1040 IEEE Transactions on Dielectrics and Electrical Insulation Vol. 8 No. 6, Dec(�mber 2001

Fundamental Investigations on Some Transformer Liquids under Various

Outdoor Conditions I. Fofana

Research Group on Atmospheric Icing Universite du Quebec a Chicoutimi

Quebec, Canada

H. Borsi and E. Gockenbach

Institute of Electric Power Systems Division of HV Engineering

Schering·Institute University of Hanover, Hanover, Germany

ABSTRACT

This paper deals with the humidity uptake of some transformer liquids and its influence on the electrical breakdown. The investigations were performed under some selective outdoor conditions on different insulating liquids, e.g. mineral oil, ester and silicone used in HV trans­formers. The temperature was at 23 or 60"( while the relative air humidity was varied between 20 and 90% to simulate climatic variation. The subject is important in a kind of HV transformer with an air breathing system and, even if in such transformer, where normally the air passes through a dehydrating apparatus with silica gel, a bad run of this apparatus can induce an accident. The scenario considered corresponds to such a defect in the breathing filter system, and thus when the insulating liquid in the transformer becomes in contact with humidity of atmospheric air. The work presented is in two parts. The first part relates the evolution of the breakdown voltage in terms of the insulating liquid humidity content. The second one describes the evolution of insulating liquid humidity content in terms of surrounding relative air humidity. This evolution is observed during 30 days and the results are discussed regard­ing the ac electric strength according to International Electrotechnical Commission (lEe) or Verband der Elektrotechnik Elektronik lnformationstechnik e. V. (VDE) standards. A correla­tion between the surrounding relative air humidity, temperature and humidity uptake rate is derived.

1 INTRODUCTION

TRANSFORMER life/ aging is related mainly to the degradation of the insulation, caused dominantly by thermal degrading of the insu­

lating paper, together with the decomposition of the paper. The by­products are water and other substances of partly polarizable and ion­izable character. For example, the life of insulating cellulose paper at 11 0"( is calculated as ten years [1]. Aging depends not just on loading, but also is influenced significantly by the type of paper, pulp compo­sition, humidity and oxygen content as well as the acidity level within the insulating liquid [2].

down. Water content increases electric conductivity, di:lsipation factor and worsens the electric strength. A mass transfer process of water results from the equilibrium imbalance, in which at higher tempera­tures moisture leaves the paper to enter the liquid. Water in the liquid originates from air moisture in the case of 'open-breather' or 'liquid­conservator' types, and the thermal decomposition of cellulose-based solid insulating materials. It has a strong influence on the life ex­pectancy and load capacity of a transformer.

The insulation liquid in the transformer changes volumetrically ac­cording to the variation in its temperature arising from changes in at­mospheric temperature and/ or transformer load. From the expansion coefficient of the liquid and the range of temperature changes (-20 to 90"C), its volumetric change is � 9% maximum. Therefore, par­ticular attention has to be given to prevent the introduction of mois-

Water is particularly detrimental to dielectric properties of both liq­uid and paper insulation systems and thus to their resistance to ag­ing. Moisture in insulating liquid may under fast decreasing temper­ature transients, result in free water that can lead to electrical break-.

1070-9878/1/ $3.00 © 2001 IEEE

IEEE Transactions on Dielectrics and Electrical Insulation

ture into the transformer during operation. Electrical transformers are principally of two types, hermetic and air breathing transformers. The hermetic transformer will always have a great advantage over the air­breathing type, for as long as it remains air proof. The vast majority of air breathing transformers are equipped with an exterior expansion ves­sel (necessary for this volumetric change) called a conservator, which is installed on the transformer main body. When contacting the atmos­phere directly; insulating liquid absorbs humidity, which deteriorates its dielectric strength as well as that of the insulating paper when mois­ture is absorbed into the insulation material in the liquid. Therefore, to prevent inhalation of moisture from the atmosphere into the conserva­tor, a dehydrating (silica gel) breather is fitted on the end of the air inlet pipe of the conservator. The atmospheric air goes in and out automati­cally via the dehydrating agent. The dehydrating breather contributes to safe and reliable operation of the transformer. In case of defects in the breathing system filter, the insulating liquid will absorb moisture from the atmosphere. The rate of moisture uptake depends on the surround­ing atmospheric conditions i.e. relative air humidity and temperature. It is therefore useful to predict the dielectric behavior of the insulation liquid under standard outdoor conditions as well as the moisture up­take of the insulated liquids under some selective conditions.

