August 29, 2019

Dielectric Breakdown Voltage

Background:

The dielectric breakdown voltage is a measure of an insulating fluids ability to withstand a high electric field stress without breaking down. It can also indicate the presence of water or other contaminants in the oil; however, a high dielectric breakdown voltage does not necessarily prove the absence of contaminants. The dielectric breakdown voltage is not a constant of the material being tested like the dielectric constant but it is a statistical process and as a result repetitive determinations have to be done. The results can also be dependent on the design of the electrodes, the spacing of the electrodes, the wave form of the applied voltage, and the rate of rise of the applied voltage. There are two methods recognized by ASTM for this method. The first method uses disk electrodes with a voltage ramp of 3000 V/s (ASTM D 877) and the second method uses spherical electrodes with a voltage ramp of 500 V/s (ASTM D 1816).

Procedure:

The details of the entire procedures for determining the dielectric breakdown voltage of oil using disk electrodes are given in the ASTM D 877 standard and for the spherical electrodes in the ASTM D 1816 standard and both are only briefly mentioned here.

The disk electrode system utilizes 25 mm diameter square-edged disks separated by 2.5 mm. The cell is filled with oil to cover the electrodes to at least a depth of 20 mm and the sample is allowed to set for at least 2 minutes without agitation. A 60 Hz sinusoidal wave voltage is applied at a ramp rate of 3000 V/s until breakdown occurs as indicated by passage of a current through the sample of 2 to 20 mA. This occurrence is used to trip a relay within 3 to 5 cycles that stops the voltage ramping and maintaining the breakdown voltage. A series of determinations are done, which are then treated statistically to yield the final value.

The spherical electrode system utilizes electrodes that have a 25 mm radius and are spaced either 1 or 2 mm apart. The cell should be filled with enough oil to cover the top of the electrodes with at least 13 mm of oil. The cell shall be equipped with a propeller to circulate the oil in a downward direction during the testing procedure. A 60 Hz sinusoidal wave voltage is applied at a ramp rate of 500 V/s until breakdown occus as indicated by a passage of current through the sample of 2 to 20 mA. This occurrence is used to trip a relay that stops the voltage ramping and maintains the value of the breakdown voltage. A series of determinations are done, which are then treated statistically to yield the final value.

Significance:

The more uniform electric field of the spherical electrode system makes this method more sensitive to the presence of water or other conducting particulate material in the fluid. It is for this reason that the oil must be circulated during the measurement to insure that any particles are uniformly suspended in the oil. The two different ASTM methods have different purposes and should be used accordingly. The ASTM D 1816 method is recommended for testing filtered, degassed, and dehydrated oil prior to and during the filling of power systems rated above 230 kV and for testing of samples from units that are in service. This method should not be used for acceptance testing of insulating fluids. The ASTM D 877 method should be used for acceptance testing and it should not be used for units in service.

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

Type of Oil/Unit Dielectric Breakdown Voltage
D-877 D-1816 D-1816
1mm gap 2mm gap
Shipment of New Oil from Refinery 30 kV min. Not Spec. Not Spec.
New Oil Received in New Equipment
< or = 69 kV
69 – 288 kV
> 345 kV
30 kV min.
30 kV min.
30 kV min.
20 kV min.
30 kV min.
30 kV min.
40 kV min.
48 kV min.
60 kV min.
New Oil for Circuit Breakers 30 kV min. Not Spec. Not Spec.
Suggested Limits for Oil used
in Circuit Breakers
25 kV min Not Spec. Not Spec.


August 29, 2019

Color and Visual Examination

Background:

The color of an oil sample is related to the deterioration of the sample. Virgin mineral oil fresh from the refinery is essentially colorless; however, as the sample ages over time or is subjected to severe conditions such as local hot spots or arcing the sample will become darker in color. The clarity of a fresh virgin sample of oil should be sparkling with no indication of cloudiness, sludge, or particulate matter. The clarity of an oil sample is determined by observation of the sample when illuminated by a narrow focused beam of light. The color of a sample is determined by direct comparison to a set of color standards.

Procedure:

The details of the entire procedures are given in the ASTM D 1500; D 1524; and D 2129 standards and are only briefly mentioned here. Most mineral oil samples will vary in color from colorless to a dark brown going through intermediate colors of light tan to dark tan. The ASTM has set up a series of color standards covering this range of colors and has assigned them numerical values ranging from 0.5 to 8.0 with intervals of 0.5. These standards are made of colored glass and the oil sample is compared side by side with the standards in a colorimeter. If the color is less than 0.5 then a different type of comparison is made using a series of platinum-cobalt standard solutions in a set of tall-form matched Nessler tubes. The platinum-cobalt color scale runs from a value of 5 to 300. These numbers reflect the number of milligrams of platinum per liter of the standard solution. The clarity of the sample is usually determined on the same sample of oil that is being used for the color determination. This is done by shining a focused beam of light through the sample and looking for signs of cloudiness, sludge, or particulate matter.

