Barrier lightning impulse test for dry type transformer main insulation design

The main barrier of a dry type transformer is the major insulation between the high voltage and low voltage windings, usually consisting of air and solid in combination, aiming to meet the dielectric requirement. The lightning impulse test is the major driver to the insulation design. The design rule is usually set by a combination of theoretical understanding, previous experience and recommendation from insulation supplier. There are several failure modes that need to be considered when studying the dielectric failure under impulse test of the barrier structure, which includes flash over, spark over and puncture. In this study, a paper testing equipment was built to investigate the above phenomena when a combination of air and solid insulation is subjected to impulse tests. The different failure modes were differentiated by varying the test parameters and high speed camera observation. The details of the discharge behavior was studied for a better understanding of the failure process. The effect of solid insulation thickness and multiple solid barriers was studied. Moreover, the barrier discharge test result was compared to the results from model coil and full coil test results to demonstrate the representativeness and applicability of small scale study to real transformer coil design.