**3.2. Neutral voltage transfer [10]**

It can be seen from Figure 3 that overvoltage can be observed not only on the line conductor but also on the neutral conductor as well. In order to understand this phenomenon we need to look at Fig. 9 where the equivalent circuit of a transformer is shown including its stray capacitances.

Power System Protection Design for NPP 11

In the presence of transformer core saturation, 3rd harmonic neutral voltage will be present on the windings through electromagnetic transfer as long as the neutrals of the respective windings are not grounded. Once the neutral voltage is established on any side of the neutrals, the stray capacitance provides a further path for it to transfer to other neutrals

where EH0 is the neutral voltage at HV side, and EL0 is the neutral voltage at LV side.

**Table 3.** Stray Capacitance of the 345kV/13.8kV/4.16kV Power Transformer

neutral voltage (through capacitive neutral transfer) of

**Table 4.** Grounding Condition for Simulating Capacitive Transfer

Item C345/Earth C13.8/Earth C4.16/Earth C345/13.8 C13.8/4.16 C345/4.16 Capacitance 4.48 nF 13.76 nF 21.92 nF 4.3 nF 214.86 pF 8.96 nF

It can be seen from Fig. 3(a) that, during 1st overvoltage the neutral voltage gradually roses to 200 kVrms while during 2nd overvoltage the neutral voltage rose to 626 kVpeak (Note: the voltage waveform became very non-sinusoidal during 2nd overvoltage, we thus use peak value instead of rms value). The source of both overvoltages in the neutral was due to motor and transformer saturation resulting in 3rd harmonic voltages at the neutral however during the 2nd overvoltage the waveform is much more distorted with higher harmonic content.

As indicated by Fig. 3(a), the neutral on 345kV side does not appear to have been effective grounded possibly due to grounding failure. The result is that very high neutral voltage was established on the 345kV neutral and if the 4.16kV neutral was not grounded it will see a

> *nF kVrms kVrms nF nF*

*nF kVpeak kVpeak nF nF*

during 1st and 2nd overvoltages, respectively. To better appreciate various grounding combination's effect on the neutral voltage transfer, 3 simulations were conducted assuming

Item 345kV side 13.8kV side 4.16kV side Case 1 Ground (direct) Ground (8 Ω) Ground (2.4 Ω) Case 2 Non-Ground Non-Ground Non-Ground Case 3 Non-Ground Ground (8 Ω) Ground (2.4 Ω)

Table 5 shows that as the neutral voltages transferred to the 4.16kV bus can be as high as 13 times the phase-to-ground peak voltage which can pose significant threat to CB#17 as well

8.96 200 \* 58.03 8.96 21.92

8.96 626 \* 181.54 8.96 21.92

grounding conditions as per Table 4 and their results are summarized in Table 5.

0 0 / /( ) *L H HL HL LE EE C C C* (1)

according to Equation (1)

*3.2.2. Neutral voltage transfer* 

**Figure 8.** Ferroresonance Phenomenon Explanation

**Figure 9.** Voltage Transfer Diagram of 345 kV/4.16 kV Transformer

#### *3.2.1. Transformer modeling*

In Fig. 9, CHE and CLE depict the stray capacitance between high voltage (HV) winding to ground, and low voltage (LV) winding to ground, respectively, while CHL depicts the stray capacitance between HV and LV windings. Typical stray capacitances for the 345/13.8/4.16kV power transformer are shown in Table 3.

In the presence of transformer core saturation, 3rd harmonic neutral voltage will be present on the windings through electromagnetic transfer as long as the neutrals of the respective windings are not grounded. Once the neutral voltage is established on any side of the neutrals, the stray capacitance provides a further path for it to transfer to other neutrals according to Equation (1)

$$\mathcal{E}\_{L0} \;/ \; E\_{H0} = \mathcal{C}\_{HL} \;/ \left(\mathcal{C}\_{HL} + \mathcal{C}\_{LE}\right) \tag{1}$$

where EH0 is the neutral voltage at HV side, and EL0 is the neutral voltage at LV side.


**Table 3.** Stray Capacitance of the 345kV/13.8kV/4.16kV Power Transformer

#### *3.2.2. Neutral voltage transfer*

10 Nuclear Power – Practical Aspects

**Figure 8.** Ferroresonance Phenomenon Explanation

**Figure 9.** Voltage Transfer Diagram of 345 kV/4.16 kV Transformer

345/13.8/4.16kV power transformer are shown in Table 3.

In Fig. 9, CHE and CLE depict the stray capacitance between high voltage (HV) winding to ground, and low voltage (LV) winding to ground, respectively, while CHL depicts the stray capacitance between HV and LV windings. Typical stray capacitances for the

*3.2.1. Transformer modeling* 

It can be seen from Fig. 3(a) that, during 1st overvoltage the neutral voltage gradually roses to 200 kVrms while during 2nd overvoltage the neutral voltage rose to 626 kVpeak (Note: the voltage waveform became very non-sinusoidal during 2nd overvoltage, we thus use peak value instead of rms value). The source of both overvoltages in the neutral was due to motor and transformer saturation resulting in 3rd harmonic voltages at the neutral however during the 2nd overvoltage the waveform is much more distorted with higher harmonic content.

As indicated by Fig. 3(a), the neutral on 345kV side does not appear to have been effective grounded possibly due to grounding failure. The result is that very high neutral voltage was established on the 345kV neutral and if the 4.16kV neutral was not grounded it will see a neutral voltage (through capacitive neutral transfer) of

$$200k\text{V rms}^\* \frac{8.96nF}{8.96nF + 21.92nF} = 58.03k\text{V rms}$$

$$1626kVpeak\,\,^\* \frac{8.96nF}{8.96nF + 21.92nF} = 181.54kVpeak\,\,^\*$$

during 1st and 2nd overvoltages, respectively. To better appreciate various grounding combination's effect on the neutral voltage transfer, 3 simulations were conducted assuming grounding conditions as per Table 4 and their results are summarized in Table 5.


**Table 4.** Grounding Condition for Simulating Capacitive Transfer

Table 5 shows that as the neutral voltages transferred to the 4.16kV bus can be as high as 13 times the phase-to-ground peak voltage which can pose significant threat to CB#17 as well as other CB's. However, if the neutral systems were properly configured, the risk can be minimized greatly.

Power System Protection Design for NPP 13

**Figure 10.** Switching Surge Measured on 4.16kV Bus by Operating EHV GIS Disconnect-Switch. (Note:

(b) GIS-DS closing

(a) GIS-DS opening

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

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

1. The VFTO transferred to the essential bus A can be as high as 28.77 kV, which is about

2. For all simulations, the restrike that causes the maximum VFTO on "Essential Bus A" does not necessarily coincide with the one that caused the max inter-contact breakdown

3. Among the 36 simulations for DS opening, the simulation that produces the highest inter-contact breakdown voltage on EHV side is not the same as the one that produces

The bandwidth of the measurement system was 2MS/s, the highest achievable in 2003)

maximum VFTO on the essential bus occurred neither at first nor at last strike.

8.47 times the rated line-to-ground peak voltage.

*3.3.2. VFTO simulation* 

voltage on EHV side.

and 7:


(b) 2nd overvoltage

**Table 5.** Capacitive Transfer Simulation Result
