Last week, we talked about how conditions inside a load tap changer may cause accelerated filming as the oil ages. Previously, we discussed the negative effects that filming has on the efficient operation of the device, and how filming may create a need for a shorter maintenance interval. This week, we are going to talk about some of the tests that we use to monitor that process. Specifically, we are going to look at the liquid screen, moisture content analysis, and dissolved gas analysis.
The liquid screen test package for LTCs includes the same analyses that we perform for a liquid screen test package for transformers. They are also both run for similar reasons. Neutralization number (acid number) and interfacial tension (IFT) are both good, direct measures of oil oxidation. As we look at these key oil tests, we know that the acid number increases while IFT decreases as the oil ages and oxidizes. , as we discussed in a previous article, when these values are within the ranges that we classify as unacceptable in a load tap changer, the oil has oxidized to the point where filming starts to advance much more rapidly. The D877 dielectric breakdown voltage (flat disk electrodes) is a useful test for several contaminants such as very high moisture and particles. While color changes as the oil ages, this is usually not quantitative enough to be of diagnostic value on its own. Color and, more importantly, the visual examination for appearance and sediment, are much more useful for evaluating contamination by free water, excessive particles, and heavily carbonized oil. As with the acid number and IFT, unacceptable classification for color, appearance, or sediment also indicates a need for corrective action.
Testing for moisture content by coulometric Karl Fischer titration is essential for monitoring the water content of the oil within the load tap changer. Moisture values that are classified as unacceptable indicate that there is enough moisture present to greatly accelerate the filming of the oil onto the contacts and mechanism. In addition, unacceptable moisture content greatly increases the risks of electrical tracking and even dielectric failure within the device. Unlike transformers, where we are very concerned with both the level of saturation of moisture in the oil and with the moisture content of the solid insulation, in LTCs we are strictly concerned with the moisture content in parts per million. Moisture content confirmed to be over 60 ppm indicates a need to perform maintenance and reduce the hazards that high moisture presents.
The utilization of dissolved gas analysis has been a useful tool for evaluating the condition and operation of transformers since the mid-1970’s. The use of this analysis to evaluate LTCs does not have a lengthy history, but is proving to be of considerable value for indicating needs for inspection and maintenance. An example of this tool is that we expect generation of acetylene and hydrogen as a load tap changer operates normally. As the unit changes positions, arcs between contacts occur and are quenched by the insulating liquid. Acetylene and hydrogen are formed by these transient arcs. Many LTCs have breathers to allow for the escape of the hydrogen, which is not very soluble in insulating, to prevent buildup of explosive conditions in the gas space of the device. If the arc is being quenched efficiently, dissolved gas analysis will indicate increasing values for acetylene. Increases for the other gases should be small in comparison if the LTC is operating normally.
Abnormal operation of an LTC will cause the unit to have abnormal gassing. The gases of concern are ethylene, ethane, and methane. Ethylene is formed in insulating oil within an energized LTC by temperatures exceeding 300 degrees C. Significant generation of ethylene starts at around this temperature, and increases up to a poorly defined peak temperature. Generation of ethylene starts to decline at temperatures higher than this peak. Under normal conditions, where normal arcing between contacts is occurring and being quenched in a timely fashion, there will be much more acetylene dissolved in the oil than there will be ethylene. If the arc is sustained for an extended period, the resulting hot spot will cause additional ethylene to be generated, and the relative amount of ethylene compared to acetylene will change. Other abnormal conditions that may cause th the generation of ethylene include contacts being poorly aligned so that a smaller surface area on the contact is used to conduct the rated current of the device, resistors (in resistive type LTCs) being overloaded or overheated in excess of their design parameters, and coke formation.
Ethane and methane are also formed under abnormal conditions. When the contacts are overheated by hot spots, ethylene, ethane and methane are also generated and dissolved in the oil in unusually high quantities. Another key factor is the filming of the mechanism of the load tap changer causes it to work harder, mechanically, in order to continue to change taps. This causes the mechanism to heat up due to friction, since the film is resists movement to a greater extent than the clean mechanism normally experiences.
Analysis of dissolved gases in oil from LTCs is well established, and guidelines for interpretation of the gas profiles are being established by industry groups such as IEEE. One difficulty that these industry groups are having is that there appears to be no “universal” patterns to the abnormal gassing. Different manufactures, and even different models for the same manufacture, may have widely differing gas profiles that we would consider to be normal. SD Myers has access to a large database of results from different models of load tap changers and have developed our own system for diagnosis of conditions within the devices. We have contributed data from our database to industry groups in which we participate to aid in their efforts to put together technical guidance for LTC owners.
In summary, routine analysis for the liquid screen tests is performed to identify immediately hazardous conditions arising from contamination and other key problems and to evaluate whether the potential for rapidly worsening filming exists due to the condition of the service aged oil. Moisture content determination evaluates the risks for accelerated filming as this indicates increased risks of dielectric failure or moisture tracking. Dissolved gas analysis is useful for evaluating whether overheating of the contacts or the mechanism is an indication of abnormal operation. Abnormal results for any of these tests may indicate a need to perform preventative maintenance and an internal inspection of the device.
Next week, we will discuss another tool that we have for testing LTCs and improving system reliability – particles and filming compounds analysis.
