**3. Stress mechanism and modeling**

It can be seen from the above that Taipower's 3rd NPP was under sigificant and multiple stresses before and during the Level 2 event. This section explains the mechanisms working behind these stresses and provide basic principles how to model them.

Power System Protection Design for NPP 7

3(a)). This implied that the power transformers have saturated. As a result, a lot of harmonics were produced and the zero sequence components of them would be integrated into the neutral voltage resulting in unexpected high neutral voltage. This period ended at

**Figure 5.** Relationships between Motor Terminal Voltage, Magnetization Curve, and External

Table 1 shows the harmonic contents of B phase voltage between t1 and t2 in Fig. 3(a). The even order harmonics and DC component could be treated as the slight magnetic bias caused by asymmetric fault. At this stage, there was no ferromagnetic resonance in the

Order DC 1 2 3 4 5 6 7 8 % 9.3 100 7.8 8.0 3.6 13.5 1.6 6.1 1.2

Figure 6 shows the four essential conditions for a ferroresonance to occur: voltage source, capacitance, nonlinear inductance (ferromagnetic and saturable), and low losses. The R in the RLC resonant circuit in Fig. 6 is very large due to the "low losses" condition and can often be ignored. The nonlinear inductance L is the magnetizing curve of the motors and

At "t3" in Fig. 3(a), all the motors on the 4.16 kV system were tripped by undervoltage relay. Between t3 and t4, the flashover grounding of phases A and B were cleared and the motor generating effects mentioned above picked up again gradually re-establishing the line voltage. At "t4" in Fig. 3(a), most motors in 13.8 kV system were also tripped by undervoltage relay with the exception of two largest ones. With the capacitance provided by the transmission line now need only to support the terminal voltage of 2 motors, we would

transformers in the system and the capacitance is provided by the transmission line.

**Table 1.** Voltage Harmonic Contents of Phase B between t1 and t2

*3.1.2. Second overvoltage (45 Hz) - Nonlinear resonance* 

Capacitance

island system.

"t2" in Fig. 3(a) when the flashover grounded both phase A and B.
