Voltage transformer is the necessary equipment to satisfy the considerable voltage of each node in the substation． It is difficult to realize on － line monitoring and real － time state assessment economically and reliably because of its large number，wide distribution and closed structure． Capacitor voltage transformers are widely designed and applied in the newly － built substations over 220 kV through the country． By analyzing the acquisition bias model of capacitor voltage transformers，the acquisition bias is decomposed and clustered，and an assessment method for running state of capacitor voltage transformers is proposed which is driven based on wide － area grid operation data．