Transferring of VFTO From an EHV to MV System as Observed in Taiwan's No. 3 Nuclear Power Plant

The severe insulation breakdown on one medium-voltage (MV) switchgear led to a complete blackout of Taipower's 3rd nuclear power plant on March 18th, 2001. This has been one of the most serious incidents during the past 30 years of Taipower's nuclear generation and while the cause of the event was a combination of design, maintenance, and equipment aging issue, it is apparent that repeated switching operation on the extra-high-voltage (EHV) side eventually caused this severe insulation breakdown on MV switchgear. Very-fast transient overvoltage (VFTO) is the phenomenon of transient overvoltage generated during switching operation characterized by a very short rise time of 4 to 100 ns. Prior research on this topic focused on VFTO's impact on the EHV side of the transformer and this paper presents the field measurement result together with long-duration multiple-restrike simulation for the VFTO transferred from the EHV side to the MV system through the power transformers. It is shown that this VFTO can be as high as 7 times the rated line-to-ground peak voltage and the maximum VFTO often occusr neither at the first nor at the last strike due to the superposition of restriking voltage on top of oscillation voltage created by prior strikes.

I. Introduction

On march 18th, 2001, Taipower's 3rd nuclear power plant had a Level 3A event (see the Appendix for the definition), and the whole plant went into blackout from 00:45 to 02:58. While the event revealed a number of design, maintenance, and equipment aging issues, it is apparent that the severe insulation breakdown of one 4.16-kV switchgear responsible for the cooling water pump had triggered a series of subsequent switching operations leading to the blackout.