Proposals for protection of return cables on SwePol Link

Abstract

The return cable on the SwePol Link has been introduced as an alternative forced by environmentalists due to lack of social acceptance of other solutions. This is why in the proposed solution water and earth have been replaced by two return cables, although from a technical point of view such a solution is less effective. The last, eleventh fault of the return cable took place on 15 October 2012. In eight earlier cases the faults were caused by electrical failures in the cable in the sea and were located between ten and twenty kilometers from the Polish shore and triggered by disturbances in the northern part of the Polish power grid. In this situation it has been suggested to analyze and introduce one or two solutions shown below which may significantly limit the effects and lower the costs caused by return cable faults: a) assembly of additional surge arresters b) return to electrodes – lack of return cables c) ”partial” electrodes working with one return cable d) operation of the link only with earthings on converter stations. To sum up it needs to be stated that: • a relatively cheap way of protecting return cables against electrical failures is installing surge arresters in the cabinet located next to the cable container on the Polish shore • from the suggested preventive measures it seems reasonable to introduce the above mentioned solutions a) and d) simultaneously, as both of them are simple solutions which require neither considerable financial expenditure nor authorizations and may quickly show the expected results. DOI: 10.12736/issn.2300-3022.2013113 1. Reasons for introducing return cables The return cable on the Sweden-Poland link appeared as an alternative forced by ecologists. The decision on laying the return cables was made only at the stage of laying the main cable. In the original projects the construction of an anode and cathode was assumed, and the electrical circuit was supposed to be closed through the earth. In the proposed solution water and earth were replaced by the return cable; however, from a technical point of view, this solution is worse as the efficiency of the whole connection is lower (it is sufficient to build electrodes of proper low resistance values when closing the circuit through the ground, and the ground resistance practically equals zero). At the time of making the decision to lay the return cable there was no cable with a cross section of at least 1100 mm2 and insulation strength of 20 kV on the market that would be suitable for laying at the bottom of the Baltic Sea. It was decided to lay two cables at the marine section with a cross section of 630 mm2 each, and to lay one cable at the Polish land section with a cross section of 1100 mm2, all with an insulation strength of 20 kV. The first failure of the return cable showed the relevance of the decision to lay two return cables; in a failure scenario the damaged cable is disconnected and the other operates with the load of 473 MW [1, 2, 3, 4]. 2. Damage to return cables The damage to the return cable connecting Sweden and Poland that happened on 15.10.2012 was the eleventh in the history of cable operation (almost ten years had passed since the earlier tenth damage). Previous instances of damage occurred in 2001–2003 (that is, during the guarantee period). The cause of the last damage has not been clearly established yet; however, the final insulation breakdown was certainly of an electrical nature. The damage repair procedure for the return cable of the SwedenPoland link is always pretty much the same. First, it must be established which of the two cables is damaged, then the damaged cable is disconnected from the terminals and using mobile Murray bridges the approximate damage spot is located, which T. Szczepanski | Acta Energetica 1/14 (2013) | 147–152