Design Aspects of Hybrid HVDC System

Hybrid high voltage direct current transmission(Hybrid HVDC)is a new type of HVDC technology developed in recent years.It combines the characteristics of large-capacity and low cost of the line commutated converters(LCC)and non-commutation failure and dynamic reactive power support of the voltage sourced converters(VSCs)in one HVDC system.It has technical advantages in the fields of unidirectional power transmission and in the application of improving the stability of multi-infeed HVDCs,giving it broad application prospects.This paper defines the basic form of hybrid HVDC first and discusses the characteristics and challenges of different hybrid HVDCs.Then,it analyzes the matching characteristics between LCC and VSC in the station-station hybrid HVDC,studies the key technologies,such as VSC topologies and control strategies for clearing the DC overhead line faults,and introduces the key technologies for analyzing and suppressing the high-frequency resonances between VSC and the power grid.In this paper,using the Wudongde UHV multi-terminal HVDC as an example,the proposed key technologies are analyzed and verified.

Handling query skew in large indexes： a view based approach

Indexing is one of the most important tech- niques to facilitate query processing over a multi-dimensional dataset. A commonly used strategy for such indexing is to keep the tree-structured index balanced. This strategy reduces query processing cost in the worst case, and can handle all different queries equally well. In other words, this strategy implies that all queries are uniformly issued, which is par- tially because the query distribution is not possibly known and will change over time in practice. A key issue we study in this work is whether it is the best to fully rely on a bal- anced tree-structured index in particular when datasets be- come larger and larger in the big data era. This means that, when a dataset becomes very large, it becomes unreasonable to assume that all data in any subspace are equally important and are uniformly accessed by all queries at the index level. Given the existence of query skew and the possible changes of query skew, in this paper, we study how to handle such query skew and such query skew changes at the index level without sacrifice of supporting any possible queries in a well- balanced tree index and without a high overhead. To tackle the issue, we propose index-view at the index level, where an index-view is a short-cut in a balanced tree-structured index to access objects in the subspaces that are more frequently accessed, and propose a new index-view-centric framework for query processing using index-views in a bottom-up man- ner. We study index-views selection problem in both static and dynamic setting, and we confirm the effectiveness of our approach using large real and synthetic datasets.