Negative sequence directional element is widely used in the pilot protection of transmission lines. With the rapid development of wind power generation

a large scale of doubly-fed induction generator(DFIG)is connected to the grid and the negative sequence equivalent impedance of power source is no longer fixed

which destroys the theoretical basis of the traditional negative sequence directional element. This paper derives the analytic expressions of the negative sequence equivalent impedance of DFIG under two situations

which are the uninterrupted control by the rotor-side converter and the crowbar resistance being thrown into

respectively. Subsequently the change rules of phase angles of negative sequence equivalent impedance have been obtained under two situations. It has been found that the traditional negative sequence directional element faces the problems of sensitivity and reliability due to the influences of DFIG. Therefore

an improvement measure of the negative sequence directional element has been proposed as follows: before faults

the maximum sensitivity angle under different operation states is determined by DFIG parameters in the mode on line; after faults

the action criterion with higher sensitivity and reliability is adopted according to DFIG operation states. Digital simulation results show that the improved negative sequence directional element has higher sensitivity and can be well adapted to various operation states of DFIG.