DETECTION OF FAILURE SECTION ON MIXED LINES

The development plans of the high-voltage network in the EES of Serbia in several places in the system include the construction of lines consisting of an overhead (transmission line) and an underground (cable) section. Such a line is called mixed or hybrid in the literature. In this paper, the term mixed is used in accordance with the Cigre B5.23 [1] report. Therefore, the mixed line consists of at least two sections and the question arises of how to implement relay protection and automation, that is, the implementation of the AR (automatic reclosure) technique. According to the current EMS technical regulations, in the event of a cable line failure, reconnection is allowed only after cable testing (high-voltage testing) and therefore AR is not applied. It raises the question of how to treat a mixed line that may have a significant part of the above-ground section where there is a high probability of transient faults that could be successfully eliminated by applying AR. Should the mixed line be treated as a purely cable line and at each outage disconnect the above-ground and underground section and conduct tests or implement a system that could reliably detect a fault on the section and, depending on that, apply the AR. The paper gives an overview of some solutions that are applied in world practice in order to determine failure on the section.