*3.3.3. Characteristic of VFTO transferring to MV system*

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

14 Nuclear Power – Practical Aspects

voltage on EHV side (354.2 kV).

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

Max. Inter-contact Breakdown Voltage

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

Item

Max. Inter-contact Breakdown Voltage Case # 18 among 36

Max. VFTO at 4.16 kV Bus Case #28 among 36

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

(a) GIS-DS opening

Among the Multiple Restrikes Total Num.

Max. VFTO at 4.16 kV

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

(b) GIS-DS closing

354.2 467 23.76 444 468

323.6 447 28.77 432 447

of Restrikes on EHV Side per Φ

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

### *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 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*

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 DS closing (thus higher VFTO) than opening.

Power System Protection Design for NPP 17

Explosion of CB#17 took down the adjacent CB#15 as well. (2) "Independent sources" are not

For various reason such as space requirement, ease of maintenance, etc, switchgear panels are usually installed in the same room side by side. If this cannot be changed, during the risk evaluation process one must consider the N-1 condition being loss of "one group of equipment" instead of "one equipment" unless sufficient separation are provided between

The "independence" of power sources need then be examined closely. If multiple sources or multiple buses can be taken down by a single failure such as permenant fault to ground, etc, they cannot be considered as independent sources and more backup needs to

It should be noted that during the "318 Event", after the explosion of CB#17 the plant utility room was filled with smoke which makes the manual starting of other diesel generators extremely difficult. Not only were equipments under significant stress but also the human operators. It is thus recommended that the feasiblity of starting backup sources under utility room smoke condition be checked and that any manual operation required during this stage be as simple and straightforward as possible with proper interlock to reduce the chance of

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

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

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.

always independent due to improper bus configuration.

human error which may further escalate the event.

**4.2. Nonlinear resonance prevention** 

must be taken to prevent it from initiating.

motor generating effect.

the equipments.

be added.

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