To reduce the probability of commutation failure for DC systems

the key factors to promote the commutation process are obtained by analyzing the mechanism of commutation failure so as to prevent the commutation failure by means of fuzzy control.For improving the accuracy of fuzzy controller and balancing the number of control rules and the accuracy

a variable universe fuzzy control method based on characteristic vectors of phase plane is proposed.This method can alter the universe dynamically and revise the control rules to improve control accuracy and convergence.It measures the change of voltage dynamically

increases the firing angle and decreases the commutation area to inhibit commutation failure when a fault occurs.Simulation based on the standard CIGRE HVDC model verifies the sensitivity and correctness of the proposed variable universe fuzzy control method

and indicates that this method can reduce commutation failure probability.