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Voltage sags are the leading cause of unscheduled downtime in industrial plants with high levels of automation. Zero-energy sag correctors (ZESCs) that utilize energy from those phases that have sufficient remaining voltage on them promise levels of protection that are comparable with conventional energy storage based sag correctors, but in a more compact package. This paper presents a ZESC topology with reduced device count using readily available device packages that can realize similar performance as the original ZESC circuit. The proposed ZESC behaves like a 'thin ac converter,' and can be integrated with existing assets such as transformers and switchgears. This paper presents details of the operation, design, and experimental results for such a ZESC circuit. Index Terms--AC-AC boost, power disturbance statistics, power quality, voltage sags, zero-energy sag corrector.

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).

Offshore wind farms using HVDC links can be positioned a large distance from shore, opening up new opportunities for wind generation. Conventional approaches using 60 Hz generators and transformers are not appropriate in such applications, as they are heavy and result in expensive and complex installation and maintenance issues. This paper proposes an alternative architecture for such wind farms, using permanent magnet generators, medium frequency transformers and simple power converters to realize a compact and light system. It is possible that in the long term, the proposed approach may prove attractive for land-based wind farms as well. Index Terms--DC power systems, HVDC, offshore wind farm, permanent magnet (PM) machine, wind power generation.