurement provides the information for all frequencies which should be measured. This principle provides a way to save measurement time: Conventional conversion from time domain to frequency domain uses a static correlation between the measurement time and the calculated frequency range. If, for example, the time domain measurement was performed for 1000 seconds, the frequency response was always calculated down to the inverse frequency of 0.001 Hz. 3.3 Time-improved PDC+ measurement method A new algorithm called PDC+ uses a dynamic approach and calculates up to 20 times more data in frequency domain from the same time domain information (Figure 6). Figure 6: Conversion from time to frequency domain for conventional PDC and PDC+ Thus, the PDC+ technique allows much shorter measurements for the same frequency range than FDS or conventional PDC techniques (Figure 7). Modern measurement devices combine FDS for high frequencies and PDC+ for low frequencies. Where conventional FDS measurements take more than a day to measure from 1 kHz to 10 μHz, modern FDS&PDC+ devices can achieve this in 30 minutes to 1 hour 45 minutes. Figure 7: Test time for a frequency sweep from 1 kHz to 10 μHz There is also yet another way to reduce the test time. In many cases, it is not necessary to measure the whole frequency range down