We present here some fundamental investigations concerning hu­midity uptake rate of different insulating liquids used in HV transform­ers under selective conditions. To the best of our knowledge, such in­vestigations have not yet been performed.

The temperature was chosen at 23 and 60"C while the relative air humidity was varied between 20 and 90% to simulate climatic varia­tion, since the relative humidity during the winter can decrease below 30% and increase above 75% in summer days. Water content of liquid specimens were controlled during 30 days. The results are discussed regarding the ac electric strength according to lEe (International Elec­trotechnical Commission) or Verband der Elektrotechnik Elektronik In­

formationstechnik e. V. (VDE) standards.

2 DESCRIPTION OF THE

LIQUIDS

The investigations were carried out using three different insulating liquids, commonly used in HV transformers, and a mixture of two of them recently proposed as insulating liquid for transformers [3]. A short description of them is presented below.

2.1 MINERAL OIL Mineral oil, so called 'transformer oil', is made by refining a frac­

tion of the hydrocarbons collected during the distillation of petroleum crude stock. The crude oil stock and the refining process used in pro­ducing these oils are typical of those used in producing many common petroleum lubricating oils. Chemically, the mineral oil consists of a complex mixture of basic hydrocarbon liquids such as paraffin (40 to 60%), naphthene (30 to 50%), aromatic (5 to 20%) and olefin (roughly 1 %). The chemical structures are described in the literature [4-7].

The early transformer oils were paraffin based but, in about 1925, they were replaced with naphthene oils because of the high pour point of paraffin oils. It was soon recognized that paraffin oils are prone

Vol. 8 No. 6, December 2001 1041

to sludging, and this resulted in the development of oil reclamation techniques [2].

The relatively good aging behavior and low viscosity make mineral oil a good insulating and cooling liquid. Its electrical and dielectric properties are strongly temperature and moisture content dependent. The main advantages of this petroleum-based oil are its wide availabil­ity and low cost. However, mineral oils posses a relativel� low permit­tivity, a low flash point and are slightly toxic. Mineral oil also has the disadvantage of endangering the environment in case of a transformer leakage [9].

When searching substitutes for poly chlorinated biphenyls (PCB), ecological considerations raised the problem of searching for incom­bustible and non-toxic insulating liquids, and ester liquids and silicone fluids were proposed. They belong to the high fire point (HFP) 'liq­uids also known as less inflammable' liquids. It is to noted that, by definition, a HFP liquid must have a minimum fire point of 300"C [4]. Their relatively high cost and availability has limited their use to special transformer applications.

2.2 ESTER LIQUID

The ester liquid used for these investigations consists of pentaery­thritol tetraester and different fatty acids. The chemical structure of it can be found in the literature [4-7]. Ester liquids are not toxic, are well digested by micro-organisms and posses low vapor pressure at oper­ating temperature of IIV transformer. In fire they generate no dioxins or toxic products and possess a good biodegradability [8]. Previous work [9] has shown that ester liquids can be used for the retro-filling of mineral oil filled transformers. Ester liquids posses good properties, and this feature, coupled with the ability of drying the solid insulation (impregnated paper), is considered as a positive one. However, they are also prone to possible hydrolytic detachment through their high moisture saturation limit.

2.3 SILICONE FLUID

Silicone transformer fluid is one of a family of fluids known chem­ically as poly-dimethyl siloxanes [10]. The chemical structure can be found in the literature [6,7]. Silicone filled transformers meet most en­vironmental and fire protection requirements. Indeed, silicone fluid is environmentally neutral and flame retardant. It posseses good aging and oxidation properties. Moreover, it does not endanger living or­ganisms in case of emission [11]. As far as its cooling and insulating properties are concerned, silicone can be compared with mineral oils. Its stability is related to the energy of the Si-O bond (374 instead of 245 kJ/mol for C-C bond). Silicone fluid is colorless, and characterized by a very low pour point compared to that of mineral oils, even if their viscosity at 20"C is higher.

