Capacitor DC blocking devices are usually used for suppressing DC-bias of transformers. If there is no full consideration before installation

owing to the complex flow routes of geomagnetically induced currents(GIC)

the blocking device installed at the neutral point of a certain transformer may lead to more severe DC bias on adjacent transformers. Therefore the optimal position of installation for the capacitor blocking device is studied. This paper introduces the transformer effective GIC to describe the effect of geomagnetic storms on transformers by considering the wiring of autotransformers and the topology of power networks. After investigating the influence of DC blocking devices on transformer effective GIC

this paper proposes an optimization method of all transformers effective GIC being less than the allowable limiting value

with the least quantity of DC blocking devices as optimization objective

and the genetic algorithm to solve the problem. As an example

the GIC equivalent model of Gansu main power grid is built

which includes 47 substations and 101 nodes. Based on the model

the optimal quantity and installing location of DC blocking devices are calculated with the optimization method and genetic algorithm

and the outcome is compared with that derived from the un-optimized method. The results of calculation and comparison have verified the feasibility and superiority of optimization methods proposed in this paper. This work is supported by National Natural Science Foundation of China(No. 51177045)and National High Technology Research and Development Program of China(863 Program)(No. 2012AA121005).