**4.3. Neutral voltage transfer**

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 current in the NPP.

Power System Protection Design for NPP 19

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

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

In summary, to ensure the proper design of NPP power protection system, the following 3

1. **Check Independence of Equipment and Protection Zone for Various Scenarios:** The "318 Event" was caused by a single CB failure (CB#17) but leading to a complete NPP blackout for over 2 hours. This is mainly due to (1) the breakdown of CB#17 took down CB#15 at the same time due to their physical proximity. (2) The bus configuration cause none of the 2nd, 3rd, or 4th backup power to be available when both CB#15 and #17 both fails and CB#15 created a permanent line to ground fault. (3) The last resort (DG5) was located in a building filled with smoke caused by the CB#17's breakdown making manual starting extremely challenging. (4) The sustained overvoltage in the system could have been avoided should the tripping of the EHV cable be done on both ends of the line instead of just the remote end. All of the above suggest that the independence of equipment and protection zone have failed and needs to be taken into consideration

2. **Accumulated Equipment Stress Must be Monitored and Considered**: Particularly relevant for NPP's located on the seashore and subject to frequent line switching, equipment stress manifested in the form of reduced insulation level must be subject to more frequent and detailed monitoring. This would include not only absolute value measuring but also trending the measurement so that early signs of equipment

3. **Use System Protection Design Approach**: The cause of "motor generating effect", "neutral voltage transfer", "VFTO", and "ferroresonance" occurring during the "318 Event" cannot be addressed one by one and need to be taken into consideration from a system protection perspective. This would include the consideration for using different tripping scheme (such as Direct Transfer Trip on the EHV line) , adding additional protection device (such as installing surge absorbers on the MV bus), as well as

weakness can be identified and proper measures be adopted to address it.

the lessons learned from Taipower's 3rd NPP "318 Event" be properly addressed.

**5. Conclusion** 

the previous section.

considerations must be incorporated:

when improving existing or future designs.

reconfiguring the bus connections.

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 system.