2.4 MIXTURE OF MINERAL OIL AND

ESTER LIQUID

The dielectric and aging properties of mineral oil are well known to depend strongly on moisture content [11, 12]. High relative humidity enhances a high water content in insulating paper that increases the ag­ing rate of the associated solid insulation. Experience shows that dou­bling the water content in paper accelerates the aging rate by a factor

10412 Fofana et at.: Fundamental Investigations on Some Transformer Liquids under Outdoor Conditions

ten [12). In a recent work [3), we have shown that through the mix­ture of mineral oil with various amounts of an hygroscopic insulating liquid such as ester liquid, the dielectric and aging properties of the solid/liquid insulation of the HV apparatus were improved. Moreover, the water solubility of the mixture were improved compared to that of the mineral oil itself. When coupled with solid insulation, it increases its drying compared to pure mineral oil. For the present investigations we will use the mixture of mineral oil with 20% of ester liquid, which seems to be more suitable for use in HV transformers when taking into account economic aspects as well as electrical properties [3).

Some of the properties of the investigated liquids are summarized in Table 1.

3 THE RELATIVE HUMIDITY

By definition, the relative humidity of a liquid is the dissolved water content of the liquid relative to the maximum capacity of moisture that it can hold at that specific temperature.

The relative humidity Wr (%) at a given absolute temperature T (K), is defined in terms of the absolute water content in liquid Wabs (ppm) VS. the saturation limit WL(T) such as

W - Wabs (1) r - WdT) The saturation limit of the insulating liquids depend on the insulat­

ing liquid type, its chemical composition and mean molar weight [7). The temperature dependence of maximum water solubility of the in­sulating liquids is described by the following well-known exponential equation [7, 12)

(2)

where Wo (ppm) and H (K) are material paramaters which have to be determined experimentally for each insulating liquid [4,7,11). In a logarithmic diagram, Equation (2) depicts a linear curve. Therefore, only two points are necessary to describe the insulating liquid behav­ior. For this reason, we only performed investigations at three different temperatures (20,60, 88"C) and extrapolated the results from 0 to 100"C.

l000������----��������

1 '0000

" '---l00�------�==��----==-=------�

37E-3 3.5E-3 3.1E-3

20 30

2.9E;.-3 2.7E-3

1rr[1/K] +--------

60 100

T['C] __

Figure 1. Water saturation limit W L as a function of the absolute temperature T for different insulating liquids.

Table 2. Material constants of the investigated liquids.

Insulating liquid K(ppm) H(K) Mineral oil 19.2x106 3805 Silicone Fluid 6.58x105 2372 Mineral oil, 20% ester 6.25x105 2229 Ester liquid 2.61x105 1340

The material variables Wo and H of the investigated insulating liquids can be taken from Table 2. Figure 1 shows the water saturation limit of the investigated liquids for an absolute tempera hire range of 0 to 100"C. As illustrated, the water solubility of the ester is much higher than that of mineral oil, followed by the mixture of mineral oil with 20% ester liquid, and silicone fluid. The difference in the behavior of silicone and ester liquids compared to mineral oil can be explained by the difference in water absorption in these liquids. Ester or silicone liquid can take up water in chemically bounded and in dissolved form, while in mineral oil, water is only dissolved [7,11). Indeed, it is well known that water contamination in the liquid may be present in three different states, namely dissolved, emulsified, and dispersed [6,7,11). Moisture can accumulate chemically in the oxygen-containing liqUid. Especially, ester molecules contain chemically bond oxygen of � 20% by weight [13) and water linkages of the polar side valences lead to a high water saturation limit [6,14). However, investigations in the field of chemical species diffusion would be very helpful in understanding the process.

