The ground current during monopole operation of HVDC transmission system can lead to DC bias in the transformers near the grounding pole

which affect the safe operation of AC power grid. Due to the multiple flow paths of the bias current in the power grid

the DC blocking device installed in the neutral point of a transformer may cause more severe DC bias in the other adjacent transformers while obviously suppressing the DC current in one transformer. We introduced the transformer effective bias current to describe the effect of the grounding current in the transformers after considering the wiring characteristic of auto-transformers and the topology of the power networks

and proposed an optimization method to find the least quantity of DC blocking devices which satisfied the constraint conditions that effective bias current of all transformers was less than the allowed limiting value.Moreover

we applied a genetic algorithm based on improved roulette selection method to solve the optimal configuration problem. Using the test case of Xizhe HVDC transmission receiving-end grid near the Jinsi grounding pole

we calculated the optimal quantity and installed location of capacitor blocking devices under different effective bias current limiting values. The results in comparison with the un-optimized method show that the required quantity of the blocking devices can be reduced by 50% or more.