Life Extension of the Transformer Insulation withan Innovative Online Drying and Filtering system

H. Borsi, E. Gockenbach, V. Wasserberg

University of Hannover, GermanyInstitute of Electric Power Systems, Division of High Voltage EngineeringSchering-Institute Callinstrasse 25A D-30167 Hannover, Germany

Keywords: Life Extension, Transformer Insulation, Transformer Drying

Abstract

Water and Oxygen are the most important enemies of the insulation of oil filedtransformers leading to an accelerated ageing of the insulation. Thus dryingprocedures and reducing the influence of oxygen are required to extend lifetime andoperation reliability.In this contribution an innovative procedure for the dehydration of mineral insulatingoils and insulating paper in transformers is presented, working online duringtransformer operation. This procedure not only performs a complete desiccation, it alsopurifies the liquid without influencing the dissolved gas analysis (DGA) or any otherphysical property. Furthermore a Zeolite filled air filtering is introduced which not onlyprevents water molecules from penetrating the transformer but also oxygen. Herewithpresented investigations on the operation parameters like temperature and flow ratehave shown, that the procedure is beneficial for insulating liquids as well as for solidinsulation immersed therein.

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Introduction

Transformers are one of the key components of electric power distribution andtransmission systems. This statement is especially valid for power transformers, wherean outage possibly endangers the reliability of electrical energy supply and alwaysaffords unpredictable expenses. In consequence every transformer operator mustintend to increase the dependability of his devices.The trustiness of a power transformer is especially limited by the reliability of thewinding insulation system which additionally has to operate as a heat transfer mediummoving the losses out of the core and the windings. Even today the most frequentlyused insulation systems in these devices are therefore the liquid immersed paper andpressboard insulations. Due to cost aspects a combination of cellulosic paper andmineral oil has shown to be the favorite choice, although the lifetime of this insulationdepends on the operating conditions, the presence of catalysts, oxygen and especiallywater. To reduce the insulation aging it is therefore essential to avoid or to remove thementioned influences.

Parameters affecting the aging

Water may diffuse slowly inside the transformer during service (for breathingtransformers) or may be generated inside the device due to the depolymerization ofthe cellulose (Figure 1). Even small contents of water in the mineral oil reduce theelectrical strength of the insulation. Moreover the cellulosic paper and pressboardinsulation collects the water out of the liquid which leads to a severely acceleratedaging [1]. The aging of the cellulose can be accelerated by water, oxygen and heat.The degree of polymerization (DP) decreases until the material becomes brittle and asafe transformer operation is jeopardized.

Conventional Drying of

Due to the costs of repaunrecoverable failure whiAn effective drying of thedestroyed therefore enlamost of the actually availa

Figure 1:

C O

C C

CH CH

O

H2COOH

H

OH

H

H

OH

CH CH

C O

H2COOH

H

C C

OH

H

H

OH

H2O

Water generation by the scission of cellulose molecules.

the insulating system

ir a destroyed paper insulation represents in most cases anch leads to the retirement of the transformer.

cellulosic insulation before it has already been completelyrges the transformer operation lifetime and reliability. Withble systems only a drying of the liquid insulation is performed

within short times. If only the liquid insulation is dried or replaced most of the collectedwater is still inside the paper [2]. An additional drying of the paper insulation affordsnormally, that the transformer has to be drawn out of service. By heating the cellulosicmaterial with Low Frequency Heating (LFH) or Vapor Phase technologies like they arevery often applied for new, non impregnated transformers it is tried to remove thewater.But there is a difference between new and aged transformers: the solid insulation isalready impregnated, thus the cellulose fibers are covered with insulating liquid whichimpedes the extraction of the water from the cellulose. Due to the heating of theinsulation above the normal operating temperature such a treatment also causes anextended "lifetime consumption".An alternative thereto is the continuous extraction of the water covered by theinsulating liquid. When the water saturation level of the fluid decreases due to thedrying thereof water is extracted from the solid insulation. The insulating liquid acts asa “water transfer medium”. The advantages of such a cure are, that it needs notransformer outage and, what is even more important, that it does not endanger thesolid insulation during drying. Regarded over the time of its application such aprocedure is also more effective than a short time treatment with heat and vacuum.Actual systems, working according to this principle, extract the water either by vacuumor with the application of a hygroscopic material. The vital disadvantage of thesesystems is, that they affect the Dissolved Gas Analysis (DGA) because they extract oraffect, beside the water, also the indicating gases, that are used for DGA. DGA isactually the most important tool for the surveillance of the insulation condition, thus anymeasure, that affects this tool, also has impact on the monitoring of the transformerinsulation condition.With the novel system presented in this contribution the mentioned disadvantages areavoided while the transformer operation management is not affected at all.

