Recent developments in energy policies and prices have directed an increasing amount of interest at exploiting small energy resources. This small-scale power generation evidently needs to be connected to the present power distribution system with simple manners in order to be economically competitive. As the distribution system has initially been built for simple one-way power delivery, interconnecting small generators requires new thinking and new methods for operating the network. Small-scale generators located on the distribution level are generally referred to as distributed generation (DG).

There are several clear consequences for locating DG in the distribution network that need to be taken into account. Probably the most critical concern is the operation of network protection. This can be stated as protection malfunctions can result in safety hazards. The presence of DG affects for instance short-circuit current amplitudes, which can further disturb the operation of feeder protection. DG may also result in failing reclosings or situations in which the DG unit maintains the voltage alone in the network. These situations must be avoided.

This thesis focuses on the network protection impacts of DG. It aims to provide new tools and methods for including the impacts of DG correctly in network planning methods. This can be achieved by bringing the research observations towards the practical level of network planning. To help form a precise image on the subject, the typical DG-related protection problems are described and analysed. Simulation tools have been used to study the phenomena. A perspective of distribution network planning is maintained throughout the thesis. The information systems applied for planning purposes are also considered. Development needs created by DG are especially focused on.

The most important observations of this thesis relate to the contradictions between protection selectivity and sensitivity; to the problematic nature of differentiating between faults that require rapid actions and other disturbances that should not result in any action. At the present situation, it may be possible to trip the DG units too sensitively in the name of network safety. However, as the DG becomes more significant for the power system, more coordination will be needed in the future. An efficient coordination of protection devices during all possible situations is beneficial to all parties.

The thesis shows the significance of performing certain studies during the interconnection process of a new DG unit. The sequence of the studies performed can also be essential depending on the case. The thesis proposes a procedure for gathering the studies as one entity. A new approach for presenting protection requirements for the new DG unit is also presented. This method is intended especially for facilitating the dialogue between network utility and power producer. A new method for extending the short-circuit calculation of a typical planning system is also defined. This extension enables more accurate results.

The ideas presented for integrating the DG impacts in planning systems and methods form the most important contribution of this thesis. The knowledge gathered and case studies conducted should also be useful for the interest groups of DG.

According to many forecasts, the amount of DG will rapidly increase in the near future. At the same time, managing the DG impacts will become an essential question for the network utilities. It can be seen that the planning process for a new DG unit interconnection can become a part of daily network planning activities. Thereby it needs to be handled efficiently.