Improving voltage sag tolerance of three-phase ASDs through addition of a neutral connection

This paper presents a new method to improve ride-through capability of adjustable speed drives during voltage sags. The method consists of connecting the three-phase neutral to the mid-point of the DC bus when the voltage dips. This changes the front-end from a six-pulse configuration to a series connection of two half-wave connections. The modified configuration provides a higher DC bus voltage and can avoid undervoltage tripping of the drive. The principle of the proposed approach is discussed in detail, and verified through computer simulations. A modification of the method for application in three-wire systems without a neutral is also discussed briefly. An additional function that the proposed circuit can perform is balancing of the DC bus capacitor voltages.