Zero-energy sag correctors--optimizing dynamic voltage restorers for industrial applications

Plants and processes with a high level of industrial automation are most severely affected by short-duration power disturbances. Such plants are frequently fed from highly reliable utility grids, with measured reliability levels that are consistently in excess of 3-5 nines, yet face hours of unscheduled downtime resulting from short-duration voltage sags. Extensive data confirm that such plants experience, on average, 30 power disturbances annually, of which over 95 % are short-duration disturbances lasting for no more than six cycles on average. Data also confirm that over 90% of these disturbances are asymmetrical faults. This paper shows a new approach to protecting industrial plants from power disturbances by building on the fact that it is prohibitively expensive to protect against all possible power disturbances and that the cost-effective protection is based on a statistical assessment of disturbances recorded and equipment susceptibility. This paper also shows that it is possible to use a new family of zero-energy sag correctors to realize protection against over 96 of all power disturbances. The design guidelines for such sag correctors, along with simulation and experimental results for the new dynamic voltage restorer system, are provided. Index Terms--AC-AC boost, power disturbance statistics, po?>wer quality, voltage sags, zero-energy sag corrector (ZESC).