The nine-zone graph principle is widely accepted in the reactive power optimization, but hard to treat with three-coil transformers. This paper presents a method of identifying the VQC regulation unit based on topology analysis, which looks on the three-coil transformer as two double-coil transformers and then finds the parallel connection transformers. So the transformers in substation are turned to devices in regulation units. By the connection mode of transformers, the paper establishes the correlation and priority of the regulation units in a same substation. According to the voltage level and reactive power of every regulation unit and the relationship of the regulation units, the paper give reasonable regulation policy, resulting in turning adjusting the voltage and reactive power in substation to adjusting the unit's voltage and reactive power. The method acquires greater flexibility and comprehensive application, regardless of the connection manner and running mode of the substation. The software based on the theory gained successful and wide use. (8 pages)