Installing small generation in the distribution grid has become popular since the mid eighties and the protection problems caused by DG has been studied accordingly (Dugan et al., 1984; Rizy et al., 1985). In the literature for the problems mentioned in the previous section a wide pallet of solutions is offered. These solutions vary between a simple change in relay settings to a complete new adaptive protective system. In this section an overview of possible solutions is given.
2.1 Prevention of detection and selectivity problems
Fault detection problems do have a relation with the amount of generation connected to the distribution grid and the local short-circuit power. To prevent fault detection problems a first attempt is to modify the relay settings of the relays and reclosers (Baran & El-Markabi, 2004; Hadjsaid et al., 1999; Kumpulainen et al., 2005; Maki et al., 2004). The generator contribution leads to a reduction of the grid contribution to the fault current hence the pick-up current of the relays has to be reduced. However, fault detection problems might be solved by reducing the pick-up current, the sensitivity and security of the protective system is decreased and might lead to false tripping in case of a fault in an adjacent feeder. In (Maki et al., 2004) an example of a weak network is given where blinding of protection occurs due to the connection of a small wind farm. By reducing the pick-up current blinding of protection is solved but at the same time it introduces for faults in a certain area false tripping. A proposed solution is to install protection devices with an additional time delay to give the feeder including the wind farm a longer fault clearing time. These type of solutions also discussed in (Deuse et al., 2007). Another example of changed protection settings is discussed in (Baran & El-Markabi, 2004). Here an adaptive overcurrent relay is proposed which decreases the pick-up current as the output of the local generation increases. This is also studied in (Vermeyen, 2008) where it is stated that a continuous adaptation of the pick-up current as function of the generator output results in less superfluous disconnection of the feeder.