Page 11 - Power Electronic Control in Electrical Systems

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Power electronic control in electrical systems 3

PSSs to ensure trouble-free operation between the Scottish power system and its
larger neighbour, the English power system (Fairnley et al., 1982). Deregulated
markets are imposing further demands on generating plant, increasing their wear
and tear and the likelihood of generator instabilities of various kinds, e.g. tran-
sient, dynamic, sub-synchronous resonance (SSR) and sub-synchronous torsional
interactions (SSTI). New power electronic controllers are being developed to help
generators operate reliably in the new market place. The thyristor-controlled
series compensator (TCSC) is being used to mitigate SSR, SSTI and to damp
power systems' oscillations (Larsen et al., 1992). Examples of where TCSCs have
been used to mitigate SSR are the TCSCs installed in the 500 kV Boneville Power
Administration's Slatt substation and in the 400 kV Swedish power network.
However, it should be noted that the primary function of the TCSC, like that of
its mechanically controlled counterpart, the series capacitor bank, is to reduce the
electrical length of the compensated transmission line. The aim is still to increase
power transfers significantly, but with increased transient stability margins.
A welcome result of deregulation of the electricity supply industry and open access
markets for electricity worldwide, is the opportunity for incorporating all forms of
renewable generation into the electrical power network. The signatories of the Kyoto
agreement in 1997 set themselves a target to lower emission levels by 20% by 2010.
As a result of this, legislation has been enacted and, in many cases, tax incentives
have been provided to enable the connection of micro-hydro, wind, photovoltaic,
wave, tidal, biomass and fuel cell generators. The power generated by some of these
sources of electricity is suitable for direct input, via a step-up transformer, into the
AC distribution system. This is the case with micro-hydro and biomass generators.
Other sources generate electricity in DC form or in AC form but with large, random
variations which prevent direct connection to the grid; for example fuel cells and
asynchronous wind generators. In both cases, power electronic converters such as
VSCs provide a suitable means for connection to the grid.
In theory, the thyristor-based static var compensator (SVC) (Miller, 1982) could be
used to perform the functions of the PSS, while providing fast-acting voltage support
at the generating substation. In practice, owing to the effectiveness of the PSS and its
relative low cost, this has not happened. Instead, the high speed of response of the
SVC and its low maintenance cost have made it the preferred choice to provide
reactive power support at key points of the transmission system, far away from the
generators. For most practical purposes they have made the rotating synchronous
compensator redundant, except where an increase in the short-circuit level is required
along with fast-acting reactive power support. Even this niche application of rotating
synchronous compensators may soon disappear since a thyristor-controlled series
reactor (TCSR) could perform the role of providing adaptive short-circuit compen-
sation and, alongside, an SVC could provide the necessary reactive power support.
Another possibility is the displacement of not just the rotating synchronous com-
pensator but also the SVC by a new breed of static compensators (STATCOMs)
based on the use of VSCs. The STATCOM provides all the functions that the SVC
can provide but at a higher speed and, when the technology reaches full maturity, its
cost will be lower. It is more compact and requires only a fraction of the land
required by an SVC installation. The VSC is the basic building block of the new gener-
ation of power controllers emerging from flexible alternating current transmission