Dec 14, 2019 - 05:36 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?
Dielectric strength is defined as the minimum applied electric field (i.e., the applied voltage divided by electrode separation distance) that results in a materials' insulating breakdown. The breakdown voltage (which is the number typically shown on data sheets in kilo volts (kV)) is the point at which the electric field frees the bound electrons and the fluid conducts electricity.
In layman's terms, Dielectric Strength is a measurement of the capacity of a material, in our case - a fluid, to prevent an electrical arc of a given strength in volts from traveling through a certain distance of intervening fluid, where the distance is measured in millimeters.
What often confuses people who are learning about Dielectric Strength for the first time, is why product data sheets only list the breakdown voltage and not the electrode distance? This is because the measurement standard, which defines how methodology of how the measurement will be taken and under what conditions defines the measurement distance, the amount of fluid used, the type and shape of the electrodes, all of which then determine the breakdown voltage. So what we do on our data sheets is describe two important pieces of information relative to Dielectric Strength a) what is the breakdown voltage, and b) what standard do we use to measure dielectric strength.
Unfortunately, unlike EF many of our competitors only provide the breakdown voltage, so in order to actually compare different Dielectric Coolants true ability to arc quench, you will have to ask them not for the distance, but rather for the ASTM/ISO standard they used to conduct the test.
2) Why is Dielectric Strength a critical characteristic of liquid Immersion Coolants?
Since electronics are directly immersed in the coolant with liquid immersion cooling, its very important that you use a coolant that has a high dielectric strength to ensure that even during the most stressful operating conditions your electrical devices will not short out during operation. This is especially important during a potential failure situation where you could have power ICs in particular able to contain voltages upwards of 10kVs during failure. Even though your electronics are only operating at 12VDC, 120VAC, or 220VAC, some of the capacitors and power ICs in electronics can store energy on the board and while very low amperage, these components can store 5-50kV in operation. When these units fail catastrophically they will often dump all of their stored energy at one time which can cause an electrical arc which may then ground out to other working equipment in your tank or to the steel or aluminum tank itself. This poses both an electrical shock hazard and potentially a fire hazard depending on how you've engineered your system.
In addition, the dielectric strength of your coolant can be impacted by the amount of impurities such as moisture and dust in your coolant. While these types of impurities are common, and in many cases not harmful to the operation of the coolant itself, their presence in the coolant can cause a dramatic reduction in dielectric strength. If you use a coolant with a low dielectric strength when new, you can be assured that over time its dielectric strength will continue to diminish, and depending on the chemistry of the coolant this can lead to disastrous results where the breakdown voltage drops below the operating parameters of power supplies and other high voltage components.
Also, the action of the coolant itself is important to understand, because in the case of 2-phase coolants, their boiling action can actually cause erosion of the soft metals on the electronics through cavitation, thereby releasing these metallic particles into the coolant and rapidly destroying the 2-phase fluids' strength.
3) What are the standards bodies that set the measurement standards for Dielectric Strength?
The ISO (International Standards Organization) is the global organization responsible for publishing technical standards, of which IEC (International Electrotechnical Committee) in Europe and ASTM (American Society for Testing and Materials) in the USA are contributing members. While there is not simple way of cross-referencing the standards published by the IEC and the ASTM the standards for most materials tests are often very similar. This is certainly the case as regards the testing of Dielectric Strength of fluids, however the ASTM standards in this area tend to be more detailed in their methodology and description of the test apparatus to be used than the IEC standards. Given this specificity of methodology, most vendors will often use the ASTM testing standards when publishing their dielectric strength characteristics.
4) What are the standards for used for measuring Dielectric Strength and how do they differ?
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)
What are the differences between the 3 major standards?
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|
|Type of electrodes||Disk||VDE|
Ball (Fig. I) or VDE (Fig. II)
|Electrode gap||2.54 mm||1 or 2 mm||2.5 mm|
|Stirring||None||Continuous with impeller||Optional with magnetic stirring bar|
|Voltage rate of rise||3 kV/s||0.5 kV/s||2 kV/s|
|Breakdown value||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, the DTA 100 C would be recommended because it can apply a voltage up to 100 kV.
The DPA 75 C, 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.
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