Finding minimum number of connecting lines is as important as locating power quality monitors (PQMs) for full observability of power system. Therefore, a PQM placement method should determine both optimum buses and lines, since utilities can make better decisions for monitoring of power system with this information. This paper attempted to propose a new method to locate the PQMs at various unobservability depths. In this method, the problem of placement is solved on three levels, taking into account the limited number of channels required for measuring each bus. At the first level of this method, the optimum combination of zero injection buses (ZIBs) is achieved. At the second level, different combinations of connecting lines are produced, while the third level determines the best location of monitors in different nuobservability depths. In addition, the third level identifies the critical buses and their effects on the number and location of monitors. Moreover, the best location for installing monitors and the number of channels required by equipment in each depth will be specified. The results of applying the new method on 14-bus IEEE network demonstrate the ideal performance. پرونده مقاله