Impulse Voltage Distribution and Transferred Surge in Transformer Winding with Series Reactor – a Case Study

Transformers are critical component within the power system and are often subjected to transient overvoltage due to atmospheric lightening and switching surges. They are subjected to dielectric tests in line with international standards i.e., IEC/IEEE. These transient surges generate oscillatory overvoltage within the winding under impulse and produce transferred overvoltage to untested windings on account of inductive and capacitive coupling [1]. The transferred overvoltage to lower voltage winding can reach remarkably higher values than designed insulation level particularly in case of network transformers with high turns ratio and lead to potential failure. The transferred surge heavily depends on the type of connection of non-tested terminals of untested winding [1] [2]. Usually, the non-tested terminals of untested winding are connected to ground during the test. However, presence of series reactor changes this situation significantly and can lead to very high transferred surge and insulation stress in the untested winding. This paper presents study and analysis of transferred surge on untested low voltage winding connected to a series reactor in a transformer during impulse testing on intermediate voltage winding. Further the paper shows that the nature and magnitude of the transferred surge to untested low voltage winding depends heavily on connection of non-tested winding terminals and presence of reactor using a simplified capacitive network. The calculation results are further verified with the detailed simulation in a commercial impulse distribution program and the experimental measurement using RSG. The results highlight the importance of design consideration of the untested low voltage winding series capacitance, design of the series reactor, and its connection to the low voltage winding to mitigate the insulation stress and failure.