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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).
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.
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|
|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.|