When the DC current flows into inside of transformer

it will cause DC bias of transformer

and such problems as big noise and increase of reactive power loss. Various problems brought by the DC current into the transformer can be reduced by way of adding the blocking device in the neutral point of transformer

but the problem is how to perform the preventive test to the device. The preventive test items for the device is set up in accordance with field experience and the blocking device for 13 units of transformers operated at Guangzhou grid is inspected and the fault as well as hidden trouble are found. Verification of preventive test has a definite practical value and reference.