The new drying and filtering system

Water and impurities in the liquid significantly reduce the electrical capabilities of theliquid. For a safe transformer operation optimal properties of the liquid are required,thus it should be clean and dry.The life expectancy of the paper insulation is influenced by water, oxygen and heat. Allthese parameters can cause depolymerization of the paper insulation, thus influencingthe mechanical and electrical properties of the paper and resulting in a reduction of thelife expectancy of the insulation. For an optimal lifetime therefore the paper should bekept dry during operation. The influence of oxygen should be prevented or reducedand overheating of the cellulose should be avoided. It has been shown by manyinvestigations, that water in the liquid and cellulose insulation is the main cause for thereduction of electrical properties and acceleration of the aging. Therefore one of themain tasks of the new system is to keep the insulating system dry.The basis of the conceived procedure for the drying system is the water solubility in theinsulating liquid and the equilibrium between the water content in the insulating liquidand the solid insulation immersed therein. As the combination of a cellulosic paper and

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pressboard immersed in a mineral based insulating oil is the most commoncombination of materials the operating principle is illustrated for these materials.The water solubility of an insulating liquid can be expressed by the following formula[3]

T

H

S eKW−

= *

Where K and H are constants specific for each insulating liquid and T the temperature.For a very common mineral oil these constants are e.g. K = 1.918 * 107 andH = 3.807 * 103. The water solubility therefore increases exponentially with thetemperature.The insulating oil of any transformer in operation, which always has a highertemperature than its environment due to the different losses inside the vessel,therefore intends to collect water. For a sealed transformer the only source for water isthe paper and pressboard insulation. For breathing transformers additionally water canpenetrate from outside the transformer into the vessel.The equilibrium between the water content in the mineral oil and the cellulosic paper isdiverse for different temperatures. This effect, which has been reported by severalauthors like [4], can be used to dry the insulation of liquid and paper insulated HVapparatuses. The equilibrium between the moisture contained in the paper and themoisture content in the surrounding mineral oil is depicted in figure 2.This diagram shows, that for high temperatures the water content of the mineral oil isrelatively high while the water content of the paper insulation is low. For lowtemperatures this relation is reverse, the water content in the paper is high while thewater in the oil is low.

0

1

2

3

4

5

6

7

8

9

11

0 10 20 30 40 50 60 70 90

Wat

er C

onte

nt in

the

Pap

er

Water Content in the Oil

20 °C 30 °C 40 °C

50 °C

60 °C

80 °C

100°C

ppm

%

Figure 2: Dependency between the water content in mineral oil and cellulosic paper [5].

This behavior can be used for a continuous drying of the paper insulation in a liquidimmersed insulation system. The warm and "wet" oil inside the transformer vessel isslowly drained from the transformer and pressed through a cooled cellulosic filter in acircular flow.

In this filter a very porous cartridge offers a large surface for the oil to transfer its waterto the cellulose fibers. The mineral oil has previously been cooled down to atemperature of about 3 °C when it enters the cartridge, thus it is in almost any casessupersaturated with water and therefore dispenses it easily.Additionally to the drying the filter purifies the oil and removes particles of dirt andimpurities out of the liquid. As such particles are possible sources for partial dischargesor even breakdowns of the liquid insulation the operation reliability is additionallyenhanced. The described procedure is actually protected by German and internationalpatents.For sealed transformers oxygen usually is not in contact with the transformer insulationand therefore the influence of oxygen by the aging is negligible. For breathingtransformers the influence of the oxygen is reduced by the installation of a vessel filledwith special Zeolite (a synthetic mineral), which can be installed preceding the silicagelcontainer for breathing air desiccation. With this not only the amount of oxygen,coming in contact with the transformer insulation from the environment, is reduced butalso the amount of water to be adsorbed by the silicagel as Zeolite is an effectivedesiccant, too.

