Northern Technology & Testing began metals testing in-house in 1997. Metals testing is used to provide vital information about transformer operation and in locating potential fault sites within the transformer. Not only can the method provide information needed to pin-point fault locations but it can also detect excessive wear in oil circulation pumps. We routinely analyze for seven metals: iron, copper, aluminum, lead, silver, tin and zinc. Analysis for other metals can be done on request.
NTT has recently introduced state-of-the-art equipment to analyze your insulating fluids for metals at low concentration levels. As a result, significant price reductions are now available. As always, we provide rapid turnaround with NTT’s well-established QA/QC program.
High energy faults can generate metal particles, which are then dispersed in the oil. Identifying the types of metal particles and their concentrations by atomic absorption (AA) provides answers that help localize the fault within the tank.
Several methods exist for determining low concentrations of metals present in a liquid matrix such as transformer oil. Among them include electromechanical, nuclear-activation analysis, X-ray fluorescence, emission spectrometry, and atomic absorption spectroscopy (AA).
AA analysis depends on the selective absorption of discrete frequencies in the radiation spectrum by free metal atoms. During the analysis, an oil sample is burned, and the metals are turned into free metal atoms by this high-temperature incineration. During incineration, the instantaneous intensity of the radiation spectrum is measured and compared against standards to determine the presence and concentration of any free metal atoms. The most practical way to do this is to burn the oil sample in a high-temperature graphite furnace. The advantages of this method are that the oil sample can be placed directly in a disposable graphite sample tube and burned in the tube without any further preparation and, more importantly, the detection levels of this method are better able to determine lower concentrations of metals. Another, less suitable method is to dilute the oil with hexane and then burn it.
Metals analysis by AA spectroscopy with a graphite furnace:
Metals and their possible sources in transformers
||Windings, corona shields, ceramic bushings
||Windings, bronze & brass components
||Core and tank
|Lead, Tin, Silver, Zinc
||Connectors, lugs, bolts and peripheral components