Theoretical Review and Precautions on Surge Testing Medium and High Voltage Stators in Fully Assembled Rotating Electrical Machines

The steep-front impulse (surge) test is widely recognized as an effective method for assessing interturn insulation integrity in form-wound stator coils and bars. Its application to fully assembled stator windings, however, remains technically controversial and frequently misunderstood in field practices. This paper examines the physical principles, limitations, and risks associated with surge testing when applied to complete stator windings of medium- and high-voltage rotating electrical machines. The work reviews insulation systems, ageing mechanisms, and interturn failure modes, explaining why surge testing is highly reliable for individual coils but fundamentally constrained when extended to complete windings. The differing positions of IEC 60034-15 and IEEE Std 522 are analysed, clarifying why IEC restricts surge testing to manufacturing stages, while IEEE allows its controlled use for field diagnostics. Beyond classical phase-to-phase comparison, the paper discusses alternative evaluation approaches enabled by modern test equipment, such as stepped-voltage testing of individual phases. While not eliminating intrinsic limitations, these methods may provide additional insight when applied within a preventive, physics-based maintenance strategy.