Technical realization

Basic investigations on mineral oil as well as on ester liquid have shown, that theinnovative procedure is applicable for drying transformer insulating liquids andtherewith impregnated solid insulations [6].The basic characteristic of online treatments is their profitableness over long timeperiods. Although power transformers contain several tons of liquid and solid insulatingmaterials the drying of small volumes of insulating liquid is therefore sufficient.To estimate the water inside a transformer the following example is given: Inside a 10MVA transformer for voltages up to 123 kV 8600 kg of insulating oil are contained [7].About between 1/8 and 1/7 of this weight is the mass of the solid insulation. For theappraisal presented here a weight of 1200 kg is therefore rated. With an oil humidity of15 ppm and an operation temperature of 60 °C the oil dissolves 130 g of water whilethe solid insulation envelopes 2 % of its weight, thus being 24 kg (values gatheredfrom figure 2).Therewith the vital problem of any short time treatment becomes obvious. The watercovered by the solid insulation is captured all over the material. If the insulating liquid isremoved and vacuum is applied the water originating from the inner parts of theinsulation has to pass the surrounding cellulose material, thus still being impregnatedwith the insulating liquid. The speed of the water transfer out of the core of the solidinsulation is thus limited by the transfer speed through the impregnated material. Asthis speed is quite low short term procedures can only be effective for the outerinsulation layers close to the surface as long as the material is still impregnated.The new system is directly applied to the transformer in operation as depicted in figure3.

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Upgraded insulating liquid

Cooling circuit

Cooler

Hot, wet oil

Cellulose filter cartridge

Pump

Tran

sfor

mer

ves

sel

Figure 3: Schematic of the application of the presented system on a transformer in operation.

The insulating liquid removed from the transformer via the oil sample extraction tubespasses at first the pump that transports the fluid through the system. Afterwards the oilis injected into the cycle stream through the filter vessel and the cooling unit where it iscooled down to 3 °C before it enters the filter cartridge. As the filter cartridge is a flowresistance this part of the system is continuously under a pressure of about 1 bar plusthe hydrostatic pressure originating from oil level of the transformer. After passing thefilter cartridge the dried and purified mineral oil is refilled into the transformer vessel viaanother oil sample extraction tube or directly into the conservator.

Primary tests

For the tests performed with this facility a transformer model has been used. It consistsof a heatable oil barrel containing 200 liters of mineral oil plus 30 kg of therewithimpregnated paper and pressboard samples. The samples had previously beenaerated with water, thus the oil humidity at the start of the experiment was about 60ppm at 60 °C. A direct measurement of the paper samples water content is possible,but as the transformer model is sealed it is time consuming and implies the risk ofleakage. The water content of insulating liquid samples extracted via a cannula into thebarrel and from the outlet of the filter has been measured using a Karl-Fischer-Coulometer 684KF manufactured by Metrohm. The temperature of the transformermodel and of the filter cartridge has been controlled using a Pt 100 thermometer.As the most interesting parameter the maximum attainable drying has been tested atfirst. Therefore the filter cartridge has been cooled down to -4 °C and the oil flow hasbeen reduced to 4 liters per hour. The water content of the mineral oil leaving thissetup has continuously been around and below 1 ppm. In comparison to theconventional method where the oil is spread over large surfaces under vacuum andhigh temperatures this is a significant improvement. Additionally the oil is not thermallystressed during the desiccation process.For a practical application on power transformers there is no need for such an extremedryness but it may be interesting for laboratory use. During the test time of 12 days thewater content of the transformer model liquid has been reduced from 56.7 to 22.6 ppmand the water content of the solid insulation from about 4.5 % to 2.8 % respectively.

For transformers the flux of dried insulating liquid is more important than the maximumattainable dryness as the volume of insulating liquid to be dried is very large and with ahigher throughput of the filter unit the drying is accelerated. The system flow-rate for apower transformer is actually rated to 240 liters per day but the basic limit for this valueis the capacity of the cooling system as transformer insulating liquids are, bydestination, good thermal transfer media carrying a lot of thermal energy that has to beremoved.For an application on power transformers it is essential to detect the saturation state ofthe filter as in the case of a saturated purifier the drying stops and the cartridge has tobe replaced. For this purpose the resistance of the impregnated filter material ismeasured as schematically depicted in figure 4. The filter cartridge consists of severallayers of filter material that are glued together. The liquid flows in radial directionthrough this cartridge, thus a measurement of the resistance between the twoelectrodes covering one of the filter slices is representative for the whole filtercondition.

Flow direction

Wat

er c

onte

nt

mea

sure

men

t

Filter cartridgeElectrodes

Figure 4 – Schematic view of the filter cartridge saturation detector.

Alternatively a drying of the filter cartridge inside the filter vessel is possible butcomprises several disadvantages. The particles, removed from the insulating liquid,remain on the surface of the filter and hamper the subsequent drying by vacuum. Thedrying of the impregnated cellulose of the filter cartridge is a time consuming processduring which the continuous drying has to stop. Each drying cycle worsens thecondition of the filter due to the material burden. And finally the transformer gasbalance and therewith also the DGA may be influenced by the application of vacuum.For these reasons the filter cartridge used in the presented system has been specifiedfor single use. To nevertheless assimilate large amounts of water until a replacementis necessary the filter dimensions have been enlarged. The actually implantedcellulose filter cartridge is capable for covering up to 15 liters of water.

