Adverse impact on transformer protection performance will occur when commutation failure happens. First

the mechanism of transformer saturation caused by commutation failure is researched based on the addition switching function. Then

the effect of commutation failure on the transformer differential protection is studied in detail. The analysis result shows that the differential protection is seldom affected by commutation failure while there is only an external fault in the transformer

but it may cause trip failure during an internal fault. The existing traditional differential protection with second harmonic restraint cannot well adapt to the AC/DC interconnected network. In order to solve this problem

a synthetic restraining scheme with a new logic is proposed

which is capable of ensuring the reliability of differential protection during an internal fault

while the differential protection is blocked during an external fault by a novel restraining scheme. The correctness and effectiveness of the proposed scheme is verified by simulation results. This work is supported by National Basic Research Program of China (973 Program) (No. 2012CB215206) and the Fundamental Research Funds for the Central Universities (No. 2015XS16).