In thyristor at the time of turn ON anode current increases rapidly.This rapid variation of anode current does not spread across the junction area of the thyristor. This create the local hotspots in the junction and increases the junction temperature.

If the junction temperature exceeds permissible value then the tyristor is damaged.

The rapid variation of the thyristor current are also called $d_i/d_t$.

The thyristor can be protected from excessive $d_i/d_t$ by using an series inductor $L_s$ as shown in below figure.

$d_i/d_t$ can be calculated by using the formula

$$\frac{d_i}{d_t} ={v_s}{L_s}$$

$$L_s=\frac{v_s}{d_i/d_t}$$