The paper introduces the principle of cells transverse differential protection,and makes analysis on the reason of large unbalance current of Longyangxia hydropower plant cells transverse protection,as well as proposes solving methods and its importance.

According to the test results the HV capacitor banks which the unbalance current is larger have been adjusted.

However,unbalanced differential current can occur at normal conditions when traveling-wave differential protection is applied.

It is shown that the unbalanced differential current of the non-fault phase for the double-circuit line is larger than that for the single-circuit line.

New energy-saving equipments for adjusting unbalanced currents and compensating power factor;

Based on the loop equations of transformer model, utilizing the unbalanced current in the delta windings to establish the relations among the three-phase loop equations, a winding unbalanced parameter loop equation under the normal operation is obtained.

When there is no fault within protection range,unbalanced current inside differential relay is near to zero;but on some cases,there does still big unbalanced current exist.