Reliable Moisture Determination in Power Transformers

Home , Moisture analysis

Preface – Electra offers students or young engineers the possibility of publishing articles of a scientific nature. The present paper is a sum- mary of a PhD thesis published at the University of Stuttgart, Germany.

Maik Koch, personal member of Cigré

Abstract – This article describes and compares methods for reliable assessment of moisture in oil-paper-insulated power transformers. The work concentrates on the use of equilibrium diagrams and dielectric response methods for on-site moisture determination. Case studies illus- trate the practical application of the achievements.

I. MOISTURE IN TRANSFORMERS Among aging phenomena of power transformers, moisture in the liquid and solid insulation in recent years became a frequent- ly discussed issue because of uncertainties of traditional measurement methods, the availability of new methods and its detrimen- tal effects on the insulation condition. Moisture decreases the dielectric withstand strength of oil and paper, accelerates paper aging and causes the emission of bubbles at high temperatures. For describing the moisture concentration in materials, today two measures are used. This is firstly relative moisture saturation (RS in %), as the ratio of the actual water vapor pressure to the saturation water vapor pressure, having the same physical meaning like the well-known relative humidity in gases. Secondly, water content is used, calculated by the ratio of water mass to insulation mass and given in % for cellulose materials and ppm for oils. The decision about maintenance actions as for example on-site drying requires dependable knowledge about the actual moisture concentration. Though the bulk of water resides in the solid insulation (pressboard, paper), it cannot be readily assessed in transformers and indirect methods are needed. State of the art are equilibrium diagrams, where one tries to derive the moisture content in paper/pressboard from moisture content in oil (ppm), [1]. In recent years, dielectric response methods were developed, which deduce moisture in paper and pressboard from dielectric properties of the insulation. Dielectric response analysis promises higher accuracy and is designed for onsite moisture determination, [2]. This background lead to the initiation of research work at the University of Stuttgart, partly financed by the European research project REDIATOOL (Reliable Diagnostics of HV Transformer Insulation for Safety Assurance of Power Transmission Sys- tem), whose outcome is summarized in this article.

II. MOISTURE MEASUREMENT THROUGH MOISTURE EQUILIBRIUM A. Moisture Equilibrium Moisture equilibrium has been used for decades by measuring moisture in oil and concluding on moisture in paper. Physically, moisture equilibrium is based on three conditions: thermal equilibrium (temperature), mechanical equilibrium (e.g. pressure) and chemical equilibrium. In the condition of moisture equilibrium, the migration of water molecules inside materials and between oil and cellulose has ceased; the thermodynamic property "water potential" becomes equal throughout the system. Because of load and temperature changes, equilibrium is never fully reached for operating power transformers. Still for locally limited areas as e.g. the oil flowing around paper-insulated conductors, equilibrium is established. In this case, relative saturation can replace the term water potential, and the equilibrium law can be written as: Equilibrium is reached in adjacent materials, if moisture saturation RS becomes equal (1). The material might be cellulose, oil, air or even a plastic. In other words, differences in relative saturation are the driving force for moisture migration.

RSCellulose  RSOil  RH Air (1) B. Conventional Equilibrium Diagrams It is a standard procedure for operators of power transformers to derive moisture content in cellulose from moisture content in oil (ppm) using equilibrium diagrams, [1]. However, various errors affect this procedure: - Sampling, transportation to the laboratory and moisture measurement by Karl Fischer titration. Water titration by the Karl Fischer method suffers from different procedures releasing water from the sample lead to unsatisfying comparability of the results, [3]. - Equilibrium conditions are rarely achieved (depending on temperature after hours/days/months), - A steep gradient and high uncertainty in the low moisture region compounds the accuracy, - Diagrams from various literature sources lead to different results,

- Equilibrium depends on moisture solubility in oil and moisture adsorption capacity of cellulose. The validity of equilibrium diagrams is restricted to the original materials that were used to establish the diagrams. Particularly oil aging changes the moisture adsorption capacity substantially; shifting the equilibrium curves towards the oil, Figure 1. Because of these errors, traditional equilibrium diagrams (ppm-based) tend to overestimate moisture content in paper/pressboard.

