Compensation strategies in cabled rural networks
Permanent address of the item is
Maaseudun kaapeliverkkojen kompensointistrategiat
The medium voltage network of Finland is experiencing major changes in the near future as the overhead lines are replaced with cables. The reason for the extensive cabling is the new suggestions for distribution network operators' quality of supply, which were set by the Ministry of Employment and the Economy. Cabling sets a number of practical and technical challenges to distribution network operators. From the technical point of view, cables generate a significant amount of reactive power and earth fault current, which were the main research interests of this thesis. The purpose of the thesis was to find an optimal way to compensate both reactive power and earth fault currents in extensively cabled networks. The thesis studied, what is the actual need for compensation and how should it be implemented optimally in a techno-economic way. The study was carried out by using PSCAD-simulation software and making calculations based on the results. The study revealed that there is a certain need for reactive power compensation. The main reason is Fingrid's fee for inputting reactive power in the main grid, which can cause major costs as the amount of cabling increases. Another reason is the power losses due to reactive power flow, which can be effectively limited with the correct selection and placement of shunt reactors. The reactive power compensation should be implemented by using centralized shunt reactors. Distributed shunt reactors did not appear to be profitable in the normal branched feeders. Instead, they should be used with over 50~km straight feeders. The distributed earth fault current compensation should be implemented with distributed Petersén coils. The centralized earth fault compensation should be carried out with a combination of adjustable Petersén coils and centralized shunt reactors. It is possible to use a shunt reactor for earth fault compensation if its neutral point is grounded. However, substations should always first have an adjustable Petersén coil before grounding the shunt reactors in order to keep the correct compensation degree.