Which Oil Testing Standard Should You Choose to Determine Dielectric Breakdown Voltage?
- October 11, 2018
- Posted by: Dominique
- Category: Dielectric Oil Testing
All dielectric oil testers that HVT provides include one test cell under standard delivery. Many times, customers do not know which cell they should choose or are not aware which standard they should adhere to when testing their dielectric insulating liquids.
Why use a standard for testing oil?
Determining the dielectric breakdown voltage of insulating liquids is important to understand the insulating liquid’s ability to withstand electric stress without failure. A low breakdown voltage value can be a clear indication of contamination within the liquid from the degradation processes that occur during the lifetime of a transformer. Such contaminants can include humidity, acids, and foreign particles (cellulose fibers, metallic particles, sludge, etc.). See a related blog post on Protecting the Workhorses of our Power Grid.
Different international standards have been created to provide 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 D1816 Standard Test Method for Dielectric Breakdown Voltage of Insulating Liquids Using VDE Electrodes (USA)
- ASTM D877 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 D877||ASTM D1816||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 D877 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 D877 is recommended for routine acceptance of new, unprocessed oil from a supplier for use in circuit breakers.
ASTM D1816 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 D1816 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 D877 to D1816 Section 5.2.1 in IEEE C57.106 states:
“The electrodes in D877 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 D1816 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, D1816 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.
IEC 60156 Fig. I Test Cell
IEC 60156 Fig. II Test Cell
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 D1816 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.
Advantages of the Testers and Test Cells HVT Provides:
- All standards and their specific test routines are pre-programmed on the DPA 75 C and DTA 100 C for fully automatic measurements
- Test cells ergonomically fit the DPA 75 C and DTA 100 C oil testers
- The electrodes are manufactured to exactly adhere to the respective standard requirements. All are made of polished brass material.
- The test cells are made of glass material, which has been shown to have the best chemical characteristics for testing insulating liquids. Glass is nonabsorbent to moisture and non-soluble to testing liquids or cleaning solvents. Glass also does not allow cross-contamination of testing samples with leaking plasticizers, which can be the case from synthetic test cells.
- Test cells are compatible to test mineral oil, natural and synthetic esters, and silicone oils.
- All cells have an incorporated Vernier gauge (micrometer), which allows setting of the gap separation without having to use gap setting gauges. When the electrodes contact each other a short-circuit is caused producing a high-pitched ring. By ever so slightly backing off, the ring will stop and the zero position can be set. One revolution of the Vernier gauge corresponds to exactly 1 mm in gap separation.
- The ASTM D1816 has a built-in motor on the lid to drive the impeller. When the lid of the instrument is closed, contact is made to the motor electrodes to drive the motor.