فشار ولتاژ در یک سیم پیچ ترانسفورمر طی جریان های لحظه ای خیلی سریع ناشی از رویداد بسته شدن مدار شکن Voltage Stress in a Transformer Winding During Very Fast Transients Caused by Breaker Closing Event

THE lightning phenomenon is considered to be the source of the highest and fastest transients, which electric apparatuses in power systems, such as transformers, are exposed to. The generated voltage pulses have a very steep front in the order of microseconds ( s) and can reach a magnitude many times higher than the system’s rated voltage. High overvoltages produced by lightning are mitigated by the use of surge arresters that keep the voltage limited within a range that is not harmful to the protected equipment. However, SF and vacuum interrupters can produce voltages where the rise time is more than 100 times shorter than the lightning impulse. Furthermore, it is reported worldwide that many transformer insulation failures have occurred possibly by switching operations of vacuum circuit breakers (VCBs), even though the transformers have previously passed all of the standard tests and complied to all quality requirements [1]–[۴]. This phenomenon is not related only to VCBs since another study showed that SF insulated breakers and disconnectors can also generate such transients [5]. The breakdown in the SF medium can have a rise time between 2–۲۰ ns, where SF interrupters can also generate voltage restrikes and prestrikes during their operation [5]. Studies of the insulation failures of the motors, caused by the switching phenomenon, showed that the surges generated during switching of the air magnetic CBs are very similar to the surges created with vacuum devices [6], [7]. According to one of these studies, the air magnetic CBs generated surges with a magnitude of 4.4 p.u. and a rise time of 0.2 s, while the vacuum breaker-generated surges have a 4.6-p.u. magnitude but with a longer rise time of 0.6 s [6]. A 10-year-long study that included the investigation of failures of thousands of transformers conducted by Hartford Steam Boiler earlier concluded that the high-frequency transients are the main cause of the transformer insulation failure [8]. Furthermore, many studies give a description of the phenomenon that can produce high overvoltages internally in transformer windings [9]–[۱۶]. Although some authors claim that the direct proof of the negative impact of the high-frequency transients on the transformer insulation is not yet found [5], the high-frequency internal overvoltages are considered as a likely cause of transformers insulation failures [9]–[۱۴]. Moreover, even though the voltages at the terminals of the transformer, produced by the closing of the breaker, are well within the limits and do not exceed the magnitude of the peak grid voltage, it is reported that some transformer failures occurred even during such energizing transients [1]–[۳]. The goal of this paper is to investigate interturn voltages that appear during winding energization of transformers with interleaved winding with a very fast transient voltage. Moreover, energization of transformers with different winding configurations and different insulation types, using a test reactor, is analyzed. The obtained interturn voltages are then compared with the interturn voltages measured during the lightning impulse test at the basic lightning impulse (BIL), which is considered to be the highest and fastest voltage stress by the transformers standards [17]–[۲۱].