**4.4. VFTO transferring to MV system**

According to field measurement and numerical simulation, the VFTO transferring to MV system is usually underestimated by literatures. As demonstrated by both the field measurement and simulation result, peak voltage of VFTO in MV system could be as high as 8.47 times the rated line-to-ground peak voltage with an average 466 times restrike during DS operating [1,23]. Though the peak VFTO voltages transferred to the MV side are usually still within the basic impulse insulation level (BIL) tolerances of the equipment, this does not mean that repeatedly striking the equipment with 8.47 times the rated line-toground peak voltage would cause no damages to the equipment. In fact, this can accelerate equipment ageing and cause quick degradation of the insulation material and eventually leading to equipment breakdown.

After the "318 Event", a recommendation was made to Taipower No. 3 Nuclear Power Plant in 2003 for the installation of surge absorbers (0.8μF capacitor specially designed for surge absorption installed right close to the start-up transformer for each of the three phases) on the MV side in Fig. 1 [9,23]. The recommendation was adopted by Taipower in March 2005 and a subsequent measurement in March 2006 plus one-year monitoring indicated that there were no further VFTO exceeding rated line-to-ground peak voltage on the MV system.

## **4.5. Maintenance testing of in-service equipments**

The damaged circuit breaker (CB#17) in Taipower 3rd NPP has been put into service for 20 years at the time of event. Maintenance testing history showed that insulation condition of this circuit breaker was good prior to the event however that being the case the circuit breaker should not have exploded when faced by transient voltage no higher than its BIL of 60kV. This shows that the current diagnostic method of insulation degradation (insulation resistance measurement, dielectric power factor measurement) may not be sensitive enough to detect insulation degradation due to ageing or repeated VFTO strikes. It is recommended that the reliability of such tests, including both the tool used, methodology employed, and interpretation of testing results (including monitoring the trend of measurement results) be further improved. For equipment subject to repeated switching surges, a higher standard should be applied.
