IMPORTANCE OF REVIEWING EXCITATION CURRENT (mA) PATTERN DURING TRANSFORMER VOLTAGE (TURNS) RATIO TESTING
This month’s Featured Technical Paper is from the Proceedings of the 2024 International Conference of Doble Clients. The paper is titled: Importance Of Reviewing Excitation Current (mA) Pattern During Transformer Voltage (Turns) Ratio Testing.
Abstract
The transformer turns ratio (TTR) is one of the most common tests used to assess the condition of a transformer. This test is performed as routine during Factory Acceptance Test (FAT) and performed during commissioning and maintenance of the transformer in the field.
Many manufacturers perform additional ratio testing during the manufacturing process as part of their quality assurance program. During the manufacturing process, ratio is performed on coil assemblies and after wiring is completed. These results are compared with design turns ratio whereas for FAT and field testing, results are compared with nameplate voltage ratios. IEEE Standard C57.12.00 states that the voltage ratios between windings of a transformer at no load shall be within 0.5% of the nameplate voltages. An error of less than 0.5% is understood as the golden rule for accepting test results (except in certain special cases).
Most commercial ratio test instruments display and/or record excitation current (milliamps or mA) on each phase but rarely is it given the attention it deserves.
As part of quality assurance practice, Waukesha records and reviews the mA values and mA pattern using a comparison of mA between phases. Over the years, Waukesha has identified winding defects using mA pattern that did not meet internal standards but did meet the IEEE ratio error tolerances. These defects include shorted turns, both disc-to-disc and turn-to-turn failures, or even missing turns. Proceeding with manufacturing or further testing of these transformers would have caused major failures during FAT or later in the field, resulting in significant rework, cost, and delays.
This paper will include case studies from Waukesha’s experience showing where defects were identified based on high mA or abnormal mA patterns.
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