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.
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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
Shipment of New Oil from Refinery
D-1816
D-1816
1mm gap
2mm gap
30 kV min. Not Spec. Not Spec.
New Oil Received in New Equipment
< or = 69 kV
69 - 288 kV
> 345 kV
30 kV min. 20 kV min. 40 kV min.
30 kV min. 30 kV min. 48 kV min.
30 kV min. 30 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.
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