Analyzing Moisture in Paper and Pressboard of Power Transformers-A New Approach

Power transformers are critical, capital-intensive assets for the utility industry.

Nov 1st, 2010
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Power transformers are critical, capital-intensive assets for the utility industry. As an asset manager reviewing the life expectancy of a transformer or a substation operator responsible for determining the loading capabilities of transformers, the transformer's water content should be a concern.

One of the most important aging indicators of transformers is the water content in the solid part of the insulation-paper and pressboard. Accurate diagnostic tools are critical for determining the health of transformers. The Omicron Dielectric Response Analyzer (DIRANA) is an efficient device that determines the water content in the solid insulation.

Moisture entering oil-paper insulations can cause three dangerous effects in transformers: It decreases the dielectric withstand strength, accelerates cellulose aging-de-polymerization-and causes the emission of gas bubbles at high temperatures.

Water in transformers comes from four sources: residual water after drying, water from cellulose and oil aging, water through leaky seals or repairs, and water due to breathing. Even in the case of a non-breathing transformer, therefore, the moisture can reach a critical level.

The DIRANA measures the dielectric response of solid insulation in equipment. The dielectric response is a unique characteristic of the particular insulation system. The increased moisture content of the insulation results in a changed dielectric model and, consequently, a changed dielectric response. By measuring the dielectric response of the equipment in a wide frequency range, the moisture content can be assessed and the insulation condition diagnosed. For the dielectric response tests, the test performed is a traditional ungrounded specimen test (UST) made from the high voltage winding to the low voltage winding (Chl) in a two winding transformer. The Chl test is the one of most concern, because this is the measurement that contains the most cellulose insulation material. The test connections and modes are the same as those used in a traditional transformer insulation power factor test; the difference is the test is performed at a low voltage and at frequencies from 1 kHz to 10 μHz.

Figure 1: Response Curve for Oil-impregnated Paper

Figure 1 shows the response curve for oil-impregnated paper. This curve shows a frequency vs. dissipation factor relationship. With increasing moisture content, temperature or aging, the curve shifts towards the higher frequencies. Moisture influences the low and high frequency areas. The linear, middle section of the curve with the steep gradient reflects oil conductivity. Insulation geometry conditions determine the hump that is located to the left side of the steep gradient.

The DIRANA combines the polarization current measurement method in time domain with the frequency domain spectroscopy (FDS), significantly reducing the testing time compared to existing techniques. Time domain measurements can be accomplished in a short time period but are limited to low frequencies. The extended measurement range of 5 kHz down to 50 μHz allows the DIRANA to discriminate between the oil, insulation geometry and paper. The result is independent from moisture equilibrium.

DIRANA's patented technique combines the advantages of both principles. It acquires data in the time domain from 10 uHz to 0.1 Hz and in the frequency domain from 0.1 Hz to 5 kHz. This reduces the measuring duration by up to 75 percent compared to exclusive frequency domain measurements.

DIRANA's moisture determination is based on a comparison of the transformer's dielectric response to a modeled dielectric response. A fitted algorithm compares the measured data and modeled data and calculates the geometry data, moisture content and oil conductivity. The moisture assessment is based on IEC 60422. The software is easy to use, and the user only needs to enter the oil temperature. Figure 2 shows the measurement assessment results displayed by Omicron's software.

Aged transformer oils often have increased values of conductivity through acids and other aging by-products. This can lead to incorrect water content results. The insulation model in the DIRANA's software compensates for this influence.

Figure 2: Measurement Assessment Results

Benefits of the DIRANA:

  • Dielectric Reponse Analyzer: Combined polarization and depolarization current (PDC) and FDS method,
  • Non-intrusive, reliable test of any HV insulations: transformers, bushings, cables and generators,
  • Two input channels: reduces outage time and measurement effort to a minimum,
  • Flexible curve representation: PDC results can be displayed either as FDS or PDC curves,
  • Easy-to-use software with intuitive user interface: facilitates automatic determination of the water content, even without expert knowledge,
  • Advanced import function: allows the importation of external PDC or FDS results and their analysis with the DIRANA software, and
  • Built-in processor: makes it possible to run DIRANA, even without personal computer (PC).

Other DIRANA applications:

  • Bushings,
  • Instrument transformers: current transformers and potential transformers,
  • Monitoring of transformer drying, and
  • Paper-mass insulated cables.

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