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Application on site

The system has been applied to a 30 MVA transformer as it is depicted by Figure 5.During the application for 3 months the results of the DGA have not been changed as

analyses of the

The water coefficiency of ttransformer. Tvoltage net buonly the losses

The ordinarytransformers oequilibrium betoil moves to thsmall amountsduring desiccathe drying spe

Actually a sim80 °C. As thewater are avail

Another possibhygroscopic inincreases thethis method ca

Conclusions

The innovativedrying the solioxygen . The m

Drying setup inwater-proofhousing

Figure 5: Drying setup applied to a transformer on site.

oil before and after the application of the setup have shown.

ntent of the transformer has been reduced by 300 g although thehe drying has been limited by the relatively low temperature of thehe transformer is used as a backup, which is connected to the hight transmits energy only if the consumer needs extra power. Thereforeduring idle operation led to a warming of the transformer

operation temperature has been around 30 °C while usual powerperate at temperatures above 60 °C. Due to the lower temperature theween the water in the cellulosic insulation and the water dissolved in thee cellulose side (see Figure 2) and the insulating oil contains relativelyof water. Even if this water is almost completely removed from the liquidtion the poor supply with water by the transformer insulating liquid limitsed.

ilar setup is applied to a transformer operating at temperatures arounddrying has just started no values concerning the amount of removedable yet.

ility for increasing the drying performance would be the addition of asulating liquid like ester liquids. Even the addition of 20 % ester

water solubility of the liquid by a factor of 10. Further information aboutn be found in [6].

method for the lifetime extension of oil filled transformers is based ond and liquid insulation, filtering of the oil and reducing the influence ofain tasks of the system are as follows:

- allows a continuous, gentle and efficient drying of the insulating system and therebyincreases the operation reliability and residual lifetime.

- purifies the insulating liquid by filtering out solid contaminants and therewithincreases the breakdown and partial discharge inception voltage of the liquid.

- does not influence the gas balance of the transformer, thus the Dissolved GasAnalysis (DGA) is not affected.

- is efficient over long time periods and can be applied on new and aged apparatuswithout the need for long time outages.

- allows an almost complete desiccation of mineral insulating oils. The drynessattainable with this method is below 1 ppm. The insulating liquid is not stressedduring this procedure.

- Reduces the influence of oxygen

Actually the system is in the beginning of a serial production and is already applied onsample transformers throughout Germany. The experiences gained during these finaltests will lead to a “fine tuning” of the setup and finish the development.

Acknowlegements

The authors gratefully acknowledge the Minna and James Heinemann foundation fortheir grant to Mr. Wasserberg in supporting his Ph.D. thesis.Also special thanks to the Karberg & Hennemann CJC Feinfilter corporation,Hamburg, Germany (info@kh.filter.de) for their interest in the performed work and thesupport of a technical realization.

References

[1] C. Krause, H.P. Gasser, J. Hochspannungstechnik – Theoretische und praktischeGrundlagen für die Anwendung", Springer-Verlag 1986, pp. 145 - 148

[4] Y. Du, M. Zahn, B.C. Lesieutre, A.V. Mamishev and S.R. Lindgren: "MoistureEquilibrium in Transformer Paper-Oil Systems", IEEE Electrical Insulation MagazineVol. 15, No. 1 January/February 1999

[5] J. Fabre and A. Pichon: "Deteriorating Processes and Products of Paper in Oil.Application to Transformers", 1960 International Conference on Large High VoltageElectric Systems (CIGRE), Paris, France, paper 137Huser and A. Sidler: "Effects ofMoisture in Transformerboard Insulation and the Mechanism of Oil Impregnation ofVoids", transform´98 Forum der Technik, Munich, Germany, 20.-21.04.1998

[2] F. Flottmeyer, U. Sundermann and A. Möllmann: “Alterungsverhalten vonTransformatorisolierölen”, Elektrizitätswirtschaft Jahrgang 95 (1996), No. 8 , pp. 496- 502

[3] M. Beyer, W. Boeck, K. Möller and W. Zaengl: "

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[6] V. Wasserberg, H. Borsi, E. Gockenbach: “A New Method for Drying the PaperInsulation of Power Transformers during Service”, 13th Intern. Symposium onElectrical Insulation (ISEI 2000), Anaheim, USA, paper 9-2

[7] N.N.: “Power Engineering Guide – Transmission and Distribution”, Publication ofthe Siemens AG, p. 4/19