Protection of Double Circuit Transmission Lines

I. INTRODUCTION

Parallel transmission lines have been extensively utilized in modern power systems to enhance the reliability and security for the transmission of electrical energy. The different possible configurations of parallel lines combined with the effect of mutual coupling make their protection a challenging problem. Different types of protection and algorithms have been proposed in the past to overcome some of the performance related issues. In addition to the effect of mutual coupling, fault resistance and pre-fault loading consideration, as well as cross-country faults have to be considered in order to determine the protection response. In the case of distance relays, the determination of the final fault impedance is therefore not that straight forward as a number of system conditions affect it:

1. The dynamic changes of the characteristics of the power system, such as in generation capacity, load and in the network topology
2. Inaccuracies in the measuring instruments, relay measuring inputs, line parameters and protection algorithms

The effect of the dynamic characteristics of the system on protection is normally dealt with by traditionally determining the relay settings according to worst-case scenario studies. Inaccuracies in the electric power system need to be considered in the process. For example, a sequence components based model for a protection coordination study of a transmission line that is not completely transposed, will result in a difference between the results from the simulation and real fault conditions.

Inaccuracies in the image of the primary system conditions seen by the protective relays due to the errors in the measuring circuit can be limited by using properly selected current and voltage transformers and more advanced relay input modules.

To compensate for the above described effects, the relay settings require large safety margins. A typical Zone 1 distance setting is 80% of the line impedance. These settings have to be appropriate for extremely weak system conditions, as well as in the cases where over or under tripping could further weaken the system and lead to stability problems.
 
Since distance relay settings may therefore not be appropriate for all system conditions in terms of security and dependability, adaptive protection based on different setting group selection are often considered in practice.

Consideration of different types of protection for double circuit lines is important in order to achieve improved performance of the protection system and reduction in the effect of the fault on power system stability and sensitive loads. The issues with the limitations in the performance of distance relays on parallel lines can be easily overcome by the use of modern line differential protection relays based on digital communications technologies. Line differential schemes using GPS for data synchronization are a very secure and dependable solution for such applications in case when a communications channel is available. They offer significant benefits, but have an important deficiency - lack of remote backup functionality and dependence on the state of the communications channel. However, modern relays have increased processing power that allows them to run dual high-performance schemes based on distance and line differential protection respectively thus giving the users the benefit of both technologies in a single relay hardware.

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.

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8 Comments
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#4 B. Katz wrote at 08.04.2008 22:09

Big Grins Very good. Written clear and useful for young engineers.

#2 Bhupendra Badiya wrote at 15.08.2007 23:47

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#1 J.P.SHETH wrote at 13.08.2007 02:40

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#3 Joseph Quartey wrote at 11.10.2007 05:34

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