In Xibei-Huazhong LingBao BtB expansion project

the corresponding rated DC current is the largest in the world at present

the thyristor trip caused by overhigh junction temperature occurred frequently

during system commissioning test.These events have been investigated in this paper

and the real trip reason is not from thyristor valve cooling system

but from the design unreasonable of thyristor junction temperature protection criterion in AREVA′s valve based electronic equipment（VBE）.The improvement suggestion is cancel the repeated temperature protection from VBE

or adjust the temperature protection criterion and only makes the backstage record to its application.