Leading current interruption

When the leading current is interrupted at current zero, it occurs at a voltage peak assuming that the current waveform is leading the voltage waveform 

Zero-missing phenomenon with sequential switching. 

Overvoltage caused by leading current interruption and restrike. 

Considering the severe overvoltage that can be caused by the leading current interruption, the leading current interruption capability of circuit breakers is specified in lEC 62271-100 (see Table 3.7). 

When the charging capacity of a long EHV cable line is not compensated by shunt reactors directly connected to the cable, the leading current interruption capability requires careful attention [31], Considering a typical capacitance of 0.2 pF/km, the maximum line length for 400 kV cable line is limited approximately below 26 km without shunt reactors directly coimected to the cable. Here, it is assumed that the leading current is interrupted by one end, and the other end is open before the interruption. 

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Other issues, such as the zero-missing phenomenon, the leading current interruption, and the cable discharge, also stem from the large charging capacities of cables. The effects of these issues on the cable system design are discussed in Section 3.5. The discussion includes countermeasures for the problems and suggestions for equipment selection. 

Various transient studies have been performed on the 400-kV cable that will connect Sicily to mainland Italy [28,29]. In addition to switching transients, these studies include the harmonic overvoltage caused by line energization, leading current interruption, and zeromissing phenomena. The studies identified the resonant condition at the second harmonic when the cable is energized from Sicily s side under a particular condition. The harmonic overvoltage caused by the resonant condition is avoided by the operational constraint. 

It is recommended to trip at least half of the shunt reactors of healthy phases as shown in Figure 3.25 as the tripping will normally lower the compensation rate below 50%. The remaining shunt reactors will be useful in maintaining the charging current within the leading current interruption capability of the line breakers. 

Usually, long EHV cable lines are compensated by shunt reactors that are directly connected to the cable. When the compensation rate is high enough, the leading current interruption capability is not a concern. If sequential switching is applied to a cable line as a... 

Shunt reactors for other cables are often connected to buses, as the area compensation is applied at these voltage levels. When shunt reactors are connected to buses, the zero-missing phenomenon does not occur. In this case, however, the inductive VT connected to the cable needs to have enough discharge capability, and the line breaker needs to have sufficient leading current interruption capability. 

Requires higher leading current interruption capability... 

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