**5. Conclusion**

18 Nuclear Power – Practical Aspects

current in the NPP.

system.

**4.3. Neutral voltage transfer** 

**4.4. VFTO transferring to MV system** 

leading to equipment breakdown.

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

on the MV system.

should be applied.

Neutral voltage transfer can occur via either electromagnetic or capacitive transfer. Based on simulation result the risk can be significantly reduced with proper grounding of the neutral. This, however, must be carefully implemented in order not to increase the short circuit

Again, any manual operation during event would introduce extra risks therefore should be desigined to be as simple and straightforward as possible with proper interlock or checking

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

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

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 Most NPP's in the world have been designed in such way that their local power loads are provided by "multiple independent sources" to ensure continuous power supply even during faulted periods. However, unless the NPP's local power grid is properly configured and its protection system properly designed, all these "multiple independent sources" can failed at same time as exemplified by Taipower's "318 Event". In view of the many similarities in design and other risk factors for world NPP's, it is of utmost importance that the lessons learned from Taipower's 3rd NPP "318 Event" be properly addressed.

This Chapter examines the Taipower "318 Event" in detail to demonstrate the various possibilities that could leads to NPP blackout. The possibilities investigated include: NPP's location factor, NPP local power grid configuration, cable parameters, switching events, switching surges propagating to MV circuits, ferroresonances, remote tripping, and manual starting difficulties. The lessons learned and proposed countermeasures are summarized in the previous section.

In summary, to ensure the proper design of NPP power protection system, the following 3 considerations must be incorporated:

