Challenges in Extensive Cabling of the Rural Area Networks and Protection in Mixed Networks

Abstract

Despite of the careful building and maintenance of the electricity network, faults like short-circuit and earth-faults take place from time to time. Majority of the faults customers experience in low voltage networks are caused by the faults in the medium voltage networks. Therefore minimizing faults in medium voltage networks contributes to the quality of delivery.

A way of improving the quality of delivery is to sectionalize the protection areas into smaller units with network reclosers. Reclosers include short-circuit and earth-fault protection and auto-reclosing functionalities. The best results are gained by placing the recloser so that the majority of the faults stay behind the device and the customers on the other side, so that they are not affected by the outage caused by the fault occurring behind the recloser. The quality of delivery indexes are improved by using reclosers.

After the great storms in the Nordic countries the network owners started to replace over-head line networks with underground cable. The cable characteristics, however, are very different from the characteristics of an over-head line. Cabling increases the capacitive earth-fault current for it may be considered as a cylindrical capacitor. Due to this, also the reactive power generation is increased in cabled networks.

Cable can be represented with a pi-section in which the series impedance consists of reactive and resistive parts. Because an underground cable has a zero sequence series impedance which is non-negligible on contrary to over-head line, cabling long feeders produces a resistive earth-fault current component. This cannot be compensated with the usage of a Petersen coil, which is used to compensate purely capacitive earth-fault current. Increase in earth-fault current may cause hazards for human safety because the earth-fault current can energize network equipment and thereby cause dangerous over-voltages. Therefore the contact voltages have to be limited also in terms of regulations.

The studies regarding earth-fault current behavior were carried out with a program namely Power System simulator for Engineering. The studies show that as the cabling increases, the zero sequence resistance becomes more dominating. When using only centralized compensation the zero sequence resistance produces resistive earth-fault current, which may cause dangerous over-voltages and causes voltage drops in zero sequence network. This may lead to difficulties in detecting high impedance earth-faults. The fault detection can be contributed with the usage of distributed compensation. The best results are gained by compensating first 10-15 kilometers centrally and the rest locally. The distributed Petersen coil should be dimensioned according to the produced earth-fault current in order to avoid over-compensation which may lead to false relay functions. The cable zero sequence impedance, however, is not an unambiguous matter. Therefore some field tests should be performed in the future in order to achieve even better knowledge regarding these issues. /Kir10