4 EXPERIMENTAL

PROCEDURES

The liquids were dried and degassed in a two-stage drying unit [4, 11] to ensure a very low initial water content. The investigations were performed at relative air humidity of 20,50 and 90% for various temper­atures (23 and 60"C) to simulate operating conditions as well as critical ones.

i 100

I 90 so

� 70

5 60 � . 50 � � 40 a 30

20 10

I I

---...., f"--. � r--�

'"', '"

-� ! I 10 20 30 40 50 60 70 80 90 100

Air relative humidity RH (%) -_

Figure 2. Relative air humidity in a closed environment using a water-glycerin mixture according t6 [10]. The relative humidity is ob­tained with an uncertainty of 1% for air temperatures between +15 and 60'C.

Relative humidity for air is the water vapor content of air relative to its content at saturation at the specific temperature Ill]. To obtain an expected relative air humidity, water has been mixed with a certain amount of glycerin (C3Hs(OHh), a water white colorless liquid. The mixture ratio can be taken from Figure 2 [15).

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 8 No.6, December 2001 1043

Table 1. Some important properties of the investigated insulating liquids compared to the limits according to lEe and VDE . • The data are mean values obtained from many manufacturers.

Property I Unit I Oil I Oil, 20%ester I Silicone I Ester General physical properties

Density, 23'C kg/m3 856 890 960 960

Density, 90C( kg/m3 810 851 - 915

Pour point c( -40 - -33 -50

Toxicity slightly slightly non·toxic non·toxic Biodegradability high high high very high Water solubility, 20C( ppm 45 310 200 2700

Water solubility, IOOC( ppm 650 1600 1100 7200

Heat transfer Cinematic viscosity, 20C( mm'/s 16 19.43 50 63

Cinematic viscosity, lOOC( mm'/s 2.3 3.45 16 7.7

Heat capacity, 20C( W((mK) 0.135 - 0.151 0.165

Heat capacity, 90C( W((m K) 0.125 .

-- 0.155

Specific heat, 20C( kJ(kg.K 1.85 - 1.55 1.81

Fire properties Flash point 'C 150�175 - >335 310

Flame point c( 130�135 - >300 257

Combustion heat WkJlkg 40 - 32.2 36.8

Self ignition c( 330 - - 405 Electrical properties (at 23 'C)

Breakdown strength (ac) Uv JjJ<: kV >60 >55 >50 >55

Permittivity Or (25C(, 50 Hz) 2.2 2.35 2.9 3.3

Dissipation factor t"" " (90q <lOXlO-4 <20xlO-4 1.6xlO-4 1xlO-4

Volume resistivity Dcm lOOxlO-12 8xlO-14 20x10-'2

Scaled vessel containing

Io--_-I!-_insuluting liquid specimens and a hygrometer

• ""mm��""",,,,,,,_��;;;;;Jt- water-glycerine mixture - ors.ilicagC!1

Figure 3. Experimental procedure used to asses the moisture uptake rate of different insulating liquids under selective conditions,

For the relative air humidity of 20%, the amount of glycerin was extrapolated from Figure 2 or a certain amount of silica gel was used in place of the water-glycerin mixture. The uncertainty for a linear interpolation between experimental points is <1% [15]. This mixture was placed in a sealed vessel containing samples of the different insu­lating liquid, where a hygrometer controlled continuously the relative humidity, Indeed, the water-glycerin mixture changes in case of water absorption by the insulating liquids, It is therefore of prime impor­tance to check regularly the relative air humidity. Figure 3 depicts the experimental procedure,

The uncertainty of the relative humidity was 1%. The tempera­ture was controlled by a proportional integral differential (PID) system, with an uncertainty of lOC. The water content of the liquid specimens were measured with a microprocessor controlled Karl-Fischer titration' method. The relative uncertainty of the measuring device is <0.5%, In contrast to the iodometric method, this procedure can be used to deter­mine the physically dissolved water as well as bonded water [11].

