۱- Introduction

۲- System modelling

۳- Surge protective device

۴- Stochastic analysis

۵- Conclusion

References

Abstract

Vacuum circuit breaker operation causes transient overvoltages that may lead to severe damages to transformer insulation. Since the parameters affecting these overvoltages have stochastic nature, a statistical analysis may provide detailed insight into the overvoltages from the point of view of insulation coordination applications. In this paper, a statistical analysis of the overvoltage variations of the step-up transformer during vacuum circuit breaker operation is conducted. Some variables, including switching angle, current chopping, high-frequency current quenching capability, and wind turbine power, are assumed as random variables. Besides, some indicators representing the switching overvoltage characteristics such as the amplitude, the number of restrikes, and the rate of rise are extracted. By performing a probabilistic analysis, the destructiveness due to switching overvoltages on typical transformer insulation can be investigated. The sensitivity of different surge protective devices and their effectivities are also analyzed and statistically evaluated. The applied methodology is very useful because of the stochastic nature of the switching overvoltages. In this work, the impact of protective equipment on each of the overvoltage indicators is analyzed. It is also found that considering only the amplitudes of transient overvoltages is not enough to draw conclusions about the safety of the transformer when exposed to frequent switching surges.

Introduction

VCBs are commonly used in offshore wind farms due to their ability to eliminate high fault currents and their low maintenance costs. Due to wind speed variations, numerous switching actions (energization and de-energization) commonly occur in wind farms every day. VCB switching transients reach the step-up transformer through the short cable connected to the wind turbine. In case, the amplitude of a switching transient voltage rises above the BIL; it can lead to transformer winding failures. Previous statistical studies related to VCB opening were only focused on the amplitude of transient overvoltages regardless of their rise times [1–۴] However, some transformer failures due to VCB operations have indicated that the leading cause of the failure was the high du/dt [5]. Recently performed detailed research showed that due to fast transients and during resonance conditions, the voltages with high amplitudes are non-linearly distributed along the windings [6], and the high du/dt accelerates failure mechanisms because of high interturn overvoltages in the transformer windings; causing coil-to-coil failures in the top or in the middle of the winding. Besides, the critical voltage of the transformer BIL remarkably decreases with the reduction of the rise time [7,8] and the amplitude [7] of the transient voltage applied. Moreover, the experimental studies confirm the effects of lightning impulses on oiled paper [9] and very fast transients on cast-resin insulation [10]. Accordingly, the rate of rise and the number of transient surges should be considered alongside their amplitude in a stochastic analysis in order to accurately assess the effects of the switching overvoltages on transformer insulation lifetime. The characteristics of the transient overvoltages occurring due to VCB operation depend on various parameters. The current chopping phenomenon in VCBs being one of the key parameters that contribute to the reignition overvoltages. Experimental tests conducted in Ref. [11] however, showed the low dependency of this parameter on the operation instant.