Oxidation Stability of Insulating Oil

The insulating oil reacts with oxygen and generates organic acids, sludge, water, and other matter. These contaminants significantly degrade the electrical characteristics of the insu­lating oil. The organic acid and the moisture advance corro­sion and other deterioration of materials in contact with the oil, and sludge lowers its cooling efficiency. Therefore, oxida­tion stability of the insulating oil is important for the life and reliability of oil-filled electrical equipment. It is known that the copper used for conductors in electrical equipment can catalyze the oxidative deterioration of the insulating oil (5,6).

The oxidation stability of the insulating oil is evaluated from the amount of sludge, total acid number, and electrical characteristics after heating the insulating oil in contact with excess oxygen and copper. Test methods are described in IEC Publications 74, 474, 813; ASTM D 1313, 1934, 2112, 2440; BS 148; DIN 51554; and JIS C 2101. Tests are carried out at 100°C to 120°C. Since ASTM D 1313 uses no catalyst, that test is done at the highest temperature (140°C).

The oxidation stability of mineral oil is influenced by its degree of refinement. However, higher refinement does not necessarily mean higher stability. Research on improved oxi­dation stability of insulating oil is often done from the view­point of optimum aromaticity.

tan 8 of Insulating Oil in Relation to Oxidative Deterioration

The insulating oil of oil-immersed electrical devices that are equipped with oxidation deterioration prevention devices does

Operation (year)

0 5 10 15 20 25

Figure 4. tan S behavior for actual transformer oil in the field (A) and insulating oil in laboratory data (B).

Aging time (h)

not come in direct contact with the atmosphere. Therefore the oxidative deterioration of the insulating oil is slow. As shown in Fig. 4, the temperature dependence of tan S for insulating oil shows peculiar behavior.

In Fig. 4, the band marked A shows the deterioration of tan S in the insulating oil in a nitrogen-enclosed transformer and in a sealed transformer, at 80°C, gathered from many transformers over years of operation. The time dependence of tan S yields an N-shaped curve. A peak is observed at 5 to 7 years after the start of operation. This behavior is confirmed in laboratory experiments. It implies that oxygen and copper play an important role.

Curve B shows laboratory data on the deterioration of in­sulating oil where the copper surface area was 44.8 cm2 per 100 mL of oil, the oxygen volume was 5 mL per 100 mL of oil, and the oil temperature was 95°C (7).

From comparison of curves A and B, one hour of deteriora­tion as accelerated in the laboratory is seen to be equivalent to about one year in operation.

MOISTURE EFFECT Moisture in Insulating Oil

Moisture in insulating oil leads to a decrease of volume resis­tivity and to dielectric breakdown. The moisture content is related to the humidity in the atmosphere and also changes with the oil temperature. Standards for it are given in JIS C 2101, BS 2511, IEC Publ. 733, and also ASTM D 1533. One method of measuring it uses Karl Fischer’s reagent, which reacts sensitively with very small quantities of moisture. The reactions between Karl Fischer’s reagent and water are as follows:

I2 + SO2 + 3C5H5N + H2O ^ 2C5H5N • HI + C2H5N • SO3 C2H5N • SO3 + CH3OH ^ C2H5NH • OSO2 • OCH3

A liquid mixture of chloroform and methanol is used for the titration solvent.

Dissolved water content in liquids in general is determined by the relative humidity and the temperature of the atmo-

40-

(17)

(18)

<D

TO

~ 80

60

-40

0

20

40

60

Figure 3. Temperature dependence of dielectric breakdown voltage in transformer oil. О = Nitrogen sealed-off transformer; • = trans­former with rubber-bag conservator.

Temperature (°C)

-20

о

0

о

-a

я

ф

20

Figure 2. Relation between moisture in insulating oil and relative humidity of atmosphere.

RESISTANCE TO IGNITION

Because of increasing environmental problems due to the bio – accumulative nature of polychlorinated biphenyls (PCBs), the production and use of PCBs have been prohibited throughout the world. Almost all substitutes for PCBs are more flamma­ble than the PCBs, and the evaluation of flammability of those liquids becomes very important.

There are many test methods for the evaluation of the re­sistance to ignition and fire propagation of insulating liquids.