5 RESULTS AND DISCUSSIONS

5.1 SIMULATION OF A DEFECT IN THE BREATHING SYSTEM

The methods for evaluating the properties of insulating liquids that guarantee their quality and life duration are standardized. All the prop­erties listed in the literature [2, 16] are important, but some have a spe­cial merit for the characterization of an insulating liquid. The important ones are those that are likely to vary significantly with liquid purity and composition as well as with external parameters, such as temper­ature and electric field. T he dielectric properties of insulating liquids are well known to depend on water content [2, 5, 8, 9, 12]. Water con­tent increases electric conductivity and dissipation factor and worsens dielectric strength. Since the two parameters of our investigations are temperature and moisture, we investigated their ac electric strength (or the dielectric breakdown voltage at power frequency), which is the most often controlled parameter describing the liquid's function as an insulant [2]. The obtained results are then used to asses insulating liq­uid insulant behavior under the selective conditions,

To determine the electric strength at various temperatures and for various water contents, the respective specimens were placed in the vir­tually homogeneous field of the Verband der Elektrotechnik Elektronik

Informationstechnik e, V. (VDE) hemispherical setup [17] and subjected to a 50 Hz voltage, increasing at a rate of 2 kV /s to breakdown, The time interval between individual measurements was set at 5 min. Be­tween individual measurements, a glass stirrer was used to remove any

.

solid decomposition products appearing between the electrodes, and to take away any gaseous decomposition products back in the liquid. Be­fore and after each measurement series, the electrodes were purged of the decomposition products of preceding breakdowns. The basic dia-

1044 Fofana et al.: Fundamental Investigations on Some Transformer Liquids under Outdoor Conditions

Figure 4. Measuring circuit for breakdown tests. 1: Motor actuating transformer, 2: manual transformer, 3: HV transformer, 4: protective resistor, 5: capacitive voltage divider (100 pF /100 nFl, 6: breakdown recognition (optional), 7: test vessel with VDE electrodes, 8: heating device.

gram for dielectric testing of the insulating liquids is shown in Figure 4. In the VDE guidelines [17], six individual measurements in series are recommended. When investigating silicone fluid, a problem arose at electric breakdown with the formation of gelatinous crosslinked poly­meric siloxanes which impair the breakdown strength of the fluid con­siderably, leading to individual breakdown. A modification in the test apparatus has already been proposed by Borsi [11].

Figure 5. Schematic circuit diagram for measuring breakdown volt­age of silicone fluid. 1: Series resistor in primary circuit, 2: current limiting resistor, 3: coupling resistors , 4: electronic switch off.

The arcing energy was reduced by limiting the magnitude and duration of the short-circuit current, so that subsequent readings are no longer influenced by decomposition products. Figure 5 shows a schematic diagram of the circuit design developed for this purpose.

The investigations were performed at different water content and temperatures. The temperatures was varied between 20 and 120"( to simulate normal operating conditions as well as critical ones, while the water content of each insulating liquid has been varied with respect to their saturation limit at room temperature. More particularly, water content has been varied from 5 to 80 ppm for mineral oil, from 30 to 5000 ppm for ester liquid, from 20 to 460 ppm for silicone fluid and from 20 to 120 ppm for the mixture of mineral oil with 20% ester liquid. These conditions allowed reaching relative water contents of > 100%.

Because the saturation mixing ratio is also a function of pressure, and especially of temperature, the relative humidity is a combined in­dex of the environment and reflects more than water content [12]. The breakdown voltage is thus described as a function of relative water con­tent. Figures 6 to 8 summarize the results of the ac electric strength

40 60 80 100

Relallli& water content [%]

�-------, o Minera!oU I • Esoorliquid

I - L�mltrorunagedliQuid ,-,� lIlflllror agt!o liqUId

120 140 160

Figure 6. Breakdown voltage vs. insulating liquid relative water content for mineral oil and ester liquid.

60

�'m�

J o Mn!'Jraloil 4J� .- MineraloU.j.20%Esterli':juid

- limit for u1aged 011 �,- limitforaged oit

�u � ______ ��� ____________________ � F � r[� 20

10

o . ---------.- " 20 R,

��-1 eo R2 80 100 120 140 160 Rel2uI.ewAierconmnt 1%] -_

Figure 7. Breakdown voltage us. insulating liquid relative water content for mineral oil and the mixture of mineral oil with 20% ester liquid.