Significance:

The color of an oil sample is used mainly as a guide to the degree of refinement of the oil when it is new. If the sample is from a unit that has been in service then the color can be followed over a period of time to indicate the possible condition of the oil. It should be pointed out that the color of the oil by itself should never be used to indicate the quality of the oil; however, it can be used to determine whether more definititive tests should be done to determine specfic characteristics of the sample that are more related to the performance of the oil.

The clarity of the sample can also give possible suggestions for further tests. Cloudiness of the sample can indicate the presence of water, which in turn will lower the dielectric strength of the sample. Particles of sludge material can indicate the products of oxidation such as acids that will raise the acidity and lower the IFT. Particles of carbon and or metal can indicate severe local over heating and or arcing. All of these can suggest further testing to determine the source of these materials.

The IEEE has suggested guidelines for color and clarity 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 Color Clarity
Shipments of New Mineral Oil
as Received from Refinery
0.5 max. Bright and Clear
New Oil Received in New Equipment
< or = 69 kV
69 – 288 kV
> 345 kV
1.0 max.
1.0 max.
0.5 max.
Bright and Clear
Bright and Clear
Not Specified
New Oil for Circuit Breakers 0.5 max. Bright and Clear
Suggested Limits for Oil used
in Circuit Breakers
2.0 max. No excessive Carbon in Circuit Breakers


August 29, 2019

Acidity

Background:

The acidity of an oil sample is related to the deterioration of the sample. The mineral oil insulating fluid is essentially a non-polar saturated hydrocarbon; however, when the sample undergoes oxidative degradation there are formed carboxylic acids, which are acidic in nature. The presence of these acidic materials can be quantitatively determined by a procedure called titration. The amount of a standardized base that is needed to neutralize the acidic materials present in a known quantity of an oil sample is determined. The result is referred to as the acidity or the neutralization number of the sample and is reported in terms of the milligrams of potassium hydroxide per gram of the oil sample. The titration procedure can be done either volumetrically or gravimetrically and the end point can be determined either colorimetrically or potentiometrically.

Procedure:

The details of the entire procedure are given in the ASTM D 974 standard and are only briefly mentioned here. The standard alcoholic potassium hydroxide solution is made up by dissolving 6 g of potassium hydroxide in 1 L of anhydrous isopropyl alcohol. The resulting solution is treated with barium hydroxide and standardized against pure potassium acid phthalate. The titration solvent is a mixture of about equal volumes of isopropyl alcohol and toluene containing about 0.5% water. The colorimetric indicator used is p-naphtholbenzein and the orange sample solution is titrated to a green or green-brown end point.

Significance:

The mineral oil is a non-polar material while the acidic components formed by the deterioration of the oil are highly polar materials. The result of this large difference in polarity is that the two are mutually insoluble in each other. Since the insulating fluid in a new unit would have little or no acidic materials initially present, as the acidic materials start to form the small amounts present would be soluble. However, as more of the acidic material forms it would reach a saturation point and further formation would result in the separation of solid material. This material would settle to the bottom of the unit as a sludge. Another source of sludge would be from the reaction of the acidic materials in the insulating fluid with various metals present in the unit to give salts, which would also tend to be insoluble in the insulating fluid.

The buildup of sludge inside of a unit can drastically affect the operation of the unit. The presence of the polar materials dissolved in the oil can reduce the dielectric strength of the fluid as well as increasing its dissipation factor. The presence of the solid material can interfere with the circulation by plugging up pumps or cooling radiators, which in turn affect the fluids function as a heat transfer medium.

The IEEE has suggested guidelines for neutralization numbers 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 Neutralization Number
Shipments of New Mineral Oil as Received from Refinery Maximum 0.03 mg KOH/g Oil
New Oil for Units Rated at 345 kV and above Maximum 0.03 mg KOH/g Oil
Limits for Continued Use
< or = 69 kV
69 – 288 kV
> 345 kV
Maximum 0.2 mg KOH/g Oil
Maximum 0.2 mg KOH/g Oil
Maximum 0.1 mg KOH/g Oil
Limits for Oil to be Reclaimed
Group II
Group III
Maximum 0.2 mg KOH/g Oil
Maximum 0.5 mg KOH/g Oil
New Oil for Circuit Breakers Maximum 0.03 mg KOH/g Oil


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