Table 3. Physical Properties of Insulating Oils

Property

Naphthenic Oil

Paraffinic Oil

Silicone Liquid

High-Molecular – Weight Hydrocarbon

Heat conductivity (W/m • K)

25°C

0.120

0.132

0.136

0.134

100°C

0.109

0.116

0.128

0.119

Specific heat (kJ/kg • K)

25°C

2.05

1.96

1.49

1.63

100°C

2.33

2.33

1.65

1.90

Density [kg/(0.1 m)3]

25°C

0.87

0.86

0.96

0.88

Thermal expansion coefficient (K—:):

25°C

7.8 X 10—4

7.8 X 10—4

1.04 X 10—4

8.0 X 10—4

Kinematic viscosity (m2/s)

25°C

11 X 10—6

12 X 10—6

50 X 10—6

350 X 10—6

100°C

2.1 X 10—6

2.2 X 10—6

16 X 10—6

16 X 10—6

Flash Point and Fire Point

Determination of the flash and fire points of petroleum prod­ucts by the Cleveland open cup method is described in ISO 2592, and that of the flash points of petroleum products and lubricants by the Pensky-Martens closed cup method is de­scribed in ISO 2719. These are normative and very easy methods, now being used worldwide and specified by various standards for the evaluation of flammability of insulation liq­uids of almost all types.

Oxygen Index

Determination of the oxygen index of insulating liquids is de­scribed in IEC 61144. This method is an adaptation for liq­uids of ISO 4589, which has the same principles and is appli­cable to solids. The oxygen index is defined as the minimum concentration of oxygen by percentage volume in a mixture of oxygen and nitrogen that will just support combustion of a material. The smaller the index, the more combustible a tested liquid is.

Net Calorific Value

Determination of the net calorific value or net heat of combus­tion of liquid hydrocarbons with a bomb calorimeter described in ASTM D 240 is specified in ISO 1928. This quantity repre­sents the rate of heat generation by the liquid during its com­bustion.

Other Fire Tests

The pool fire test (large scale and small scale), trough test, spray mist test, and heat release test, developed by Factory Mutual Research, are attractive and practical methods to

Table 4. Ratios of AT and ф of Silicone Liquids and High- Molecular-Weight Hydrocarbon Oils to Those of Mineral Oilsa

High-Molecular-

Property

Mineral Oil

Silicone Liquid

Weight Hydrocarbon

AT

1.00

2.75

2.99

Ф

1.00

1.18

1.26

a Except for the density and thermal expansion coefficient, characteristics at 100°C were used for the calculations.

evaluate the ease of ignition and behavior of liquids during combustion, but the required equipment is bulky and requires ample space.

Linear Flame Propagation Test

The linear flame propagation test method, using a glass fiber tape, is described in IEC 61197. A glass fiber tape impreg­nated with the sample liquid is ignited at one end, and the time for the flame to travel between two lines is measured. This method is very easy to set up and useful for impreg­nated systems.

The method to be selected for the evaluation of fire hazard depends on the kind of machine and conditions of use. No single method is always effective, and in many cases some combination of the methods mentioned above is necessary. For example, classification of insulating liquids according to fire point and net calorific value is given in IEC 61100.

THERMAL TRANSFER CHARACTERISTICS Cooling Method

Insulating Oil

Dielectric Constant

tan S (%)

Mineral oil

2.18

0.02

5-Chlorobiphenyl

4.25

0.03

3-Chlorobiphenyl

5.20

0.04

Chloroalkylene

5.05

<0.01

Trixylenyl phosphate

6.00

1.0

Diethylhexylphthalate

4.55

0.10

Castor oil

4.00

0.30

Alkylbenzene

2.17

0.005

Alkyl naphthalene

2.48

0.01

Alkylbiphenylethane

2.51

0.10

Silicone oil

2.52

0.008

Insulating liquids (oils) have not only excellent dielectric characteristics but also significant cooling effects. Insulating oils are important coolants in apparatus in which much heat is produced. Nevertheless, external cooling systems must sometimes be added, depending on the capacity of the appara­tus and the load.

In the case of transformers, there are several types with different cooling methods, such as the oil-immersed self­cooled type, forced-oil self-cooled type, direct-oil-flow self­cooled type, forced-oil forced-air-cooled type, direct-oil-flow forced-air-cooled type, oil-immersed forced-water-cooled type, and forced-oil forced-water-cooled type. Oil-immersed self­cooling is the simplest method. In this method transformers are cooled by natural convection. In other methods heated in­sulating oils are cooled by forced air, forced oil, or water using coolers or heat exchangers. In high-voltage and high-power transformers the latter methods are widely used.

