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The common-mode voltage (CMV) in voltage source inverter may seriously affect the motor drive system. It can be restrained with proper PWM method if only nonzero space vectors are used. In this paper, a nonzero vector PWM technique (NZPWM) is proposed. Using this method, three odd/even active vectors in low voltage region, or three adjacent active vectors in high voltage region, are used to build the reference voltage vector. So in the entire modulation index range, the peak-to-peak value of CMV can be reduced to one third of the traditional SVPWM, which is the minimum theoretically. Some simulation and experimental results will be presented, which show that the proposed method is effective in CMV reduction.

With parallel gap, by controlling the motion path of the electric arc, effectively avoid burning and damaging the insulator during lightning strike so that its importance in measures of lightning protection for transmitting line is continually improved. Initial discharge position is key consideration during design of parallel gap and depends on discharge characteristics of parallel gap under the action of lightning surge when the lightning strikes the tower or line. This paper gives three types of parallel gap used to protect the composite insulator, sets up the impulse discharge test and observation platform including high-speed camera, studies the impulse discharge characteristics of parallel gap and analyzes the influence of structural parameters on the discharge characteristics. This study showed that there is randomness for discharge path of parallel gap; three types of parallel gap given in this paper can achieve 100% discharge rate of parallel gap; the determinant of discharge rate in the presupposition discharge area (PDA) is the distance that the presupposition discharge area stands out of the whole electrode plane. When this distance exceeds 20mm or more, it can ensure that most of the discharge occurs in the PDA; the parallel gap can reduce the insulation distance of 4000mm insulator about 40mm, U 50 about 1%, which may approximately consider that the parallel gap has no influence on the insulating property of the insulator.

With parallel gap, by controlling the motion path of the electric arc, effectively avoid burning and damaging the insulator during lightning strike so that the effect to reduce the accident rate of line is obvious. Because of simple structure, convenient installation and good economical efficiency, its importance in measures of lightning protection for transmitting line is continually improved. Electric field characteristics on the surface of parallel gap electrode have obvious influence on corona generation and the gap breakdown position and are the key parameters that must be considered in the design. Based on the finite element method, this paper sets up the analysis model of electric field of parallel gap including the tower, the conductor and the insulator, uses ANSOFT software for simulation, studies the surface electric field characteristics of parallel gap electrode, compares the electric field of four types of parallel gap put forward in the paper and analyzes the law of influence of parallel gap electrode parameters on the electric field characteristics. The results show that, when the short circuit distance is increased, the maximum values of electric field of ball end and snap ring body are obviously increased; the opening angle of snap ring is larger and the maximum value of electric field of ball end is higher; when the pipe diameter is increased, the maximum value of electric field of snap ring body is reduced considerably; the ball end is bigger and the maximum value of electric field of ball end is less; the ring pipe diameter is the structure parameter having the largest influence on the surface electric field; for the ball end at the end of ring electrode, the structure parameter having the largest influence on the electric field is the diameter of ball end. Reducing the distance that the ball end stands out of the ring plane can decrease the maximum value of electric field of ball end.