Table 3. Values of relative water content related to the limits sug­gested by lEe or VDE standards [18-20J for different insulating liquids depicted in Figures 6 to 8.

Liquid Unused specimens Aged s� ecimens VI (kV) RI(%) V2 (kV) R2(%)

Ester liquid 45 20-+30 30 22�40

Mineral oil 50 43 30 6(1 Silicone fluid 40 >15 30 >25 Mineral oil, 20% ester 50 40-�,45 30 55-->80

investigations, showing clearly the limits (Ul and U2) suggested by lEe (International Electrotechnical Commission) or VDE [18-20]. On these Figures, U1 and U2 correspond respectively to the limits related to unused and aged specimens, while Rl and R2 are the relative water content accordingly related to these limits (Table 3). The investigations were only performed for new liquid specimens. However they can be useful for serviceable insulating liquids since experimental evidence indicates that liquids which are in serviceable condition or liquids sub­mitted to accelerated oxidation tests show little change in their water solubility characteristics [6,14]. Only when the liquid is severely aged or contaminated does the saturation level increase significantly.

Polar compounds present in severely aged liquids are thought to

IEEE Transactions on Dielectrics and Electrical Insulation

1= 70

�.6(] .. so-\! jU1 •

m U,

20

10-

0,

0 Rl R1 50 100 150 200 250 RelatNe water content 1%J

Figure 8. Breakdown voltage 115_ insulating liquid relative water content for mineral oil and silicone fluid.

influence the water solubility characteristics since water molecules can be captured by hydrogen bonds with carboxyl groupings [6].

For mineral-based oil, the breakdown voltage U B may be described as a function of relative water content (very small scatter) and thus all breakdown data at the different temperatures and different absolute humidity contents can be shown in mutual dependence, This does not apply to either silicone fluid or ester liqUid, Contrary to mineral oil, there is a relatively wide scatter range for ester liquid and a wide scat­ter range for silicone fluid, Especially, investigations were performed for high relative water content in case of silicone fluid in order to check if its dielectric behavior decays with increasing relative water content (>100%) since its behav ior is totally different from those of other inves­tigated insulating liquids.

For ester and silicone liquids, the twin dependency of breakdown voltage on temperature and water content cannot be explained by the dependence on relative water content. Thus the breakdown of these two liquids is evidently not due to physical dissolving of the water.

From Table 1, it can be seen that both liquids posses a high viscos­ity compared to mineral oil, but results reported by Beroual et al. [21] indicate that viscosity has little effect on the breakdown process_

The difference in their behavior compared to mineral oil can be ex­plained by the difference in water absorption and/or water solution states in these liquids, and consequent change in their molecular struc­ture, depending on the temperature, Ester or silicone liquid can take up water in chemically bonded form and in dissolved form, while in min­eral oil, water is only dissolved [7, 11]. Because the electric breakdown of insulating liquids is determined by electronic conduction processes [6,7,11], the molecular structure has a major effect on the level of the breakdown voltage.

However, the behavior of the mixture of mineral oil with 20% ester shows an intermediate behavior between mineral oil and ester liquid (Figures 6 and 7).

Figures 9 to 12 depict the results of the investigations concerning the moisture uptake rate of the different insulating liquids vs. time, The temperature and ambient relative air humidity acted as parameters.

It can be seen that when testing the liquids as new ones, the limits suggested by VDE or IEC standards are respected to a 30 day exposition

Vol. 8 No. 6, December 2001 1045

Figure 9. Moisture uptake of mineral oil vs. exposition time. The temperature and ambient relative air humidity acted as parameters,

- - •.. �-.-.-.. -.--. t- ------i !

-!

ExposlhOrldurahool(diI�)

• RH=20%-1=23"C • RH=50%-T=23"C • RH=1I0'!o-T=23"C o RH"'20%- T.ooO·C o RH=50%- r=so·c D. R�=90%-T"'60'C

Figure 10. Moisture uptake of silicone fluid us, exposition time, The temperature and ambient relative air humidity acted as parameters.

