August 29, 2019

POLYCHLORINATED BIPHENYL (PCB) ANALYSIS

Background:

Polychlorinated biphenyls are a group of related compounds or congeners made by reacting biphenyl with chlorine and replacing anywhere from one to ten of the original hydrogens of the biphenyl. In so doing there are a total of 209 congeners that are collectively referred to as polychlorinated bihenyls (PCB’s). The prime manufacturer of PCB’s in the United States was the Monsanto Chemical Company, which first marketed them in the late 1920’s. They designated them with the trade name Aroclor followed by a numerical designation of four digits. The first two digits designated the ring system, e.g. 12 was for derivatives of biphenyl. The last two digits designated the average weight percent chlorine in the mixture. Thus Aroclor 1221, 1242, 1254, and 1260 all represent chlorinated biphenyls that contain 21, 42, 54, and 60 % chlorine respectively. The higher chlorine containing Aroclors are solids and to make them useful for an insulating fluid in a transformer they are blended with various chlorinated benzenes to give mixtures known as Askarels, which are then marketed under a variety of trade names, such as Inerteen (Westinghouse) and Pyranol (General Electric).

Procedure:

The details of the entire procedure for determining the PCB content of an insulating fluid are given in the ASTM D 4059 method and are only briefly mentioned here. A sample of the fluid to be analyzed is diluted with a suitable solvent and the resulting solution is treated to remove any substances that could interfere with the determination. A sample of the treated solution is then analyzed by gas chromatography utilizing an electron capture detector, which is very sensitive for chlorine containing species. The resulting chromatogram is compared against standard chromatograms for the various known Aroclors. This data then allows one to identify which Aroclor or mixture of Aroclors is present and to quantitate them. The results are usually reported in terms of parts per million (ppm) of each Aroclor present.

Significance:

The PCB content of a unit is usually required for regulatory purposes in order to establish whether a unit is free of PCB’s, contaminated with PCB,s, or does it have to be treated as a PCB filled unit. The values of the PCB,s that must be present to classify the unit as described above will depend on the location of the unit and the regulatory agency that has jurisdiction.

PCB’s have been shown to have a low order of toxicity in humans. There is no adverse effect on the reproductive process, there is no teratogenicity risk to offspring, no mutagenicity, and no carcinogenic effect in humans. There is an occasional chloracne that disappears after exposure ceases. These materials are very stable and do not biodegrade easily. They do tend to bioconcentrate and have a more serious effect on the lower life forms, thus they are of concern in the environment.



August 29, 2019

OXIDATION INHIBITOR

Background:

Mineral oil insulating fluids undergo oxidative degradation in the presence of oxygen to give a number of oxidation products. The final products of oxidation are acidic materials that can affect the characteristics of the insulating fluid as well as cause damage to the components of the electrical unit. Oxygen is a diradical species and the reactions of the oxidative process are complex but they do involve free radical reactions. One way to prevent these types of reaction is to incorporate an oxidation inhibitor that will interupt and terminate the free radical process of oxidation. Phenolic materials are quite good for this purpose and the two most commonly used inhibitors are 2,6-di-tertiary-butylphenol (DBP) and 2,6-di-tertiary-butyl-4-methylphenol or 2,6-di-tertiary-butyl-para-cresol (DBPC).

Procedure:

The entire details of the procedure for the determination of DBP and DBPC in mineral oil insulating fluids are given in the ASTM D 2668 method and are only briefly mentioned here. The method used for the determination of the amount of DBP and DBPC in mineral oil is by using infrared spectrometry. Both of the inhibitors exhibit infrared bands at about 3650 cm-1 due to the O-H stretching frequency of the phenol group. It is this band that is used to quantitate the amount present. In order to determine which inhibitor is present the spectrum is scanned in the region from about 900 to 700 cm-1. The DBP exhibits a band at about 745 cm-1 due to the out-of-plane bending motion of the hydrogens attached to a 1,2,3-trisubstituted benzene ring. The DBPC exhibits a similar band at about 860 cm-1 due to the out-of-plane bending motion of the hydrogens attached to a 1,2,3,5-tetrasubstituted benzene ring.

