**1. Introduction**

One of the key purposes of NPP power system protection is to ensure that NPP's local power demand (such as cooling pumps, control systems, etc.) are met under all circumstances even during faulted periods. To achieve this goal, NPP power system protection must ensure that it can supply these local loads using either (1) power from the grid (via the transmission connection, which in most time, however, are used for exporting the excess power generated by the NPP after supplying its local loads) or (2) power from local generations such as diesel generators, batteries, etc. at all times and under all circumstances.

On the first power source (grid power), many NPPs worldwide have been built along the seashore for cooling water availability reasons. Overhead transmission lines are thus built in the vicinity of the seashore to transport the large amount of power generated from the NPP to the grid economically. As these overhead lines are exposed to salt contamination, flashover will occur when contamination becomes excessive. In the event of flashover, which is equivalent to a line-to-ground fault, the plant's protection system will need to initiate a series of switching operation to redirect the large power output from the NPP to a backup route in order to avoid reactor emergency shut-down. However, such switching has the adverse effect of causing undesirable transient overvoltages to propagate in the plant's local power grid [1-4]. Dealing with the frequent switching actions of these overhead lines while mitigating their adverse effects thus becomes the first challenge of designing NPP power system protection.

Once the NPP loses its connection to the grid, it will need to rely on the local generation to continue supplying its local loads. Most NPP use multiple "independent" sources as backup power. However, unless NPP's local power grid is properly configured and its protection system properly designed, these "independent" sources can all fail at the same time as manifested in Taipower's 18 March, 2001 Level 2 event ("318 Event") [5].

© 2012 Lee and Chen, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Lee and Chen, licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

In the following sections, we will examine Taipower's "318 Event" in detail to demonstrate the various possibilities that could lead to NPP plant blackout. Moreover, as these possibilities are not mutually exclusive, we will use this example to illustrate how multiple or cascaded problem can present further challenges to the overall NPP power system protection design. Recommended preventive measures are then summarized in the final section of this chapter.

Power System Protection Design for NPP 3

**Figure 1.** System Configuration of the NPP

**Figure 2.** 345kV and 161kV Overhead Line Switching Event Log
