Transformer Testing Course – e-lesson #11: Line terminal AC withstand voltage test (LTAC)

This chapter of the Transformer Testing Course details the line terminal AC withstand voltage test of power transformers. You will learn about IEC and IEEE standard requirements, test connections, testing techniques, and failure cases.

This lesson provides an overview of the line terminal AC voltage withstand test for power transformers. The test is specified by IEC standards and is mandatory for non-uniformly insulated transformers above 170 kV. Examples of non-uniform insulation levels in transformers are discussed, highlighting the need for this additional test beyond the standard applied voltage test. The typical test connection and excitation methods are explained, including the importance of maintaining a 1/3 to 2/3 voltage distribution along the winding. Special considerations around test frequency, duration, and capacitive current are covered. The test procedure, including preparing the transformer and evaluating the results, is outlined. Reporting requirements and some unique test cases, such as transformers with non-uniform medium voltage windings, are also presented.