Transformer Testing Procedures Flashcards

According to NETA standards, what is the maximum acceptable deviation for a transformer turns-ratio (TTR) test result when compared to the calculated nameplate ratio?

Answer: 0.5%

The NETA Acceptance Testing Specifications (ATS) and Maintenance Testing Specifications (MTS) state that the turns-ratio test results should not deviate by more than 0.5% from the calculated ratio. This tolerance ensures that there are no significant issues like shorted turns, incorrect winding connections, or core problems.

A technician performs a winding resistance test on a three-phase, delta-connected transformer. The temperature-corrected results are: H1-H2 = 0.452 ohms, H2-H3 = 0.455 ohms, H3-H1 = 0.479 ohms. Based on NETA guidelines, what is the appropriate action?

Answer: Investigate the results, as the deviation between the highest and lowest reading is greater than 1%.

NETA standards suggest that temperature-corrected winding resistance values should compare within one percent of previously obtained results. When comparing phase-to-phase readings without prior data, a deviation greater than 1-2% warrants investigation. The deviation between 0.479 ohms and 0.452 ohms is over 5%, indicating a potential issue like a loose connection or internal winding problem that requires further investigation.

When performing an insulation power-factor test on a liquid-filled transformer, which of the following is the primary source for determining the maximum acceptable value according to NETA?

Answer: The manufacturer's published data.

The ANSI/NETA standards specify that the maximum winding insulation power-factor values for liquid-filled transformers should be in accordance with the manufacturer's published data. NETA tables are to be used in the absence of manufacturer's data. Different insulating materials and designs result in a wide variation of acceptable power-factor values.

A NETA technician is preparing to perform an insulation resistance test on a dry-type transformer rated at 13.8 kV. What is the standard test duration for applying the DC voltage?

Answer: 1 minute

For standard insulation resistance tests on transformers, NETA specifications require the test voltage to be applied for one minute. While a ten-minute reading may be taken to determine the polarization index (PI), the one-minute reading is the standard duration for the basic insulation resistance value.

Which of the following electrical tests is specifically intended to detect issues with the magnetic core of a transformer, such as shorted laminations or core heating problems?

Answer: Excitation Current Test

The excitation current test is performed by applying a voltage to one winding with the others open-circuited. The amount of current drawn is a measure of the magnetizing characteristics of the core. Abnormally high excitation current can indicate problems like shorted turns, core defects, or improper core grounding. NETA standards specify that for a three-legged core, the typical pattern is two similar current readings and one lower reading.

During acceptance testing of a new liquid-filled transformer, a dissolved gas analysis (DGA) is performed. The presence of which key gas would most strongly indicate overheating of the paper insulation?

Answer: Carbon Monoxide (CO) and Carbon Dioxide (CO2)

Carbon monoxide (CO) and carbon dioxide (CO2) are generated by the thermal decomposition of cellulosic materials, such as the paper insulation used in transformers. An elevated ratio of CO to CO2 can indicate overheating of the paper insulation. Other gases indicate different fault types, such as corona (Hydrogen) or arcing (Acetylene).