New

Aged

Figure 1: Oil aging changes moisture partitioning between oil and paper and makes traditional equilibrium diagrams unreliable, [1] C. Measurement via Moisture Saturation of Oil Instead of moisture content in oil (ppm), the relative saturation in oil (%) is utilized for assessing moisture in paper. Based on equation (1), the moisture content in cellulose is derived from moisture relative to saturation of the surrounding oil. Figure 2 was measured at aged paper and illustrates the use of such equilibrium diagrams, [5]. This leads to several advantages: Oil aging and its influence on moisture saturation level becomes negligible. With relative saturation the graphs become less temperature de- pendent (Figure 2). Errors due to sampling, transportation to the lab and titration are excluded. Continual, accurate measurement and easy implementation into a monitoring system become possible. 65 10 5

60 % / 55 Oil temperature 8 4

50 paper 45 40 6 Kraft 3 35 2,2 RS in oil aged 30

4 % / saturation Relative 2 Top oil temperature / °C / temperature oil Top 25 Aged KP 21°C 20 RS in cellulose Aged KP 40°C 15

2 in Moisture 1 10 Aged KP 60°C 5 4,1 Aged KP 80°C 0 0 0 01.06.2003 05.06.2003 09.06.2003 13.06.2003 17.06.2003 0 10 20 30 40 Time, date Moisture relative to saturation / % Figure 2: Determination of water content in paper via moisture saturation in oil D. Measurement and Implementation of Moisture Saturation in Cellulose Moisture saturation is a critical factor that determines the amount of water available for interactions with materials. The destructive effects of water in insulation systems depend on water molecules that are available for interactions with materials. This is not the case for molecules that are strongly bound, e.g. by hydrogen bonds to OH-groups of cellulose molecules forming a monolayer. Just water content, as measured by Karl Fischer titration, reflects the bound and therefore less active water as well. In contrast to this, moisture saturation determines the available water for destructive effects. Conclusively, using relative saturation in oil and paper gives the following advantages: Neither oil, nor paper aging effect the accuracy, conversion via equilibrium charts becomes unnecessary, a direct relation to the destructive impacts of water is given and the possibility for drying of the insulation system is directly observable. Figure 3 illustrates the application of a relative saturation measurement using a capacitive probe in a power transformer equipped with an online monitoring system. A long term average equates the relative saturation in oil with the relative saturation of the surrounding cellulose and comes to 4.1 %. Using a moisture isotherm as Figure 2 one can derive the moisture by weight in cellulose as well, that would be 2.2 % in this case.

65 10 5

60 % / 55 Oil temperature 8 4

50 paper 45 40 6 Kraft 3 35 2,2 RS in oil aged 30

4 % / saturation Relative 2 Top oil temperature / °C / temperature oil Top 25 Aged KP 21°C 20 RS in cellulose Aged KP 40°C 15

2 in Moisture 1 10 Aged KP 60°C 5 4,1 Aged KP 80°C 0 0 0 01.06.2003 05.06.2003 09.06.2003 13.06.2003 17.06.2003 0 10 20 30 40 Time, date Moisture relative to saturation / % Figure 3: Top oil temperature, relative saturation in oil and in cellulose measured resp. calculated by an online monitoring system