10 15 20 &positilmdurSMI1t(c!ays)

Figure 11. Moisture uptake of mineral oil mixed with 20% ester liq­uid vs, exposition time, The temperature and ambient relative air humidity acted as parameters.

only for a relative air humidity of 20%. For higher relative humidity (? 50%) the limit is respected only for a few days. But when testing the liquids as an aged one, the limits remained respected up to a 30 day exposition at an relative air humidity of 50% for mineral oil and the mixture of mineral oil with 20% ester liquid. Table 4 shows a compari­son of the investigated liquids according to the limits suggested by IEC or VDE. It can be seen that the coupling moisture uptake rate and the ac electric strength limit suggested by lEe or VDE standards [18-20], the mixture of mineral oil and 20% ester liquid seems to be more suitable, since it will mostly retard possible failure under operating condition.

5.2 FUNDAMENTAL

INVESTIGATIONS

As can be seen in Figures 9 to 12, the higher relative air humidity, the higher moisture uptake rate of the insulating liquids. Also, the temperature has a very significant influence on the moisture uptake of each insulating liquid. For a given relative air humidity, the higher the

1046 Fofana et al.: Fundamental Investigations on Some Transformer Liquids under Outdoor Conditions

• RH"'2C%· T=23·C

• RH=50%· T="J�G ... RH::90%.T=23"C

o RI-l�20%. T .. 6C�C C RH�50%. T;:60°C t. RH�90%. T=fl[]"C

Figure 12. Moisture uptake of ester liquid vs. exposition time. The temperature and ambient relative air humidity acted as parameters.

Table 4. Exposition duration time (in days) of the investigated insu­lating liquids at which the limit suggested by the standards (lEe or VDE [18-20]) are no more respected. Note that these limits were calcu­lated taking into account the lower value of the relative water content as depicted in Table 3.

��������

50

90

20

50

90

>2

>1

>30

>2

>1

Table 5. Parameters A and k of the investigated insulating liquids, depicted in Equation 3.

RH(%)I Oil I Oil, 20%ester I Silicone I Ester A(%)

20

I

28±13 ,I 18±5.21

I

18.6±2.5 1 16±2.4

50 50±7.6 1l±2.3 47±10.3 35±3.7

90 83±17.5 79±3 . 9 73±13.5 70±15.3 k (day)

20

I

3

I

3

I

3

I

5

50 3 3 3 5

90 3 3 3 5

temperature, the higher the moisture absorption rate. The reason is the saturation limit of the insulating liquid which is highly temperature­dependent and already shown in Figure 1.

On Figures 9 to 12, it can be observed generally that, during an ex­position under some selective conditions (ambient relative air humidity and temperature), insulating liquids uptake moisture and saturate after a few days depending on the kind of liquid (moisture uptake veloc­ity, saturation limit, temperature and initial water content), and on the outdoor conditions (relative humidity and temperature).

The investigations show that mineral oil takes moisture to a rela­tive water content of � 9 3% (Figure 9 ) while silicone fluid and ester liqUid reach � 70% as relative water content (Figures 10 and 12). Out of Figure 11, it can be seen that the mixture of mineral oil and ester liq­uid reaches � 80% of relative water content, i.e. an intermediate value between mineral oil and ester liquid.

Apart from mineral oil, a close relationship seems to exist between the relative moisture uptake rate of the different liquids and the exposi. tion duration, information concerning the temperature being included

Table 6. Parameters a and b of the investigated insulating liquids, depicted in Equation 4 .

Este� 0.77±0.18

-l.l±3

in the relative liquid humidity. The reason of the large scatter in the values obtained for mineral oil could be related to the difference in the initial relative water content used for the investigations performed at 20 and 60'C. But additional investigations are needed before drawing more general conclusions.

A mathematical approach of data suggests the existence of some kind of relationship between relative moisture content of liquid and ex­position time. Indeed, relative liquid humidity Wr can be described as a mathematical function of the exposition time t (in days) representing the solid·line curves, which fits the experimental data plotted in Figures 9 to 12

Wr = A(l - exp[-t/k]) (3)

The constants A (in %) and k (in days) depending on the liquid itself, are deduced from Figures 9 to 12.

On Table 5, it can be seen that k depends only on the insulating liq­uid while A depends on the liquid and on ambient relative air humidity RH(in%).

