Abstract: The tracing methods determine the contribution the
power system sources have in their supplying. The methods can be used
to assess the transmission prices, but also to recover the transmission
fixed cost. In this paper is presented the influence of the modification of
commons structure has on the specific price of transfer. The operator
must make use of a few basic principles about allocation. Most
tracing methods are based on the proportional sharing principle. In this
paper Kirschen method is used. In order to illustrate this method, the 25-
bus test system is used, elaborated within the Electrical Power
Engineering Department, from Timisoara, Romania.