Protection of parallel (double) circuit transmission lines in modern power systems

Distance protection performance problems

This paper describes different cases of parallel transmission lines and analyzes some well known application problems associated with their protection. Distance protection performance problems are in the focus due to the fact that they are the most commonly used protection type for parallel transmission lines.

The behavior of ground overcurrent protection on double circuit applications is also discussed in the paper.

Finally, some advanced features of communications assisted schemes, non-communications protection schemes and fault location for double circuit lines are discussed.

Application considerations

Mutual coupling and the possibility of the occurrence of cross country faults are possibly the main sources of problems and complexity when faults in multi-circuit lines are to be studied.

The use of single line settings are often only considered without analyzing the mutual coupling effects. Also, cross-country faults may be deemed a low risk, but in the event of such a fault may cause incorrect tripping operation which could lead to stability problems on already weak-end networks.

Mutual Coupling Effects

Double circuit lines on the same towers (Figure 1) or on the same right-of-way are very common.

They can have many different configurations based on the system topology and how they are connected at the two ends of the double circuit line.

For example:

1. They are not connected to the same bus at either end of the line.
2. They are connected to the same bus at only one end of the line.
3. They are connected to the same bus at each end of the line.

Figure 2 shows the last case that will be the focus of the analysis in the paper.

The parallel line configuration is further complicated when the two (or more) lines on the same tower or right-of-way are at different voltages.

The main difference between a single transmission line and a transmission line on the same tower or parallel along the same right-of-way with one or more other lines is the mutual coupling.