In this paper, beginning with the basic equations of hydrodynamics, turbine and governing system, the author deduced the basic equations of transient process caused by small fluctuation in the inclined ceiling tailrace, then analyzed and calculated the basic equations.

Hydraulic calculation for tailrace tunnel with variable crown heights for underground powerhouse in rightabutment at Xiangjiaba Hydropower Station;

Analysis on tailrace system with varying crown elevation of TGP s right bank underground power station;

Study on the configuration of hydropower station tailrace tunnel with sloping ceiling;

In light of the actual hydrological characteristics of a certain hydropower station, small fluctuation stability of the two kinds of tailrace layout, tailrace tunnel with sloping ceiling and tailrace surge tank, of hydropower station under extremely low head is compared and analyzed.

According to the characteristic of Xiangjiaba Underground Hydropower Station at the right bank of Jinsha River, one alternative option that the tailrace tunnel with sloping ceiling is adopted has been studied comprehensively, by method of model test and numerical calculation.