**3.3. Switching surges and Very Fast Transient Overvoltage (VFTO)**

Switching operations are the most prominent phenomenon in the "318 Event". During the 48 hours prior to the Level 2 event, there were 37 switching operations and each could cause switching surges. Switching surges caused by GIS switching is characterized by its nanosecond wavefront and is commonly referred to as Very Fast Transient Overvoltage (VFTO) [11].

VFTO is the phenomenon of transient overvoltage generated during switching operation characterized by very short rise-time of 4 to 100 ns and has been covered by various literatures [3,12-22]. The phenomenon is particularly significant during Disconnect Switch (DS) operation due to multiple-restriking in the DS due to lack of arc-suppressing chamber.

## *3.3.1. Field measurement*

In the past, VFTO was not considered to be possible to transfer from EHV through power transformer to medium voltage (MV) system [12-14]. However, in light of the "318 Event", a field test was conducted in Taipower 3rd NPP during plant overhaul by switching the DS of EHV GIS and measure the voltage on "Essential Bus A".

Field test result [9] shows that after switching the DS of EHV GIS, multiple 25 kV-level restrikes (approximately 7 times the rated line-to-ground peak voltage) were measured on the 4.16kV bus indicating VFTO can be transferred from the EHV side to MV side. It also indicates that the maximum peak voltages measured on the 4.16kV bus occur neither on the first strike nor on the last strike, and this behaviour is quite different with that in EHV system. The measurement results are shown in Fig. 10.

**Figure 10.** Switching Surge Measured on 4.16kV Bus by Operating EHV GIS Disconnect-Switch. (Note: The bandwidth of the measurement system was 2MS/s, the highest achievable in 2003)

### *3.3.2. VFTO simulation*

12 Nuclear Power – Practical Aspects

345kV side Phase Voltage

345kV side Phase Voltage

EHV GIS and measure the voltage on "Essential Bus A".

system. The measurement results are shown in Fig. 10.

**Table 5.** Capacitive Transfer Simulation Result

minimized greatly.

(VFTO) [11].

chamber.

*3.3.1. Field measurement* 

as other CB's. However, if the neutral systems were properly configured, the risk can be

Case 1 457.55 kVpeak 0 kVpeak 0 kVpeak Case 2 450.6 kVpeak 275.88 kVpeak 42.29 kVpeak Case 3 448.93 kVpeak 44.58 kVpeak 0.002 kVpeak (a) 1st overvoltage

Case 1 428.83 kVpeak 0 kVpeak 0 kVpeak Case 2 418.7 kVpeak 263.43 kVpeak 41.08 kVpeak Case 3 420.94 kVpeak 46.25 kVpeak 0.002 kVpeak (b) 2nd overvoltage

Switching operations are the most prominent phenomenon in the "318 Event". During the 48 hours prior to the Level 2 event, there were 37 switching operations and each could cause switching surges. Switching surges caused by GIS switching is characterized by its nanosecond wavefront and is commonly referred to as Very Fast Transient Overvoltage

VFTO is the phenomenon of transient overvoltage generated during switching operation characterized by very short rise-time of 4 to 100 ns and has been covered by various literatures [3,12-22]. The phenomenon is particularly significant during Disconnect Switch (DS) operation due to multiple-restriking in the DS due to lack of arc-suppressing

In the past, VFTO was not considered to be possible to transfer from EHV through power transformer to medium voltage (MV) system [12-14]. However, in light of the "318 Event", a field test was conducted in Taipower 3rd NPP during plant overhaul by switching the DS of

Field test result [9] shows that after switching the DS of EHV GIS, multiple 25 kV-level restrikes (approximately 7 times the rated line-to-ground peak voltage) were measured on the 4.16kV bus indicating VFTO can be transferred from the EHV side to MV side. It also indicates that the maximum peak voltages measured on the 4.16kV bus occur neither on the first strike nor on the last strike, and this behaviour is quite different with that in EHV

**3.3. Switching surges and Very Fast Transient Overvoltage (VFTO)** 

345kV side Neutral Voltage

345kV side Neutral Voltage

4.16kV side Neutral Voltage

4.16kV side Neutral Voltage

> To further appreciate VFTO transfer mechanism, numerical simulation model was built [23]. To validate this simulation model, the field test condition for Fig. 10 was reconstructed and the simulation result is shown in Fig. 11. It can be seen from Fig. 11 that the waveform envelope are consistent with measurement for both DS opening and closing and that the maximum VFTO on the essential bus occurred neither at first nor at last strike.

> We then change the DS operation angle for each 5° intervals to simulate different closing/opening condition and Table 6 and 7 summarizes the maximum EHV inter-contact breakdown voltage vs. maximum MV VFTO. The following can be observed from Table 6 and 7:


the maximum VFTO on "Essential Bus A". This is also true for DS closing. E.g., Case #28 (δoper = 135°) of DS opening produces the highest VFTO in MV system (28.77 kV) while it was Case #18 (δoper=85°) that produces the highest inter-contact breakdown voltage on EHV side (354.2 kV).

Power System Protection Design for NPP 15

of Restrikes on EHV Side per Φ

Among the Multiple Restrikes Total Num.

Mag. (kV) Seq. Num. Mag. (kV) Seq. Num.

284.8 1 23.27 12 460

278.0 1 26.41 26 455

Max. VFTO at 4.16 kV

Item

Max. Inter-contact Breakdown Voltage Case #14 among 36

Max. VFTO at 4.16 kV Bus Case # 23 among 36

Max. Inter-contact Breakdown Voltage

**Table 7.** Max. Inter-contact Breakdown Voltages vs. Max. VFTO in MV for DS Closing

VFTO and the oscillation voltages VOSC (voltages created by preceding restriking that can be superimposed to the following restrike) on the EHV side can be transferred to MV system through the start-up power transformer via capacitive coupling. The transfer ratio is mainly dependent on transformer's EHV-to-MV interwinding capacitance, transformer's MV winding-to-enclosure capacitance, and the bus-to-ground capacitance of MV system [23]. From both our measurement and simulation result, it was observed that the VOSC, which is of several tens kV in the EHV GIS, could still be of several kV in the MV system, and this will be superimposed to the VFTO coupled from the EHV side causing up to 7 ~ 8.47 times

*3.3.3.2. Superposition of oscillations initiated by a prior strike on top of subsequent restrikes* 

Figure 12(a) shows two consecutive restrikes from a multiple-restrike simulation and Fig. 12(b) shows its counterpart single-strike simulation. It can be seen from Fig. 12(a) that the VOSC initiated by the first restrike is superimposed to the second restrike resulting in a

During DS opening the contact distance becomes wider and wider leading to longer intervals between two consecutive restrikes while that during DS closing is the opposite. As a result, there is a higher probability of superposition of VOSC to subsequent restrike during

*3.3.3. Characteristic of VFTO transferring to MV system* 

*3.3.3.1. Capacitive coupling of high-turn-ratio transformer* 

the rated line-to-ground peak voltage on MV side.

DS closing (thus higher VFTO) than opening.

higher peak voltage (10.72 kV vs. the single strike one of 9.88kV).

*3.3.3.3. Maximum VFTO transferred to MV for DS closing vs. DS opening* 

**Figure 11.** Simulation of VFTO at the Field Measurement Point


**Table 6.** Max. Inter-contact Breakdown Voltages vs. Max. VFTO in MV for DS Opening


**Table 7.** Max. Inter-contact Breakdown Voltages vs. Max. VFTO in MV for DS Closing
