Page 263 - Power Electronic Control in Electrical Systems

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Power electronic control in electrical systems 251

There are a number of significant advantages gained from the application of PWM
VSC technology into the HVDC transmission systems. These can be summarized as
follows:
1. Independent control of both real and reactive power almost instantaneously.
2. Minimum contribution of the converter to the short-circuit power.
3. Transformerless applications may be possible if the voltage handling capacity of
the semiconductors is high enough to be connected directly to the AC system.
4. The system can be connected to an AC weak grid without the presence of
generators, as the voltages are not required for the commutation of the thyristors
which are replaced by fully controlled devices.
5. Reduction of the size of the installation since the AC filters are smaller, and the
reactive power compensators are not required.
The converter is a typical six-switch three-phase VSC as shown in Figure 6.32 where
transistors are used to represent the switches. A number of IGBTs of course are
connected in series to make up one switch.
There are a number of potential applications for the HVDC systems based on
IGBT or similar technology to build the converter. For instance with the continuous
push for renewable technology systems, it is important that these applications are
exploited further. Wind farms can be located either off-shore or away from commu-
nities for various reasons including minimum environmental impact on local com-
munities, maximization of the resource available, building of large plants, etc. Such
relatively large-scale power generation can be connected to the grid via an HVDC
system with VSCs. This will potentially reduce the cost of the investment by cutting
down transmission costs. Of course the same concept can be applied to other sources
of power such as hydro. In the case of small islands, like Greece for instance, where
diesel generators are used, VSC-based HVDC technology can be used to connect
them to the mainland's grid and this way the dependency on non-renewable energy
sources can be eliminated.
Moreover, upgrading AC lines for the same power capacity can be difficult these
days mainly for environmental reasons. The AC cables from existing transmission
lines can be replaced by DC cables to increase the transmission capability of the line
without adding extra cable towers that would be difficult to build. Furthermore,
upgrading AC lines into cities' centres is costly and permits for the right of way may
be difficult to obtain. If more high-rise buildings cause power demand increases,
HVDC can be used as an alternative in competitive terms against AC power trans-
mission.
This technology is relatively new. Specifically, the first VSC based PWM HVDC sys-
tem has been in operation since March 1997 (Helljso È n project, Sweden, 3MW, 10km
distance, 10 kV). The installation of this project is shown in Figure 6.81. Another one is
the Go È tland project (Sweden) commissioned recently (50MW, 70km distance, 80 kV).
There are currently three similar projects under construction or just completed
recently as follows:

1. The HVDC connection between Texas and Mexico at Eagle Pass.
2. The DirectLink between Queensland and New South Wales in Australia
(180 MW, 65 km distance).