III. MOISTURE DETERMINATION BY DIELECTRIC RESPONSE ANALYSIS Dielectric response analysis measures dielectric properties of insulation systems over a very wide frequency or time range and calculates condition variables like moisture content and oil conductivity by use of mathematical modeling. It is applied as a non- intrusive technique for quality control in the factory and periodical assessments of the aging condition. A dielectric response measurement involves a three terminal measurement circuit that includes output voltage, sensed current and guard. The guarding technique insures for an undisturbed measurement even at onsite conditions with dirty insulations and electromagnetic interfer- ences. For measuring the most important insulation between high and low voltage winding, the voltage output is connected to the HV winding, the current input to the LV winding and the guard to the tank. The test can be performed in time domain while applying a DC voltage for a time of typically 1-10'000 s (Polarization and Depolarization Currents PDC) or in Frequency Domain while applying an AC voltage across a frequency range of 1 kHz – 0.1 mHz (Frequency Domain Spectroscopy FDS). Both test techniques reflect the same fundamental polarization and conduction mechanisms and can be combined for uniting the advantages of both principles. Moisture, temperature and conductive aging products influence the dielectric response. The discrimination of moisture from other effects is a key quality feature for the analysis of dielectric measurements, where the historically used Recovery Voltage Method RVM failed, [4]. The total dielectric response of oil-paper-insulated power transformers consists of the single contributions of oil and paper/pressboard superimposed with interfacial polarization. The upper row of Figure 4 displays the single contributions of cellulose and oil. Together with interfacial polarization, the dielectric response shows a typical s-shaped curve when (left drawing in lower row of Figure 4). The very low and the high frequencies reflect the cellulose, the steep slope the oil and the local maxi- mum the interfacial polarization effect, i.e. the insulation geometry. Water content and oil conductivity shift the dielectric response towards higher frequencies; however the shape remains similar (middle drawing in lower row of Figure 4). Temperature further increases the losses of cellulose and the oil conductivity (right drawing in lower row of Figure 4). Since moisture increases the losses especially at low frequencies, reliable moisture calculation requires also data at lower frequencies than the area influenced by the interfacial polarization effect. Oil 0,3 Cellulose 10 1 %, 20°C 1 pS/m, 20°C

0,003 0,0001 Dissipation factor Dissipation 0,0001 1000 0,0001 1000 Frequency / Hz

30 Cellulose 3 % Cellulose 3 %

Oil 10 pS/m Oil 43 pS/m

Cellulose

Cellulose Interfacial Polarisation Oil at 20°C 0,3 Temperature 50°C

Dissipation factor Dissipation 0,003 0,0001 1000 0,0001 1000 0,0001 1000 Frequency / Hz Figure 4: Superposition of dielectric properties: Single impacts of the materials (upper row) and superposition at a multilayer insulation with influences of interfacial polarisation, oil conductivity, water and temperature (lower row) Moisture analysis using the dielectric response is based on a software-aided comparison of the transformers dielectric response to a modelled dielectric response. A fitting algorithm rearranges the modelled dielectric response and automatically calculates moisture content and oil conductivity. In this work, the data pool for the model consists of measurements on new pressboard at

various temperatures, moisture contents, and oils used for impregnation, [3]. The dielectric properties of aged pressboard were investigated as well in order to compensate for the influence of aging.

IV. APPLICATION FOR ON-SITE MEASUREMENTS A. Two new transformers With the awareness of the hazardous effects of moisture the demand to receive new transformers in dry condition increases. Therefore the moisture content in the solid insulation of two new transformers was evaluated with dielectric response measurements in the factory. Transformer A shows much lower losses than transformer B, so from a first glance at the dissipation factor vs. frequency one may conclude that the second transformer is in a worse condition (Figure 5). However, a closer look at the dielectric response of transformer B reveals low losses at the very low frequencies below 1 mHz, the moisture-sensitive region (Figure 4). Analysis software actually determined the moisture content in the cellulosic insulation of both transformers to be 0.4 %, which is a very low value. The difference in the dissipation factor curve comes not from moisture but from different conductivities of the insulation oils (0.05 pS/m for transformer A, 0.94 pS/m for transformer B). The manufacturer of transformer B filled used and recycled oil into the new transformer for factory tests, the final, new oil would be filled in on-site only. This example illustrates that particularly for new transformers the very low frequencies, reflecting the condition of the solid insulation, are important for moisture analysis using dielectric response methods. Limiting the frequency range to e.g. 2 mHz would make the discrimination between the contributions of oil and cellulose impossible and lower the accuracy of moisture analysis. 0.7

