**2.3. Electrical stress in plant power system**

4 Nuclear Power – Practical Aspects

two remaining motors on the 13.8kV bus.

**Figure 3.** Overvoltages at 20:38 in March 17, 2001

Figure 2 shows the switching event log of the four 345kV and two 161kV lines connecting to the NPP. After reviewing the event log, it was found that CB#17 on Essential Bus A broke down when one GIS switching operation was occurring. The event log also showed that there were 37 EHV switching operations during the 48-hour period prior to the event due to salt-fog influence in the plant area. Because of the unstable offsite power, the GIS switched

Figure 3 shows the transient recording of overvoltages for both the 345kV bus and one MV (medium voltage) bus at 20:38 in March, 2001. At t0, the flashover on the 345kV line occurred leading to its subsequent tripping at t1. The tripping took place on the remote end of the 345kV line thus overvoltage can still be observed at the NPP between t1 and t2 due to "motor-generating effects" to be explained in the following section of this Chapter. The overvoltage on the 345kV line eventually caused flashover from Phases A and B to ground pulling down the line voltage and all motors on the 4.16kV bus were tripped by their respective under voltage relays at t3. At t4, the flashover from Phases A and B to ground was cleared and the "motor-generating effects" start to build up the voltages again with the

(a) Transient Recording at 345 kV GIS

(b) Line-to-Line Voltage Transient Recording of one 13.8 kV motor, Recycle Colling Pump B

between different offsite power to acquire the stable power sources.