90 80 70

-60 e: -:: 50 � E 40 �

0. 30 20 10

-

-

-

�. o Mineral 011

� • Silicone fluid /' 6 Mineral oil + 2D% ester I / V: / • ester liquid / � V

� � V -----r-....-7 �

'� I:/"

I I 10 20 30 40 50 60 70 ao 90

Ambiant air relative humidity RH (%)

Figure 13. Parameter A VS. ambient relative air humidity RH for different insulating liquids

The dependency of the constant A VS. RH can be taken from Fig­ure 13. A close linear relationship exists between parameter A and the ambient relative humidity RH. The curves can be linearly fitted such as

A= aRH+b (4) where a and b are dimensionless parameters that depend on the liquid itself as seen in Table 6.

The behavior of insulating liquids under these selective conditions (different exposition temperatures and ambient relative air humidity) can thus be modeled combining Equations (3) and (4) such as

Wr = (aRH + b)(l - exp[-t/k]) (5) The absolute moisture content Wabs can be deduced knowing the satu· ration level of the insulating liquid at a given temperature T (K).

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This relationship can be useful to predict the moisture uptake rate of an insulating liquid submitted to some selective conditions (tem­perature, ambient relative air humidity) provided the initial relative humidity of the insulating liquid (to be calculated according to Equa­tion 1) is known. Indeed at a given ambient relative air humidity RH and temperature T, moisture uptake rate of the insulating liquid VS. the time t can be deduced such as

with

Wabs Wr = WdT) 100

= (aRH + b) (1 _ exp [_ t: to]) . [ W · 100 ] to = -k In 1 - WdT)

'(aRH + b)

(6)

(7) where a, b, k and W L (1') depend on the kind of liquid. The standard deviation of Equations (6) and (7) can be deduced from the parameters a and b from Table 6. The parameter to contains information related to the initial absolute moisture content Wi of the liquid itself before the exposition.

These relationships are of prime importance since they allow to de­scribe the moisture uptake rate of an insulating liquid under selective conditions (temperature, relative humidity), provided the liqUid pa­rameters (a, b, k, WdT)) are known. However, the relatively large standard deviation associated to the parameter a and b (Table 6) shows that additional investigations will be very useful for refining these val­ues.

6 CONCLUSION

THE work presented dealt with some fundamental investigations concerning insulating liquids actually used in HV transformers and

a mixture of two of them, recently proposed as insulating liquid.: It is generally found that the less hygroscopic the insulating liquid is, the higher the moisture uptake rate is. The mixture of mineral oil with 20% of ester liquid seems to be promising. lndeed, when adding ester liquid to mineral oil, an intermediate behavior of the two insulating liquid is obtained: the ac electric strength remain almost unchanged while the moisture uptake velocity is reduced compared to that of the pure mineral oil.

When considering the scenario of a defect in a breathing system (for air-breathing type transformers) it was found that at a very low ambient relative air humidity (20%), the ac breakdown voltage limit suggested by IEC or VDE remains respected to a 30 day exposition. For a relative humidity >50% these limits are respected only for a few days depending on the kind of insulating liquid (moisture uptake velocity, saturation limit, temperature and initial water content) as well as on the outdoor conditions (relative humidity and temperature). These inves­tigations can provide practical information for monitoring conservator type transformer in case of a bad run in the breathing system.

Also, the mathematical approach depicted empirical relationships between moisture uptake velocity, outdoor conditions (temperature and ambient relative air humidity). These relationships can be use­ful also for serviceable insulating liquids since experimental evidence indicates that liquids which are in serviceable condition or liquids sub­mitted to accelerated oxidation tests show little change in their water

Vol. 8 No.6, December 2001 1047

solubility characteristics [16]. Only when the liquid is severely aged or contaminated, does the saturation level increase significantly. Po­lar compounds present in severely aged liquids are thought to influ­ence their water solubility characteristics. However, the relatively large standard deviation associated to the parameters a and b shows that additional investigations are needed to refine these values.

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Manuscript was received on 27 November 2000, in final form 16 Tillie 2001.