A new model and associated algorithm of power flow calculation with constraints of changing range of variables by inequality equations are presented. Theory and applications of generalized inverse of matrices are introduced and used in resolving the new model. The new model can not only simplify the power flow calculation， but also reveal the relationship between the variation of voltage and reactive power clearly. Hence it can be used to determine the reactive power compensation capacity to be installed to satisfy the requirement of system voltage. The validity of the algorithm is demonstrated by case studies in the paper.