factor 0.3

0.1 Dissipation Dissipation 0.03

A B 0.01

0.003

0.0001 0.001 0.01 0.1 1 10 100 1000 Frequency / Hz Figure 5: Dissipation factor over frequency for two new transformers having identical water content in paper of 0.4 % but different oil conductivity of 0.05 pS/m for transformer A and 0.94 pS/m for transformer B B. Moisture Determination of an Heavily Aged Transformer A heavily aged transformer with 30 MVA, built in 1950, was designated for scrapping. Paper and oil samples were taken out after measuring the dielectric response with various methods. Figure 6 compares the results of the moisture determination techniques. Karl Fischer titration on paper samples yielded 2.6 % moisture by weight (KFT). Results of the modeling of the dielectric response measurements by means of different software differ from each other: Two algorithms (DA1, DA2) had no compensation for the influence of conductive aging products and came to 3.8 and 4 % moisture by weight. Another algorithm (DA3) with built-in compensation for conductive aging products [5] indicates 2.9 % moisture relative to weight. In the oil sample the moisture saturation was measured directly onsite and the moisture content in ppm by Karl Fischer titration in a laboratory. An equilibrium diagram based on relative saturation [5] led to 2.5 % of moisture in cellulose (RS), which well agrees with the KFT analysis of the paper samples and the dielectric response analysis with compensation for conductive aging products. At the same time, direct application of equilibrium curves, based on moisture content in oil in ppm [1], indicated much too high water content in paper – 6.0 %. To conclude, the findings at this very aged transformer show, that a compensation for aging products is necessary both for the measurement based on moisture equilibrium and those based on dielectric properties.

6,0 6

5 4,0 3,8 4 2,9 3 2,6 2,5

2 Moisture content / / % contentMoisture 1

0 KFT DA1 DA2 DA3 PPM RS Paper Dielectric Equilibrium sample response Figure 6: Moisture content in the solid insulation obtained from Karl Fischer titration of paper samples (KFT), dielectric response analysis (DA1, DA2, DA3) and from equilibrium diagrams for moisture content in oil (PPM) and from the relative saturation (RS). C. Validation of the Results 61 power transformers have been measured with the aim to compare the different moisture determination methods. This large number of transformers allows for a comparison and, with paper samples as benchmark, a validation of the methods. Figure 7 shows the water content in paper/pressboard vs. age of the investigated transformers as obtained by traditional equilibrium diagrams (ppm), equilibrium diagrams based on relative saturation (RS) and dielectric response methods (DA1-DA4). New transformers typically have water contents of 0,3-1 %, whereas a transformer of 30 years age ranges from 1 to 4 %. 7 Dielectric DA1 DA2 6 response DA3 DA4 5 Equilibrium ppm RS 4

3 Moisture content / % / content Moisture 2 1 0 0 10 20 30 40 50 Age / years Figure 7: Water content in paper vs. age for 61 transformers Traditional equilibrium diagrams tend to substantially overestimate moisture in paper, particularly for aged insulation systems. New equilibrium diagrams based on relative saturation provide good agreement with the results obtained from dielectric response tests and paper samples taken from the insulation. All methods need algorithms to compensate for the influence of insulation aging, particularly on the background of the large number of aged equipment in today's power grids.

REFERENCES [1] Y. Du, M. Zahn, et al. “A Review of Moisture Equilibrium in Transformer Paper-Oil Systems” IEEE Electrical Insulation Magazine, Vol. 15, No. 1, pp. 11- 20, January-February 1999 [2] S. M. Gubanski, et al.: “Dielectric Response Analysis for Transformers Windings” CIGRÉ Task Force D1.01.14, Technical Brochure 404, Paris, 2010 [3] M. Koch, S. Tenbohlen, I. Hoehlein and J. Blennow: “Reliability and Improvements of Water Titration by the Karl Fischer Technique” Proceedings of the XVth International Symposium on High Voltage Engineering, ISH, Ljubljana, Slovenia, 2007 [4] M. Koch, S. Tenbohlen, M. Krüger and A. Kraetge: “A Comparative Test and Consequent Improvements on Dielectric Response Methods” Proceedings of the XVth International Symposium on High Voltage Engineering, ISH, Ljubljana, Slovenia, 2007 [5] M. Koch: "Reliable Moisture Determination in Power Transformers”, Dissertation, IEH, University of Stuttgart, Sierke Verlag Goettingen, Germany, 2008 OMICRON is an international company serving the electrical power industry with innovative testing and diagnostic solutions. The application of OMICRON products allows users to assess the condition of the primary and secondary equipment on their systems with complete confidence. Services offered in the area of consulting, commissioning, testing, diagnosis, and training make the product range complete.

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