Dec 14, 2019 - 10:36 AM
There are several test methods available for measuring the Dielectric Strength of insulating fluids.
Engineered Fluids' uses the ASTM D-1816 standard for all measurements of Dielectric Strength which is done over a 2mm gap. We believe this is the most comprehensive and relevant test method for liquid immersion coolants of the three available test methods.
In the remainder of this article we'll address four important questions around Dielectric Strength and the various measurement methods.
1) What is Dielectric Strength, what does it actually measure?
2) Why is Dielectric Strength a critical characteristic of liquid Immersion Coolants?
3) What are the standards bodies that set the measurement standards?
4) What are standards for measuring Dielectric Strength and how do they differ?
1) What is Dielectric Strength, what is it measuring?
All of the generally accepted standards for measuring dielectric strength were originally developed to test the breakdown voltage of mineral oils used in power transformers. These standards have been further refined to provide standard methods of measuring the dielectric strengths for any insulating fluid.
The different international standards provide the recommended practices when determining the dielectric breakdown voltage of insulating liquids. Although there are many standards throughout the world, the majority of them are derivatives from three main standards:
- ASTM D-1816 Standard Test Method for Dielectric Breakdown Voltage of Insulating Liquids Using VDE Electrodes (USA)
- ASTM D-877 Standard Test Method for Dielectric Breakdown Voltage of Insulating Liquids Using Disk Electrodes (USA)
- IEC 60156 Insulating Liquids – Determination of the breakdown voltage at power frequency – Test Method (Europe)
Below is a table that gives an overview of the similarities and differences between the ASTM and IEC standards. As can be seen, there are quite some differences between the 3 major standards.
|ASTM D-877||ASTM D-1816||IEC 60156|
Ball (Fig. I) or VDE (Fig. II)
|2.54 mm||1 or 2 mm||2.5 mm|
|None||Continuous with impeller||Optional with magnetic stirring bar|
|3 kV/s||0.5 kV/s||2 kV/s|
|Mean of 5 measurements||Mean of 5 measurements||Mean of 5 measurements|
The ASTM D-877 is a fairly old standard that specifies the use of flat disk electrodes of polished brass material, 25.4 mm (1 in.) in diameter, at least 3.18 mm (1/8 in.) thick, sharp edges with no more than 0.254 mm (0.01 in.) radius, and with parallel faces and axes in a coincident horizontal line when mounted in the test cell. The electrode gap separation is fixed at 2.54 mm. This standard does not call for stirring and has a relatively fast voltage rate of rise. Due to this, the standard is not very sensitive to the presence of moisture. Two different procedures can be chosen when running this standard test:
- Procedure A: 5 breakdown measurements are made in one test cell filling with a 1-minute interval between the breakdowns. The mean of the 5 fillings is considered the breakdown voltage. This is typically used for liquids in which insoluble breakdown products easily settle during the interval between breakdowns (petroleum oils, hydrocarbons, natural and synthetic esters).
- Procedure B: 1 breakdown measurement is made on each of 5 successive fillings of the test cell. The mean of the 5 breakdowns is considered the breakdown voltage. This is typically used for liquids in which insoluble breakdown products do not completely settle during the interval described in Procedure A. This is also used for silicone liquids or to establish the breakdown value of a liquid where an ASTM specification does not exist (new liquids, R&D, etc.).
According to IEEE C57.106, ASTM D-877 is recommended for routine acceptance of new, unprocessed oil from a supplier for use in circuit breakers.
ASTM D-1816 is the most widely used standard in North America. The electrodes are mushroom-shaped and made of polished brass and adhere to the specifications given in a VDE standard, a German standards organization. Stirring is constant during the test, even during the intervals between breakdown measurements, with a two-bladed impeller having a defined pitch and operating speed between 200 and 300 rpm. The impeller directs liquid flow towards the bottom of the test cell for a uniform flow throughout the cell. Two different electrode gap separations can be chosen – either 1 or 2 mm. In general, if breakdown cannot be achieved at 2 mm, then the gap should be decreased to 1 mm. 5 sequential breakdowns are conducted and the mean is taken as the breakdown voltage.
IEEE C57.106 recommends ASTM D-1816 for testing liquids being processed into transformers or contained in transformers or load tap changers, as it allows for a more sensitive evaluation of changes occurring in the dielectric properties of insulating liquids. In fact, when comparing D-877 to D-1816 Section 5.2.1 in IEEE C57.106 states:
"The electrodes in D-877 are thin flat disks, which are not representative of the electrodes in transformers. Although the rounded electrodes in D1816 do not duplicate the characteristics of insulated electrodes in transformers, they more closely approximate transformer applications. However, the D-1816 electrodes are more responsive to particles and dissolved water in oil, both of which are detrimental to the electrical strength of oil in transformers. Therefore, D-1816 test results furnish a better evaluation of changes that may occur in the oil from transformers."
IEC 60156 is an international standard that incorporates the use of either brass ball electrodes (Fig. I) or brass VDE electrodes (Fig. II, same as ASTM D1816). VDE electrodes are most commonly used and the main difference to ASTM D1816 is that the electrode gap separation is fixed at 2.5 mm. Also, stirring the liquid is optional and this is achieved with a magnetic stirring bar. The use of this stirrer is only permitted when there is no risk of removing magnetic particles from the liquid under test. This standard also asks for 5 sequential measurements to be conducted with the mean being the breakdown voltage value.
Differences in Breakdown Voltages:
Generally speaking the IEC 60156 standard using Fig. II electrodes will produce the highest breakdown voltages values when compared to the ASTM D-1816 standard. This is due to the larger gap separation and the higher voltage rate of rise. Therefore, when wanting to use this standard, thewould be recommended because it can apply a voltage up to 100 kV.
The, allowing voltage application up to 75 kV, is more than sufficient to provide the necessary voltages to cause breakdown when using the ASTM D1816 and D877 standards. For ASTM D1816 this is mostly due to the lower allowable gap separations. Since the ASTM D877 standard utilizes flat disk electrodes with sharp edges, the enhanced electric field at these edges causes breakdown to occur at lower voltages.