In variable-speed constant-frequency doubly-fed wind-energy generation system， over voltage is found at the rotor side and transformer in grid when doubly-fed asynchronous generator is fed by PWM converter through long cable， which will damage the roll and rotor current-collection-ring prematurely.For solving the problem，firstly，this paper studies the over voltage occurring mechanism based on transmission line theory and the influence from d u /d t ; then it discusses the design of d u /d t restrain network in details， and analyzes the difference between the normal PWM output filter and d u /d t restrain network. Finally， simulation results show the validation of the theory analysis.