## 3 Results
### Switching Transient Voltages
#### 3.2.2 Typical Measuring Results  
(1) Transient voltages during switching on the Tang-Xiang transmission line  
Tang-Xiang transmission line connects the 500 kV Batang substation and the 500 kV Xiangcheng substation. The length of this line is about 190 km with the common-tower double-transmission scheme. Before the energization, the line is disconnected at both sides with a suspended potential. The sensors are arranged under the conductors on the Batang substation. The energization process is  
Figure 8: Lightning transient voltage waveforms. (**a**) Field measured results in Y-substation; (**b**) field measured results in Z-substation. Figure 7: Voltage waveforms recorded on 13:58:27, May 14 th, 2017. (**a**) Field measured results in Y-substation; (**b**) field measured results in Z-substation. accomplished by switching on the breakers on the Xiangcheng substation to connect the line to the 500 kV bus bar. The typical measuring result after the decoupling process for this type of switching transient voltage is shown in Figure 9. The maximum overvoltage (\(U_{\text{max}}\)) of 679.6 kV appeared on the Phase C which was 1.66 times as the rated value. While the conductor of Phase C was energized, a negative edge of the voltage formed on Phase B at the Xiangcheng side which then reached the Batang side (sensor installed) at \(t_{1}\) (18.226 ms). Then this edge further transmitted back to the Xiangcheng substation and then reflected back to the Batang side at \(t_{2}\) (19.596 ms). During the period between \(t_{1}\) and \(t_{2}\), the traveling wave had passed through the transmission line twice. Thus, the actual average velocity of the traveling wave on this line was estimated to be about \(2.77\times 10^{8}\) m/s. Moreover, this estimated velocity from field measured data can be used in the fault determination in the future. (2) Transient voltages during switching on a power transformer to the bus bar  
The transient voltages on switching the #3 power transformer onto the 500 kV bus bar on Batang substation were measured and the typical measured results are shown in Figure 10. To avoid involving overvoltage caused by magnetizing rush current during switching on a no-load power transformer, the closing resistors, and phase selection closing strategy were used during the operations. According to Figure 10, the breaker of Phase A were closed firstly and the voltage on the Phase A conductor reached the rated value immediately. Though the breaker for Phase B and Phase C were still open, the voltage on these two phases rose to about half of the rated value. This is because the flux linkage in the power transformer had been established by the winding of Phase A and the winding for the three phases were in the same flux linkage. About four and a quarter periods later (85.84 ms), the breakers of Phase B and Phase C were closed, and a transient overvoltage of approximately 554.2 kV (1.36 p.u.) can be found on Phase C.  
Figure 10: Transient voltage during switching on the #3 power transformer on Batang substation. Figure 9: Transient voltage during switching on the Tang-Xiang transmission line. (3) Transient voltages during switching off a power transformer from the bus bar  
While switching off the no-load power transformer under non-zero running current, since the current flowing through an inductance never suddenly changes, the residual running current will charge the capacitance connecting to the transformer. This energization process would produce transient overvoltage. The amplitude of the overvoltage mainly depends on the current upon switching off the power transformer. Theoretically, a larger current leads to a higher overvoltage. The transient voltage while switching off the #3 power transformer from the 500 kV bus bar on Batang substation is shown in Figure 11. The breaker of Phase C was opened while the voltage just crossed the zero level with a low value.