The rapid growth of small-scale photovoltaic (PV) units in distribution grids brings new challenges in the protection coordination of overcurrent relays. So far, various adaptive techniques have been addressed to solve DG units’ mis-coordination problem, however, the un-predictable penetration of installed PV units highlights the importance of a penetration-free methodology. This study presents a novel offline, quick, applicable and cheap solution which guarantees protection coordination for any penetration and location of PV units. To do so, first, under different PV penetration levels and locations, the conventional protection performance is studied to discover the worst mis-coordination cases. Next, according to relays standards, the characteristic curve of the back-up relay is modified such that it can maintain coordination in all worst cases. The proposed methodology is applied to a practical power distribution network equipped with numerous small-scale PV systems. The results successfully prove the effectiveness of the proposed method. Since this technique only applies some changes in characteristic curves of the back-up relays, lack of programmable relay or adaptive protection requirements cannot decrease its performance. This fact makes this method an attractive option for distribution systems with PV system.