The structure and characteristics of HVDC (High-Voltage Direct Current) converters have
remained practically unaltered for the first 40 years of commercial operation. Restricted by
the switching characteristics, first of the mercury arc valve and later of the silicon-controlled
rectifier, this technology requires substantial extra support at the link terminals to ensure
stable operation.
More recently the development of power semiconductors with improved characteristics
has provided the basis for a flexible AC transmission system (FACTS) technology. This
technology covers a variety of power electronics controllers created to enhance the performance
of the traditional grid. The individual members of the FACTS family are designed
to solve a specific problem, e.g. active or reactive power flow control, short-circuit current
limitation, etc. So it is the complete family that provides transmission flexibility, rather than
the individual controllers.
The new power semiconductors have also, in the past decade, changed the attitude towards
HVDC transmission, and a variety of converter configurations have been developed to take
advantage of the higher controllability and switching frequencies of the new devices.
Although the main market for HVDC is still thyristor based, a transistor-based technology
has recently been developed, and is already being used throughout the world. The new HVDC
technology can provide most of the enhancements of the individual FACTS controllers,
i.e. permit large stable power transfers, deliver or absorb the required reactive power to
maintain the specified voltages at the interconnected buses, contain fast emergency controls
to avoid large fault current levels, be designed (if required) to control sub-synchronous
resonances, etc. Moreover, the DC link is the only practical way of connecting asynchronous
systems and systems of different frequencies. For a given HVDC configuration, all these
tasks can be achieved purely by control action.
Therefore, a modern HVDC interconnection is potentially the most flexible power transmission
system. However, the provision of greater HVDC transmission flexibility comes at
a price, in terms of either reduced efficiency or increased structural complexity. Thus, when
considering a new scheme, it is important to decide on the degree of flexibility required
for the particular application (i.e. taking into account power ratings, transmission distances,
extent of ancillary services expected, etc.).
A critical review of the HVDC options already available and under consideration constitutes
the purpose of this book, which therefore complements recent titles describing the
FACTS technology to help power system engineers to make informed decisions on the
planning, design and operation of future power transmission systems. It is also a useful
reference text for students taking advanced courses in power transmission.
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发表于 2011-12-28 15:20:43
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