Thus one can identify which inhibitor is present by scanning the low frequency range of the spectrum and then it can be quantitated by scanning the high frequency band at 3650 cm-1. A series of standard solutions of the inhibitors in an uninhibited oil such as Diala A are made up and a calibration curve is determined for each inhibitor. The standards are covered over a range up to 0.5 % by weight.

Significance:

The presence of inhibitors in the oil will increase the useful life of the oil with respect to oxidative degradation in the presence of oxygen. As the oil is exposed to this type of oxidative degradation, the oil will be protected as long as there is inhibitor present. However, as the process proceeds the inhibitor will be used up and when it is gone the oil will degrade at a much higher rate. Thus the determination of the amount of inhibitor present can be used to estimate the useful life of the oil. It can also be used to determine whether or not new oil has been properly inhibited prior to its use. As the inhibitor is used up its concentration can be monitored and additional inhibitor added as needed to maintain a proper concentration in the unit. Typical values for fresh oil are in the range of 0.25 to 0.35 % DBP or DBPC by weight.



August 29, 2019

DISSIPATION FACTOR,

POWER FACTOR,

AND

RELATIVE PERMITTIVITY (DIELECTRIC CONSTANT)

Background:

There is a relationship between the dissipation factor, the power factor, and the permittivity or dielectric constant. They all relate to the dielectric losses in an insulating fluid when used in an alternating electric field. The permittivity is represented as a complex quantity in the following manner: e* = e – j e ; where e* is the complex permittivity, e is the real or measured permittivity, and e is the imaginary permittivity. In the presence of an alternating field there is created a capacitance current and a resistive current that are 90o out of phase with respect to each other. The vector sum of these two currents represents the total current and the angle between the capacitance current vector and the resulant total current vector is defined as the loss angle, d. The ratio of the imaginary to the real part of the permittivity is equal to tan d ; i.e. tan d = e” / e’. The factor tan d is defined as the dissipation factor, D, and represents the dielectric loss in the insulating fluid. The power factor, P, is defined as sin d. The relationship between D and P is the following: D2 – F2 = D2 F2 , thus if you know one value you can calculate the other. Furthermore for small values of d, tan d ~ sin d, thus for values of tan d up to 0.05 the power factor and the dissipation factor are the same within one part in a thousand.

Procedure:

The details of the entire procedure are given in the ASTM D 924 standard and are only briefly mentioned here. The measurements are made in specially designed cells that are machined to precise dimensions. The measurements are done at precise temperatures, usually 25 and 100 oC, thus the cells have to be kept at a constant temperature. The actual measurement is one of comparing the capacitance of the cell filled with the insulating fluid sample in a sensitive electronic bridge circuit. The result is usually expressed as a percentage for the dissipation factor or power factor.

Significance:

The dielectric loss factor relates to the inability of molecules in the insulating fluid to reorient themselves with an alternating electric field. This ability is dependent on the temperature of the sample, the size of the molecules involved, and their polarity. It is also dependent on the frequency of the alternating field. The dissipation factor and the permittivity are both affected by the molecular size, composition, and relative orientation of functional groups within the molecules. In general within a series of similar molecules, the permittivity will increase as the molecular weight increases. The above described factors are electrical characteristics of the insulating fluid and can be used to monitor the quality of the oil with regard to deterioration in use and for the presence of contaminents.

The IEEE has suggested guidelines for Power Factors depending on the type of oil and the unit it is being used in (IEEE C57,106-1991). Some representative values are given below:

 

Type of Oil/Unit Power Factor
@ 25 oC @ 100 oC
Shipment of New Oil from Refinery max. 0.05% max. 0.3%
New Oil Received in New Equipment
< or = 69 kV
69 – 288 kV
> 345 kV
max. 0.15%
max. 0.10%
max. 0.05%
max. 1.50%
max. 1.00%
max. 0.30%
New Oil for Circuit Breakers max. 0.05% max. 0.30%
Suggested Limits for Oil used
in Circuit Breakers
max. 1.0% Not Spec.

 


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