**4.2. Nonlinear resonance prevention**

16 Nuclear Power – Practical Aspects

Subsequent Restrike Voltages.

**design** 

**Figure 12.** Oscillation Voltage (VOSC) Initiated by a Strike or Restrike Can Be Superimposed to

lessons learned from this event and their recommended preventive measure.

**4.1. Bus configuration and fault area isolation** 

**4. Lesson learned and important aspects of NPP power system protection** 

Events like the "318 Event" were seldom caused by one single reason. It can be seen from the above discussion that Taipower 3rd NPP was under multiple stresses before the event and there were mutiple mechanisms for the generation, amplification, and transfering of overvoltages which, combined with the operation practices and equipment history, eventually led to the explosion of CB#17 and total blackout of the NPP. Below are the key

The "318 Event" was essentially triggered by a single equipment failure but leading to a complete blackout of the power plant. There are 2 key lessons learned from this event: (1) Among all the scenarios considered in this Chapter, nonlinear resonance is the most difficult one to be detected. In view of the potential hazard it could cause, precautionary measure must be taken to prevent it from initiating.

The first step is to prevent motor-generating effect from ever occuring (thus removing the key source of initiation.) As explained above, the essential conditions of motor-generating effect are (1) rotating motor with large inertia, (2) large capacitor bank in an isolation system to support the terminal voltage. Since a rotating motor with large inertia can not be stop immediately, the focus is to remove the capacitive support. In the case of Taipower 3rd NPP, the capacitive support came from the long transmission line who were tripped only on the remote end. It is recommended that Direct Transfer Trip (DTT) function be implemented for transmission line protection to greatly reduce the risk of motor generating effect.

The second step is to ensure effective grounding of transformer neutrals as designed. Due to the objective of minimizing short circuit current, the neutral groundings in NPP are usually multi-configured: arrestor grounded under normal condition and direct grounding when in islanding operation. The switching from one grounding scheme to another often requires manual operation and this increases the risks of leaving the islanded system ungrounded as well as nonlinear resonance of power. Proper interlock or checking mechanism should be implemented to ensure proper grounding as designed at all times.
