Zero-Missing Phenomenon

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. 

Zero-missing current in the underground cable energization. 

When a line breaker is to be operated to interrupt this current without the zero crossing, the arc current between the contacts cannot be extinguished within a couple of cycles and may continue for an extended duration. The extended duration may lead to the failure of the line breaker depending on the arc energy generated during this duration. The duration is mainly determined by the magnitude of the dc component and the relationship between arc resistance and arc current inside the line breaker. Typical durations for EHV cables can be several hundreds of milliseconds in severe conditions. 

The zero-missing phenomenon can theorehcally be avoided by limiting the compensahon rate to lower than 50%, but it is not a common way to address the problem. Normally, a compensahon rate near 100% is adopted, especially for 500/400 kV cables, and the countermeasures listed in Table 3.6 are applied to the cables. All of these countermeasures, except for Coxmtermeasure (4), have already been applied to the cable line in operahon. Countermeasure (1), which in particular has a number of applicahon records to long EHV cable lines, is discussed in detail later in this sechon. 

Countermeasure (3) will be applied to the 400 kV Sicily-Mainland Italy cable [30]. This countermeasure can be implemented rather easily as long as a cable line is installed together with single-phase circuit breakers and current differential relays. For this reason. Countermeasure (3) is more suited for EHV cable lines than high-voltage (HV) cable lines. 

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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. 

The amount of shunt reactors (that is, the compensation rate of a cable) is a key figure that has a major impact on the following studies. A compensation rate close to 100% is often preferred since it can eliminate the reactive power surplus created by the introduction of the cable. It also offers a preferable condition for the TOV but causes a severe condition for the zero-missing phenomenon. The negative effect on the zero-missing phenomenon is not a primary concern as there are countermeasures established for tackling this effect. 

Zero-missing phenomenon with sequential switching. 

Michigami, T., S. Imai, and O. Takahashi. 1997. Theoretical background for zero-miss phenomenon in the cable network and field measurements. lEEJ General Meeting 1459, Tokyo, Japan (in Japanese). 

Figure 3.23 shows an example of current waveforms when an EHV cable is energized with its shunt reactors. It can be seen that the ac component of the energization current is very small since the compensation rate is close to 100%. The simulation was run for 0.2 s, but the energization current did not cross the zero point during this duration. Since the Zero-missing phenomenon is caused by a dc component of an energization current, it is most severe... 

Usually, long EHV cable lines are compensated by shimt reactors directly connected to the cable. When the compensation rate is high enough, the leading current interruption capability is not a concern. If the sequential switching is applied to a cable line as a countermeasure to the zero-missing phenomenon, however, the tripping of shunt reactors makes the compensation rate lower. This is the only occasion that requires a careful attention. 

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