Page 10 - Power Electronic Control in Electrical Systems

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2 Electrical power systems ± an overview

Clearly it has not been easy for the power industry to reach its present status.
Throughout its development innumerable technical and economic problems have
been overcome, enabling the supply industry to meet the ever increasing demand
for energy with electricity at competitive prices. The generator, the incandescent
lamp and the industrial motor were the basis for the success of the earliest schemes.
Soon the transformer provided a means for improved efficiency of distribution so
that generation and transmission of alternating current over considerable distances
provided a major source of power in industry and also in domestic applications.
For many decades the trend in electric power production has been towards an inter-
connected network of transmission lines linking generators and loads into large integ-
rated systems, some of which span entire continents. The main motivation has been to
take advantage of load diversity, enabling a better utilization of primary energy resour-
ces. It may be argued that interconnection provides an alternative to a limited amount
of generation thus enhancing the security of supply (Anderson and Fouad, 1977).
Interconnection was further enhanced, in no small measure, by early break-
throughs in high-current, high-power semiconductor valve technology. Thyristor-
based high voltage direct current (HVDC) converter installations provided a means
for interconnecting power systems with different operating frequencies, e.g. 50/60 Hz,
for interconnecting power systems separated by the sea, e.g. the cross-Channel link
between England and France, and for interconnecting weak and strong power
systems (Hingorani, 1996). The rectifier and inverter may be housed within the same
converter station (back-to-back) or they may be located several hundred kilometres
apart, for bulk-power, extra-long-distance transmission. The most recent develop-
ment in HVDC technology is the HVDC system based on solid state voltage source
converters (VSCs), which enables independent, fast control of active and reactive
powers (McMurray, 1987). This equipment uses insulated gate bipolar transistors
(IGBTs) or gate turn-off thyristors (GTOs) `valves' and pulse width modulation
(PWM) control techniques (Mohan et al., 1995). It should be pointed out that this
technology was first developed for applications in industrial drive systems for
improved motor speed control. In power transmission applications this technology
has been termed HVDC Light (Asplund et al., 1998) to differentiate it from the well-
established HVDC links based on thyristors and phase control (Arrillaga, 1999).
Throughout this book, the terms HVDC Light and HVDC based on VSCs are used
interchangeably.
Based on current and projected installations, a pattern is emerging as to where this
equipment will find widespread application: deregulated market applications in
primary distribution networks, e.g. the 138 kV link at Eagle Pass, interconnecting the
Mexican and Texas networks (Asplund, 2000). The 180 MVA Directlink in Australia,
interconnecting the Queensland and New South Wales networks, is another example.
Power electronics technology has affected every aspect of electrical power
networks; not just HVDC transmission but also generation, AC transmission,
distribution and utilization. At the generation level, thyristor-based automatic
voltage regulators (AVRs) have been introduced to enable large synchronous
generators to respond quickly and accurately to the demands of interconnected
environments. Power system stabilizers (PSSs) have been introduced to prevent
power oscillations from building up as a result of sympathetic interactions between
generators. For instance, several of the large generators in Scotland are fitted with