Effect of DG on Distribution Grid Protection

Abstract

Up till recently past electric power systems were characterized by centralized production units, a high voltage transmission grid for the bulk energy transmission and medium and low voltage distribution grids to bring the energy to the consumer. Traditionally no generation sources were connected to the distribution grid, however, this has changed significantly the past decade. Nowadays various types of small generation sources, better known as distributed generation (DG), are connected to the distribution grid. Due to CO2 reduction goals many of the small units integrated in the distribution grid are renewable energy sources, such as wind turbines, small scale hydro plants and photovoltaic panels but also high efficient non-renewable energy sources, such as small Combined Heat and Power (CHP) plants are implemented. Connection of DG not only alters the load flow in the distribution grid but can also alter the fault current during a grid disturbance. Most distribution grid protective systems detect an abnormal grid situation by discerning a fault current from the normal load current. Because DG changes the grid contribution to the fault current, the operation of the protective system can be affected. This is reported in (Deuse et al., 2007; Doyle, 2002; Kauhamieni & Kumpulainen, 2004), however, in these papers the protection problems are discussed in general terms. In this chapter a detailed analysis of possible protection problems is given. It starts with an analytical description of fault currents in distribution grids including DG. With the aid of the analytical equations the effect of DG on the fault current is studied and key parameters are identified. This chapter also provides an equation to calculate the location where the DG-unit has the most effect on the grid contribution to the fault current. During the design stage of the protective system for a distribution feeder including DG this equation can be applied to determine if protection problems are to be expected. The application of the derived equations are demonstrated on a generic test feeder. An overview of all possible protection problems is presented and a classification of the protection problems is given. Furthermore these protection problems are linked to the theoretical background which is discussed in the beginning of the chapter. In this part of the chapter solutions for the possible protection problems are presented as well as new developments in protective systems which enables a further integration of DG in distribution grids. 5