Heat Transfer Characteristics of Insulating Oils

When insulating oil cools insulating solids that cover a heat source, such as the paper on transformer windings, the heat flux through the solid to the oil is expressed in (2)

In the expression for Nu, Re always appears in the form Re" (n > 0). Therefore, generally speaking, the higher the ve­locity is, the higher is a. For instance, in the case of a flat plane exposed to forced convection of liquid

DIELECTRIC CONSTANT AND LOSS

Dielectric polarization occurs when an electric field is applied to insulating oil. When there is a time delay in the formation of polarization, dielectric loss arises from the phase delay of polarization under an alternating electric field. The loss due to this dielectric polarization is proportional to the dielectric loss tangent tan S, which is equal to the ratio of the dielectric loss factor e" to the dielectric constant e’:

tan S = є "/є’ (1)

The temperature dependence of tan S and e’ in three mineral insulating oils is shown in Fig. 1. At a frequency of 1 kHz, the maximum of tan S is observed at —40°C to —50°C. The maximum value moves to higher temperatures as the mea­surement frequency is increased.

The maxima in tan S and in the dielectric loss factor e" are caused by asymmetry of the molecular structure leading to dipole moments in, for example, aromatic compounds. These maximum values increase as the insulating oil deteriorates.

The frequency fm (Hz) at which tan S or Є are a maximum has the following temperature dependence:

fm = f, exp (—AH /RT)

1.2

1.0

0.8

0.6

0.4

0.2

0

2.20

«Ж» ^

I Oil A

. Oil B

-A * Oil C

Oil A

X Oil B

Oil C

2.00 f

<D

1.80 a

с

я

-80 -60 -40 -20 0 20

120 140 160 180

Figure 1. Temperature dependence of di­electric constant and tan S at 1 kHz in mineral insulating oil.

40 60 80 100

Temperature (°C)

ln

(3)

AH =

(4)

(5)

Table 2. Dielectric Constant and tan S in Various Insulating Oils”

1 60 Hz 80°C.

where f0 is a constant (Hz), AH is the activation energy (cal/ mol), R is 1.987 cal/mole • K, and T is the absolute tempera­ture (K). The value of AH is about 20 kcal/mol for insulating oil with 15% of aromatic compounds, and about 40 kcal/mol for insulating oil without aromatic compounds.

Equation (2) applies over a rather narrow temperature range. On the other hand, it is known that the following equa­tion, due to Williams, Landel, and Ferry (WLF), applies over a wider temperature range:

fmO _ Cl (T T0) fm C2 + (T — T0)

where fm0 is the frequency that yields the maximum value of tan S at temperature T0, fm is the frequency that yields the maximum value of tan S at temperature T, and C1 and C2 are the WLF constants. AH is expressed by the following equa­tion, deduced from Eqs. (2) and (3):

2.303RC1C2T2

C*+T-Ta

which applies over a wide temperature range. C1 and C2 are obtained from the slope and intercept of 1/ln( fm0/fm) plotted against T — T0, in view of Eq. (2).

There is no effect due to the orientation of dipoles above room temperature at low frequencies, such as those of com­mercial power. However, the mobility of the charge carriers is high, and electric conduction losses can be large. The electric conduction loss is caused by the conduction due to ions that arise for impurities and from the dissociation of the insulat­ing oil itself. Therefore, tan S can be related to the volume resistivity p that is obtained from dc measurements:

tan S = 100/ax ‘e0p

where tan S is in percent, w (=2nf, where f is the frequency of the voltage) is the angular frequency, e’ is the dielectric constant, e0 is 8.85 X 10—14 F/cm, and p is in ohm-centimeters.

Dissipative current is observed on applying a dc voltage to the electrode that measures the volume resistivity of the insulating oil. The volume resistivity p in Eq. (5) is calculated from the current i0 that is obtained at time t = 0.

In recent years, a measuring device that can read tan S directly has been marketed. In its absence, however, tan S can be calculated from Eq. (5).

Values of the dielectric constant and tan S of various insu­lating oils are shown in Table 2.